Why does a seal swim with its abdomen and a sea lion with its front fins? Go forward!
Very funny, intelligent, but where does this difference in swimming technique come from in very similar animals? The difference is so remarkable that biologists have long suspected that seals and sea lions had different ancestors. But genetics refuted that: the species are so genetically related that they must come from the same lineage.
Seals and sea lions are pinnipeds. They are considered by biology to belong to the predator family and evolved from predators that lived on land and at one point moved their hunting grounds to coastal waters. This has resulted in such a variety of species (and swimming techniques) that this walk into the water may not have been a one-time event, but has occurred many times; in different places, under different conditions, with different results. But we don’t know exactly how.
To help solve this evolutionary puzzle, researchers at Monash University in Australia took a closer look at the swimming techniques of marine predators. And biologists were smart enough to call in engineers with experience in aerodynamics. From their joint research, which has just been published in the specialized journal Current biology, it appears that the swimming techniques of pinnipeds are less strictly separated than previously thought. There hasn’t been a class of butterfly swimmers here and a class of caterpillars before there; all pinnipeds started out as foot swimmers, using their hind fins to move forward.
This has remained the main driving force behind the seals. Their front fins still show where they came from; they look like claws, like a bear. And the seals can still spread their fingers. Useful for holding prey, but not ideal for swimming performance The seal primarily uses the front fins for cornering and braking, but they have too much resistance to generate a lot of speed.
It’s different with sea lions. Their front fins no longer look like a bear claw. They still have five fingers, but they are connected by a hard cloth and can hardly move relative to each other. The metacarpals have become straight and the differences in length are greater, ranging from the index, which is the longest, to the little finger. This produces front fins that resemble airplane wings, with the characteristic teardrop-shaped cross section: thick in the front, thin in the rear.
Catching prey is much more difficult, but the front fins have become a more powerful stroke engine than the rear fins. Development in this branch of the pinniped family must have been driven by an evolutionary advantage, the researchers say. And they mainly think of fast prey, such as penguins and schools of fish, which were within reach of sea lions due to their faster swimming speed. Under certain living conditions, this advantage may be greater than the absence of claws.
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