Hydrodynamic perception in seals and sea lions

Abstract

Marine mammals often forage in dark and turbid waters. While dolphins use echolocation under such conditions, pinnipeds seem to lack this sensory system. Instead, species of the families Phocidae (true seals) and Otariidae (eared seals) both possess richly innervated whiskers (synonymously “vibrissae”) representing highly sensitive hydrodynamic receptors that enable these animals to detect fish-generated water movements. The third family of pinnipeds, the Odobenidae (walruses), is less well studied. As water movements in the wake of fishes persist for several minutes, they constitute hydrodynamic trails that should be trackable by piscivorous predators. Hydrodynamic trail following has indeed been shown for the harbor seal (Phoca vitulina) and the California sea lion (Zalophus californianus). However, in experiments with a sea lion aging of the trails resulted in an earlier decrease in performance. This difference in tracking performance most likely is due to differences in the structure of the respective vibrissal hair shaft. In the harbor seal the high sensitivity and excellent tracking performance is ascribed to the specialized undulated structure of the whiskers that largely suppresses self-generated noise in the actively moving animal. In contrast, the whiskers of a swimming California sea lion, which are smooth in outline, are substantially affected by self-generated noise. However, in the sea lion such self-generated noise contains a characteristic carrier frequency that might allow hydrodynamic reception by being modulated in response to hydrodynamic stimuli impinging on the hair. Thus, in the course of pinniped evolution at least two types of whiskers evolved that realized different mechanisms for the reception of external hydrodynamic information.