The hippopotamus according to MammalMAP
The hippopotamus (Hippopotamus amphibius), or hippo, from the ancient Greek for "river horse" , is a large, mostly herbivorous mammal in sub-Saharan Africa, and one of only two extant species in the family Hippopotamidae (the other is the pygmy hippopotamus). After the elephant and rhinoceros, the hippopotamus is the third largest type of land mammal and the heaviest extant artiodactyl. Despite their physical resemblance to pigs and other terrestrial even-toed ungulates, their closest living relatives are cetaceans (whales, porpoises, etc.) from which they diverged about 55 million years ago. The common ancestor of whales and hippos split from other even-toed ungulates around 60 million years ago. The earliest known hippopotamus fossils, belonging to the genus Kenyapotamus in Africa, date to around 16 million years ago.
The hippopotamus is semi-aquatic, inhabiting rivers, lakes and mangrove swamps, where territorial bulls preside over a stretch of river and groups of 5 to 30 females and young. During the day, they remain cool by staying in the water or mud; reproduction and childbirth both occur in water. They emerge at dusk to graze on grass. While hippopotamuses rest near each other in the water, grazing is a solitary activity and hippos are not territorial on land. Hippos are recognizable by their barrel-shaped torso, enormous mouth and teeth, nearly hairless body, stubby legs and tremendous size. They are the third largest type of land mammal by weight (between 1½ and 3 tonnes): the only heavier species on average are the white and Indian rhinoceroses, typically 1½ to 3½ tonnes, and the elephants, typically weighing 3 to 9 tonnes. The hippopotamus is one of the largest quadrupeds and, despite its stocky shape and short legs, it can easily outrun a human. Hippos have been clocked at 30 km/h over short distances. The hippopotamus is one of the most aggressive creatures in the world and is regarded as one of the most dangerous animals in Africa. Nevertheless, they are still threatened by habitat loss and poaching for their meat and ivory canine teeth.
- Novak, R. M. 1999. Walker's Mammals of the World. 6th edition. Johns Hopkins University Press, Baltimore.
The Hippopotamus is a massive aquatic mammal ranging from 431 to 516 cm long (14 to17 ft). The eyes and nostrils protrude, allowing the animal to see and breathe while submerged in water. The hippo’s rust-colored perspiration contains pigments that act as both a sunscreen and an antibiotic.
Hippopotamusi (Hippopotamus amphibius) are found exclusively in the Ethiopian region of the world. Hippos occur in rivers throughout the savanna of Africa and the main rivers of Central Africa. Known populations are found in countries including: Angola, Benin, northern Botswana, Burkina Faso, Burundi, Cameroon, Central African Republic, southern Chad, Côte d’Ivoire, northern Eritrea, Ethiopia, Equatorial Guinea, Gabon, Gambia, Ghana, Guinea, Guinea Bissau, Kenya, Liberia, Malawi, Mozambique, Namibia, Niger, Nigeria, Republic of Congo, Rwanda, Senegal, Sierra Leone, Somalia, South Africa (only in northern and eastern Limpopo Province, eastern Mpumalanga Province, and northern KwaZulu-Natal), Sudan, Swaziland, Tanzania, Togo, Uganda, Zambia, and Zimbabwe.
Biogeographic Regions: ethiopian (Native )
- Estes, R. 1992. The Behavior Guide to African Mammals: Including Hoofed Mammals, Carnivores, Primates. Los Angeles, California: University of California Press.
- Lewison, R., W. Oliver. 2008. "Hippopotamus amphibius" (On-line). IUNC Red List of Threatened Species. Accessed February 01, 2012 at http://www.iucnredlist.org/apps/redlist/details/10103/0.
- Pushkina, D. 2007. The Pleistocene easternmost distribution in Eurasia of the species associated with the Eemian Palaeoloxodon antiquus assemblage.. Mammal Review, 37/3: 224-245.
- Stevenson-Hamilton, J. 1912. Animal Life in Africa. New York: E.P. Dutton and Company.
The Common Hippopotamus was already rare in Egypt by the time of the Renaissance. From the end of the Roman Empire up until towards 1700 at the latest, the Hippo was still present in two well disjunct zones in the Nile Delta and in the upper Nile. Through the 1700s, records become increasingly scarce, and the latest definite records are from the early 1800s (Manlius 2000).
Weighing between 1,300 and 3,200 kg, hippopotamusi measure between 209 and 505 cm in length, including a tail of about 35 cm in length. They stand between 150 and 165 cm tall. Hippopotami have skin tones of a purple gray or slate color, with brownish pink coloring around their eyes and ears. Their bodies are covered with a scarce amount of thin hair, except for thick bristle like hair on their heads and tails. Outer epidermal skin layers are extremely thin, making them vulnerable to wounds from fighting.
Hippos lack scent and sweat glands. Instead, mucous glands secrete a thick oily layer of red pigmented fluid. For years this fluid was thought to be a mixture of blood and sweat, giving it the nickname “blood sweat.” It is now known that this fluid is a combination of hipposudoric acid and norhipposudoric acid. These compounds create a sunscreen effect by absorbing ultra violet rays from the sun and prevent the growth of disease causing bacteria. The secretion originates colorless and turns an orange-red within minutes of being exposed to the sun.
Bulky and barrel-shaped, it would seem hippos would be clumsy on both land and water. However, adaptations to their semi-aquatic environments have allowed them to move swiftly on both water and land. On land, they are able to move at speeds up to 30 km per hour and can maintain these speeds for several hundred meters. In shallow waters their short legs provide powerful propulsion through water, while their webbed feet allow them to navigate on shallow river bottoms. Placement of eyes, ears, and nostrils high on their head allows them to remain mostly submerged while still being able to breathe and stay aware of their surroundings. When completely submerging, the nostrils close and ears fold to prevent water from entering them. The jaws of hippopotamusi are capable of opening up to 150 degrees, showing enormous, sharp, incisors and canine tusks. Canines grow to 50 cm and incisors grow to 40 cm, sharpening themselves as they grind their mouths together during grazing.
Sexual dimorphism is present in hippopotami. Males tend to be about 200 kg larger than females at maturity, but can grow to be almost several thousand kg larger with age. Males appear to continue growing throughout their life, while females reach their maximum weight around age 25. Males reach a maximum length of about 505 cm long, while females usually only reach around 345 cm. The largest hippo ever recorded was a captive male hippo in Munich, Germany, weighing 4,500 kg. Aside from size, male muzzles are larger and have a more developed jowl area than females. Tusks are also twice as long in males than in females.
Range mass: 1300 to 4500 kg.
Range length: 290 to 505 cm.
Sexual Dimorphism: male larger
Other Physical Features: endothermic ; bilateral symmetry
- Coughlin, B., F. Fish. 2009. Hippopotamus underwater locomotion: Reduced gravity movements for a massive mammal. Journal Of Mammalogy, 90/3: 675-679.
- Nott, J. 1886. Wild Animals Potographed and Described. London: S. Low, Marston, Searle, & Rivington.
Hippos are a semi-aquatic mammal, usually inhabiting shallow lakes, rivers, and swamps. The water must be deep enough for the hippo to submerge its entire body in; usually water about 2 meters deep is preferred. During the daytime, herds prefer to sleep in shallow water, or occasionally on a mud bank, grouped closely together. It is in these waters that mating and childbirth occurs. When shallow waters are not present hippos reside in deeper water, leaving only their nostrils above the surface to breathe. Hippos emerge from water at dusk and go ashore to feed, and travel individually down familiar paths usually less than 1.6 km to dense, grassy grazing areas along the banks of the water.
Average elevation: 2000 m.
Range depth: 1.5 to 14 m.
Average depth: 2 m.
Habitat Regions: temperate ; tropical ; freshwater
Aquatic Biomes: lakes and ponds; rivers and streams; temporary pools
- 2011. Hippopotamus. Pp. electronic database in P Lagassé, ed. Columbia Electronic Encyclopedia, Vol. 1, 6th Edition. Columbia University: Columbia University Press. Accessed February 09, 2012 at http://www.encyclopedia.com/topic/hippopotamus.aspx.
- Roomer, A. 1872. Anecdotal and Descriptive Natural History: The Hippopotamus. London: Groombridge.
Habitat and Ecology
The water body must be large enough to accommodate a number of animals for the Common Hippo is highly gregarious when resting by day. The social habits of the species have been studied by Klingel (1991), who found that the "schools" are unstable groups of females and bachelors. The social system is based on mating territoriality. Common Hippos are gregarious, social, polygymous animals. Females become sexually mature between the ages of 7–9, and males 9–11. Females typically bear a single offspring every other year as lactation can extend for 18 months. Territorial males monopolize a length of the shoreline of the river or lake but tolerate bachelors within the territory provided they behave submissively. Non-breeding males also settle outside territorial areas, especially seasonal wallows. Fights for the possession of a territory can be fierce and the animals may inflict considerable damage on each other with their huge canines but minor conflicts are usually settled by threat displays, of which the "yawn" is the most conspicuous. Territorial males do not normally fight each other and severe fights usually occur only when a bachelor challenges a territorial male for control of its territory. There is little association between animals when they are feeding at night, except between females and their dependent young, and the males do not then behave in a territorial fashion.
The male Common Hippopotamus, rarely the female, spreads its dung by wagging its tail vigorously while defecating, both in the water and on land, where it is thought to have a signalling rather than a territorial function. The dung piles may serve for orientation.
Vocalizations take the form of complex bellows and grunts, which presumably have a signalling function. Sounds may be made either on land or in the water and may be transmitted simultaneously through air and water. This is the only known case of amphibious calls in a mammal.
It is probable that the need to avoid the direct rays of the sun has determined the nocturnal feeding habits of the animal. It leaves its wallow soon after sunset and spends the night grazing on short grass swards for up to several kilometres from water. These swards, which are kept short by the activities of the hippopotamus, are known as hippo lawns. Although the hippopotamus grazes every night, except for mothers with very young calves, there are usually animals present in the water all night, as some return after a few hours and others leave later. The animal feeds by plucking the grass with its wide, muscular lips and passing it to the back of the mouth to be ground up by the molars. The front teeth (incisors and canines) play no part in feeding. The amount of food ingested is small relative to the size of the animal but its resting habits by day reduce its energetic demands. The stomach is a complex four-chambered structure with a ruminant type digestion although the animal does not chew the cud.
The ecological requirements for hippopotamus, therefore, include a supply of permanent water, large enough for the territorial males to spread out, and adequate grazing on open grassland within a few kilometres of the daytime resting sites.
Hippopotamuses were once found throughout sub-Saharan Africa, but most populations have been reduced or exterminated. Currently, the only large populations occur in the Nile River Valley of East Africa. Their preferred habitat is deep water with adjacent reed beds and grasslands.
Hippopotami leave their resting waters at dusk, moving down familiar paths, "hippo paths," to grassy areas surrounding their waterbeds. Hippos prefer to remain close to water beds, however they will travel several kilometers when food is scarce. Grazing lasts between four and five hours each night, covering between 3 and 4 km in circular patterns. Their mainly folivorous diet consists of small shoots, grasses, and reeds. They do not dig for roots or fruits. However, they will consume many other types of plants if they are present. Muscular lips, 50 cm wide, are ideal to pull up grasses. Hippos do not use their teeth to chew their meals, and instead they tear and soften the grasses to prevent any nutritive loss. While their sedentary lifestyle allows for a simple diet, they are known to consume an enormous amount of food each night, 1-1.5% of their body weight (usually around 40 kg of food). Hippos enter and exit their water pools at the same spot, returning from grazing before dawn. Occasionally, if the hippo has traveled too far it will seek a nearby pool to rest in until the following nightfall. Hippos have been observed, on a few occasions, consuming dead animals near their resting pools. However, their stomachs are not suited to digest meat. It is possible that this carnivorous behavior is induced by illness or nutritional deficiencies.
Animal Foods: carrion
Plant Foods: leaves; wood, bark, or stems; flowers
Primary Diet: herbivore (Folivore )
Because of their massive size, hippos play an important role in their ecosystems. Daily activities both in and out of water create habitats for smaller organisms. The formation of hippo paths from water to land clears avenues that water can flow through during wet seasons. Flooding of these paths creates most of the lagoons and side pools that small fish retreat to during droughts.
As with all mammals, several forms of parasites affect hippopotami on both the inside and outside of their body. Ectoparasites are parasites found feeding outside of the hippo’s body. Oculotrema hippopotami, monogene flukes, live on the outer surface of a hippopotami’s eye. They attach to the inner edge of the nictitatine membrane and under they eyelid of hippopotami. Although the fluke does not cause serious harm to the eye, the areas affected become extremely irritated. Leeches and ticks are commonly found around the anal region of the hippo. This is not unusual due to their aquatic lifestyle. Other than the loss of blood and irritation around the attachment site, there is no serious damage caused by these parasites. Endoparasites are many. Flatworms and Buxifrons buxifrons are commonly found in the stomach and first 1.5 meters of the small intestine. A tapeworm parasitizes the gut as adults, but occur as cysts in the muscles when in the larval phases. Liver flukes are most commonly found in the livers of young hippopotami, suggesting hippos acquire immunity to the parasite with age. Members of the blood fluke family inhabit the hepatic portal system and the pelvic veins of hippopotami. These flukes lay their eggs in capillaries, which then migrate to the intestine or bladder to be passed outside of the body. Once hatched, the "miracidium" larva penetrates multiple species of freshwater snails and transform into "cercaria" larva. These larva are then released back into the water, where they are then able to penetrate the skin of the hippo.
Ecosystem Impact: creates habitat; keystone species
- Eltringham, S. 1999. The Hippos. Soho Square, London: T and A.D. Poyser Ltd.
- Mosepele, K., P. Moyle, G. Merron, D. Purkey, B. Mosepele. 2009. Fish, floods, and ecosystem engineers: Aquatic conservation in the Okavango Delta, Botswana. Bioscience, 59/1: 53-64.
Occasionally lions, hyenas, and crocodiles will prey on young hippopotami. Besides humans, adult hippopotami have no known predators.
Anti-predator Adaptations: aposematic
Life History and Behavior
Hippos are extremely social and have a large repertoire of surface and underwater sounds. The honking call made by submerged hippos is the most common type of communication, using exhaled breath to alarm their herd of threats. This honking can reach up to 115 decibels, equivalent to a loud thunder, and when conducted by the lead male can create a chorus from other males up to a 1.6 km away. Vocalizations can be made on land or in water and are transmitted simultaneously through both. This is the only known case of amphibious calls in mammals. Sound heard above the surface comes from the hippos nostrils, but is made in the larynx underwater. Hippos have a large fat layer across the larynx that vibrates when the vocalization is made, sending sound throughout the water. Underwater vibrations shake jaw tissues connected the skull and ear of other hippopotami, transmitting the sound.
Visual displays such as wheezing, yawning, honking, and dung showering are common territorial displays. Male hippos often emerge from the water to spread dung along the shoreline or along their grazing paths to mark their territory. Besides smell reception of urine and dung showering, hippos also use the vomeronsal organ, operating like an underwater syringe to draw in urine, to communicate the reproductive status of a male or female
Communication Channels: visual ; tactile ; acoustic ; chemical
Other Communication Modes: choruses ; pheromones ; vibrations
Perception Channels: visual ; tactile ; acoustic ; vibrations ; chemical
Although not strictly nocturnal, hippopotamuses typically forage for grasses at night along river banks and spend the day in water digesting their food, sleeping, and socializing. Although their diet mainly comprises grasses, isolated incidences of scavenging have been observed.
Hippopotamus herds of 10-15 individuals are primarily made up of females and their young, headed by adominant adult male or bull. Bulls vie for control of these herds, using their long canine teeth in intense battles that, not in frequently, result in the serious injury or death of a combatant.
Hippopotami have average life expectancies around 55 years in both captivity and the wild. The longest living hippopotamus exceeded 61 years in captivity. Infant mortality rates are low, 0.01 deaths per year.
Status: captivity: 61 years.
Status: wild: 55 years.
Status: captivity: 55 years.
Status: wild: 45.0 years.
Status: captivity: 49.0 years.
Status: wild: 50.0 years.
Status: captivity: 40.0 years.
Status: captivity: 54.3 years.
Status: wild: 54.5 years.
Lifespan, longevity, and ageing
Hippopotami are polygynous, meaning that one bull mates with several females in the social group. Although breeding is not strictly seasonal, conception usually occurs during the dry season, between February and August, and births usually occur during the rainy season, between October and April.
Mating System: polygynous
When searching for a mate, the dominant male wanders through a resting or grazing herd, smelling each female’s posterior end. He acts unusually submissive towards the females as to avoid being attacked by the herd. The goal of the submissive male is to find a female in heat. Once a mate has been found the courtship begins. The male taunts the female, by pushing her out of the herd. He pursues her into deeper waters, until she becomes frustrated and lashes out and clashing jaws with him. He forces her into submission and mounts her, forcing her head under the water. It is unclear why her head must be under the water. If the female tries to raise her head to breathe, the male will usually snap at her and force her head back down. During mating the males release a wheezy honking to announce that mating has occurred. Hippopotami usually mate every other year, due cost of parental investment into the young. Although breeding can occur year round, it is most common between February and August. Gestation lasts nearly a year, 324 days, and yields just one offspring. The young are not weaned for nearly another year, and maturity is achieved at 3.5 years.
Breeding interval: Hippopotami mate every other year.
Breeding season: Breeding occurs year round, but peaks between February and August.
Average number of offspring: 1.
Average gestation period: 324 days.
Average weaning age: 341 days.
Average age at sexual or reproductive maturity (female): 1,279 days.
Average age at sexual or reproductive maturity (male): 1,279 days.
Key Reproductive Features: iteroparous ; seasonal breeding ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; viviparous
Average birth mass: 40000 g.
Average gestation period: 234 days.
Average number of offspring: 1.
Average age at sexual or reproductive maturity (male)
Sex: male: 1279 days.
Average age at sexual or reproductive maturity (female)
Sex: female: 1279 days.
Prior to birth, pregnant females become very protective and aggressive to those who encounter her. Pregnant hippos isolate themselves on land or in shallow water and do not rejoin the herd until 14 days after giving birth. At birth, calves weigh between 22 and 55 kg. Mother and calf have a very close relationship. They are often seen cleaning, cuddling, and presumably showing affection to each other. Most attacks on humans and other animals are by females prior to birthing or when protecting their calves. Calves are adapted for underwater nursing; their ears fold back and their nostrils close during sucking, gripping the nipple between the tongue and roof of the mouth. Because hippos live in a social family environment, males are very protective over both the females and calves in the herd and will often attack anything that poses a threat.
Parental Investment: precocial ; male parental care ; female parental care ; pre-weaning/fledging (Protecting: Male, Female); pre-independence (Protecting: Male, Female); post-independence association with parents; inherits maternal/paternal territory
- Estes, R. 1992. The Behavior Guide to African Mammals: Including Hoofed Mammals, Carnivores, Primates. Los Angeles, California: University of California Press.
- Roomer, A. 1872. Anecdotal and Descriptive Natural History: The Hippopotamus. London: Groombridge.
- Stevenson-Hamilton, J. 1912. Animal Life in Africa. New York: E.P. Dutton and Company.
- de Magalhaes, J., J. Costa. 2009. A database of vertebrate longevity records and their relation to other life-history traits. Journal of Evolutionary Biology, 22/8: 1770-1774.
Evolution and Systematics
A secretion of the hippopotamus protects its skin from the sun and bacteria thanks to two pigments that absorb UV light and have antibiotic properties.
"The rust-colored perspiration of the hippopotamus does more than keep the animal cool. Hippo sweat contains pigments that act as both sunscreen and antibiotic. Researchers at Kyoto Pharmaceutical University in Japan identified two such pigments, the red hipposudoric acid, and the orange norhipposudoric acid. Both are conjugated three-ring structures. The two compounds absorb light in the UV-visible range (200-600 nm) and so are thought to protect the hippo's dermis from the sun. Additionally, low concentrations of hipposudoric acid inhibit the growth of bacteria. Both compounds are highly reactive, and tend to polymerize when removed from the hippo and/or a water source. An unknown agent in hippo mucus keeps the compounds from polymerizing for several hours, even after the hippo sweat dries." (Courtesy of the Biomimicry Guild)
"The efficient sunscreen activity of NH and HP stems from their broad absorption in the UVA and UVB regions of the spectrum." (Galasso and Pichierri 2009:2543)
"Although the fluid secreted by the hippopotamus (Hippopotamus amphibius) is not strictly sweat as it is produced by the subdermal glands, it acts like sweat in helping to control body temperature. It is also thought to be antiseptic…What is the function of these pigments as far as the hippopotamus is concerned? Their spectra in the ultraviolet/visible range (200–600 nm; see supplementary information) indicate that they may act as sunscreens. The red pigment 2 also has antibiotic activity: at concentrations lower than that found on the hippopotamus’s skin, it inhibits the growth of the pathogenic bacteria Pseudomonas aeruginosa A3 and Klebsiella pneumonia." (Saikawa et al. 2004:363)
Learn more about this functional adaptation.
- Galasso, V; Pichierri, F. 2009. Probing the molecular and electronic structure of norhipposudoric and hipposudoric acids from the red sweat of hippopotamus amphibius: A DFT Investigation. Journal of Physical Chemistry A. 113(11): 2534-2543.
- Saikawa, Y.; Hashimoto, K.; Nakata, M.; Yoshihara, M.; Nagai, K.; Ida, M.; Komiya, T. The red sweat of the hippopotamus. Nature. 429(6990): 363.
Molecular Biology and Genetics
Barcode data: Hippopotamus amphibius
Below is a sequence of the barcode region Cytochrome oxidase subunit 1 (COI or COX1) from a member of the species.
See the BOLD taxonomy browser for more complete information about this specimen and other sequences.
-- end --
Download FASTA File
Statistics of barcoding coverage: Hippopotamus amphibius
Public Records: 3
Specimens with Barcodes: 4
Species With Barcodes: 1
According to the IUNC Redlist, there has been a 7 to 20% decline in hippopotamus populations over the past 10 years. It was estimated that in the 29 countries within its geographic range, there are only between 125,000 to 148,000 individuals. Although hunting and exploitation of hippopotami are illegal, it remains the main reason for declines in populations. This exploitation is most commonly found in areas where hippo populations are on unprotected land. Habitat loss is another reason for population declines. Hippopotami rely heavily on freshwater bodies, making them vulnerable to drought, agricultural and industrial development, and rerouting of natural water flows. There are few conservation efforts aimed at protecting hippo habitat and populations specifically. Countries where hippopotami are most common have strict hunting regulations and protected habitats, including national parks, reserves, and conservation areas.
US Federal List: no special status
CITES: no special status
State of Michigan List: no special status
IUCN Red List of Threatened Species: vulnerable
IUCN Red List Assessment
Red List Category
Red List Criteria
- 2006Vulnerable(IUCN 2006)
Status in Egypt
IUCN Red List Status: VULNERABLE
The species is not common in West Africa and the population is split into a number of small groups totalling about 7,000 spread over 19 countries. Populations most at risk are those in West Africa, where the distribution is particularly fragmented.
Hippopotamus are absent from the rain forests except near large rivers. They are most abundant in estuarine habitats and on the lower reaches of rivers. Some are found in the sea in the Archipelago of Bijagos off Guinea Bissau. Guinea, Guinea Bissau and Senegal probably contain the bulk of the West African Common Hippopotamus, with total numbers likely to be in the region of a few thousand. Although small in area, Guinea Bissau supports a substantial population, which is particularly abundant on the islands of the Bijagos Archipelago and along the numerous inland rivers. The species is common on most of the rivers in Guinea and in the east and south of Senegal with an estimated country-wide population of between 500 and 700. The Gambia contains no more than about 40 animals. There are probably less than 200 in Sierra Leone or Mali and none at all in Liberia or Mauritania.
The group of contiguous countries, Côte d’Ivoire, Ghana, Togo, Benin and Burkina Faso, contain a total of, at most, two thousand Common Hippopotamus with the majority in Burkina Faso. There have been no recent counts except on the Comoe River on the border with the Côte d’Ivoire, where 720 were recorded in 1989. A further group is found on the Pendjari River system bordering Benin. This numbered about 500 in 1979 but only some 280 remained in 1987. The Mono River between Benin and Togo supported a small but stable population of 53 in 1986. Only remnant populations remain in Ghana.
Nigeria and Niger between them contain at least 400. No recent information was obtained for Chad but according to Sidney (1965) the species was common in the vicinity of Lake Chad during the 1950s. Common Hippopotamus were also once numerous in Cameroon but the only information obtained during the present survey was from the Korup National Park, where signs of the species are common around the confluence of the Miri and Bake Rivers although sightings are few. It is likely that the species does not occur in the Bake River much further upstream than Bajo although some traces were found as far up as Bakut. At least 150 Common Hippopotamus (possibly as many as 1,500) are known to exist in the Central African Republic in addition to an unknown number in Bamimgui-Bangoran National Park, where 136 were counted in 1973 although now there are probably only 20 to 30 present. Common Hippopotamus occur along most of the coastline of Gabon and for a considerable distance up the Ogooue River and although there are no recent estimates of numbers, they are said to be abundant in places. A few are found in neighbouring Equatorial Guinea on the Campo River. No counts have been made in the Congo, but the species is reported by one correspondent to be widely distributed and numerous on suitable rivers but another reports its presence on only one, the Nyanga River. The entry for the Congo in the IUCN Directory (IUCN/UNEP, 1987) lists Odzala National Park, Lefini Reserve (Louna and Lesio Rivers), and Nyanga North Reserve as containing hippopotamus. Zaire will be considered with East Africa as most of the hippopotamus are in the east of the country.
The total number of Common Hippopotamus in the nineteen west African countries considered here cannot be assessed with any accuracy because of the absence of recent counts but the figure is likely to be in the region of 7,000.
East Africa holds substantial numbers with 30,000 in eastern DR Congo and populations numbering tens of thousands in Ethiopia, Sudan and Tanzania. Several thousand also occur in Kenya and Uganda bringing the total for East Africa as a whole to about 70,000.
Many of the Common Hippopotamus in Africa are found in the east, especially in Ethiopia, Kenya, Sudan, Tanzania, Uganda and DR Congo. The Common Hippopotamus occurs in the southern Sudan on the Rivers Nile, Sobat and Jur south of Malakal and in several national parks and reserves. Other localities include the Sudd and tributaries of the Nile. There is no information on population sizes but it is said to occur in good numbers in most places. The species is also abundant between altitudes of 200 and 2,000 m in neighbouring Ethiopia, where its main strongholds appear to be the Omo, Awash and Great Abbi (Blue Nile) Rivers. It also occurs in most of the larger lakes and as isolated populations in smaller swamps and pools. The few that occur in the dry south-east are confined to the Webi, Shebeli and Ganale Rivers. The northern limit of the species is the Setit River. No precise counts have been made recently but the Common Hippopotamus is said to be numerous throughout its range. The total for the two countries combined is probably to be numbered in tens of thousands. Very few animals remain in neighbouring Somalia although some small groups have been reported on the lower Shebeli River and along the Juba River, where they are rather more numerous. No Common Hippopotamus have been reported from Djibouti.
The species occurs in most of the many suitable habitats throughout Kenya and some recent counts have been made in the Mara River area (2,132 in 1980), Lake Naivasha (220 in 1988) and along part of the Tana River between Osako and Adamson's Falls (220 in 1983) (Coe and Collins 1986, Karstad et al. 1980, Smart, in litt). The Mara figure includes some from over the border in Tanzania. Elsewhere in Tanzania, Common Hippopotamus are common in the Selous Game Reserve, where 1,894 were counted on 115 km of the River Rufigi in 1987 (Samuels, in litt). An estimate for the total population of the Selous in 1986 was 16,900 (with a standard error 6,307) from an aerial sample count made by I. Douglas-Hamilton. Independent aerial counts in the Selous reported by Games (1990) returned figure of 15,483 in 1986, 24,169 in 1989 and 20,589 in 1990. The last total is a rather crude extrapolation from an observed figure of 6,866. A large population occurs on the Akagera River and associated lakes on the border between Tanzania and Rwanda, but no recent count has been made. The total counted from the air in 1969 was 671 (Spinage et al. 1972). Common Hippopotamus are found in most other national parks and reserves of Tanzania and although not present anywhere in large numbers, the total probably amounts to several thousand more.
The principal concentrations of the species in Uganda are in the two large national parks, Murchison Falls and Queen Elizabeth. At one time the population in the latter park reached 21,000, but this was reduced to about 14,000 in the culling programme of the 1950s. Counts in the early 1970s returned about 11,000 but heavy poaching during the Amin years had left only a couple of thousand by 1989 when 2,172 were estimated from an aerial sample count. Similar numbers were found in the Murchison Falls National Park in the past but there, too, heavy poaching has reduced the population to remnant numbers although a recent count has not been made. The latest appears to have been in 1980 when 1,202 were recorded on the Nile between the falls and Paraa Lodge. The total for the whole park is probably about the same as in Queen Elizabeth National Park i.e., a few thousand. Other regions in Uganda where substantial numbers of hippopotamus occurred include the Semliki River and lakes Victoria and Kyoga. An educated guess of about 7,000 for the present total population of hippopotamus in the whole country is probably not far wrong.
Common Hippopotamus have a wide distribution in DR Congo including some in the north-west of the country although the bulk is in the east, where they occur around Epulu and Wamba and along some of the larger rivers in the Ituri Forest. Other populations occur on the Zaire River (Yangabi), Bomu River and elsewhere in several national parks including Garamba, Kundelungu, Salonga, Upemba and Virunga. The latter contains the greatest concentration with a total of 22,875 estimated from a 1988 aerial count made by C. Mackie, who with K. Hillman Smith also recorded 2,851 in Garamba National Park in 1988. In round figures, these counts suggest a total of some 26,000 Common Hippopotamus for the two parks. Numbers elsewhere in DR Congo probably do not amount to more than a few thousand, perhaps bringing the country-wide total up to about 30,000.
There are not many Common Hippopotamus in the remaining East African countries of Rwanda and Burundi. Numbers on the Akagera River have been mentioned above in the section on Tanzania and there are probably still a few in wallows within the Akagera National Park or Mutara Game Reserve but no recent information has been received. Common Hippopotamus occur in Burundi on the Malagarazi, Ruvubu and Rusizi Rivers but there are conflicting reports over numbers. P. Chardonnet reports good populations numbered in hundreds and P. C. Trenchard puts the total on these rivers as over 1,000 as a conservative estimate. K. M. Doyle, however, casts doubt on these figures, for along a 120 km stretch of the Ruvubu River where several hundred were reported by P. Chardonnet, he recorded only 39 animals, all but two within the Ruvubu National Park, although there may have been more in wallows etc. away from the river, which were not surveyed.
Although there are many gaps in the data, the above analysis suggests that there could be as many as 70,000 Common Hippopotamus in the east African countries.
Southern Africa also has flourishing populations, with Zambia containing the biggest population, 40,000, of any country in Africa. Others with large numbers include Mozambique (16,000–20,500), Malawi (10,000), Zimbabwe (6,900) and South Africa (5,000). The total in the whole of the region may be around 80,000.
No information has been received from Angola. According to Sidney (1965), the Common Hippopotamus was widespread throughout Angola particularly in the east on the Cunene, Cubango, Cuando, Cuanza, Longa and Zambezi Rivers.
There are probably more Common Hippopotamus in Zambia than in any other single country. F. E. C. Munyenyembe puts the country-wide total at 40,000 with 20-25,000 in the Luangwa Valley according to R. H. V. Bell. They are reported to be widespread on the Kafue Flats and in Lochinvar National Park. Neighbouring Malawi, although small, is also densely populated with Common Hippopotamus, which occur on all rivers and lakes of sufficient size. The main concentrations are at Elephant Marsh (lower Shire River), the south-west arm of Lake Malawi, Upper Shire River and Lake Malombe in Liwonde National Park. R. H. V. Bell makes a guess that there are some 10,000 Common Hippopotamus in the whole of Malawi. Further south in Zimbabwe, the species is still common. It is found on most of the large rivers particularly the Limpopo. Zambezi and the Sabi/Lundi systems. It is also found in smaller rivers and dams where there is permanent water. Some wander over long distances to provide isolated records. The only estimate for the country-wide total is that made by R. B. Martin on the basis of some limited counts, which have revealed some dense populations e.g. 2,000 on a 50-km section of the Zambezi. His estimate is 6,900, of which 5,530 occur in national parks or reserves, 1,020 on communal lands and 350 elsewhere.
A surprising number of Common Hippopotamus appear to have survived in Mozambique, at least up to 1986, despite the recent civil strife. The species is still widely distributed throughout the country and is present on most river systems. Several national parks and reserves contain hippopotamus although only Gorongosa, with about 2,000, has a sizeable population. L. Tello's estimate made in 1986 year puts the total at between 16,000 and 20,500 for the country as a whole with most (10,000 -12,000) in the Zambezi Wildlife Utilization Area, which includes Marromeu Reserve and four safari hunting blocks. It is also contiguous with the Gorongosa National Park. This is the only region where numbers have increased (by some 20% since 1974). Elsewhere there has been a decline, except in Tete Province, whose population of between 1,500 and 2,500 is said to be stable.
Namibia is too dry to support many Common Hippos except in the north, where the species is present in some numbers on the Cuando and Zambezi Rivers in the Caprivi Strip. Elsewhere it occurs along the boundary with Angola on the Okavango River. Botswana is also too dry, except in the north of the country, where some animals occur in the Okavango Delta and in the Chobe/Linyati River system. A few (18+) exist on the Limpopo in the east. Outside this area, a small population may still exist near Ghanzi although some observers think this is unlikely. C. A. Spinage puts the total in northern Botswana at 1,600 in the wet season and 500 in the dry.
Common Hippopotamus are confined to the north-east of the country in the Republic of South Africa, mainly in the Limpopo, Mpumalanga and North West provinces and the northern tip of KwaZulu-Natal. Most of them are in the Kruger National Park in perennial rivers, dams and the larger pools of seasonal rivers. The total counted in the park in 1989 was 2,761 with 2,575 in rivers and 191 in dams and pools. R. H. Taylor gives a total (for 1986) of 1,264 for KwaZulu-Natal, with the largest concentration (595) on Lake St Lucia, but he suggests a better estimate of 1,423 averaged over the five years 1982-1986. Those in kwaZulu-Natal outside the Kruger National Park are mainly confined to the large rivers in the eastern and northern regions of the province. These figures suggest that there are approaching 5,000 Common Hippopotamus in the country as a whole.
It is not possible to provide a total for the whole of southern Africa because of the lack of data from Angola, which used to support large populations and may do so still, although the disturbed political situation in the country makes it more likely that most hippopotamus have been shot. Assuming the worst and that only a few hundred remain in Angola, a very rough estimate for the regional total would be 80,000.
Follow link below for Table 1: country information including population status, trend, etc.
Estimates of the amount of Common Hippo ivory illegally exported have also increased. A 1994 assessment by TRAFFIC, the monitoring agency of international trade for the IUCN, reported that illegal trade in hippo ivory increased sharply following the international elephant ivory ban in 1989. Between 1991-1992, approximately 27,000 kg of hippo canine teeth were exported, an increase of 15,000 kg from 1989–1990 estimates (Weiler et al. 1994). In 1997, more than 1,700 hippo teeth en route from Uganda to Hong Kong were seized by customs officials in France (TRAFFIC 1997). Five thousand kilos of hippo teeth (from an estimated 2,000 hippos) of unknown origins were exported from Uganda in 2002 (New Vision 2002).
Common Hippo’s reliance on fresh water habitats appears to put them at odds with human populations and adds to their vulnerability, given the growing pressure on fresh water resources across Africa (WWC 2004). Habitat loss stems from water diversion related to agricultural development (Cole 1992; Jacobsen and Kleynhaus 1993; Viljoen 1995; Viljoen and Biggs 1998) as well as larger-scale development in and around wetland areas (Jacobsen and Kleynhaus 1993). Reports of human mortalities from Common Hippo interactions have also increased in recent years. Ten countries reported growing numbers of hippo-human conflicts, in several cases exacerbated by drought conditions.
Although there are several ongoing research projects in captive facilities and with wild populations, little research has focused directly on common hippo conservation. Mwanika et al. (2003) considered the genetic consequences of the intense unregulated hunting that occurred in Uganda in the late 1970s and early 1980s. Based on both nuclear and mitochondrial data, they conclude that although populations were reduced to 70% of initial population size, their current levels of genetic diversity are substantial and not a cause for concern. This suggests that for some populations, once the hunting disturbance is removed, recovery from intense hunting is likely and may not result in detrimental long-term population effects.
Lewison (2007) evaluates the relative impacts of the known threats to persistence—habitat loss (from agricultural or larger-scale development) and hunting pressure—on a model population. While accounting for rainfall variability and demographic stochasticity, the model results suggest that combinations of habitat loss and even moderate levels of adult mortality from hunting (1% of adults) can lead to a relatively high probabilities of population declines over the next 30–40 years.
Follow link below for Table 1: country information including population status, trend, etc.
Major threats to this species include loss of wetlands and grazing lands to agriculture and settlement, and poaching. In the Democratic Republic of Congo, populations have been decimated by unregulated hunting for meat and hippo teeth, which are used as ivory.
There are numerous protected areas across the countries where Common Hippos are found. Although in most countries the official level of protection is good, the level of enforcement of these regulations is poor in many countries. In some countries, Common Hippos are still found outside of protected areas.
Relevance to Humans and Ecosystems
Hippopotami are usually very docile animals. However, when threatened, especially during drought-burdened seasons, attacks on humans occasionally occur. Accidental encounters with hunters, boaters, and developers occurs usually because the individuals are unaware of a population's presence and can leads to human fatality.
Negative Impacts: injures humans
Hippopotami are extremely valuable to native hunters, providing a substantial amount of meat and valuable tusks and hide. Their thick hide is used in making shields and elastic whips. Canine tusks contain ivory, and is illegally sold on the black market.
Positive Impacts: food ; body parts are source of valuable material
- Wilson, A. 1882. Wild Animals and Birds: Their Haunts and Habits. London, Paris, New York: Cassell, Petter, Galpin & Co..
The common hippopotamus (Hippopotamus amphibius), or hippo, is a large, mostly herbivorous mammal in sub-Saharan Africa, and one of only two extant species in the family Hippopotamidae, the other being the pygmy hippopotamus (Choeropsis liberiensis or Hexaprotodon liberiensis). The name comes from the ancient Greek for "river horse" (ἱπποπόταμος). After the elephant and rhinoceros, the common hippopotamus is the third-largest type of land mammal and the heaviest extant artiodactyl. Despite their physical resemblance to pigs and other terrestrial even-toed ungulates, their closest living relatives are cetaceans (whales, porpoises, etc.) from which they diverged about . The common ancestor of whales and hippos split from other even-toed ungulates around . The earliest known hippopotamus fossils, belonging to the genus Kenyapotamus in Africa, date to around .
Common hippos are recognizable by their barrel-shaped torsos, wide-opening mouths revealing large canine tusks, nearly hairless bodies, columnar-like legs and large size; adults average 1,500 kg (3,300 lb) and 1,300 kg (2,900 lb) for males and females respectively. Despite its stocky shape and short legs, it is capable of running 30 km/h (19 mph) over short distances. The hippopotamus is a highly aggressive and unpredictable animal and is ranked among the most dangerous animals in Africa. Nevertheless, they are still threatened by habitat loss and poaching for their meat and ivory canine teeth.
The common hippopotamus is semiaquatic, inhabiting rivers, lakes and mangrove swamps, where territorial bulls preside over a stretch of river and groups of five to 30 females and young. During the day, they remain cool by staying in the water or mud; reproduction and childbirth both occur in water. They emerge at dusk to graze on grasses. While hippopotamuses rest near each other in the water, grazing is a solitary activity and hippos are not territorial on land.
- 1 Etymology
- 2 Taxonomy and origins
- 3 Description
- 4 Distribution
- 5 Behavior
- 6 Hippos and humans
- 7 References
- 8 External links
The word "hippopotamus" is derived from the ancient Greek ἱπποπόταμος, hippopotamos, from ἵππος, hippos, "horse", and ποταμός, potamos, "river", meaning "horse of the river". In English, the plural is hippopotamuses, but "hippopotami" is also used; "hippos" can be used as a short plural. Hippopotamuses are gregarious, living in groups of up to 30 animals. A group is called a pod, herd, dale, or bloat.
In Africa, the hippo is known by various names, including seekoei (Afrikaans), mvuvu (Venda), kubu (Lozi) and mvubu (Xhosa, Siswati and Zulu) in the south; kiboko (Swahili), ensherre (Nkore), tomondo (Turu), nvubu (Luganda), ifuru (Luhya), emiria (Ateso), magawit (Sebei), kibei (Kalenjin) and olmakau (Maasai) in the African Great Lakes region;:256 and ጉማርረ/gumarre (Amharic) and jeer (Somali) in the Horn of Africa.
Taxonomy and origins
The hippopotamus is the type genus of the family Hippopotamidae. The pygmy hippopotamus belongs to a different genus in Hippopotamidae, either Choeropsis or Hexaprotodon. Hippopotamidae are sometimes known as hippopotamids. Sometimes, the subfamily Hippopotaminae is used. Further, some taxonomists group hippopotamuses and anthracotheres in the superfamily Anthracotheroidea.:39 Hippopotamidae are classified along with other even-toed ungulates in the order Artiodactyla. Other artiodactyls include camels, cattle, deer and pigs, although hippopotamuses are not closely related to these groups.
- H. a. amphibius – (the nominate subspecies) which stretched from Egypt, where they are now extinct, south up the Nile River to Tanzania and Mozambique
- H. a. kiboko – in Kenya in the African Great Lakes region, and in Somalia in the Horn of Africa. Broader nasals and more hollowed interorbital region
- H. a. capensis – from Zambia to South Africa, most flattened skull of the subspecies
- H. a. tschadensis – throughout Western Africa to, as the name suggests, Chad, slightly shorter and wider face, with prominent orbits
- H. a. constrictus – in Angola, the southern Democratic Republic of Congo and Namibia, named for its deeper preorbital constriction
The suggested subspecies were never widely used or validated by field biologists; the described morphological differences were small enough that they could have resulted from simple variation in nonrepresentative samples.:2 Genetic analyses have tested the existence of three of these putative subspecies. A study examining mitochondrial DNA from skin biopsies taken from 13 sampling locations, considered genetic diversity and structure among hippo populations across the continent. The authors found low, but significant, genetic differentiation among H. a. amphibius, H. a. capensis, and H. a. kiboko. Neither H. a. tschadensis nor H. a. constrictus has been tested.
Until 1909, naturalists grouped hippos with pigs, based on molar patterns. Several lines of evidence, first from blood proteins, then from molecular systematics and DNA  and the fossil record, show that their closest living relatives are cetaceans – whales, dolphins and porpoises. The common ancestor of hippos and whales branched off from Ruminantia and the rest of the even-toed ungulates; the cetacean and hippo lineages split soon afterwards.
The most recent theory of the origins of Hippopotamidae suggests that hippos and whales shared a common semiaquatic ancestor that branched off from other artiodactyls around  This hypothesized ancestral group likely split into two branches around . One branch would evolve into cetaceans, possibly beginning about , with the protowhale Pakicetus and other early whale ancestors collectively known as Archaeoceti, which eventually underwent aquatic adaptation into the completely aquatic cetaceans. The other branch became the anthracotheres, a large family of four-legged beasts, the earliest of which in the late Eocene would have resembled skinny hippopotamuses with comparatively small and narrow heads. All branches of the anthracotheres, except that which evolved into Hippopotamidae, became extinct during the Pliocene without leaving any descendants..
A rough evolutionary lineage can be traced from Eocene and Oligocene species: Anthracotherium and Elomeryx to the Miocene species Merycopotamus and Libycosaurus and the very latest anthracotheres in the Pliocene. Merycopotamus, Libycosaurus and all hippopotamids can be considered to form a clade, with Libycosaurus being more closely related to hippos. Their common ancestor would have lived in the Miocene, about . Hippopotamids are therefore deeply nested within the family Anthracotheriidae. The Hippopotamidae are believed to have evolved in Africa; the oldest known hippopotamid is the genus Kenyapotamus, which lived in Africa from 16 to . While hippopotamid species spread across Asia and Europe, no hippopotamuses have ever been discovered in the Americas, although various anthracothere genera emigrated into North America during the early Oligocene. From 7.5 to , an ancestor to the modern hippopotamus, Archaeopotamus, lived in Africa and the Middle East.
While the fossil record of hippos is still poorly understood, the two modern genera, Hippopotamus and Choeropsis (sometimes Hexaprotodon), may have diverged as far back as . Taxonomists disagree whether or not the modern pygmy hippopotamus is a member of Hexaprotodon – an apparently paraphyletic genus, also embracing many extinct Asian hippopotamuses, that is more closely related to Hippopotamus – or of Choeropsis, an older and basal genus.
Three species of Malagasy hippopotamus became extinct during the Holocene on Madagascar, one of them within the past 1,000 years. The Malagasy hippos were smaller than the modern hippopotamus, likely through the process of insular dwarfism. Fossil evidence indicates many Malagasy hippos were hunted by humans, a likely factor in their eventual extinction. Isolated members of Malagasy hippopotamus may have survived in remote pockets; in 1976, villagers described a living animal called the kilopilopitsofy, which may have been a Malagasy hippopotamus.
Two species of hippopotamus, the European hippopotamus (H. antiquus) and H. gorgops, ranged throughout continental Europe and the British Isles. Both species became extinct before the last glaciation. Ancestors of European hippos found their way to many islands of the Mediterranean during the Pleistocene. The Pleistocene also saw a number of dwarf species evolve on several Mediterranean islands, including Crete (H. creutzburgi), Cyprus (H. minor), Malta (H. melitensis), and Sicily (H. pentlandi). Of these, the Cyprus dwarf hippopotamus, survived until the end of the Pleistocene or early Holocene. Evidence from an archaeological site, Aetokremnos, continues to cause debate on whether or not the species was encountered, and was driven to extinction, by man.
Hippopotamuses are among the largest living land mammals being only smaller than elephants and some rhinoceroses. Head-and-body length is from 2.8 to 4 m (9 ft 2 in to 13 ft 1 in), a tail of about 35 to 50 cm (14 to 20 in) and shoulder height averages about 1.4 to 1.5 m (4 ft 7 in to 4 ft 11 in). Mean adult weight is around 1,500 kg (3,300 lb) and 1,300 kg (2,900 lb) for males and females respectively, very large males can reach 2,000 kg (4,400 lb) and an exceptional male weighting almost 2,700 kg (6,000 lb) has been reported. Male hippos appear to continue growing throughout their lives while females reach maximum weight at around age 25.
Different from all other large land mammals, hippos are of semiaquatic habits, spending the day in lakes and rivers.:3 The eyes, ears, and nostrils of hippos are placed high on the roof of their skulls. This allows these organs to remain above the surface while the rest of the body submerges.:259 Their barrel-shaped bodies have graviportal skeletal structures,:8 adapted to carrying their enormous weight, and their specific gravity allows them to sink and move along the bottom of a river. Hippopotamuses have small legs (relative to other megafauna) because the water in which they live reduces the weight burden. Though they are bulky animals, hippopotamuses can gallop at 30 km/h (19 mph) on land but normally trot. They are incapable of jumping but do climb up steep banks. Despite being semiaquatic and having webbed feet, an adult hippo is not a particularly good swimmer nor can it float. It is rarely found in deep water; when it is, the animal moves by porpoise-like leaps from the bottom.:3 The testes of the males descend only partially and a scrotum is not present. In addition, the penis retracts into the body when not erect. The genitals of the female are unusual in that the vagina is ridged and two large diverticula protrude from the vulval vestibule. The function of these is unknown.:28–29
The hippo's jaw is powered by a large masseter and a well-developed digastric; the latter loops up behind the former to the hyoid.:259 The jaw hinge is located far back enough to allow the animal to open its mouth at almost 180°.:17 On the National Geographic Channel television program, "Dangerous Encounters with Brady Barr", Dr. Brady Barr measured the bite force of an adult female hippo at 8,100 newtons (1,800 lbf); Barr also attempted to measure the bite pressure of an adult male hippo, but had to abandon the attempt due to the male's aggressiveness. Hippopotamus teeth sharpen themselves as they grind together. The lower canines and lower incisors are enlarged, especially in males, and grow continuously. The incisors can reach 40 cm (1.3 ft), while the canines reach up to 50 cm (1.6 ft). The canines and incisors are used for combat and play no role in feeding. Hippos rely on their broad horny lips to grasp and pull grasses which are then ground by the molars.:259, 263 The hippo is considered to be a pseudoruminant, it has a complex three- or four-chambered stomach but does not "chew cud".:22
Unlike most other semiaquatic animals, the hippopotamus has very little hair.:260 The skin is 15 cm (6 in) thick, providing it great protection against conspecifics and predators. By contrast, its subcutaneous fat layer is thin.:3 The animals' upper parts are purplish-gray to blue-black, while the under parts and areas around the eyes and ears can be brownish-pink.:260 Their skin secretes a natural sunscreen substance which is red-colored. The secretion is sometimes referred to as "blood sweat", but is neither blood nor sweat. This secretion is initially colorless and turns red-orange within minutes, eventually becoming brown. Two distinct pigments have been identified in the secretions, one red (hipposudoric acid) and one orange (norhipposudoric acid). The two pigments are highly acidic compounds. Both pigments inhibit the growth of disease-causing bacteria; as well, the light absorption of both pigments peaks in the ultraviolet range, creating a sunscreen effect. All hippos, even those with different diets, secrete the pigments, so it does not appear that food is the source of the pigments. Instead, the animals may synthesize the pigments from precursors such as the amino acid tyrosine. Nevertheless, this natural sunscreen cannot prevent the animal's skin from cracking if it stays out of water too long.
A hippo's lifespan is typically 40–50 years.:277 Donna the Hippo was the oldest living hippo in captivity. She lived at the Mesker Park Zoo in Evansville, Indiana in the US until her death in 2012 at the age of 61. The oldest hippo ever recorded was called Tanga; she lived in Munich, Germany, and died in 1995 at the age of 61.
Hippopotamus amphibius was widespread in North Africa and Europe during the Eemian and late Pleistocene until about 30,000 years ago. Archaeological evidence exists of its presence in the Levant, dating to less than 3,000 years ago. The species was common in Egypt's Nile region during antiquity, but has since been extirpated. Pliny the Elder writes that, in his time, the best location in Egypt for capturing this animal was in the Saite nome; the animal could still be found along the Damietta branch after the Arab Conquest in 639. Hippos are still found in the rivers and lakes of the northern Democratic Republic of the Congo, Uganda, Tanzania and Kenya, north through to Ethiopia, Somalia and Sudan, west to Gambia, and south to South Africa. They inhabit both savanna and forest areas.
Genetic evidence suggests that common hippos in Africa experienced a marked population expansion during or after the Pleistocene epoch, attributed to an increase in water bodies at the end of the era. These findings have important conservation implications as hippo populations across the continent are currently threatened by loss of access to fresh water. Hippos are also subject to unregulated hunting and poaching. In May 2006, the hippopotamus was identified as a vulnerable species on the IUCN Red List drawn up by the World Conservation Union (IUCN), with an estimated population of between 125,000 and 150,000 hippos, a decline of between 7% and 20% since the IUCN's 1996 study. Zambia (40,000) and Tanzania (20,000–30,000) possess the largest populations.
The hippo population declined most dramatically in the Democratic Republic of the Congo. The population in Virunga National Park had dropped to 800 or 900 from around 29,000 in the mid-1970s. The decline is attributed to the disruptions caused by the Second Congo War. The poachers are believed to be Mai-Mai rebels, poorly paid Congolese soldiers, and local militia groups. Reasons for poaching include the belief that hippos are harmful to society, as well as financial gain. The sale of hippo meat is illegal, but black-market sales are difficult for Virunga National Park officers to track. Hippo meat is considered a delicacy in some areas of central Africa and the teeth have become a valued substitute for elephant ivory.
In the late 1980s, Pablo Escobar kept four hippos in a private menagerie at his residence in Hacienda Nápoles, 100 kilometres (62 mi) east of Medellín, Colombia, after buying them in New Orleans. They were deemed too difficult to seize and move after Escobar's death, and hence left on the untended estate. By 2007, the animals had multiplied to 16 and had taken to roaming the area for food in the nearby Magdalena River. In 2009, two adults and one calf escaped the herd and, after attacking humans and killing cattle, one of the adults (called "Pepe") was killed by hunters under authorization of the local authorities. As of early 2014, 40 hippos have been reported to exist in Puerto Triunfo, Antioquia from the original four belonging to Escobar. The National Geographic Channel produced a documentary about them titled Cocaine Hippos.
With the exception of eating, most of hippopotamuses' lives – from childbirth, fighting with other hippos, to reproduction – occurs in the water. Hippos leave the water at dusk and travel inland, sometimes up to 10 km (6 mi), to graze on short grasses, their main source of food. They spend four to five hours grazing and can consume 68 kg (150 lb) of grass each night. Like almost any herbivore, they consume other plants if presented with them, but their diet in nature consists almost entirely of grass, with only minimal consumption of aquatic plants. Hippos are born with sterile intestines, and require bacteria obtained from their mothers' feces to digest vegetation. Hippos have (rarely) been filmed eating carrion, usually close to the water. There are other reports of meat-eating, and even cannibalism and predation. The stomach anatomy of a hippo is not suited to carnivory, and meat-eating is likely caused by aberrant behavior or nutritional stress.:84
Hippo defecation creates allochthonous deposits of organic matter along the river beds. These deposits have an unclear ecological function. Because of their size and their habit of taking the same paths to feed, hippos can have a significant impact on the land across which they walk, both by keeping the land clear of vegetation and depressing the ground. Over prolonged periods, hippos can divert the paths of swamps and channels.
Adult hippos move at speeds up to 8 km/h (5 mph) in water; typically resurfacing to breathe every three to five minutes. The young have to breathe every two to three minutes.:4 The process of surfacing and breathing is automatic. A hippo sleeping underwater rises and breathes without waking. A hippo closes its nostrils when it submerges into the water. As with fish and turtles on a coral reef, hippos occasionally visit cleaning stations and signal, by opening their mouths wide, their readiness for being cleaned of parasites by certain species of fishes. This is an example of mutualism in which the hippo benefits from the cleaning, while the fish receive food.
Studying the interaction of male and female hippopotamuses has long been complicated because hippos are not sexually dimorphic; thus females and young males are almost indistinguishable in the field. Although hippos lie close to each other, they do not seem to form social bonds except between mothers and daughters, and they are not social animals. The reason they huddle close together is unknown.:49
Hippopotamuses are territorial only in water, where a bull presides over a small stretch of river, on average 250 m (270 yd) in length, and containing 10 females. The largest pods can contain over 100 hippos.:50 Other bachelors are allowed in a bull's stretch, as long as they behave submissively toward the bull. The territories of hippos exist to establish mating rights. Within the pods, the hippos tend to segregate by gender. Bachelors lounge near other bachelors, females with other females, and the bull on his own. When hippos emerge from the water to graze, they do so individually.:4
Hippopotamuses appear to communicate vocally, through grunts and bellows, and they may practice echolocation, but the purpose of these vocalizations is currently unknown. Hippos have the unique ability to hold their heads partially above the water and send out a cry that travels through both water and air; individuals respond above and under water.
Female hippos reach sexual maturity at five to six years of age and have a gestation period of eight months. A study of endocrine systems revealed that female hippopotamuses may begin puberty as early as three or four years of age. Males reach maturity at around 7.5 yr. A study of hippopotamus reproductive behavior in Uganda showed that peak conceptions occurred during the end of the wet season in the summer, and peak births occurred toward the beginning of the wet season in late winter. This is because of the female's estrous cycle; as with most large mammals, male hippopotamus spermatozoa is active year round. Studies of hippos in Zambia and South Africa also showed evidence of births occurring at the start of the wet season.:60–61 After becoming pregnant, a female hippopotamus will typically not begin ovulation again for 17 months.
Mating occurs in the water, with the female submerged for most of the encounter,:63 her head emerging periodically to draw breath. Baby hippos are born underwater at a weight between 25 and 50 kg (55 and 110 lb) and an average length of around 127 cm (4.17 ft), and must swim to the surface to take their first breaths. A mother typically gives birth to only one calf, although twins also occur. The young often rest on their mothers' backs when the water is too deep for them, and they swim under water to suckle. They suckle on land when the mother leaves the water. Weaning starts between six and eight months after birth, and most calves are fully weaned after a year.:64 Like many other large mammals, hippos are described as K-strategists, in this case typically producing just one large, well-developed infant every couple of years (rather than many small, poorly developed young several times per year as is common among small mammals such as rodents).
Hippopotamuses are aggressive animals. Hippos that attack other animals are often either territorial bulls or females protecting their calves. Hippopotamus coexist with a variety of formidable predators. Nile crocodiles, lions and spotted hyenas are known to prey on young hippos.:273:118 However, due to their aggression and size, adult hippopotamus are not usually predated by other animals. Cases where large lion prides or cooperating groups of Nile crocodiles have successfully preyed on adult hippopotamus have been reported, however, this is exceptionally rare. Crocodiles are frequent targets of hippo aggression, probably because they often inhabit the same riparian habitats; crocodiles may be either aggressively displaced or killed by hippopotamuses. Hippos are also very aggressive towards humans, whom they sometimes attack whether in boats or on land, commonly with no apparent provocation, and are widely considered to be one of the most dangerous large animals in Africa.
Hippos mark their territory by defecation. While depositing the faeces, hippos spin their tails to distribute their excrement over a greater area. "Yawning" serves as a threat display. When fighting, male hippos use their incisors to block each other's attacks and their large canines to inflict injuries.:260 When hippos become over-populated or a habitat is reduced, bulls sometimes attempt infantacide, but this behavior is not common under normal conditions. Incidents of hippo cannibalism have been documented, but this is believed to be the behavior of distressed or sick hippos.:82–83
Hippos and humans
The earliest evidence of human interaction with hippos comes from butchery cut marks on hippo bones at Bouri Formation dated around 160,000 years ago. Later rock paintings and engravings showing hippos being hunted have been found in the mountains of the central Sahara dated 4,000–5,000 years ago near Djanet in the Tassili n'Ajjer Mountains.:1 The ancient Egyptians recognized the hippo as a ferocious denizen of the Nile.
The hippopotamus was also known to the Greeks and Romans. The Greek historian Herodotus described the hippopotamus in The Histories (written circa 440 BC) and the Roman naturalist Pliny the Elder wrote about the hippopotamus in his encyclopedia Naturalis Historia (written circa 77 AD).
Zulu warriors preferred to be as brave as a hippopotamus, since even lions were not considered to match its courage. "In 1888, Captain Baden-Powell was part of a column searching for the Zulu chief Dinuzulu, who was leading the Usutu people in revolt against the British colonists. The column was joined by John Dunn, a white Zulu chief, who led an impi (army) of 2,000 Zulu warriors to join the British." 
The words of the Zulu anthem sounded like this:
"Een-gonyama Gonyama! Invooboo! Yah-bo! Yah-bo! Invooboo!
"John Dunn was at the head of his impi. [Baden Powell] asked him to translate the Zulu anthem his men had been singing. Dunn laughed and replied: 'He is a lion. Yes, he is better than a lion – he is a hippopotamus.'"
In the U.S., Representative Robert F. Broussard of Louisiana introduced the "American Hippo bill" in 1910 to authorize the importation and release of hippopotamus into the bayous of Louisiana. Broussard argued that the hippopotamus would eat the invasive water hyacinth that was clogging the rivers and also produce meat to help solve the American meat crisis. The chief collaborators and proponents of Broussard's bill were Major Frederick Russell Burnham and Captain Fritz Duquesne Former President Theodore Roosevelt backed the plan, as did the U.S. Department of Agriculture, The Washington Post, and The New York Times which praised the taste of hippopotamus as "lake cow bacon". The "American Hippo Bill" fell just short of being passed.
Attacks on humans
The hippopotamus is considered very aggressive and has frequently been reported as charging and attacking boats. Small boats can be capsized by hippos and and passengers can be injured or killed by the animals or drown. In one case in Niger, a boat was capsized by a hippo and 13 people were killed. As hippopotamuses will often engage in raiding nearby crops if the opportunity arises, humans may also come in conflict with them on these occasions, with potential for fatalities on both sides.
Hippos in zoos
Hippopotamuses have long been popular zoo animals. The first zoo hippo in modern history was Obaysch, which arrived at the London Zoo on May 25, 1850, where he attracted up to 10,000 visitors a day and inspired a popular song, the "Hippopotamus Polka". Hippos have remained popular zoo animals since Obaysch, and generally breed well in captivity. Their birth rates are lower than in the wild, but this is attributed to zoos not wanting to breed as many hippos as possible, since hippos are large and relatively expensive animals to maintain.:129
Like many zoo animals, hippos were traditionally displayed in concrete exhibits. In the case of hippos, they usually had a pool of water and patch of grass. In the 1980s, zoo designers increasingly designed exhibits that reflected the animals' native habitats. One of these, the Toledo Zoo Hippoquarium, features a 360,000 gallon pool for hippos. In 1987, researchers were able to record for the first time an underwater birth as in the wild at the Toledo Zoo. The exhibit was so popular, the hippos became the logo of the Toledo Zoo.
A red hippo represented the Ancient Egyptian god Set; the thigh is the "phallic leg of Set" symbolic of virility. Set's consort Tawaret was also seen as part hippo and was a goddess of protection in pregnancy and childbirth, because ancient Egyptians recognized the protective nature of a female hippopotamus toward her young. The Ijo people wore masks of aquatic animals like the hippo when practicing their water spirit cults. The Behemoth from the Book of Job, 40:15–24 is thought to be based on a hippo.
Hippos have been the subjects of various African folktales. According to a San story; when the Creator assigned each animal its place in nature, the hippos wanted to live in the water, but were refused out of fear that they might eat all the fish. After begging and pleading, the hippos were finally allowed to live in the water on the conditions that they would eat grass instead of fish and would fling their dung so that it can be inspected for fish bones. In a Ndebele tale, the hippo originally had long, beautiful hair, but was set on fire by a jealous hare and had to jump into a nearby pool. The hippo lost most of his hair and was too embarrassed to leave the water.
Ever since Obaysch inspired the "Hippopotamus Polka", hippos have been popular animals in Western culture for their rotund appearance that many consider comical. Stories of hippos such as Huberta, which became a celebrity in South Africa in the 1930s for trekking across the country; or the tale of Owen and Mzee, a hippo and tortoise which developed an intimate bond; have amused people who have bought hippo books, merchandise, and many stuffed hippo toys. Hippos were mentioned in the novelty Christmas song "I Want a Hippopotamus for Christmas" that became a hit for child star Gayla Peevey in 1953. They also feature in the songs "The Hippopotamus" and "Hippo Encore" by Flanders and Swann, with the famous refrain "Mud, Mud, Glorious Mud". They even inspired a popular board game, Hungry Hungry Hippos.
Hippos have also been popular cartoon characters, where their rotund frames are used for humorous effect. The Disney film Fantasia featured a ballerina hippopotamus dancing to the opera La Gioconda. Other cartoon hippos have included Hanna-Barbera's Peter Potamus, the book and TV series George and Martha, Flavio and Marita on the Animaniacs, Pat of the French duo Pat et Stanley, The Backyardigan's Tasha, The Moomins, and Gloria and Moto-Moto from the Madagascar franchise.
The hippopotamus characters "Happy Hippos" were created in 1987 by the French designer André Roche to be hidden in the "Kinder Surprise egg" of the Italian chocolate company Ferrero SpA. The Nintendo Company published Game Boy adventures of them in 2001 and 2007. In the game of chess, the hippopotamus lends its name to the Hippopotamus Defense, an opening system, which is generally considered weak. The River Horse is a popular outdoor sculpture at George Washington University in Washington, D.C.
- Lewison, R. & Oliver, W. (IUCN SSC Hippo Specialist Subgroup) (2008). Hippopotamus amphibius. In: IUCN 2008. IUCN Red List of Threatened Species. Retrieved 2009-04-05. Database entry includes a range map and justification for why this species is vulnerable.
- "ITIS on Hippopotamus amphibius". Integrated Taxonomic Information System. Archived from the original on 2014-08-26. Retrieved 2007-07-29.
- "Deadly 60: 15 Deadly Animal Facts". National Geographic. Archived from the original on 2014-10-16.
- ἱπποπόταμος, ἵππος, ποταμός. Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at the Perseus Project.
- "Hippopotamus". Merriam-Webster's Online Dictionary. Retrieved 2007-07-18.
- Harper, Douglas. "hippopotamus". Online Etymology Dictionary.
- "Plural of hippopotamus". Oxford English Dictionary. Retrieved 2007-07-18.
- Walker, C. (1997). Signs of the Wild. Struik. p. 140. ISBN 1-86825-896-3.
- Kingdon, J. (1988). East African Mammals: An Atlas of Evolution in Africa, Volume 3, Part B: Large Mammals. University Of Chicago Press. pp. 256–77. ISBN 0-226-43722-1.
- Kane, Thomas Leiper (1990). Amharic-English dictionary: H – N., Volume 1. Otto Harrassowitz Verlag. p. 1909. ISBN 3-447-02871-8.
- Saeed, John I. (1999). Somali. John Benjamins Publishing Company. p. 29. ISBN 90-272-3810-3.
- Eltringham, S.K. (1999). The Hippos. Poyser Natural History Series. Academic Press. ISBN 0-85661-131-X.
- Okello, J.B.A, Nyakaana, S., Masembe, C., Siegismund, H.R. an Arctander, P. (2005). "Mitochondrial DNA variation of the common hippopotamus: evidence for a recent population expansion.". Heredity 95 (3): 206–215. doi:10.1038/sj.hdy.6800711. PMID 16030528.
- Meijaard, Erik (ed.) (September 2005). "Suiform Soundings: The IUCN/SSC Pigs, Peccaries, and Hippos Specialist Group (PPHSG) Newsletter" (PDF). IUCN 5 (1). Archived from the original on 2008-03-08.
- Ursing, B.M., Arnason U. (1998). "Analyses of mitochondrial genomes strongly support a hippopotamus-whale clade". Proceedings of the Royal Society 265 (1412): 2251–5. doi:10.1098/rspb.1998.0567. PMC 1689531. PMID 9881471.
- Gatesy, J. (1 May 1997). "More DNA support for a Cetacea/Hippopotamidae clade: the blood-clotting protein gene gamma-fibrinogen" (PDF). Molecular Biology and Evolution 14 (5): 537–543. doi:10.1093/oxfordjournals.molbev.a025790. PMID 9159931.
- Geisler, J. H. and Theodor, J. M. (2009). "Hippopotamus and whale phylogeny". Nature 458 (7236): E1–4; discussion E5. doi:10.1038/nature07776. PMID 19295550.
- Sanders, Robert (2005-01-25). "Scientists find missing link between the dolphin, whale and its closest relative, the hippo". Science News Daily. Archived from the original on 2015-02-26. Retrieved 2011-01-08.
- "National Geographic – Hippo: Africa's River Beast". National Geographic. Archived from the original on 2013-12-26. Retrieved 2007-07-18.
- Boisserie, Jean-Renaud; Lihoreau, Fabrice and Brunet, Michel (2005). "The position of Hippopotamidae within Cetartiodactyla". Proceedings of the National Academy of Sciences 102 (5): 1537–1541. doi:10.1073/pnas.0409518102. PMC 547867. PMID 15677331.
- Boisserie, Jean-Renaud; Lihoreau, Fabrice; Brunet, Michel (2005). "Origins of Hippopotamidae (Mammalia, Cetartiodactyla): towards resolution". Zoologica Scripta 34 (2): 119–143. doi:10.1111/j.1463-6409.2005.00183.x.
- Boisserie, Jean-Renaud (2005). "The phylogeny and taxonomy of Hippopotamidae (Mammalia: Artiodactyla): a review based on morphology and cladistic analysis". Zoological Journal of the Linnean Society 143: 1–26. doi:10.1111/j.1096-3642.2004.00138.x.
- Stuenes, Solweig (1989). "Taxonomy, habits and relationships of the sub-fossil Madagascan hippopotamuses Hippopotamus lemerlei and H. madagascariensis". Journal of Vertebrate Paleontology 9 (3): 241–268. doi:10.1080/02724634.1989.10011761.
- Burney, David A.; Ramilisonina (1998). "The Kilopilopitsofy, Kidoky, and Bokyboky: Accounts of Strange Animals from Belo-sur-mer, Madagascar, and the Megafaunal "Extinction Window"". American Anthropologist 100 (4): 957–966. doi:10.1525/aa.1922.214.171.1247. JSTOR 681820.
- Petronio, C. (1995). "Note on the taxonomy of Pleistocene hippopotamuses". Ibex 3: 53–55.
- Simmons, A. (2000). "Faunal extinction in an island society: pygmy hippopotamus hunters of Cyprus". Geoarchaeology 15 (4): 379–381. doi:10.1002/(SICI)1520-6548(200004)15:4<379::AID-GEA7>3.0.CO;2-E.
- Carawadine, M. (1995) Natural History Museum: Animal Records. Guinness Publishing (Sterling). ISBN 1402756232.
- Stuart, Chris; Stuart, Mathilde (2011). Field Guide to the Mammals of Southern Africa. Struik Publishers. ISBN 978-1770074040.
- Owen-Smith, R. Norman (1995). Megaherbivores: The Influence of Very Large Body Size on Ecology. Cambridge University Press.
- Pienaar, U. de V.; Van Wyk, P.; Fairall, N. (1966). "An experimental cropping scheme of Hippopotami in the Letaba river of the Kruger National Park". Koedoe 9 (1). doi:10.4102/koedoe.v9i1.778.
- Marshall, P.J., Sayer, J.A. (1976). "Population ecology and response to cropping of a hippopotamus population in eastern Zambia". The Journal of Applied Ecology 13 (2): 391–403. doi:10.2307/2401788. JSTOR 2401788.
- "Hippopotamus Hippopotamus amphibius". National Geographic. Archived from the original on 2014-11-25. Retrieved 2013-07-10.
- Exploring Mammals. Marshall Cavendish Corporation. 2008. p. 616. ISBN 9780761477280.
- Estes, R. (1992). The Behavior Guide to African Mammals: including hoofed mammals, carnivores, primates. University of California Press. pp. 222–26. ISBN 0-520-08085-8.
- Barr, Brady. "Undercover Hippo," Dangerous Encounters, National Geographic Channel, January 20, 2008.
- Saikawa Y, Hashimoto K, Nakata M, Yoshihara M, Nagai K, Ida M, Komiya T (2004). "Pigment chemistry: the red sweat of the hippopotamus". Nature 429 (6990): 363. doi:10.1038/429363a. PMID 15164051.
- Jablonski, Nina G. (2013). Skin: A Natural History. University of California Press. p. 34. ISBN 0-520-24281-5.
- "Oldest Hippo Turns 55!". Mesker Park Zoo. 2006-06-12. Archived from the original on 2007-09-27. Retrieved 2007-06-21.
- "Celebrate with Donna". Evansville Courier & Press. 2007-07-12. Retrieved 2007-07-15.
- Fears, Danika (2012-08-03). "Goodbye, Donna: World's oldest hippo in captivity dies at 61". Today.com. Retrieved 2013-09-12.
- "Old mother hippo dies". Agence France Press. July 12, 1995.
- van Kolfschoten, Th. (2000). "The Eemian mammal fauna of central Europe". Netherlands Journal of Geosciences 79 (2/3): 269–281.
- Horwitz, Liora Kolska; Eitan Tchernov (1990). "Cultural and Environmental Implications of Hippopotamus Bone Remains in Archaeological Contexts in the Levant". Bulletin of the American Schools of Oriental Research 280: 67–76. doi:10.2307/1357310.
- Haas, Georg (1953). "On the Occurrence of Hippopotamus in the Iron Age of the Coastal Area of Israel". Bulletin of the American Schools of Oriental Research 132: 30–34. doi:10.2307/1355798.
- Pliny the Elder. "Chapter 15, Book VIII". Naturalis Historia (in Latin original or English translation). ISBN 3-519-01652-4.
- "Hippo Haven". Smithsonian Magazine. 2006-01-01. Retrieved 2007-01-23.
- "DR Congo's hippos face extinction.". BBC. 2005-09-13. Retrieved 2005-11-14.
- Owen, James (2006-10-24). "Hippos Butchered by the Hundreds in Congo Wildlife Park". National Geographic News. Retrieved 2013-09-11.
- Sundaram, Anjan (2005-09-12). "Congo's Hippos Fast Disappearing". Toronto Star.
- Pearce, Fred (2003). "Poaching causes hippo population crash". New Scientist. Retrieved April 26, 2014.
- Kraul, Chris (2006-12-20). "A hippo critical situation". Los Angeles Times. Archived from the original on 2015-03-08. Retrieved 2008-03-27.
- "Colombia kills drug baron hippo". BBC News. 2009-07-11. Archived from the original on 2015-01-05. Retrieved 2009-07-11.
- "Crece controversia en el país por decisión de cazar a hipopótamos de Pablo Escobar". El Tiempo. Archived from the original on 2015-03-08. Retrieved 2009-07-11. English translation at Google Translate
- "Hipopótamos bravos". El Espectador. 2014-06-24. Archived from the original on 2014-05-09. Retrieved 2014-06-28. English translation at Google Translate
- "The Invaders: Cocaine Hippos". National Geographic Channel. Archived from the original on 2013-06-26.
- "Hippopotamus". Kruger National Park. Retrieved 2007-06-18.
- Grey, J., Harper, D.M.; Harper (2002). "Using Stable Isotope Analyses To Identify Allochthonous Inputs to Lake Naivasha Mediated Via the Hippopotamus Gut". Isotopes in Environmental Health Studies 38 (4): 245–250. doi:10.1080/10256010208033269. PMID 12725427.
- "BBC Nature — Dung eater videos, news and facts". Bbc.co.uk. Retrieved 2011-11-27.
- Dudley, J.P. "Reports of carnivory by the common hippo Hippopotamus Amphibius". South African Journal of Wildlife Research 28 (2): 58–59.
- McCarthy, T.S., Ellery, W. N., Bloem, A (1998). "Some observations on the geomorphological impact of hippopotamus (Hippopotamus amphibius L.) in the Okavango Delta, Botswana". African Journal of Ecology 36 (1): 44–56. doi:10.1046/j.1365-2028.1998.89-89089.x.
- "Hippopotamuses". PBS Nature. Retrieved 10 July 2013.
- Balcombe, Jonathan (2006). Pleasurable Kingdom: Animals and the Nature of Feeling Good. Palgrave Macmillan. pp. 132–33. ISBN 1-4039-8602-9.
- Beckwitt, R., Shea, J., Osborne, D., Krueger, S., and Barklow, W. (2002). "A PCR-based method for sex identification in Hippopotamus amphibius". African Zoology Journal: 127–130.
- William E. Barklow (2004). "Low-frequency sounds and amphibious communication in Hippopotamus amphibious". The Journal of the Acoustical Society of America 115 (5): 2555. doi:10.1121/1.4783854.
- Graham L.H., Reid K.; Webster T.; Richards M.; Joseph S. (2002). "Endocrine patterns associated with reproduction in the Nile hippopotamus (Hippopotamus amphibius) as assessed by fecal progestagen analysis". General and Comparative Endocrinology 128 (1): 74–81. doi:10.1016/S0016-6480(02)00066-7. PMID 12270790.
- Lewison, R (1998). "Infanticide in the hippopotamus: evidence for polygynous ungulates". Ethology, Ecology & Evolution 10 (3): 277–286. doi:10.1080/08927014.1998.9522857.
- Novak, R. M. 1999. Walker's Mammals of the World. 6th edition. Johns Hopkins University Press, Baltimore. ISBN 0-8018-5789-9
- Hunter, Luke (2011). Carnivores of the World. Princeton University Press. ISBN 978-0-691-15228-8.
- Ross, S Charles A.; Garnett, Stephen (1989). Crocodiles and Alligators. Checkmark Books. ISBN 978-0-8160-2174-1.
- "Dangerous Encounters: Undercover Hippo". National Geographic Channel. Retrieved 2008-11-04.
- Hippo Specialist Group, World Conservation Union. (June 2008). In the News. Duke University. Retrieved 2009-9-04.
- National Geographic exhibit on different animals and their poop. News.nationalgeographic.com (October 28, 2010). Retrieved on 2012-05-12.
- Clark, JD; Beyene, Y; WoldeGabriel, G; Hart, WK; Renne, PR; Gilbert, H; Defleur, A; Suwa, G et al. (2003). "Stratigraphic, chronological and behavioural contexts of Pleistocene Homo sapiens from Middle Awash, Ethiopia". Nature 423 (6941): 747–52. doi:10.1038/nature01670. PMID 12802333.
- Herodotus. "Chapter 71, Book II". The Histories (in English translation). ISBN 0-19-521974-0.
- Ingonyama – he is a lion!. Scouting.org.za. Retrieved on 2011-03-29.
- Orans, Lewis P. (1997-06-17). "Scouting in South Africa 1884-1890". Pinetreeweb.com. Archived from the original on 2014-07-28. Retrieved 2011-03-29.
- Miller, Greg (2013-12-20). "The Crazy, Ingenious Plan to Bring Hippopotamus Ranching to America". Wired. ISSN 1059-1028. Archived from the original on 2015-01-17.
- Mooallem, John (2013). American Hippopotamus. New York: The Atavist. Retrieved March 7, 2014.
- Mooallem, Jon (August 10, 2014). "Lake Bacon: The Story of The Man Who Wanted Us to Eat Mississippi Hippos". The Daily Beast (The Newsweek Daily Beast Company). ISSN 0028-9604. Retrieved August 13, 2014.
- Eplett, Layla (March 27, 2014). "The hunger game meat: How hippos early invaded American cuisine". Scientific American. ISSN 0036-8733.
- Burnham, Frederick Russell (1944). Taking Chances. Los Angeles: Haynes Corp. pp. 11–23. ISBN 1-879356-32-5.
- Kendall, C. J. (2011). "The spatial and agricultural basis of crop raiding by the Vulnerable common hippopotamus Hippopotamus amphibius around Ruaha National Park, Tanzania". Oryx 45 (1): 28–34. doi:10.1017/S0030605310000359.
- Root, N. J. (1993). "Victorian England's Hippomania". Natural History 103: 34–39.
- Melissa Greene (December 1987). "No rms, jungle vu: a new group of "landscape-immersion" zoo designers are trying to break down visitors' sense of security by reminding them that wild animals really are wild.". The Atlantic Monthly.
- "Hippoquarium". Toledo Zoo. Archived from the original on February 11, 2007. Retrieved 2007-03-26.
- Cooper, J.C. (1992). Symbolic and Mythological Animals. London: Aquarian Press. p. 129. ISBN 1-85538-118-4.
- Hart, George (1986). A Dictionary of Egyptian Gods and Goddesses. Routledge. ISBN 0-415-05909-7.
- Hamilton, Janice (2003). Nigeria in Pictures. Twenty-First Century Books. p. 49. ISBN 0-8225-0373-5.
- Metzeger, Bruce M., Coogan, Michael D. f, ed. (1993). The Oxford Companion to the Bible. Oxford, UK: Oxford University Press. p. 76. ISBN 0-19-504645-5.
- Greaves, N.; Clement, R. (2000). When Hippo Was Hairy: And Other Tales from Africa. Struik. pp. 67–71. ISBN 1-86872-456-5.
- Chilvers, H.A. (1931). Huberta Goes South, a Record of the Lone Trek of the Celebrated Zululand Hippopotamus. London: Gordon & Gotch.
- "A hippo and tortoise tale". NPR. July 17, 2005. Retrieved 2007-06-18.
- Hatkoff, Isabella; Hatkoff, Craig and Kahumbu, Paula (2006). Owen & Mzee; The True Story of a Remarkable Friendship. New York: Scholastic Press. ISBN 0-439-82973-9.
- "I Want A Hippopotamus For Christmas Lyrics". Christmas-lyrics.org. Retrieved 2007-12-20.
- "Childhood Trauma: Hungry Hungry Hippos". Newcastle Herald (Australia). May 2, 2006.
- "Fred Kroll, of Trouble and Hungry Hungry Hippos games, dead at 82". Associated Press. August 5, 2003.
- Markstein, Don (2008). "Peter Potamus". Don Markstein's Toonopedia. Archived from the original on 2015-03-08.
- "Weekly Candy: Kinder Happy Hippos". Philadelphia City Paper. 2011-11-22. Archived from the original on 2015-03-08.
To request an improvement, please leave a comment on the page. Thank you!