Leopard seals are large seals, displaying sexual dimor-» phism where the females are larger than the males. Fe-1 males grow up to 3.8 m and weigh up to 500 kg, whereas males grow up to 3.3 m and weigh up to 300 kg.
I. Distribution
The main leopard seal population remains within the circumpolar Antarctic pack ice but 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 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.
The leopard seal population is estimated to be 222,000 to 440,000. 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 widi die 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 dian 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 behavior. Due to intraspecific aggression there is a greater degree of spatial separation among older seals.
II. Diet
Leopard seals take a diverse range of prey, including fish, cephalopods, sea birds, and seals. Different food sources are used when available or when opportunities to take other more sought after prey are few. Krill makes up the largest proportion of their diet, particularly during the winter mondis when other food types are not abundant. At this time the leopard seals must compete directly with krill-feeding specialists, such as the crabeater seal (Lo-bodon carcinophaga) and Adelie penguin. This is believed to be a time of potential food shortage and causes some juvenile leopard seals to move north from die pack ice during die austral winter. The leopard seal is responsible for more predation on warmblooded prey than any other pinniped. Leopard seals capture and eat juvenile crabeater seals in particular, but also prey on Wed-dell (Leptomjchotes loeddellii), Ross (Ommatophoca rossii), southern elephant (Mirounga leonina), sub-Antarctic and Antarctic fur seals (Arctocephalus tropicalis and A. gazelfo) and southern sea lions (Neophoca cinerea and Phocarctes hookeri). Newly weaned crabeater seals are the most vulnerable and are taken from November to February. Crabeater seal survivors bear characteristic parallel paired scars from leopard seal attacks, and approximately 78% of adult crabeater seals display such marks. The teedi of die leopard seal have a dual role; the large recurved 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.
III. Reproductive Biology
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 about 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. 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.
Figure 1 Leopard seal in threatening posture.
Acoustic behavior is important in the mating system of the leopard seal. Leopard seals become highly vocal prior to and during their breeding season (Fig. 1). Lone male leopard seals vocalize for long periods each day, from early November through January. Female leopard seals also use long-distance acoustic displays during the breeding season. However, female seals vocalize for a brief period only from the beginning of es-trus until mating, presumably to advertise sexual receptivity. The calls of the leopard seal are at low-to-medium frequencies and so powerful that they can be heard through the air-water interface and felt through the ice.
