Abstract

The successful return of mammals to aquatic environments presented numerous sensory challenges to overcome.  Aquatic habitats reduced the utility of vision and the type of chemoreception important in terrestrial perception. In several orders, the sense of touch assumed greater importance, especially when enhanced by the development of vibrissal (sensory hair) systems.  Species of two extant orders, Sirenia and Cetacea, lost all of their hairs except for vibrissae. In the former, these hairs cover the entire bodies of the two families, Trichechidae and Dugongidae.  Hairs in adult cetaceans are more constrained (e.g., some river dolphins and baleen whales) and are restricted primarily to rostral regions.  Pinnipeds and sea otters retained their pelage, but in addition have elaborated their mystacial and other facial vibrissae.  High numbers of vibrissal receptors, associated dense innervation, prominence of neural tracts, and hypertrophy of brain areas associated with touch suggest an importance of tactile senses for aquatic mammals.  Experimental testing has demonstrated the exquisite tactile sensitivity of many marine mammal species. Sensory hairs contribute to that tactile sensitivity in both haptic and mechanosensory contexts.  Several, if not most, pinniped species, seals and sea lions, can track prey based on mechanoreception alone.  In this review we will discuss the neurobiological and behavioral evidence for the tactile senses of marine mammals.

We examined side preferences in the Florida manatee (Trichechus manatus latirostris) through observations of limb use (right and left flipper) in 123 wild and 16 captive individuals.  We also analyzed archival data on wild manatees to develop an index of boat-caused body scars to determine lateralization of evasive action.  Wild and captive manatees displayed flipper lateralization at the individual, but not the population level for several behaviors including substrate touches, sculling, and feeding.  In contrast, manatees were lateralized at the population level for boat-scar biases with more manatees showing a left scar bias (45%) versus right (34%) or dorsal (21%).

In Experiment 1 underwater visual acuity was assessed in two Florida manatees, Trichechus manatus Latirostris , using grating stimuli in three conditions: Vertical gratings presented in freshwater, vertical gratings in saltwater, and horizontal gratings in saltwater. All stimuli were tested in a free-swimming format from a minimum distance of one meter. Substantial differences were found between the two subjects. One subject’s minimum angles of resolution (MAR) were 56 min for vertical stimuli in freshwater, 38 min for vertical stimuli in saltwater, and 24 min for horizontal grating stimuli in saltwater. When only trials under brightest light conditions were analyzed, MARs improved for vertical stimuli to 24 min in freshwater and 21 min in saltwater. No improvement was found for horizontal stimuli with brighter light conditions. The MARs of the second subject measured over a degree for all test conditions. In Experiment 2 only the first subject was tested from a closer viewing distance. He showed no improvement when allowed to approach targets to within 30 cm. The limited resolution of both subjects and absence of increased acuity at closer distances in one suggests that manatees use vision for intermediate or longer distance inspection of large objects. The disparity in visual resolution between subjects has possible implications for variability in acuity within the species.