The basic social and reproductive unit in hamadryas baboons is the one male unit (OMU). Within this OMU, there is a single adult male who mates with one or more females.
Reproductive behavior in P. hamadryas is closely tied to social organization. The basic breeding unit is the OMU, in which the leader male aggressively herds females, keeping them from straggling during the foraging march, and preventing them from socializing with other males. Females typically spend most of their social time in proximity to the leader male. Most social grooming within the OMU is focused on the leader male, with females grooming him, especially his mane, face, and buttocks. The pelage characters of males can therefore be thought of as strong mate attractants, and seem to function in the maintenance of the OMU.
Because of the division into OMUs, most females have only opportunities to mate with the OMU leader. However, males may follow a number of reproductive strategies, and females may at times "sneak" copulations with males other than their unit leader.
For males without an OMU, reproductive behavior is limited, and effort seems to be expended in attempts to establish an OMU. Establishment of an OMU can occur in one of two ways. First, a subadult male may attach himself to an already established OMU as a follower. In general, a follower male remains separated from the females of the OMU, although he travels with the OMU on the daily foraging march, and sleeps near the OMU at night. There may be some potential for such follower males to mate with females, if such copulations can be conducted without detection by the leader of the OMU. Evidence for such copulations comes from the pattern of testicular development in this species, as well as a limited number of observations of such "trysts." However, the principle goal of followers seems to be to either steal females from the OMU leader, having become familiar to these females through association with the OMU, or to depose the OMU leader and commandeer his entire harem of females.
Because OMU leaders actively restrict the interactions between their females and other males, chasing, biting, or otherwise punishing females who appear to be straying, one might wonder why a female would risk incurring his wrath by engaging in copulations with other males. One might speculate that such interactions might confuse paternity if there is a turnover in leadership of the OMU, and thereby inhibit tendencies toward infanticidal behavior on the part of the new leader male.
In general, hamadryas males "respect" the social bond between other males and their female affiliates. However, rarely within a band, there is intense physical competition between males. This seems to be associated with turnover of male OMU leaders.
The second strategy utilized by males to establish a OMU is to "adopt" a juvenile or subadult female. This strategy entails much less risk to the male, because there is no overt competition for the female in question. The male will care for the little female, grooming her, carrying her if necessary, and providing what would appear to many to be parental care. When the female reaches reproductive maturity, he will breed with her. This strategy seems especially effective because females hamadryas baboons do not readily consort with single males. Once a male has established a OMU with his "adopted" female, he may become much more attractive to other females.
Females exercise some choice in their mates. Females typically disperse from their natal group between 1.5 and 3.5 years of age. About 70% of females will change affiliation to a new OMU within a period of 3 years, often choosing to join OMUs that contain other females with whom they are already familiar. Through this type of transfer, it is possible for females to maintain bonds with one another throughout their lives.
Mating System: polygynous
Hamadryas baboons breed aseasonally. Mating is based on the occurence of estrus in females, and the reproductive condition of females is generally independent of season. However, Kummer (1968) did report a peak of births in May/June and November/December.
Females characteristically have an estrous cycle of 31 to 35 days in length. There is a noticeable menstrual flow for approximately three days per cycle if the female does not conceive. During the period around ovulation, the perineal skin of the female swells, alerting the male to her potentially fertile condition. During mating, there is generally a pattern of serial mounting initiated by the female, who presents her hindquarters to the male. The male mounts the female and thrusts several times. This mounting is followed by other mount/thrust episodes until the male ejaculates. Mating frequencies can be from 7 to 12.2 per hour while the female is receptive.
Gestation lasts about 172 days, after which the female gives birth to a single offspring. The neonate, weighing from 600 to 900 g, has a black coat, making it readily identifiable from older infants. Infants are completely dependent upon their mother for the first few months, until they begin to eat solid food and are able to walk on their own.
Puberty occurs between the ages of 4.8 and 6.8 years in males, and around the age of 4.3 years in females. Full size is attained in males around 10.3 years of age. Females, which are significantly smaller than males, reach adult size around 6.1 years of age.
Puberty in males is a lengthy process, and the timing of different developmental events reveals interesting details about the reproduction of these animals. Testicular development does not closely follow male growth in this species. Testes develop rapidly between the ages of 3.8 and 6 years, reaching full size prior to attainment of full adult body size. In contrast, body mass doubles between the ages of 7 and 8 years, after the testicles are fully developed. This pattern of development may indicate that subadult males, who do not possess OMUs of their own, may yet achieve some "sneak" copulations. Interestingly, the remainder of adult male secondary sexual characteristics, including the silver mane, white cheeks, and pink hindquarters, do not develop until after full adult size is reached. These characteristics are thought to function in the maintenance of the OMU, as they are very attractive to the females of the OMU and elicit large amounts of female grooming.
Females have an average interbirth interval of 24 months, although individual females have been known to have offspring as close together as 12 months. Some females have not given birth until 36 months after the birth of their previous offspring. It is likely, that as in anubis baboons, differences between females in the length of the interbirth interval are related to differences in nutritional status or social stress levels.
The average length of lactation is 239 days, but the timing of weaning may vary according to maternal condition, ecological variables, and social circumstances. Lactation can last from 6 to 15 months. The period of infant dependence is difficult to assess. Because this species is social, juveniles may continue to associate with their mothers until they disperse at or near adulthood. Also, because young females may be "kidnapped" by males wishing to establish an OMU, it is even more difficult to assess whether or not these individuals could survive without the quazi-parental care provided by the kidnapping male. In short, it would be reasonable to put the upper limit of the period of juvenile dependence at the mean interbirth interval (24 months), but to realize that this type of estimation is imprecise.
Breeding interval: Male hamadryas baboons can breed continuously, if females in their OMU are in reproductive condition. Females can produce offspring annually, but are more likely to produce an offspring every two years.annually
Breeding season: Hamadryas baboons are not seasonal breeders, and can mate throughout the year, provided females are in estrus.
Range number of offspring: 1 (low) .
Average number of offspring: 1.
Average gestation period: 172 days.
Range weaning age: 6 to 15 months.
Average time to independence: 24 months.
Average age at sexual or reproductive maturity (female): 4.3 years.
Range age at sexual or reproductive maturity (male): 4.8 to 6.8 years.
Key Reproductive Features: iteroparous ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); fertilization ; viviparous
Average birth mass: 814 g.
Average gestation period: 171 days.
Average number of offspring: 1.
Average age at sexual or reproductive maturity (female)
Sex: female: 1514 days.
Most parental behavior is performed by the female. Females nurse and groom their offspring. There does not seem to be cooperative care of offspring among females, although it is not uncommon for one female in an OMU to groom the offspring of another female. As is the case for all baboons, infants are very attractive to other members of the social group, and are the focus of a great deal of investigation and attention, especially while they are still displaying their black natal coat.
Females can experience deceptive estrous cycles when a new male takes control of the OMU. This may be an adaptive parental behavior with an anti-infanticidal effect.
Males offer protection to infants by keeping control of the OMU. Males exclude other males from contact with their females and offspring, potentially inhibiting infanticide. Also, adult males maintain vigilance over the group, and are therefore likely to spot potential predators, protecting their offspring from that particular threat. Males are typically very tolerant of infants and juveniles within the OMU, and will often play with them or carry them.
The caretaking behavior of males toward to juvenile females during the formation of an OMU is quazi parental. Although from the perspective of the male this behavior is reproductive, it is parental from the perspective of the juvenile female. She obtains food, protection, warmth, and is often carried by the male, much as she would be by her own father or mother.
Parental Investment: altricial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Male, Female); pre-independence (Provisioning: Female, Protecting: Male, Female); post-independence association with parents; extended period of juvenile learning