Antarctic fur seals are a strongly sexually dimorphic species with adult males being 4–5 times the mass and 1.4–1.5 times the length of adult females. The muzzle is moderately-short to medium long, wide, and tapers in width and thickness to a moderately pointed end at the nose. The rhinarium is modest in size and does not extend much past the mouth. The ear pinnae are long, prominent, and naked at the tip. The creamy white vibrissae are long and conspicuous. On adult males they are some of the longest found on any pinniped, reaching 35-50 cm.
Adult males develop a mane of thicker, coarser guard hairs on the chest, neck, and top of the head. The neck and shoulders are also greatly enlarged with fat and the development of muscles in older adult males. Adult females, subadults and juveniles are more normally proportioned, and males and females are difficult to tell apart until males begin to grow larger when they are 4–6 years old. Juveniles and subadults have dark, to mixed light and dark vibrissae, becoming lighter as they age, and become all-light when they are adults.
The fore and hind flippers are proportionately long being 28-33% and 22–28% (respectively) of the total body length at all ages. The fore flippers have a dark, sparse, short fur that extends beyond the wrist onto the middle of the dorsal surface of the flipper in a ‘V’ pattern that does not reach the rounded tip. The rest of the dorsal surface, and the palms of both fore flippers are covered with a hairless black leathery skin. The first digit is the longest, widest and thickest, and curves posteriorly, giving the flipper a swept-back look. Digits 2-5 are successively shorter. There is a small opening in the skin at the end of each digit for a claw that is usually reduced to a vestigial nodule, and rarely emerges above the skin. The claw openings are set back from the free edge of the flippers by cartilaginous rods that extend the length of each digit, and expand the size of the flippers.
The hind flippers have dark, short, sparse hair covering part of the proximal end of the flipper, and the rest of the dorsal surface, and the entire sole is covered in black leathery hairless skin. The hind flippers are long and each digit has a cartilaginous rod that adds a flap-like extension to each toe. The bones of the three central toes terminate at the position of the small nails that emerge through the skin on the dorsal surface, set back from the end of the flipper. The claws of digits 1 and 5 are vestigial, like the claws on the fore flippers, and may or may not emerge from small openings set back from the end of the flaps. All of the flaps at the end of the flipper are of relatively equal length. The first toe or hallux and the fifth toe are somewhat wider than toes 2–4.
Adult females and subadults are medium gray to brownish gray above and cream to pale gray below with shades of ginger and reddish brown. There is usually an area of pale color of variable extent on the sides between the flippers. Pale color extends from the chest variably up the neck and on the sides of the neck to as high as the throat. The dark fur on the top of the fore flippers extends into the area where the fore flipper attaches at the shoulder. Additional lighter areas often surround where the ear pinnae attach and are also on the scrolled pinnae. There is a variable amount of cream to reddish brown color on the muzzle in the mystacial area. Newly molted juveniles have the same color pattern as adult females, but are light gray to silvery gray in all lighter areas. At birth, pups are blackish, though they may be pale on the face and muzzle, and some animals are paler below.
Adult males are dark grayish brown to charcoal, with off-white to silver frosting on the guard hairs of the back and mane, and flanks. The long guard hairs of the mane often bunch up and reveal the fawn color underfur.
Hybrid Subantarctic and Antarctic fur seals are known. The few photographs available show that they share characteristics of both species. Two adult males had crests of fur on the crown and variable amounts of pale coloration on the face, neck and chest. One large male had a conspicuous grizzled mane, a pale muzzle, and pale rings around his eyes.
An unusual cream to honey colored pale morph of the Antarctic fur seal is seen at a rate of 1–2 per thousand at South Georgia. The guard hairs on these pale animals lacks pigment, and the underfur and skin are paler than in normally-pigmented animals. These animals are not albinos, as they have normally pigmented eyes and paler, but not unpigmented, skin.
Adult males on breeding territories can be up to 2 m long, but are usually around 1.8 m long. Mean weight is 188 kg, but they can weigh 133–204 kg. Adult females are 1.2–1.4 m and 22-51 kg, with a mean weight of 26 kg. Newborns are 63-67 cm long, and weigh 6-7 kg.
The dental formula is I 3/2, C 1/1, PC 6/5.
Can be Confused With
Antarctic fur seals have been recorded as vagrants at the Juan Fernandez Islands and Southern South America, and might be confused with many southern otariids that share their range or occur where Antarctic fur seal vagrants have been found. Subantarctic, New Zealand, South American, and Juan Fernandez fur seals, and South American and New Zealand sea lions are the most likely species to consider.
Subantarctic fur seals are uniquely colored. The back and rump of adult males is dark gray to brownish black, and the chest, neck, muzzle, face and area around the eyes are continuously cream colored with yellow to orange shading. The ear pinnae are short, mostly bare skin, and dark, and attach in the dark pelage of the upperparts. There is also a crest of longer pale fur in the dark crown area above the eyes. Although the adult male is enlarged and thick in the shoulders and neck, he does not have a mane. Adult females have similar coloration to the males, but have less well-defined pale facial markings that can be shaded with dull yellow orange to light brown. In both sexes the top of the fore flippers is darker as is the area where the flippers attach to the sides. The underside of the abdomen is dark ginger to reddish brown. The fore flippers and hind flippers are proportionately shorter and wider than on all sizes comparable sizes of Subantarctic fur seals.
Separating New Zealand and South American fur seals from Antarctic fur seals can be problematic depending on the age and size of the animal involved. Both are more stocky in build with a longer more pointed muzzle. The rhinarium in both species is larger than in the Antarctic fur seal. It is somewhat inflated on the top in South American fur seals giving the end of the muzzle a slight upturned look, and somewhat larger in front and slightly-drooping in adult male New Zealand fur seals. Adult male Antarctic fur seals are generally darker with more pale grizzling than on either the grayer brown New Zealand or light to dark brown South American fur seals. Separating adult female, subadult and juvenile New Zealand and South American fur seals from Antarctic fur seals of similar size and age is more problematic due to overlap of coloration. Note muzzle size and rhinarium size and shape.
Adult male Juan Fernandez fur seals have a longer muzzle, distinctive bulbous rhinarium, and unique coloration of the crown, nape and upper neck. Separation of adult female, and subadult and juvenile Juan Fernandez fur seals from similar Antarctic fur seals is problematic. In general, adult female Juan Fernandez fur seals are somewhat larger than female Antarctic fur seals, and have a longer muzzle, larger rhinarium and downward oriented nares, and a more rounded crown.
Antarctic fur seals can be differentiated from both New Zealand and South American sea lions by their distinctive coloration, short and wide pointed muzzle, proportionately large eyes, long white vibrissae, and thick pelage. Both sea lions have large blocky heads with blunt large muzzles and are paler in color than comparably sized animals with sleek fur. Adult males of both sea lions are much larger and distinctive.
Antarctic fur seals are widely-distributed in waters south, and in some areas slightly north, of the Antarctic Convergence. Most of the population breeds on South Georgia and Bird Island, but colonies are widely spread-out and can be found in the South Shetland, South Orkney, South Sandwich, Prince Edward, Marion, Crozet, Kerguelan, Heard, McDonald, Macquarie, and Bouvetoya islands. Vagrants have been found in southern South America, the Juan Fernandez Islands, and at Australia’s Mawson Station on the Antarctic Continent.
Males haul-out extensively in the mid and late summer on islands along the Antarctic Peninsula. Ashore, they prefer rocky habitats, but will readily haul-out on sandy beaches and move into vegetation zones such as tussock grass. They disperse widely at sea. Distribution and movements in winter are not well known. Males and subadults occur south to the edge of the consolidated pack ice, and can be found hauled-out on sea ice.
Ecology and Behavior
Antarctic fur seals are highly polygynous. Males arrive at the colonies in late October 2–3 weeks before the first females arrive and establish themselves on territories. Males continue to arrive and challenge for territories through much of the season. Territories are acquired and held with vocalizations, threat postures and fighting. In prime areas territories can be as small as 20 m2, and have up to 19 females. The mean length of tenure for bulls at South Georgia is approximately 34 days. Male vocalizations include a bark or whimper, a guttural threat, a low-intensity threat, possibly a full threat, and a submissive call. Females growl and have a pup-attraction call that is a high-pitched wail.
Females begin to arrive in mid-November and most pupping and breeding occurs from late November to late December. They give birth 1–2 days after arrival at the colony, attend their pup for 6–7 days, come into estrous and mate, and then depart minutes to hours after mating for their first foraging trip. Foraging trip and attendance periods vary by year depending on the availability of the lactating female’s chief prey, adult krill, but are generally 4–5 days at sea followed by 2–3 days attendance on shore. Lactating females routinely dive from 8–30 m for less than 2 minutes, but have been recorded to depths of 181 m, and to undertake dives that have lasted 10 minutes. Mean dive depth and duration increase during the lactation period.
Pups are weaned in about 4 months. After they wean their pups, females disperse widely, possibly migrating north and are not seen at the colonies much until the next breeding season. Bulls also depart breeding areas, but subadults and adult males can be seen around the rookeries at South Georgia all year.
Like other southern fur seals, Antarctic fur seals porpoise when swimming rapidly. When rafting they often assume many of the typical fur seal resting posture. At other times, they can be found busily engaged in grooming. Predators include killer whales, leopard seals, and at Macquarie Island, New Zealand sea lions. Leopard seals have had a dramatic effect on several recolonized areas in the South Shetland Islands and have caused a decline at one site due to their extensive predation on pups.
Feeding and Prey
The diet varies by season and location. Adult females at South Georgia feed heavily and selectively on adult krill. At Heard Island krill is not available and lactating females prey primarily on fish such as myctophids and mackerel icefish. In the winter, males and subadult males at South Georgia take krill and a variety of fish that eat krill, while squid and myctophids were only a small percentage of the diet. At Heard Island in the winter squid and myctophid fish dominate the diet. Foraging patterns of females in summer indicate nocturnal feeding.
Antarctic fur seals will eat penguins. Adult males have been documented chasing, killing and eating king penguins on land on Marion Island. They are also known to take macaroni and gentoo penguins in the water at Heard, Macquarie, and South Georgia Islands.
Threats and Status
As was the case for all other southern fur seals, sealers drove the species to the brink of extinction by the late 19th century. The colony at South Georgia was thought to be as small as 100 animals in the 1930s. Today the population is estimated to exceed 3 million animals and is believed to be growing and expanding at a rapid rate with 95% of these using the colony at South Georgia.
The Antarctic Treaty and the Convention for the Conservation of Antarctic Seals protect this fur seal below 60º S. Various efforts to launch commercial fisheries for krill near South Georgia have been unsuccessful. Trawling activities developing around Macquarie Island may effect the prey base of the primarily fish eating Antarctic fur seals that breed on those islands. No direct fisheries conflicts involving regular entanglements are known to exist. Antarctic fur seals become entangled in marine debris such as discarded fishing line, nets, packing bands and anything that can form a collar. It was estimated from and 1988–89 study that the numbers entangled might be as high as 1% of the total population with the majority of the impact on juvenile and subadults, particularly on males. In most cases the entangled debris was either causing injury, or was very tight and not expected to come off the animal, suggesting that most would eventually die as a result.
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