Feeling Out Your Food: A Histological Analysis of the Whisker System in Seals

The vibrissal (whisker) system is present in nearly all mammals and is especially important in deep-diving mammals. Pinnipeds (seals, sea lions, and walruses) have highly sensitive whiskers that are used to follow hydrodynamic trails created by their swimming prey. Each pinniped vibrissa is surrounded by a tripartite blood sinus system composed of an upper cavernous sinus (UCS), a ring sinus (RS), and a lower cavernous sinus (LCS). The UCS has been hypothesized to play a thermoregulatory role, insulating temperature sensitive mechanoreceptors located within the vibrissal follicle. The aim of this study was to examine this hypothesis by measuring and comparing the structures of the vibrissal system in three phocid (true seal) species: deep-diving, polar Weddell seals (Leptonychotes weddellii), deep-diving, temperate northern elephant seals (Mirounga angustirostris), and shallow-diving, temperate harbor seals (Phoca vitulina). Individual follicles were isolated and histologically processed from individuals that died in the wild or during rehabilitation efforts at the Marine Mammal Center. Total sinus lengths, absolute sinus lengths (UCS, RS, and LCS), relative sinus lengths (absolute sinus length / total sinus length), and dermal capsule widths (DC) were measured and compared among the species. Northern elephant seals had the greatest total sinus length and thus the greatest absolute sinus lengths among the species. However, polar Weddell seals had the greatest relative UCS length, suggesting that Weddell seals devote a large portion of their total sinus length to the UCS, which would be advantageous for thermoregulation. Additionally, vibrissal morphology was compared between adult and neonatal Weddell seals. The absolute UCS length and the DC width were the only two structures for which differences were observed between the age classes. The relative UCS was not significantly different between adults and neonates, which suggests that neonates devote a relatively large portion of their total sinus to the UCS, comparable to the adults. The differences in relative UCS length among the three species in this study supports the proposed idea that the UCS is a structural adaption for thermoregulation within the vibrissal system. This is the first study to make direct comparisons between adult and neonates within a single species and the first study to make direct comparisons among different phocid species to examine the UCS as a thermoregulatory structure