Seals, sea lions and walruses (Pinnipedia) use their whiskers (vibrissae) to explore their environment and locate their prey. Today they live mostly in marine habitats and are adapted for a highly specialized amphibious lifestyle with their flippers for locomotion and a hydrodynamically streamlined body. The earliest seals, however, lived on land and in freshwater habitats, much like otters and their relatives (mustelids) today.
Life reconstruction of Potamotherium valletoni. Image credit: Gabriel Ugueto.
“The shift from a terrestrial to an aquatic lifestyle in the evolution of the mammalian order Pinnipedia (seals, sea lions and walruses) is an impressive ecological transition in carnivoran evolution,” said Naturalis Biodiversity Center paleontologist Alexandra van der Geer and colleagues.
“Adaptations for underwater feeding and foraging were crucial for this transition.”
“While the feeding strategy of fossil pinnipeds can be inferred from their teeth and skeleton, their foraging behavior is more challenging to reconstruct.”
“Modern pinnipeds use their whiskers to explore their environment by detecting vibrations in the water.”
“This includes hydrodynamic prey sensing, as observed in harbor seals (Phoca vitulina), elephant seals (Mirounga angustirostris) and ringed seals (Pusa hispida).”
“We do not know, however, when this behavior first appeared during pinniped evolution,” they said.
“The earliest stem pinnipeds differed considerably from their modern counterparts. They were otter-like animals occupying freshwater environments. These stem pinnipeds occupied the ecological gap between semiaquatic freshwater and semiaquatic marine species.”
The study authors investigated the evolution of whisker-foraging behaviors in seals by comparing the brain structures of Potamotherium valletoni — an otter-like seal relative that lived in what is now Europe over 23 million years ago (Oligocene-Miocene boundary) — with those of six extinct and 31 living meat-eating mammals, including mustelids, bears, and seal relatives.
They compared the size and structure of a brain region known as the coronal gyrus, which previous research has suggested is involved in processing signals from whiskers.
They found that Potamotherium valletoni had a larger coronal gyrus than ancient and living land-based mammals that use their forelimbs to forage, but a similar sized coronal gyrus to other ancient seal relatives and semiaquatic mammals that use their whiskers to explore their surroundings.
This indicates that Potamotherium valletoni may have used its whiskers when foraging, potentially in combination with its forelimbs.
The findings suggest that whisker-based foraging was already present in ancient seal relatives before they transitioned to a fully aquatic lifestyle.
“Our results show that the reliance on whiskers in modern pinnipeds is an ancestral feature that favoured survival of stem pinnipeds in marine habitats,” the researchers said.
“Our study provides insights into an impressive ecological transition in carnivoran evolution: from terrestrial to amphibious marine species.”
“Adaptations for underwater foraging were crucial for this transition.”
The findings appear in the journal Communications Biology.
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G.A. Lyras et al. 2023. Fossil brains provide evidence of underwater feeding in early seals. Commun Biol 6, 747; doi: 10.1038/s42003-023-05135-z
