Perioral Bristles

Periorial Bristles vs. Bristle-like hairs

            Manatees possess a short muscular snout, which is covered with sinus hairs and short and thick robust modified vibrissae or bristles (Marshall et al., 2000).  They can be characterized as vibrissae because they possess a prominent blood sinus complex, dense connective tissue capsule, and substantial innervation (Reep et al., 2001).  The two types of these bristles, perioral bristles and bristle-like hairs, are considered to be facial tactile hairs, but the bristle-like hairs are found on the outer muzzle, while perioral bristles surround the oral cavity (Bachteler and Dehnhardt, 1999).  The outer muzzle, which is covered densely with these flexible bristle-like hairs, is divided into the supradisc and the oral disc.  Furthermore, hair is denser on the oral disc than on the supradisc (Reep et al., 1998).  In contrast, the perioral bristles are located in six different pads found on the upper lip, the oral cavity, and the lower jaw.  These hairs are very rigid, yet highly moveable.  The bristle-like hairs supposedly have the function of tactile scanning, while perioral bristles are usually observed in the act of feeding by being used in a prehensile grasping motion. (Bachteler and Dehnhardt, 1999) 

Perioral Bristle Morphology

            As mentioned above, the perioral bristles are divided into six different pads, or fields on the face of the manatee.  In general, they have larger diameters and great stiffness (Reep et al., 1998).  There are “four on each side of the upper lips and oral cavity (U1 – U4 on Figure 1), and two on each side of the lower lip pad (L1 – L2 on Figure 1), inside the oral cavity and rostral to the horny mandibular pad” (Reep et al., 1998).  Within each field, there is a characteristic number of bristles and a specific range of bristle length and diameter (Reep et al., 1998). 

Figure 1. (Figure 1 in Reep et al. 1998)

Used with the Permission of Dr. Roger Reep

            When different individuals were examined, it was found that there was an average of 110 (+ 19) bristles on each side of the face and that there was little variation in the distribution, diameter, and length of bristles in each field for each of the individuals (Reep et al., 1998) as shown in Table 1 below.  The U2 group of bristles, which consists of 23 long, thick bristles on each side of the upper lip, are circularly organized and were the most prominent of the groups of upper bristles, as they can reach a length of 10 mm when everted during manipulative behaviors (Reep et al., 1998).  This ability to elongate and extend the U2 bristles is facilitated by the skin thickness of the supradisk region being greater than the skin thickness of the oral disk (Marshall et al., 1998b).  The U1 group is just as long as the U2, but thinner, while the U3 group contains shorter, thinner bristles arranged in rows and the U4 group is a single row of short, thin bristles.  The lower groups, L1 and L2 are similar to group U3. (Reep et al., 1998)  Moreover, the data collected by Reep et al. (1998) suggests that the U1-U4 bristles are similar to the mystacial vibrissae of other taxa, as they innervated by the infraorbital nerve, while the L1 and L2 bristles are related to the mental vibrissae and are supplied by the inferior alveolar nerve.

Table 1. (Table 2 in Reep et al. 1998)

Used with the Permission of Dr. Roger Reep

            There are approximately 110,000 axons that innervate the Florida manatee’s facial area, and the perioral bristles and bristle-like hairs use 65,000 of these (Reep et al., 2001).  In other words, the processing and relaying of tactile information is highly important for this species.