Leopard seal
The leopard seal, also referred to as the sea leopard, is the second largest species of seal in the Antarctic. It is a top order predator, feeding on a wide range of prey including cephalopods, other pinnipeds, krill, fish, and birds, particularly penguins, its only natural predator being the orca. It is the only species in the genus Hydrurga. Its closest relatives are the Ross seal, the crabeater seal, and the Weddell seal, which are all Antarctic seals of the tribe Lobodontini.
Research history and taxonomy
, a French zoologist, described the leopard seal in 1820 from a stuffed specimen from the collection of one M. Hauville, in Le Havre. The skin that produced this work of taxidermy was sourced from "the southern seas", that he ascertained to be from around the Falkland Islands. Blainville describes the specimen as "beautiful", 7-8 pieds long and elongated, the form and features of the head resembling the "phoque moine" or monk seal, with a small number of whiskers short and simple in shape. Blainville was not able to find the ear opening. The "anterior" limbs are falciform, consisting of five fingers which decrease in size from the first to the fifth, tipped with very small claws, with that of the thumb not being "terminal". The hindlimbs are very distal and close to the short tail; these also consist of five digits, though in contrast to the forelimb, the middle toe is the shortest, with the rear flippers forming a "swallowtail". The skin he examined was described as yellow-white with a brownish dorsum; this is evidently a result of the skin discoloring, as taxidermies degrade over time. He thought that this new species, Phoca leptonyx, must correspond to the animal that "navigators of austral seas" named the "sea calf".Blainville found it comparable to a specimen he refers to as the "third skull" he examined and described in the same work, possessing the same number and shape of the incisors. This skull, unlabeled and thus of unknown locality, was noted as fresher and more bleached than the second skull he examined, being 10-12 pouces long. At first glance, he found that the skull had significant resemblance to the skull of the "phoque commun" or common seal, moreso than the previous two; though this skull was of bigger dimensions, was not as flattened nor narrowed post-orbitally, and notably had a well developed sagittal crest and enlarged nasal fossae. The robust molars of this skull possesses two roots and three cusps, pointed and prominent with the central cusp being the tallest. Five molars were present on each side of the jawbone, after the robust canines, and the two incisors, the outer pair of incisors resembling the shape of the canines.
The genus Hydrurga was erected by Johannes von Nepomuk Franz Xaver Gistel in 1848.
Description
The leopard seal has a distinctively long and muscular body shape when compared to other seals. The overall length of adults is and their weight is in the range, making them the same length as the northern walrus but usually less than half the weight. They are covered in a thick layer of blubber that helps to keep them warm while in the cold temperatures of the Antarctic. This blubber also helps to streamline their body making them more hydrodynamic, and thus able to hunt down swift prey. A seal's body condition can be ascertained through blubber thickness, along with general girth, weight, and length measurements.Females are larger than males by up to 50%; the leopard seal is sexually dimorphic. In the wild, leopard seals can live up to 26 years.
It is perhaps best-known for its massive jaws, which allow it to be one of the top predators in its environment. The seal's canine teeth are up to long. These and the incisor teeth are sharp like those of other carnivores, but their molars lock together in a way that allows them to "sieve" krill from the water. This characteristic is shared with the other species in the tribe Lobodontini such as the crabeater seal, and is the trait that gives the tribe its name.
As "true" seals of the family Phocidae, they do not have external ears or pinnae, but possess an internal ear canal that leads to an external opening. Their hearing in air is similar to that of a human, but scientists have noted that leopard seals use their ears in conjunction with their whiskers to track prey under water. The whiskers are short and clear.
Their front flippers are extremely large in comparison to other phocids. Their large front flippers are used to steer themselves through the water column making them extremely agile while hunting. They use their front flippers similarly to sea lions
The pelage is counter-shaded; consisting mainly of a blend of silver and dark-gray, with a distinctive spotted leopard-like pattern on the dorsum, and a white to light gray color ventrally.
Distribution
Leopard seals are pagophilic seals, which primarily inhabit the Antarctic pack ice between 50˚S and 80˚S. Higher densities of leopard seals are seen in West Antarctica than in other regions.Most leopard seals remain within the pack ice throughout the year and remain solitary during most of their lives with the exception of a mother and her newborn pup. These matrilineal groups can move further north in the austral winter to sub-antarctic islands and the coastlines of the southern continents to provide care for their pups. While solitary animals may appear in areas of lower latitudes, females rarely breed there. Some researchers believe this is due to safety concerns for the pups.
The estimated population of this species ranges from 220,000 to 440,000 individuals, putting leopard seals at "Least Concern". Although there is an abundance of leopard seals in the Antarctic, they are difficult to survey by traditional audiovisual techniques as they spend long periods of time vocalizing under the water's surface during the austral spring and summer, when audiovisual surveys are carried out. This habit of submarine vocalizing makes leopard seals naturally suited for acoustic surveys, as are conducted with cetaceans, allowing researchers to gather most of what is known about them.
Sightings of vagrant leopard seals have been recorded on the coasts of Geraldton, Western Australia, multiple locales in New Zealand, South America, and South Africa. Fossil evidence suggests that leopard seals were present in South Africa during the Late Pleistocene.
Behavior
Using data received from transmitters called satellite-linked depth recorders and time-depth recorders, which are attached to the seals' heads by scientists, it was determined that leopard seals are primarily shallow divers, but capable of diving deeper than in search for food. They are able to complete these dives by collapsing their lungs and re-inflating them at the surface. This is possible by increasing the amount of surfactants which coats the alveoli in the lungs for re-inflation. They also have a reinforced trachea to prevent collapse at great depth pressures.These seals feed on a wide variety of creatures; young leopard seals usually eat mostly krill, squid, and fish. Adults are able to take on more difficult but substantial prey, famously including emperor, king, rockhopper, Adélie, gentoo, and chinstrap penguins, though they also prey on other seal species such as Weddell, crabeater, Ross, young southern elephant seals, and fur seal pups.
Research shows that on average, the aerobic dive-limit for juvenile seals is around 7 minutes, which means that during the winter months juvenile leopard seals do not eat krill, which is a major part of older seals' diets, since krill is found deeper during this time. This might occasionally lead to co-operative hunting. Co-operative hunting of leopard seals on Antarctic fur seal pups has been witnessed, which could be a mother helping her older pup, or could also be female-male couple-interactions, to increase their hunting-productivity.
Around the sub-Antarctic island of South Georgia, the Antarctic fur seal is the main prey. Antarctic krill, southern elephant seal pups and petrels such as the diving petrel and the cape petrel have also been taken as prey. Vagrant leopard seals in New Zealand have been observed preying on chondrichthyans: elephantfish, ghost sharks, and spiny dogfish were recorded as prey items. Additionally, this population of leopard seals and those in Australia were noted to bear wounds from chimaeriforms and stingrays respectively.
When hunting penguins, the leopard seal patrols the waters near the edges of the ice, almost completely submerged while waiting for the birds to enter the ocean. It kills the swimming bird by grabbing the feet, then shaking the penguin vigorously and beating its body against the surface of the water repeatedly until the penguin is dead. Previous reports stating that the leopard seal skins its prey before feeding have been found to be incorrect. Lacking the teeth necessary to slice its prey into manageable pieces, it flails its prey from side to side tearing and ripping it into smaller pieces.
Krill is eaten by suction, and strained through the seal's teeth, allowing leopard seals to switch to different feeding styles. Such generalization and adaptations may be responsible for the seal's success in the challenging Antarctic ecosystem.
The only natural predator of leopard seals is the orca.
Acoustic behavior
Leopard seals are very vocal underwater during the austral summer. The male seals produce loud calls for many hours each day. While singing the seal hangs upside down and rocks from side to side under the water. Their back is bent, the neck and cranial thoracic region is inflated and as they call their chest pulses. The male calls can be split into two categories: vocalizing and silencing; vocalizing is when they are making noises underwater, and silencing noted as the breathing period at the air surface. Adult male leopard seals have only a few stylized calls, some are like bird or cricket-like trills yet others are low haunting moans. Scientists have identified five distinctive sounds that male leopard seals make, which include: the high double trill, medium single trill, low descending trill, low double trill, and a hoot with a single low trill. These cadences of calls are believed to be a part of a long range acoustic display for territorial purposes, or to attract a potential mate.The leopard seals have age-related differences in their calling patterns, just like birds. The younger male seals have many different types of variable calls, but the mature male seals have only a few, highly stylized calls. Each male leopard seal produces these individual calls, and can arrange their few call types into individually distinctive sequences. The acoustic behavior of the leopard seal is believed to be linked to their breeding behaviour. In male seals, vocalizing coincides with the timing of their breeding season, which falls between November and the first week of January; captive female seals vocalize when they have elevated reproductive hormones. Conversely, a female leopard seal can attribute calls to their environment as well; however, usually it is to gain the attention of a pup, after getting back from a forage for food.