Pinnipeds orient and control their whiskers: a study on Pacific walrus, California sea lion and Harbor seal

Abstract

Whisker touch is an active sensory system. Previous studies in Pinnipeds have adopted relatively stationary tasks to judge tactile sensitivity, which may not accurately promote natural whisker movements and behaviours. This study developed a novel feeding task, termed fish sweeping to encourage whisker movements. Head and whisker movements were tracked from video footage in Harbor seal (Phoca vitulina), California sea lion (Zalophus californianus) and Pacific walrus (Odobenus rosmarus divergens). All species oriented their head towards the moving fish target and moved their whiskers during the task. Some species also engaged in whisker control behaviours, including head-turning asymmetry in the Pacific walrus, and contact-induced asymmetry in the Pacific walrus and California sea lion: behaviours that have only previously been observed in terrestrial mammals. This study confirms that Pinnipeds should be thought of as whisker specialists, and that whisker control (movement and positioning) is an important aspect of touch sensing in these animals, especially in sea lions and walruses. That the California sea lion controls whisker movement in relation to an object, and also had large values of whisker amplitude, spread and asymmetry, suggests that California sea lions are a promising model with which to further explore active touch sensing.

Comparing vibrissal morphology and infraorbital foramen area in pinnipeds

“…Sprowls and Marshall (2019) have suggested that the sensitive caudal vibrissae may detect and localize signals, while the numerous and densely packed rostral vibrissae act as a tactile fovea-a higher resolution sampling area to aid in detailed tactile investigation. In support of this, we have previously observed that following a vibrissal contact on their more caudal vibrissae, pinnipeds tend to orient toward the stimuli with their head and then place their rostral vibrissae (or microvibrissae) toward the stimuli (Grant, Wieskotten, Wengst, Prescott, & Dehnhardt, 2013;Milne, Smith, Orton, Sullivan, & Grant, 2020). Therefore, individual caudal vibrissae are likely to be more sensitive as the animal will use information from multiple vibrissae to calculate the stimulus direction of a moving prey item from these caudal vibrissal contacts.…”

Sea lions could use multilateration localization for object tracking as tested with bio-inspired whisker arrays

“…The calculation method assumes that the tip is the first part of the whisker which is piercing the vortex ring, which is only guaranteed to be the case with the whiskers protracted forwards. This is still open to be studied in a future experiment with a new model with protracted whiskers, supported by the observation that seal lions protract their whiskers when actively tracking 18 .…”

California sea lions employ task-specific strategies for active touch sensing

“…However, comparing decision times between studies is challenging as they will be strongly affected by stimulus similarity and prior experience. Nevertheless, quickly and successfully making decisions based on information from whisker signals is likely to be important to pinnipeds, especially during foraging and navigation events in dark underwater environments ( Hyvarinen, 1989 ; Milne and Grant, 2014 ; Milne et al, 2020 ).…”

“…Pinnipeds, including seals, sea lions and walruses, have the most prominent and sensitive whiskers of any mammal ( Dehnhardt et al, 1998 ; Dykes, 1975 ; Hyvarinen, 1989 ; Marshall et al, 2006 ). Compared with other pinnipeds, California sea lions move their whiskers with larger amplitudes ( Milne et al, 2020 ) and can orient them towards moving objects ( Milne and Grant, 2014 ; Milne et al, 2020 ). California sea lions can also use their whiskers to discriminate between different object shapes and sizes ( Dehnhardt, 1990 , 1994 ; Dehnhardt and Dücker, 1996 ) with the same sensitivity as human fingertips.…”