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Ancient otter-like seal relative may have used whiskers to find food
Whiskers may have been crucial to allow seals to adopt a life in water.
New research suggests that whiskers helped to ease the transition from land to sea as seals adapted to an aquatic lifestyle.
An ancient relative of seals may have used its whiskers to forage underwater.
While modern seals rely heavily on their whiskers for finding food, these sensory hairs don’t survive in fossils. This has made it difficult to pin down exactly when whiskers first evolved.
Instead of looking for direct evidence of these hairs, a new study has taken a different approach. By looking at how the area of the brain connected to the whiskers changed over time, scientists have been given an insight into how important the hairs were.
This revealed that an early seal relative called Potamotherium valletoni probably used its hairs to forage for food underwater, potentially along with its hands.
Dr James Rule, a specialist in pinnipeds (seals, walruses and sea lions) at the Museum, says, ‘Not only do these brain casts suggest that Potamotherium depended on its whiskers, but they may also help to clarify if the animal is an early seal.’
‘Similarities in the coronal gyrus, the part of the seal brain which receives sensation from the face, between Potamotherium and two other early seals, Enaliarctos and Pinnarctidion, adds weight to the idea that Potamotherium might be an ancient pinniped after all.’
The findings of the study were published in the journal Communications Biology.
Even though they're part of the same group of animals, it's not certain exactly how bears relate to modern seals. Image © Tony Campbell/Shutterstock.
The evolution of seals
There are 34 different species of pinniped alive today, which are found all over the world. They are the descendants of an ancestor which split away from the rest of the carnivore family around 45 million years ago, but the exact path between then and now remains a mystery.
‘Seals are a member of a group of animals known as the arctoids, which include the pinnipeds, the musteloids (skunks, raccoons and otters), and bears,’ James says. ‘However, this area of the mammal family tree hasn’t been well investigated, so it’s not well known how the ancestors of this group fit together.’
‘It’s even more difficult for seals, because compared to animals like whales there aren’t very many good transitional fossils that show them moving from the land to the sea. The fossil record skips from pinnipeds being otter-like animals with distinct legs and arms to suddenly having flippers.’
As a result, it’s difficult to know where Potamotherium fits into seal ancestry. While some scientists think it’s one of the earliest lineages in the seal family tree, others think it could be a musteloid instead.
One difference between both groups is the way in which they find food. While seals primarily forage with their whiskers, otters often make use of their hands as well.
Comparing how developed the whiskers are could, therefore, help to answer some questions about early seals. However, as whiskers don’t fossilise, the researchers needed to identify another characteristic which could reveal more about these hairs.
One option was a structure in the skull known as the infraorbital foramen, where the nerves from whiskers pass into the skull.
‘Whiskers are used so that animals can touch things with their face and tell where things are in front of them,’ James says. ‘The infraorbital foramen is a gap in the skull located under the eye of mammals which allows the nerves from the face to pass through into the brain.’
‘While the size of the foramen is linked with the number of nerves in primates, it’s not certain whether this rule applies in other mammals like seals. Seals certainly have large infraorbital foramina, but this could mean they have a few large nerves, rather than many small ones.’
Scientists took what is known as a brain endocast of Potamotherium valletoni to see how the inside of the skull compared to its relatives. Image © Ghedoghedo, licensed under CC BY-SA 3.0 via Wikimedia Commons.
Getting inside the head
Instead of looking more closely at the infraorbital foramen, the researchers turned to the inside of the skull. The shape of the cranial cavity often reflects the structure of the brain in carnivorous mammals, so the team looked at an area known as the coronal gyrus, which receives signals from the whiskers.
By taking casts of the inside of the skull of a range of living and extinct mammals that live on land and in the sea, the researchers examined how the size and orientation of the gyrus changed over time. They found it gets significantly bigger in animals which use their whiskers a lot, such as otters, civets, seals and sea lions.
In modern ringed seals, for instance, it now takes up around 10% of the cerebral cortex, which is the part of the brain which interprets signals from different sensations.
For Potamotherium, however, whiskers could have been even more important as the coronal gyrus is estimated for make up about 17% of its cerebral cortex. It suggests that the semi-aquatic mammal would have depended on these to find food.
While the whiskers certainly appear to be important, Potamotherium could also have used its hands to help forage for food. Investigating this, however, will mean analysing a different section of the brain.
Research like this could help to provide some clarity on what seals were like back at the start of their family tree.
