Abstract
Streamlined flippers are often considered the defining feature of seals and sea lions, whose very name ‘pinniped’ comes from the Latin pinna and pedis, meaning ‘fin-footed’. Yet not all pinniped limbs are alike. Whereas otariids (fur seals and sea lions) possess stiff streamlined forelimb flippers, phocine seals (northern true seals) have retained a webbed yet mobile paw bearing sharp claws. Here, we show that captive and wild phocines routinely use these claws to secure prey during processing, enabling seals to tear large fish by stretching them between their teeth and forelimbs. ‘Hold and tear’ processing relies on the primitive forelimb anatomy displayed by phocines, which is also found in the early fossil pinniped Enaliarctos. Phocine forelimb anatomy and behaviour therefore provide a glimpse into how the earliest seals likely fed, and indicate what behaviours may have assisted pinnipeds along their journey from terrestrial to aquatic feeding.
Highlights
The hunt for new food resources was one of the key factors driving the repeated re-invasion of aquatic ecosystems by whales, seals and other secondarily aquatic mammals [1]
Swallowing can itself become an issue, as aquatic mammals must separate prey from surrounding seawater or bear the high physiological cost of ingesting it along with their prey. All of this is further complicated by the need to dramatically modify their body form to increase streamlining and to adapt their limbs for generating propulsion when swimming [4]. In response to these challenges, marine mammals have adopted a myriad of aquatic feeding strategies, involving behaviours which are often so highly specialized that they would be impossible to use on land [5,6]
Our results suggest that phocines have retained primitive morphology, and a suite of behaviours similar to those likely used by the earliest seals, providing important insights into how behaviour and anatomy interplay as species cross the boundary between terrestrial and aquatic existence
Summary
The hunt for new food resources was one of the key factors driving the repeated re-invasion of aquatic ecosystems by whales, seals and other secondarily aquatic mammals [1]. Air-breathing mammals need to hold their breath when diving to feed underwater, taking care not to drown while capturing prey in their mouths and processing it into pieces small enough to swallow This can be challenging, because while prey on land can be held against the ground for processing using sharp cutting teeth, aquatic prey floats freely in the water column [3]. Swallowing can itself become an issue, as aquatic mammals must separate prey from surrounding seawater or bear the high physiological cost of ingesting it along with their prey All of this is further complicated by the need to dramatically modify their body form to increase streamlining and to adapt their limbs for generating propulsion when swimming [4]. In response to these challenges, marine mammals have adopted a myriad of aquatic feeding strategies, involving behaviours which are often so highly specialized that they would be impossible to use on land [5,6]
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