Leopard seals suck (and sieve)

Leopard seals are top predators in the waters around Antarctica, where they are infamous for their relentless predation upon penguins and young seals. However, new research shows how these top predators are also able to “filter feed” on krill by using their ferocious-looking cheek teeth as a delicate sieve.

Antarctica’s leopard seals and their diet

Few predators have a more ferocious (or well earned) reputation than Antarctica’s leopard seals. Early researchers called them the “principal enemy of the penguins” and today many people know them best as the bad guy from Happy Feet. After ambushing their feathered prey, they hold it in their long curved teeth and thrash it at the water’s surface, tearing it into swallowable chunks of flesh.

Wild leopard seal scaring off an Antarctic sheathbill that ventured too close while it was resting on the ice. Photo by James Robbins (BAS) at Bird Island, South Georgia.
Wild leopard seal scaring off an Antarctic sheathbill that ventured too close while it was resting on the ice. Photo by James Robbins (BAS) at Bird Island, South Georgia.

But seabirds aren’t the only large animals on their menu. These big predators (which can reach over 3 m long and weight up to 500kg) also target other seals. Around 78% of adult crabeater seals living in the pack-ice bear long parallel scars on their backs; a badge of honour that marks those that survived leopard seal attacks in their youth. They also target fur seals and even young elephant seals, which they kill by piercing the braincase with their long canines during repeated bites to the head. In the whaling days, leopard seals were even seen scavenging whale carcasses.

But interestingly, rather than specialising solely on the large prey upon which their reputation is based, studies of their wild diet have also shown that leopard seals often eat smaller prey, in particular Antarctic Krill. The question is then, how does such a large toothy predator capture and eat such tiny prey?

To explore this we worked with with Casey and Sabine, two adult leopard seals from Taronga Zoo (Australia), where they were living in human care after being found as juvenile vagrants near Sydney a long way from their Antarctic home.

Feeding apparatus designed to test leopard seal suction feeding abilities.
Feeding apparatus designed to test leopard seal suction feeding abilities. Photo: Ady D’Ettore.

We presented the seals with small fish in a special feeding apparatus. The seals could see the fish through the sides of apparatus, but were unable to use their teeth to get hold of them because the fish were surrounded by the device. Instead, seals had to suck the fish out.

Suction feeding and the sieve

Suction feeding is where animals create lower pressure inside their mouths relative to the pressure around them (generally by retracting their tongue in mammals), causing any prey swimming in front of the mouth to be sucked in along with its surrounding water. The problem with suction feeding, though, is that animals then need to get rid of this extra water before they can swallow their catch. This is where leopard seals do something interesting.

After capturing the fish by suction, the leopard seals in our study closed their jaws and forced the water captured along with prey out via the sides of the mouth. Their flabby cheeks seemed to control the flow of water, and to exit the mouth, the water also had to pass between the seal’s trident-shaped cheek teeth. Any food captured by suction is therefore trapped behind the sieve formed by the leopard seal’s teeth.

Feeding cycle in leopard seals when targeting small prey: Seals strike out with their heads on a long neck (1-3) before the prey item is captured by suction (3). The prey item is then separated from seawater as the water is expelled via the sieve created by the complex cheek teeth (4-6). This can be seen from the cloud of bubbles exiting the sides of the mouth.
Feeding cycle in leopard seals when targeting small prey: Seals strike out with their heads on a long neck (1-3) before the prey item is captured by suction (3). The prey item is then separated from seawater as the water is expelled via the sieve created by the complex cheek teeth (4-6). This can be seen from the cloud of bubbles exiting the sides of the mouth. Photo: David Hocking.

In the wild, this feeding behaviour likely enables leopard seals to suck in a mouthful of krill-laden water, before expelling the excess water and swallowing the krill in bulk. This is similar to how baleen whales like blue whales feed, except that these use ‘engulfment’ or ‘lunging’ to capture prey inside the mouth before it is sieved, rather than suction. Watch the Video Here.

This all comes down to the shape of their highly specialised teeth. When pursuing large prey, like penguins, they use their long, sharp canines and robust front teeth to kill and break apart their catch by thrashing it at the water’s surface. But when they feed on krill or small fish, they use suction to capture prey inside their mouth, before sieving it from the seawater with their trident-shaped cheek teeth. The unique shape of their teeth allows leopard seals to feed efficiently at both the top and bottom of the Antarctic food web, increasing their feeding options and therefore, their chances of survival.

Leopard seal skull and dentition: Long and sharp front teeth (canines and incisors) are used to catch and kill large prey, while the trident-shaped cheek teeth are used as a sieve when targeting small prey.
Leopard seal skull and dentition: Long and sharp front teeth (canines and incisors) are used to catch and kill large prey, while the trident-shaped cheek teeth are used as a sieve when targeting small prey.

However, while we’ve now seen these behaviours displayed in captivity, we still haven’t observed them directly in the wild. Historically, this has been very difficult to do because of the challenges faced when observing these animals in their pack-ice habitat. But advances in research technology are now making it easier to study leopard seals in their own environment.

Future studies

Recently, Krause et al. (2015) used CRITTERCAM to film leopard seals while hunting for seals, penguins and fish. This study provides some amazing insights into how these animals find and capture their food, as well as how they interact with each other while fighting over food. Hopefully future studies can make use of these exciting new technologies to catch the first glimpses of leopard seals using suction and filter feeding to consume krill in the wild.

For more info check our our original study.

Read full article: Leopard seals (Hydrurga leptonyx) use suction and filter feeding when hunting small prey underwater.

David Hocking

David Hocking

Postdoctoral Fellow at Monash University
David Hocking is a Postdoctoral Fellow at Monash University (Australia) where he studies the evolution of feeding in aquatic mammals. Pictured here alongside archaic fossil pinniped Acrophoca. Follow him on twitter @DPHocking.
David Hocking

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