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Eat Prey, Sieve

Casey presents his teeth during a dental inspection at Taronga Zoo.

Casey presents his teeth during a dental inspection at Taronga Zoo.

By David Hocking

The ability to ambush, capture and tear apart penguins at the water’s edge has earned leopard seals a fearsome reputation. However, new research suggests that these top predators are also able to “filter feed” on krill by using their ferocious-looking cheek teeth as a delicate sieve.

Swimming to shore at the end of a feeding trip is no easy task for penguins living in the cold waters that surround Antarctica. As they dive beneath the ice, through the clear blue water, they must be constantly on the lookout for a cunning predator. Leopard seals stalk these waters. Each time a penguin enters or leaves the water, they run the risk of being ambushed by a hungry seal.

Leopard seals are one of the top predators within the Antarctic ecosystem, where they hunt for their prey among the icebergs in the cold but productive waters of the Southern Ocean. As top predators in this ecosystem, these seals hunt large prey that includes penguins and the young of other seals. Once captured, large prey is held between the sharp canines and shaken apart into bite-sized chunks at the water’s surface.

The leopard seal’s fierce reputation is not helped by reports of attacks on researchers working in Antarctica, where they have been known to launch themselves out of the water in pursuit of what they must see as giant colourful penguins standing at the water’s edge. However, as well as feeding on large prey like penguins and other seals, leopard seals also feed extensively on much smaller prey, most importantly Antarctic krill.

How can such a large predator that is adapted to feeding on large prey also take food as small as krill? There are other large predators, most notably the baleen whales, which also feed on tiny krill, but they do so using a highly specialised baleen sieve that allows them to efficiently capture their prey in bulk while separating it from the surrounding seawater prior to swallowing. In contrast, leopard seals have sharp, well-developed teeth that seem extremely well-suited to capturing and killing large prey. How can these same teeth also be used to feed on such small food items?

As unlikely as it sounds, one possibility is that leopard seals, just like baleen whales, are able to perform a type of “filter feeding” behaviour to capture small prey like krill. This idea comes from the shape of their three-cusped cheek teeth which, while sharp, also appear to form a tight interlocking sieve when the jaws are closed. By using these elaborate cheek teeth as a sieve, rather than simply as tools for piercing and holding larger prey, leopard seals may be able to efficiently feed on small prey in a way that very few other large predators can. However, because of the harsh environment in which these seals live, no researcher has ever been able to directly observe leopard seals while hunting small prey underwater in the wild to see how they use their cheek teeth.

A Trip to the Zoo

I first came across this problem as an Honours student in 2010, when I was trying to work out how seals with differently-shaped teeth are able to use these teeth to feed on various types of prey. I soon realised that in order to work out how leopard seals are capable of feeding on small as well as large prey, I would need to directly observe a live leopard seal as it performed its feeding behaviours underwater. After discussing this problem with my supervisors, Dr Alistair Evans, a zoologist with expertise in animal teeth, and Dr Erich Fitzgerald, a palaeontologist specialising in the evolution of marine mammals, we decided that rather than attempting to track these animals down in the wilds of the Antarctic, our best option might instead be to take a trip to the zoo. By studying zoo animals we would have a unique opportunity to make detailed observations of the feeding behaviours under controlled conditions. This led us to Taronga Zoo in Sydney, which is the only institution in the world to have leopard seals in human care.

It was here that we first met Casey and Sabine. Both of these seals were first brought to Taronga Zoo after they were found exhausted and in poor health on beaches in New South Wales, a long way from their Antarctic home. It is actually quite common for “vagrant” leopard seals to drift northward, where they are occasionally sighted along the southern coasts of South America and Africa, as well as Australia and New Zealand. Usually these lost seals are in such poor health by the time they reach our shores that they don’t survive in the wild, which is why Casey and Sabine were instead brought into human care at Taronga Zoo. By working with these two seals we would finally be able to make direct observations of underwater feeding in leopard seals and hopefully solve the puzzle of how they use their teeth when capturing small prey underwater.

To make our observations we designed and built a special feeding apparatus consisting of a clear plastic box with tubes set into its side. By placing fish into these tubes and presenting it to the seal, we were able to film how leopard seals capture and consume small prey underwater.

Sucking Seals

The first time the box device was presented to Sabine, an adult female leopard seal, she initially approached it with caution to explore this new and interesting object. But upon seeing the fish contained within the tubes, she immediately struck out with her head towards the device using her long curved neck, before pursing her lips and sucking the fish into her mouth with a loud slurping noise (as recorded by an underwater microphone inside the box).

She then withdrew her head, closed her mouth and squirted the seawater that had been drawn into her mouth along with the fish back out via the sides of her mouth, while the fish itself was trapped inside her mouth behind the sieve created by her cheek teeth. This act of expelling excess seawater from the mouth likely allowed her to swallow the fish with minimal seawater while also “reloading” the mouth for the next suction feeding event. None of the fish were pierced and shaken apart using the teeth prior to swallowing, as is seen when wild seals feed on large prey, with this sieving function appearing to be the main role played by the teeth during underwater feeding on small prey.

This was the main evidence we needed to finally confirm that leopard seals are indeed able to use their cheek teeth as a sieve to perform “toothed filter feeding”. By repeating this observation multiple times for both Casey and Sabine we were able to precisely measure the timing of the feeding cycle, which has allowed us to compare these feeding behaviours between both individuals and between these seals and other suction-feeding marine mammals.

Teeth Telling a Story

Having finally made these observations, we then wanted to confirm whether this same type of filter feeding behaviour was also performed during feeding in the wild. We must always be cautious when working with zoo animals to ensure that the behaviours observed are representative of those seen in the wild. Because we were not able to travel to Antarctica to watch seals hunting in the wild directly, we decided instead to look at dental wear on the teeth of skulls from wild leopard seals, to see if this information could indicate what behaviours these seals had been performing during feeding when they were alive.

When an animal uses its teeth to pierce or chew its food, the food rubbing against the tooth surface actually causes that surface to slowly wear down over time. Because we know that leopard seals capture and kill large prey using their front teeth, especially their canines, we would therefore expect these teeth to show heavy signs of this type of abrasive tooth wear. In contrast, our live observations suggested that the cheek teeth are primarily used as a sieve and should therefore show little if any signs of abrasive tooth wear.

By looking for tooth wear on the front and back teeth of 26 leopard seal skulls from Museum Victoria, we were able to show that this hypothesis was correct . While abrasive tooth wear was common on the canines and incisors – indicating that these teeth are used to pierce and hold large prey – it was almost completely absent from the cheek teeth, suggesting that these teeth are performing a different function during wild feeding. This matched well with what we observed in the behaviours performed by Casey and Sabine at Taronga Zoo, where these cheek teeth were not used to pierce or chew apart their prey, but were instead used as a sieve to separate prey items from the surrounding seawater before swallowing.

Paradoxical Pinniped Predators

These observations help us to understand how leopard seals are able to do so well in their wild environment. The ability to use their front and back teeth for different functions, with the front teeth being used to capture and kill large prey and the back teeth being used to sieve krill, allows the leopard seal to function as both a top predator and as a planktivore within the Antarctic ecosystem.

Feeding on large prey enables leopard seals to receive more energy from each prey item compared with the amount of time spent hunting. But large prey is often difficult to catch, and smaller prey may be more abundant at some times of year. This is especially true around the Antarctic convergence, where the mixing of cold water from the south and warmer waters to the north fuels large populations of planktonic animals, especially krill.

The ability to feed on krill in bulk would enable these seals to take advantage of an abundant Antarctic resource that would otherwise be unavailable to such a large predator. This dual specialisation may be one of the key factors enabling these predators to thrive in the harsh conditions of the Great Southern Ocean.

David Hocking is a PhD candidate at Monash University and Museum Victoria.