Are Vibrissae Viable Sensory Structures for Prey Capture in Northern Elephant Seals, Mirounga angustirostris?

Abstract

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. Anat Rec, 298:750-760, 2015. V C 2014 Wiley Periodicals, Inc.

Follicle Microstructure and Innervation Vary between Pinniped Micro- and Macrovibrissae

“…The remaining three pads were obtained from seals of an unknown age class. Current literature demonstrates that the innervation investment is independent of age class and sex since mammals are born with a finite number of hair follicles and axons innervating them [Szabo, 1958;Winkelmann, 1959;Scheffer, 1964;Reep et al, 2002;Setzu et al, 2004;Ito et al, 2007;McGovern et al, 2015].…”

“…Counts were conducted at this location to allow a direct comparison to previous studies. Axon branching has been shown to be minimal basal to this location in Australian water rat, bearded (Erignathus barbatus) and northern elephant seal, and sea otter (Enhydra lutris) F-SCs [Dehnhardt et al, 1999;Marshall et al, 2006Marshall et al, , 2014bMcGovern et al, 2015]. For each vibrissal pad, two lateral vibrissae, as well as multiple medial vibrissae, were processed for axon counts.…”

“…Initially, cross-sections were stained with a modified Bodian's silver stain (following Reep et al [2001], Marshall et al [2006Marshall et al [ , 2014b, and McGovern et al [2015]) to count axons. However, the staining process for the samples in this study was inconsistent and unpredictable, rendering multiple sections unquantifiable.…”

“…pinnipeds to detect the hydrodynamic and haptic cues necessary to hunt successfully in diverse foraging niches [Huber, 1930a, b;Ling, 1977;Dehnhardt et al, 2001;Marshall et al, 2006Marshall et al, , 2014aHanke et al, 2013;Marshall and Goldbogen, 2015;McGovern et al, 2015]. Among pinniped species, vibrissae vary in number, geometric arrangement, size, shape, flexural stiffness, and innervation [Ling, 1977;Marshall et al, 2006;Ginter et al, 2010Ginter et al, , 2012Hanke et al, 2013;Ginter-Summarell et al, 2015].…”

Vibrissal sensitivity in a harbor seal (Phoca vitulina)

“…The vibrotactile sense of pinnipeds relies on sturdy, specialized vibrissae and supporting hypertrophied neural architecture (Ladygina et al, 1985;Marshall et al, 2006;Hyvärinen et al, 2009;Ginter et al, 2012;Mcgovern et al, 2015), and can gather information from both terrestrial and marine environments. Pinnipeds use their vibrissae for the tactile discrimination of surfaces (Dehnhardt, 1994;Dehnhardt and Kaminski, 1995;Grant et al, 2013) and the detection and following of underwater wakes (Dehnhardt et al, 2001;Gläser et al, 2011).…”