Abstract
Modern pinnipeds (true and eared seals) employ two radically different swimming styles, with true seals (phocids) propelling themselves primarily with their hindlimbs, whereas eared seals (otariids) rely on their wing-like foreflippers.1,2 Current explanations of this functional dichotomy invoke either pinniped diphyly3–5 or independent colonizations of the ocean by related but still largely terrestrial ancestors.6–8 Here, we show that pinniped swimming styles form an anatomical, functional, and behavioral continuum, within which adaptations for forelimb swimming can arise directly from a hindlimb-propelled bauplan. Within phocids, southern seals (monachines) show a convergent trend toward wing-like, hydrodynamically efficient forelimbs used for propulsion during slow swimming, turning, bursts of speed, or when initiating movement. This condition is most evident in leopard seals, which have well-integrated foreflippers with little digit mobility, reduced claws, and hydrodynamic characteristics comparable to those of forelimb-propelled otariids. Using monachines as a model, we suggest that the last common ancestor of modern seals may have been hindlimb-propelled and aquatically adapted, thus resolving the apparent contradiction at the root of pinniped evolution.
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•The origin of forelimb swimming in fur seal and sea lions is an enduring mystery•Southern true seals (Monachinae) independently evolved sea lion-like flippers•Fluid dynamics modeling demonstrates convergence in both form and function•Like monachines, fur seals and sea lions evolved from a hindlimb-propelled ancestor
Hocking et al. combine 3D anatomy, behavior, and computational fluid dynamics to reveal how forelimb-propelled swimming evolved multiple times in pinnipeds (seals and sea lions). They show that all pinniped forelimb swimmers derive from foot-propelled ancestors, thus resolving a longstanding mystery at the heart of pinniped evolution.