Abstract

Species of the order sirenia show a distinct morphological differentiation of their facial tactile hairs, know as bristle-like hairs on the outer muzzle and perioral bristles around the oral cavity. To test for a potential functional difference between these hair types we studied active touch capabilities of an Antillean manatee by means of a texture discrimination task. In a two alternative forced choice procedure a juvenile manatee was required to discriminate perspex surfaces, each with a set of linear grooves. While the ridge width was held constant at 2 mm, grove width decreased from 10 mm to 2 mm. A difference threshold was determinded by a modified method of limits. Testing was conducted under low-light conditions. To test for visual cueing during tactual discrimination we determined the animlas visual ability to distinguish stimuli consisting of alternating black and white stripes. In the course of acquisition the animal explored the stimuli by means of the entire muzzel complex involving the highly mobile muscular lips, perioral bristles as well as the flexible bristle-like hairs of the oral disc. However, after acquisition of the discrimination it exclusively used the bristle-like hairs of the oral disc for touching the stimuli, The manatee achieved a relative tactile difference threshold of 14% groove width difference, with no apparent visual cueing. The tactile resolving power of the bristle-like hairs of the manatee is lower than that of pinnipeds, but compares well with that of the trunk of Asian Elephants.