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Ross seal (Ommatophoca rossii)The Ross seal was described during James Clark Ross' British Antarctic Expedition in 1841,lives on and around the pack ice of Antarctica. The seal has a circumpolar distribution in the Southern Ocean surrounding Antarctica, with individuals found in low densities - usually singly - in areas of medium to dense, consolidated pack ice in all regions of the continent in summer, when the seals haul out to breed, moult and rest (10). The seal almost never leaves the Antarctic Ocean; stray animals are occasionally found around sub-Antarctic islands and off the south coast of Australia (8,9). It may move north to the open ocean in autumn (7).
The male is 168-209 cm long and weighs 129-216 kg; females are slightly larger at 190-250 cm long and weighing 159-204 kg. The seal has disproportionately large eyes, up to 7 cm in diameter; Ommato- means "eye" and phoca means "seal". It has a thick neck and slender body with the shortest hair and whiskers of any seal. The coat is dark-brown in the dorsal area and silvery-white beneath; spots and streaks frequently mark the head, neck, and flank. In summer, unmoulted seals are tan to brownish, with the moult occurring in January. At the onset of winter, the coat fades gradually to become light brown.
Moulting probably occurs in January, when many Ross seals seem to fast (6). In summer most Ross seals are hauled out on the ice at midday (11-13). Some seals may move north to the open ocean in autumn (7). When a female was in the water, she dived continuously with dives averaging 110 m deep and 6.4 minutes long, with a maximum of 212 m and up to 9.8 minutes. The dives were deepest at twilight and shallowest at night and it hauled out by day (13). Ross seals are usually found singly on the ice. The seal can produce a variety of complex twittering, trilling and siren-like sounds on ice and underwater; these can carry for long distances (4). The underwater siren sound can be composed of two harmonically unrelated superimposed tones that are pulsed with the same rhythm. The vocalizations, whether on ice or in water, are made with a closed mouth - emitting no air. Their distinctive nature and long range are likely to facilitate encounters or avoidance of individuals (4).
The Ross seal feeds primarily on squid and fish, primarily Antarctic silverfish, in the pelagic zone (6,14,15), but it also eats other invertebrates including some krill (11). Seals may fast during the post-breeding moulting period. Their predators probably include killer whales and leopard seals , but these are rarely found in habitats utilized by Ross seals (14).
The seals probably mate in the water in early December, soon after the pup is weaned. Females give birth to their precocial young on the ice in early November, with a peak from early to mid-November (12). The period of pre-implantation usually takes 3.5 to 4.5 months, increasing the gestation time up to 1 year; the usual gestation is @ 9 months. Pups are @ 100-139 cm long and weigh @ 16-20 kg at birth, with some males weighting 16.5 kg. They develop quickly, gaining weight rapidly from their mother's rich milk. After 15 days of nursing pups weigh @ 75 kg. They are nursed for 4-6 weeks before weaning around mid-December (14), when the pups become independent. Nursing pups may swim between ice floes. The seals mature sexually at about 2-4 years old for females and 2-7 years for males; males reproduce for the first time at 3-4 years old. The average age of reproducing seals should be at least 10 years old. The seal reaches physical maturity at @ 9 years and is thought to live around 20 years in the wild (6,16); the oldest known male was 21 years old and the oldest female known was 19 years old (10).
The Ross seal shares a recent common ancestor with the other Antarctic lobodontine seals (tribe Lobodontini): the crabeater, leopard and Weddell seals (2). These species share teeth adaptations, including lobes and cusps useful for straining smaller prey items out of the water column. The ancestral Lobodontini likely diverged from its sister clade, Mirounga (elephant seals) in the late Miocene to early Pliocene, when they migrated southward and diversified rapidly in relative isolation around Antarctica (2). The only fossil Ross seals known date from the early Pleistocene of New Zealand (3)
The total Ross seal population is estimated at around 130,000 individuals, but reported 95% confidence intervals range from 20,000 to 227,000 (5). The seal has had limited interactions with humans, including being collected for scientific collections. Its range does not generally overlap with commercial fishing. The Red List Category is Least Concern due to the seal's widespread occurrence and large population size (1). A population reduction could occur if sea ice habitats decline due to continued climate warming (17). Ross seals are protected under the Convention for the Conservation of Antarctic Seals