How seals adapted to move through water

Have you ever wondered how seals learned to perform their skilled underwater acrobatics?

Zoologist David Hocking, formerly of Monash University and now curator of vertebrate zoology and palaeontology at the Tasmanian Museum and Art Gallery, did, and says he has now helped “to solve an evolutionary riddle at the heart of seal evolution”.

Seals and sea lions propel themselves through the water to catch their prey – but true seals (otariids) generally use their front flippers while eared seals (phocids) use their back feet, and the other limbs are used for steering.

Originally, the profoundly different swimming styles were thought to reflect separate ancestries, but genetic analysis shows the pinnipeds come from the same group. A new study led by Hocking and published in Current Biology now attributes it to environmental adaptation over millions of years.

When observing feeding behaviour in the charismatic Antarctic leopard seals (Hydrurga leptonyx), which are phocids, Hocking says he noticed how big their front flippers were and how often they used them to swim, which was surprising because they’re thought to move with their back feet.

“Instead, I was watching leopard seals using their wing-like flippers just like a fur seal or sea lion – by flapping them to literally fly through the water,” he says.

“From that moment, I became fascinated by the question of how these air-breathing mammals first learned to swim and why the different families show such different swimming styles.”

Using computational fluid dynamics simulations, Hocking and colleagues revealed that wing-like flippers evolved in leopard seals that already swam with their back feet – presumably to give them extra speed for catching their preferred prey, notes senior author Alistair Evans.

They created the models by collecting video footage of seals from zoos and aquariums and used medical CT scanning to create 3D models of different shaped flippers for four species. Comparing the shape, position and mobility of the bones gave them insights into how the limbs can be used for different tasks.

Once they had generated the 3D models, the team created high-tech engineering simulations of how water flows around the flippers during swimming – the same technology that aircraft engineers use to design the wings of airplanes and space shuttles – to explore the link between anatomy and movement.

“Interestingly, the grey seal we examined at the Natural History Museum in London, UK, has paw-like hands bearing strong claws,” says Hocking. “This allows these seals to use their limbs for tasks like grooming and holding food, but it also makes them less streamlined.

“In contrast, leopard seals have greatly reduced their claws and stretched out the length of the flipper to form a more effective paddle for swimming. In doing so they have independently evolved wing-like flippers similar to those of the other main family of seals, the Otariidae, which includes the fur seals and sea lions.”

It’s one of the first studies to closely examine seal flippers as biomechanical tools adapted for swimming, according to Hocking, and helps fill in some vast gaps left by a limited fossil record.

The findings help shine a light on how the animals evolved their body and behaviour to adapt to marine life – an incredible feat.

“The switch from life on land to life in water is one of the biggest transitions a species can undergo in its evolution,” says Hocking. “This has happened multiple times, with groups like whales, sea turtles, sea cows and crocodiles, all evolving from land-dwelling ancestors that have adapted themselves for a life at sea.”

“This is fundamental knowledge that helps us to understand how the huge diversity of life we see around us first evolved on our planet.”

This video shows different seal species in action and explains how each swims. Credit: David Hocking.

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