Australian sea lions are sexually dimorphic, with adult males reaching 1.25 times the length and 2.5–3.5 times the weight of adult females. The head is proportionately wide and long in both sexes, and becomes massive in adult males. The muzzle tapers to a blunt somewhat-rounded end. The eyes are widely-set apart. There is a slight forehead that is more prominent in adult males owing to an enlarged, but posteriorly-positioned, sagittal crest that elevates the crown. The ear pinnae are short and lie close to the head. Mystacial vibrissae are moderate in length, the longest ones reaching 18 cm. Two vibrissae are located above and behind each eye. In adult males, the neck and shoulders are greatly enlarged, and the canine teeth are thicker at the base and up to 2.5 times longer than those of adult females.
The fore flippers have a 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 2-4 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 also have cartilaginous rods that extend the length of 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 first and fifth toes are longer than the three middle toes, and the first toe, or hallux, is longer and wider than the fifth toe. The hind flippers have short 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.
Pups are dark chocolate brown to charcoal in color at birth and lighten to smoky gray. The crown is paler and there is a dark mask across the face. The postnatal molt starts when pups are 8-10 weeks old and is completed in several weeks. Pups molt to the juvenile pelage, which is like that of the adult females. Adult females and juveniles are fawn to silvery gray above and tan to pale yellow on the underside of the neck, chest, and abdomen. The underside of the abdomen can be darker in some animals. The demarcation between light and dark zones is high on the neck and dips down at the insertion of the fore flippers, remaining low on the sides between the fore and hind flippers. The coloration of the face is variable, with the light coloration of the underside of the neck rising up to include the mystacial area and side of the muzzle and face, frequently encircling the eyes, and reaching the ears. The fore flippers are often darker above. Some animals are darker overall and have little discernable contrast between coloration above and below.
Subadult males are colored like females, but darken as they mature, showing dark spots on the chest as the transition begins. The first evidence of this appears as dark spotting on the chest and darkening of the muzzle. Adult males have a mane of somewhat longer and coarser guard hairs from the crown to the shoulders. Their pelage is dark brown, with a whitish creamy crown and nape of variable extent that gradually transitions to the darker body pelage on the back and sides of the upper neck, with a scattering of light colored hairs sometimes reaching the underside of the neck. This area of pale coloration accentuates the darker "mask" across the face, which extends from ear to ear and covers the sides of the face, muzzle, lower jaw, and throat. Younger bulls are incompletely marked and can have a whitish ring around the eyes.
Very little information on sizes of adult males is available, and some values in the literature may be overestimates. Adult males reach lengths of at least 2.5 m and weights of 200 to at least 300 kg. Females are 1.3 to just over 1.8 m and weigh from 61–105 kg. At birth, pups are approximately 60–70 cm long and weigh 6.4–7.9 kg.
The dental formula is I 3/2, C 1/1, PC 5/5, and occasionally PC 6/5.
Can be Confused With
New Zealand fur seals live within the range of Australian sea lions, and the range of Australian fur seals is nearly adjacent to that of this species. Wandering sub–Antarctic fur seals periodically show up as vagrants in mainland Australia and Tasmania. Australian sea lions have a large wide head and blunt muzzle that contrasts with the tapering pointed muzzle of all southern fur seals; they also have proportionately shorter ear pinnae, and there are differences in the size and shape of the toes on the hind flippers. In addition to these features, Australian sea lions can be differentiated from these and all other southern fur seals, based on differences in pelage color patterns and paler coloration of all but adult and subadult males. Australian sea lions lack the dense underfur of fur seals, and their shorter pelage, exclusive of the mane on adult males, looks shorter and less dense than the more luxuriant pelage of dry fur seals, or the shaggy look of fur seals when they are wet.
Australian sea lions are found in southern and southwestern Australia from just east of Kangaroo Island west to Houtman Albrolhos in Western Australia. Vagrants have come ashore in Eastern Australia, as far north as the middle of New South Wales. Australian sea lions breed on at least 51 islands and at several mainland sites. Five sites account for approximately 50% of the annual pup production. At sea, Australian sea lions spend nearly all of their time in waters over the continental shelf.
Ecology and Behavior
Australian sea lions are unusual among pinnipeds in having a supra-annual pupping interval, with females producing pups every 17–18 months. Pups can be born at all times of the year, with females at a given site being loosely synchronous with each other, and pupping over an approximately 5–month period. Neighboring sites are frequently on entirely different breeding schedules. Males are sequentially polygynous, establishing territories around individual females, herding them in an effort to keep them from departing until the onset of estrous, 7-10 days after they give birth, when they mate. This pattern is repeated until the male is compelled to go to sea and forage, after which he returns and repeats the strategy. Males defend their territories with guttural clicking, growling and barking vocalizations, posturing, and by fighting with rivals. Pups can be trampled when males are fighting, or moving rapidly to confront rivals and control females, and this can be a significant source of pup mortality.
Pups are continuously attended for the first 9–10 days after birth. Over the next 5 months females make foraging trips that average 48.5 hours in length, followed by pup attendance periods that average 33 hours. Females suckle their pups for 15–18 months, usually weaning them a month before giving birth again. Some females care for their offspring for up to two years, and can be seen with a juvenile and a new pup. Adult female Australian sea lions behave aggressively toward pups that are not their own, as do adult males to all pups, and will pick them up and throw them high in the air. Pups will play at the shoreline and in tide pools while their mothers are away, and following their postnatal molt, will actively swim on their own.
Adult female Australian sea lions are benthic diurnal foragers. They routinely transit to foraging locations by swimming along the bottom. Mean depth of dives for series of lactating females ranged from 41.5–83.1 m and maximum depth of dives ranged from 60–105 m. Mean duration of dives ranged from 2.2–4.1 minutes, and the longest dive recorded lasted 8.3 minutes. Australian sea lions are fast, powerful swimmers and will porpoise when moving rapidly at the surface.
These sea lions are considered to be non-migratory and sedentary, and probably spend most of their lives near their natal colony. The greatest distance traveled by a tagged animal is approximately 250 km. Predators include great white sharks, and presumably killer whales.
Feeding and Prey
Relatively little information exists on the diet of Australian sea lions. They are thought to concentrate their efforts on shallow–water benthic prey, but take a wide variety of fishes, such as rays, small sharks, Australian salmon, and whiting. Other prey includes squid, cuttlefish, small crabs, and occasionally to rarely penguins, flying seabirds, and small sea turtles. Fishermen complain of sea lions robbing lobster traps and fishing nets. Large prey items may be taken to the surface and shaken apart into swallowable portions.
Threats and Status
The total population of Australian sea lions was estimated at 9,300–11,700 in 1993 and was considered to be stable. This species is protected. A substantial sea lion viewing industry has developed, and is regulated at sea lion colonies in parks to minimize disturbance during the breeding season. Extensive disturbance can cause Australian sea lions to abandon colony sites.
Traditionally, Australian aborigines and early colonists took sea lions for food and other products. Harvests by sealers in the 17th and 18th centuries reduced the population and extirpated them from areas around Bass Strait and Tasmania. Although now protected, the population has not rebounded fully in numbers or reoccupied all of its former range. Conflicts and interactions with fisheries exist and some sea lions are shot as a result, while others are entangled in fishing gear, although the extent of the problem is not fully understood.
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