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.
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.
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.
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.
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.
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.