Seals have evolved two different ways to swim

Despite living in the same environment and doing largely the same things, seals have evolved two distinct ways to swim.

One group of seals chiefly use their feet to propel them through the water, while the other uses their flippers to swim. This is curious, as both groups evolved from the same land-dwelling ancestor that slipped into the sea around 30 million years ago.

New research looking into how and why these two groups evolved such different swimming styles has combined anatomical and live animal observations with a team of engineers and computer simulations to figure out exactly what is going on.

The researchers, based at Monash University but also including Museum scientists, found that both groups of seals were indeed using their flippers and tails to swim in different ways, but crucially that seals which used to swim with their tails can actually evolve to swim with their flippers, as is seen with Antarctic leopard seals.    

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

The great divide

There are two main groups when it comes to seals and how they swim.

Seals that live predominantly in the northern hemisphere, such as grey and harbour seals, are from a group known as the true seals, or phocids. These have stubby, clawed paws which they can use to manipulate food and propel themselves through the water using their feet.

But it also includes one prominent outlier: the leopard seal. These huge predators live in Antarctica where they hunt down speedy penguins, and instead have incredibly streamlines front flippers.

The fur seals and sealions from the southern hemisphere like those which live around Australia, belong to a group known as the eared seals or otariids. These seals have long, paddle-like front flippers which they use to 'fly' underwater.

Dr David Hocking, also from Monash University and who led the study, explains, '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.

'But the genetics clearly shows that all living seals come from the same group of animals.'

This then got the researchers interested in why these two groups have evolved two different ways to solve the same problem.

Scanning seal flippers

By looking in detail at the anatomy of the flippers, they were able to simulate how the flippers move through water and how this water flows over the surface of the limbs. To do this, they used specimens of seals found stranded and from collections such as the Museum. 

Dr Travis Park, a researcher at the Museum, says, 'One of the key specimens from the paper is the female grey seal that was dissected at the museum in June 2018. We then took it to the Royal National Orthopaedic Hospital to have it CT and MRI scanned.

'It's obviously a tragedy that this animal was lost from nature, but through the efforts of CSIP and NHM staff, as well as the NHS staff who scanned the animal for us, we were able to turn this loss into a new understanding of how swimming in seals has evolved.'

The work has shown how both leopard seals and eared seals have evolved similar flippers despite coming from separate groups of the seal family tree, allowing them to chase and catch quick moving prey.

But for the leopard seals, this has come at a cost.

'We found that Grey seals still use their paws to hold their prey when processing it, but other seals like Leopard seals have foregone this ability to maximise their swimming speed and agility, being able to capture more mobile prey,' says Travis.

The work could also help out in the development of new technology. By figuring out how these adept swimmers have evolved the perfect shapes to move quickly through the water, it is hoped that the form could also be applied to submarines or underwater drones to improve their efficiency.