Perioral Bristles and Tactile Exploration
In the study by Bachteler and Dehnhardt (1999), active touch performance was tested in Antillean manatees. To accomplish this, they established a difference threshold for the manatee using a grooved surface that had ridges of 2 mm and groove widths varying from 10 mm to 2 mm. Manatees were able to distinguish a difference in groove width up to 1.5 mm and still make > 90% correct choices, or in other words, the comparison and standard surface had to differ by about 14% to produce a noticeable difference for the manatees (Bachteler and Dehnhardt, 1999). This difference threshold closely matches that of the Asian elephant, but is different from humans, who are able to achieve a difference threshold of about 4% (Bachteler and Dehnhardt, 1999). This would make sense, considering the manatee’s closest relative is the elephant.
Bachteler and Dehnhardt (1999), however, were also able to determine a functional differentiation between the bristle-like hairs and the perioral bristles of the manatee. Once the manatee had acquired a sense of discrimination, it resorted to using solely the bristle-like hairs on the oral disc to touch the stimuli, as compared to also using their lips and perioral bristles when first exploring the stimuli (Bachteler and Dehnhardt, 1999). As shown in Figure 1, in the first five sessions, the manatee initially swept its bristle-like hairs over the surface of the grooved plate, as it always made its first contact with the bristle-like hairs, and then grasped the stimulus using its muscular lips, perioral bristles, and bristle-like hairs. As the sessions continued, the manatee’s discrimination improved and it favored the bristle-like hairs for touching the stimuli, while perioral bristle and muscular lip use was rarely observed (Bachteler and Dehnhardt, 1999). The suggestion that bristle-like hairs are primarily used for tactile exploration is further supported by the fact that in the wild, manatees’ bristle-like hairs measure around 4-6 mm (Reep et al., 1998), while manatees in captivity have bristle-like hairs about 15-23 mm in length. In the wild, manatees must use their bristle-like hairs for active touch processes that those in captivity would not have to experience.
Figure 1. (Figure 3 in Bachteler and Dehnhardt, 1999)
Used with the Permission of Dr. Guido Dehnhardt
As it turns out, the two types of bristles work together in a two-step process to help manatees explore their environments. The bristle-like hairs are used to provide initial sensory information, and the perioral bristles are then used to grasp and manipulate for further evaluation (Bachteler and Dehnhardt, 1999). Moreover, manatees probably use tactile cues from their bristle-like hairs to first detect aquatic vegetation and may even be able to determine different type of vegetation with said hairs (Marshall et al., 1998a).