The leopard seals, Hydrurga leptonyx, are the second largest of the phocid seals living in the Southern Hemisphere. The name, Hydrurga leptonyx, pronounced, hide-rurg-ah lept-on-ix, means 'slender-clawed water-worker'.
Leopard Seals are large, spectacular hunters and are at the top of the Antarctic food chain.
Size, shape and distinctive characteristics
Long, slim body, with disproportionately large head separated from body by marked constriction at neck, they are dark grey above; lighter below; with light and dark grey spots on throat and sides. They have a characteristic 'reptilian' appearance to their head; a wide gape of jaws and characteristically three-pronged teeth, which makes identification easy. Leopard seals are sexually dimorphic, the females are larger than the males growing up to 3.8 m in length and weighing up to 500 kg, whereas males grow up to 3.3 m in length and weigh up to 300 kg.
While the majority of the leopard seal population remains within the circumpolar Antarctic pack ice the seals are regular, although not abundant, visitors to the sub-Antarctic islands of the southern oceans and to the southern continents. The most northerly leopard seal sightings are from the Cook Islands. Juveniles appear to be more mobile, moving further north during the winter. Because it does not need to return to the pack ice to breed, the leopard seal can escape food shortages during winter by dispersing northwards. Every 4 to 5 years the number of leopard seals on the sub-Antarctic islands oscillates from a few to several hundred seals. The periodic dispersal could be related to oscillating current patterns or resource shortages in certain years. By comparison, adult seals that remain in Antarctica are much less mobile and remain within the same region throughout the year.
During summer, leopard seals breed on the outer fringes of the pack ice where they are solitary and sparsely distributed. Their density is inversely related to the amount of pack ice available to the seals as haul-out platforms. Pack ice cover varies with the season, from a maximum between August and October to a minimum between February and March. Population densities are greatest in areas of abundant cake ice (ice floes of 2 to 20 m in diameter) and brash ice (ice floes greater than 2 m in diameter), whereas they are least in areas with larger floes. Densities range from 0.003 to 0.151 seals/km2, and there is an age-related difference in their spatial behaviour. Due to intra-specific aggression there is a greater degree of spatial separation among older seals.
Ecology and Behaviour
The leopard seal is responsible for more predation on warm-blooded prey than any other pinnipeds, they are catholic feeder taking a diverse range of prey, including fish, cephalopods, sea birds, and seals. They use different food sources when they become available or when opportunities to take other, more sought after prey, are few. For some individuals krill can make up the largest proportion of their diet, particularly during the winter months when other food types are not abundant, where others use penguins and fish more heavily in their diet. During the winter leopard seals must compete directly with krill-feeding specialists, such as the crabeater seal (Lobodon carcinophagus) and Adélie penguin which be a time of potential food shortage and cause some juvenile leopard seals to move north from the pack ice during the austral winter.
Leopard seals capture and eat juvenile crabeater seals in particular, but also prey on Weddell (Leptonychotes weddellii), Ross (Ommatophoca rossii), southern elephant (Mirounga leonina), sub-Antarctic and Antarctic fur seals (Arctocephalus tropicalis and A. gazelle) and southern sea lions (Neophoca cinerea and Phocarctes hookeri). Newly weaned crabeater seals are taken from November to February when they are most vulnerable. Crabeater seal survivors bear characteristic parallel paired rake scars from leopard seal attacks, and a large proportion of the adult population, approximately 78%, bear leopard seal rake marks. The teeth of the leopard seal have a dual role; the large re-curved canines and incisors are designed for gripping and tearing prey, whereas the upper and lower tricuspid (three cusped) molars interlock to provide an efficient krill sieve.
Male leopard seals are sexually mature by 4.5 years and females by 4 years of age. Females give birth to their pups and wean them on the ice floes of the Antarctic pack ice. Males do not remain with the females; only mother-pup groups are observed on ice floes. Length at birth is approximately 120 cm, with rapid growth through the first 6 months postpartum. Births are believed to occur from October to mid-November and mating from December to early January, after the pups have weaned. Lactation is believed to last for up to 4 weeks but maybe shorter than this. Mating in the wild has been observed rarely, but captive seals mount only when in the water. There is a period of delayed implantation from early January to mid-February. Implanted fetuses are found after mid-February when the corpus luteum (glandular structure in the ovary) has begun to increase in size and the corpus albicans (scar from ovarian glandular structure) from the previous pregnancy has continued to regress.
Acoustic behaviour is important part of the mating behaviour of the leopard seal. Leopard seals become highly vocal prior to and during their breeding season.
Population size from the most recent circumpolar estimates, conducted in the summer of 1999/2000, indicates a total population size of 300,000. This is a widespread circumpolar species and, similar to the other Antarctic seals that inhabit the pack ice, population assessments are very difficult and expensive to conduct and therefore infrequently undertaken. There is no indication of a declining trend in the population, but the broad-scale estimates have considerable uncertainty around them, due to the difficulties of surveying this species, and consequently trend estimates are also uncertain. Although population abundance is not precisely known, and has not been closely monitored, the maximum longevity is possibly up to 25 years, and sexual maturity thought to be attained at 4-5 years of age, so the average age of reproducing individuals should be at least 10 years. Within the last 30 years there has been no observed population reduction for the leopard seal, either observed, estimated, inferred, or suspected.
Leopard seals use different food sources when they become available or when opportunities to take other, more sought after prey, are few. For some individuals krill can make up the largest proportion of their diet, particularly during the winter months when other food types are not abundant, where others use penguins and fish more heavily in their diet. Leopard seals capture and eat juvenile crabeater seals in particular, but also prey on other seal species.
Population size from the most recent circumpolar estimates, conducted in the summer of 1999/2000, indicates a total population size of 300,000. This is a widespread circumpolar species and, similar to the other Antarctic seals that inhabit the pack ice, population assessments are very difficult and expensive to conduct and therefore infrequently undertaken. There is no indication of a declining trend in the population, but the broad-scale estimates have considerable uncertainty around them, due to the difficulties of surveying this species, and consequently trend estimates are also uncertain. Within the last 30 years there has been no observed population reduction for the leopard seal, either observed, estimated, inferred, or suspected.
The leopard seal, Hydrurga leptonyx, is currently listed as lower risk, least concern by the IUCN (version 3.1).
Leopard seals are currently listed as lower risk, least concern by the IUCN. Historically leopard seals have never been exploited commercially, however small numbers have been taken for scientific research and for use as pet food. The main conservation issue facing leopard seal populations today is reduction in sea ice habitats, which is being exacerbated by the rapid increase in climate change. Leopard Seals depend on sea ice for reproduction and loss of sufficient areas of pack ice habitat used for pupping, resting, avoidance of predators, availability of preferred prey such as penguins, other ice seals, krill, and fish could all possibly decline, and would all effect leopard seals directly or indirectly. Presently there appears to be no consistent circumpolar trend in Southern Ocean sea ice habitat, while sea ice extent presently appears to be declining in western Antarctica, it may be increasing in eastern Antarctica, and overall there appears to be no change yet in the extent of Southern Ocean sea ice.
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