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The leopard seal (Hydrurga leptonyx), also referred to as the sea leopard, is the second largest species of seal in the Antarctic (after the southern elephant seal). Its only natural predator is the killer whale. It feeds on a wide range of prey including cephalopods, other pinnipeds, krill, birds and fish. It is the only species in the genus Hydrurga. Its closest relatives are the Ross seal, the crabeater seal and the Weddell seal, which together are known as the tribe of Lobodontini seals. The name hydrurga means “water worker” and leptonyx is the Greek for “small clawed”.

French zoologist Henri Marie Ducrotay de Blainville described the leopard seal in 1820.

The leopard seal has a distinctively long and muscular body shape, when compared to other seals. This species of seal is known for its massive reptilian-like head and jaws that allow it to be one of the top predators in its environment. A notable key feature of leopard seals is its counter-shaded coat. A counter-shaded coat is when the dorsal side of the coat is darker than ventral side. Leopard seals have a silver to dark gray blended coat that makes up its distinctive “leopard” coloration with a spotted pattern, whereas the ventral side of the coat is paler in color—ranging from white to light gray. Females are slightly larger than the males. The overall length of this seal is 2.4-3.5 m (7.9-11.5 ft) and weight is from 200 to 600 kilograms (440 to 1,320 lb). They are about the same length as the northern walrus, but usually less than half the weight.

Another notable characteristic of leopard seals are their short, clear whiskers that are used to sense their environment. Leopard seals have an enormous mouth relative to their body size. The front 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, in the manner of the crabeater seal. Since leopard seals are “true” seals, they do not have external pinnae, or ears, but they do have 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.

Leopard seals are pagophilic, “ice-loving” seals, which primarily inhabit the Antarctic pack ice between 50˚S and 80˚S. Sightings of vagrant leopard seals have also been recorded on the coasts of Australia, New Zealand, South America, and South Africa. In August 2018 an individual was sighted in Geraldton on the west coast of Australia. Higher densities of leopard seals are seen in the Western Antarctic 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. Lone male leopard seals hunt other marine mammals and penguins in the packed ice of antarctic waters. The estimated population of this species ranges from 220,000 to 440,000 individuals, which puts leopard seals at “least concern”. Although with an abundance of leopard seals in the antarctic, they are difficult to survey by traditional visual techniques because they spend long periods of time vocalizing under the water during the austral spring and summer when visual surveys are carried out. This trait of vocalizing underwater for long periods has made them available to acoustic surveys, allowing researchers to gather most of what is known about them.

Leopard seals are very vocal underwater during the austral summer.
The male seals produce loud calls (153 to 177 dB re 1 mPa at 1 m) 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 (the chest) is inflated and as they call their chest pulses. The male calls can be split into two categories: vocalizing and silencing, in which 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. However, 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 cadence of calls are believed to be a part of a long range acoustic display for territorial purposes, or the attraction of a potential mate. The leopard seals have age-related differences in their calling patterns, just like birds. Where the younger male seals have many different types of variable calls – the adult 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 (or songs). 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.

Since leopard seals live in an area difficult for humans to survive in, not much is known on their reproduction and breeding habits. However, it is known that their breeding system is polygynous, meaning that males mate with multiple females during the mating period. A sexually active female (ages 3-7) can give birth to a single pup during the summer on the floating ice floes of the Antarctic pack ice, with a sexually active male (ages 6-7). Mating occurs from December to January, shortly after the pups are weaned when the female seal is in estrus. In preparation for the pups, the females dig a circular hole in the ice as a home for the pup. A newborn pup weighs around 66 pounds and are usually with their mother for a month, before they are weaned off. The male leopard seal does not participate in taking care of the pup, and goes back to its solitary lifestyle after the breeding season. Most leopard seal breeding is on pack ice.

Five research voyages were made to Antarctica in 1985, 1987 and 1997-1999 to look at leopard seals. They sighted seal pups from the beginning of November to the end of December, and noticed that there was about one pup for every three adults, and they also noticed that most of the adults were staying away from other adults during this season, and when they were seen in groups they showed no sign of interaction. Leopard seal pups mortality rate within the first year is close to 25%.

Vocalization is thought to be important in breeding, since males are much more vocal around this time. Mating takes place in the water, and then the male leaves the female to care for the pup, which the female gives birth to after an average gestation period of 274 days.

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.

The only natural predators of leopard seals is the killer whale and possibly the elephant seal. Its canine teeth are 2.5 cm (1 in). It feeds on a wide variety of creatures. Young leopard seals will usually eat mostly krill, squid and fish. Adult seals probably switch from krill to more substantial prey, including king, Adelie, rockhopper, gentoo, emperor and chinstrap penguins, and less frequently, Weddell, crabeater, Ross, and young southern elephant seals. Leopard seals are also known to take fur seal pups.

Around the sub-Antarctic island of South Georgia, the Antarctic fur seal (Arctocephalus gazella) is the main prey. Other prey include penguins and fish. Antarctic krill (Euphausia superba), southern elephant seal (Mirounga leonina) pups and seabirds other than penguins have also been taken as prey.

When hunting penguins, the leopard seal patrols the waters near the edges of the ice, almost completely submerged, 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 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 meanwhile, 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.

Leopard seals’ heads and 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 (otariids) and leopard seal females are larger than males. They are covered in a thick layer of blubber that helps to keep them warm while in the cold temperatures of the Antarctic. This layer of blubber also helps to streamline their body making them more hydrodynamic. This is essential when hunting small prey items such as penguins because speed is necessary. Scientists take blubber thickness, girth, weight, and length measurements of leopard seals to learn about their average weight, health, and population as a whole. These measurements are then used to calculate their energetics which is the amount of energy and food it takes for them to survive as a species. They also have incredible diving capabilities. This information can be obtained by scientists by attaching transmitters to the seals after they are tranquilized on the ice. These devices are called satellite-linked time depth recorders (SLDRs) and time-depth recorders (TDRs). Scientists attach this device usually to the head of the animal and it records depth, bottom time, total dive time, date and time, surface time, haul out time, pitch and roll, and total number of dives. This information is sent to a satellite where scientists from anywhere in the world can collect the data. This is how we are currently learning so much about leopard seals diet and foraging habits. With this information we are able to calculate and better understand their diving physiology. They are primarily shallow divers but they do dive deeper than 80 meters 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 surfactant which coats the alveoli in the lungs for re-inflation. They also have a reinforced trachea to prevent collapse at great depth pressures.

Leopard seals are apex predators presenting a potential risk to humans. However, attacks on humans are rare. Most human perceptions of leopard seals are shaped by historic encounters between humans and leopard seals, that have occurred since the Heroic Age of Antarctic exploration.

Examples of aggressive behaviour, stalking and attacks have been documented. Notable incidents include:

In a report read at the inquiry into Brown’s death, Professor Ian Boyd from St. Andrews University stated that the seal may have mistaken her for a fur seal or may have been frightened by her presence and attacked in defense. Professor Boyd said that Leopard Seal attacks on humans were extremely rare but warned that they may potentially become more common due to increased human presence in Antarctica. The coroner recorded a verdict of an accidental death caused by drowning due to a leopard seal attack.

Leopard seals have shown a predilection for attacking the black, torpedo-shaped pontoons of rigid inflatable boats, necessitating researchers to equip their craft with special protective guards to prevent them from being punctured.

Paul Nicklen, a National Geographic magazine photographer, captured pictures of a leopard seal bringing live, injured, and then dead penguins to him, possibly in an attempt to teach the photographer how to hunt.

From a conservation standpoint, the only known predators of the leopard seals are killer whales and sharks. Because of their limited subpolar distribution in the Antarctic, they may be at risk as polar ice caps diminish with global warming. In the wild, leopard seals can live up to 26 years old. Leopard seal hunting is regulated by the Antarctic Treaty and the Convention for the Conservation of Antarctic Seals (CCAS).