Are vibrissae viable sensory structures for prey capture in northern elephant seals, Mirounga angustirostris?
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Little is known about the tactics northern elephant seals (NES) use to capture prey due to the difficulties in observing these animals underwater. NES forage on vertically migrating prey at depths >500 m during day and at night where light levels are negligible. Although NES have increased visual sensitivity in deep water, vision is likely a limited sensory modality. Still images of NES foraging show that the mystacial vibrissae are protracted before prey capture. As a representative phocid, harbor seals can follow hydrodynamic trails using their vibrissae, and are highly sensitive to water velocity changes. In lieu of performance data, vibrissal innervation can be used as a proxy for sensitivity. Although comparative data are few, seals average 1,000 to 1,600 axons per vibrissa (five to eight times more than terrestrial mammals). To test the hypothesis that NES have increased innervation as other pinnipeds, vibrissae from the ventral-caudal mystacial field from nine individuals were sectioned and stained for microstructure (trichrome) and innervation (Bodian silver stain). Follicles were tripartite and consisted of lower and upper cavernous sinuses separated by a ring sinus containing an asymmetrical ringwulst. The deep vibrissal nerve penetrated the follicular capsule at the base, branched into several bundles, and coursed through the lower cavernous sinus to the ring sinus. Axons in the ring sinus terminated in the ringwulst and along the inner conical body. NES averaged 1,584 axons per vibrissa. The results add to the growing body of evidence that phocids, and perhaps all pinnipeds, possess highly sensitive mystacial vibrissae that detect prey.
