Engineers and biologists join forces to reveal how seals evolved to swim

Seals and sea lions are fast swimming ocean predators that use their flippers to literally fly through the water. But not all seals are the same: some swim with their front flippers while others propel themselves with their back feet.

In Australia, we have fur seals and sea lions that have wing-like front flippers specialised for swimming, while in the Northern Hemisphere, grey and harbor seals have stubby, clawed paws and swim with their feet. But the reasons why these two different ways of swimming evolved has perplexed biologists for generations. Is one style better than the other?

But now a Monash University-led interdisciplinary study published in Current Biology has used cutting-edge computer simulations alongside footage of live seals to finally answer this evolutionary mystery.

"The difference in swimming style between forelimb and hindlimb propelled seals is so great that these groups were originally thought to have evolved from separate land-dwelling ancestors," said lead study author Dr David Hocking from the Monash University School of Biological Sciences, "but the genetics clearly shows that all living seals come from the same group of animals."

The question is, how could such different swimming styles evolve within the one animal group?

To reveal the answer, Dr Hocking teamed up with Monash University engineer Dr Shibo Wang from the Department of Mechanical and Aerospace Engineering, and used advanced Fluid Dynamics simulations to show how water flows around seal flippers of different shapes.

"Our analysis showed that some Antarctic seals, like leopard seals, actually have very streamlined, wing-like forelimbs, despite being from the 'foot-propelled' seal family," said Dr Wang. This is supported by footage of live leopard seals swimming at Taronga Zoo, where they used their front limbs to swim in a similar way to the fur seals and sea lions.

This discovery shows how wing-like flippers can evolve in seals that already swim with their back feet, providing a pathway for the evolution of forelimb swimming in the fur seals and sea lions.

"Wing-like flippers help leopard seals to surge forward and ambush fast-swimming penguins," said Associate Professor Alistair Evans who also collaborated on the study, "and it seems likely that the earliest sea lions also needed this extra speed to capture their preferred prey: schooling fish."

"We finally have a window into the early evolution of swimming in seals," Dr Evans said.

But as well as explaining the origin of seals, this study also has a more practical outcome: improved human designs. "Seals have had millions of years to perfect their swimming, and they can teach us a thing or two about underwater grace and elegance," said Dr Hocking. "Learning from them may help us to improve the design of human-built machines like underwater drones and submersibles, increasing their speed, manoeuvrability or energy efficiency."

Who knows? Perhaps in future we will be able to know how it feels to swim like a seal thanks to seal-shaped submarines.