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A seal who filters

The tiger seal is a super predator that filters tiny food from the water with special teeth, similar to whales

Yonat Ashhar and Noam Levithan Galileo

A tiger seal (Hydrurga leptonyx) growls. From Wikipedia.
A tiger seal (Hydrurga leptonyx) growls. From Wikipedia.

Think of the killer whale (Orcinus orca), black and white and elegant, bursting out of the water in pursuit of an unlucky seal, tearing its flesh with its sharp, strong teeth. And now, think of his relatives, from the same series, the minke whales (Mysticeti). The seal can swim near them without fear, because they have no teeth at all, and in their place there are long bristles that serve as a filter. Although this group includes the largest animal that ever lived - the blue whale (Balaenoptera musculus), members of the group feed on tiny creatures such as krill (tiny crustaceans). The whales hunt them by pumping a large amount of sea water and small creatures into their mouths, then expelling the water through the blowpipes. The krill remain in the whales' mouths and are swallowed.

Apparently, the two hunting methods could not be more different from each other. The two types of whales "chose" each, millions of years ago, A unique evolutionary path, which led to adaptations of the oral cavity, which seem not only different from each other but also contradicting each other. The killer whale's teeth would be useless if it tried to filter the water like a minke whale and of course the minke whales' noises would make the killer whales' prey laugh.

But it turns out that there are animals that at the same time specialize in hunting at both ends of the spectrum - they hunt large prey with large, sharp teeth, and krill with filtering. These animals are seals - and more precisely, tiger seals (Hydrorga tiger, Hydrurga leptonyx).

David Hocking and his colleagues from Monash University in Melbourne, Australia, studied the hunting methods of the tiger seal. This seal lives in Antarctic waters and on the southern coasts of Australia and South America, and is one of the main predators of penguins and smaller seals. It is equipped with impressive fangs, 2.5 centimeters long, and fang-like cutting teeth complete the look of a super predator ready for action. But penguins and seals are only part - and in some cases, a very small part - of the tiger seal's diet. They also hunt fish, large and small, and when there is no large prey around they hunt krill, which may make up more than 80% of their diet. And as krill hunters, they are not bad at all: in the stomach of one seal were found no less than 10,000 krill that had just been hunted.

How do seals hunt such small prey? After all, their long fangs cannot help them with this. In an article published in a magazine Polar Biology Hawking and his colleagues studied the method by which seals hunt small prey. They lowered tubes into the ponds in which there were small fish, so that the seals had to "suck" them into their mouths from the tube. The photographs of the seals in action showed that they are able to press their lips together and move their tongue back to create a vacuum, thus sucking the prey - and a large amount of water - into their mouths. Then came the really sophisticated part - the seals took the water out through the molars on the sides of the mouth, which served as a filter, and left the prey inside the mouth.

Researchers have speculated for some time that certain types of seals are capable of hunting small prey in this way. When their mouth is closed, the molars create a dense "wall" that even the smallest creatures are unable to pass through. Another clue to the original use of the seals' molars is that these teeth did not show the wear that was seen in the fangs and incisors, which suggests that the seals do not use these teeth for biting. Hawking's research shows for the first time that this method of suction and filtration is indeed carried out. The next step, according to an article published inNature news, would be to go to the shores of Antarctica and check if in the wild the seals actually hunt krill with this method.

Are the molars of tiger seals specially adapted to allow filtering of tiny animals? It is difficult to answer this question, but there is some evidence of this in the fact that not all seals and sea lions are able to hunt in this way. When the researchers gave the tubes containing the small fish to Californian sea lions, they were able to suck the fish into their mouths - but because their teeth are not able to create such a dense "sieve" as the tiger seals, when they removed the excess water from their mouths, they left with it, through the spaces between the teeth , the fish too.

Evolution of prey methods
Most predators take one of two approaches: either they specialize in a narrow niche - that is, they are able to hunt one type of prey and do it well, or they hunt a wide variety of animals without specializing in a particular type, and therefore are not as effective at hunting each of these animals as the specialists . The division of duties between the tiger seal's tusks, which are used to hunt large animals, and the molars, which are used to filter small prey, allows it to specialize in both types of prey, without paying the price of a decrease in the hunting efficiency of each type.

The filtering method of the seals is also interesting from the evolutionary point of view. Today, as mentioned, there are whales that have completely given up hunting large animals and have specialized in filtering, to the extent that they have lost their teeth. Was the first step in the evolutionary path that led to the sperm whales filtering with the help of the teeth, like the tiger seals?

Fossils of ancient whales suggest the answer is yes. The team of researchers who studied the way the seals ate also studied a fossil of An ancient whale named Janjocetus (Janjucetus, "Whale of Jean Jacques"), who lived about 27 million years ago. Janjocetus had teeth and no beaks, but various structures found in its skeleton, such as an extremely wide upper jaw, indicate that it is closer to the beaked whales than the toothed whales. The wide upper jaw allows modern sperm whales (and at one time also allowed Janjocetus) to pump a large amount of water and tiny prey into their mouths. The researchers hypothesize, then, that the filtering method was already developed in the Janjocetus, which, like the tiger seal, hunted both large and small prey. This was, apparently, the first step on the way to specializing in only tiny prey, which led the minke whales to lose their teeth and develop the impressive structures they are named after.

The original article:

Hocking, DP, Evans, AR & Fitzgerald, EMG Leopard seals (Hydrurga leptonyx) use suction and filter feeding when hunting small prey underwater. Polar Biol. 36, 211-222. doi: 10.1007/s00300-012-1253-9

The original article appears in the Galileo magazine issue 173, January 2013

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