Perioral Bristles and Feeding

      In general, it is harder for an aquatic herbivore to forage and feed on plants than for a terrestrial one.  It is more difficult to gather large quantities of plants within a short amount of time and movement is also constrained (Marshall et al., 2000).  Having discrete fields of modified vibrissae allows manatees to have better control of their environment.  When feeding upon submerged vegetation, the U2 and L1 bristle fields alternate temporally, with the U2 bristles acting in a prehensile manner and the L1 bristles acting to push the vegetation into the oral cavity.  The U1 bristles seem to be involved in tactile exploration during feeding, while the U3, U4, and L2 bristles look as if they aid passively in moving the food further into the oral cavity.  (Marshall et al., 1998a).  Each region of bristles maintains the same function when manatees feed on floating vegetation, such as water hyacinth, with the exception of the U2 bristles, which tend to move in a raking fashion (Marshall et al., 1998a).  Manatees are also able to use their U2 bristles in a non-prehensile manner, such as dragging vegetation, demonstrating the versatility of these bristles. 

Figure 1. (Figure 3 in Marshall et al., 1998a)

Used with the Permission of Dr. Roger Reep

       For each step of the feeding cycle, the manatee uses different muscles to control their perioral bristle movement.  Figure 1 shows a schematic of these different stages, while Figure 2 shows a summary of which muscles are used for each stage.  In stage A, also known as the “flare,” the oral disk is pulled dorsally and the U2 bristles are exposed to the vegetation with the use of the longitudinal muscles, M. levator nasolabialis and the M. maxillonasolabialis.  In stage B, the U2 muscles evert forward and sweep the plant towards the midline of the body as the lower jaw begins to open.  The muscles responsible for this behavior are the M. buccinatorius, the M. levator nasolabialis, and the M. centralis nasi.  The M. masseter, M. temporalis, and M. pterygoideus internus are responsible for both the opening and closing of the lower jaw.  Stage C is characterized by the right and left U2 bristles pushing the vegetation into the oral cavity with the lower jaw fully open.  At this point, the L1 bristles also evert.  The muscles involved with these actions are the M. buccinatorius, the M. centralis nasi, and the M. mentalis.  During stage D, the U2 bristles move apart due to the contraction and relaxation of the M. SCPO and M. centralis nasi, respectively, while the lower jaw begins to close.  In the final stage, E, the L1 bristles sweep the plant into the mouth and the lower jaw closes, while the U2 bristles go back to their original position.  The M. orbicularis oris and M. centralis nasi are involved with these movements. (Marshall et al., 1998b) 

Figure 2. (Figure 3 in Marshall et al., 1998b)

Used with the Permission of Dr. Roger Reep

As found by Reep et al. (1998), facial area is strongly correlated with body weight and body length of manatees.  Therefore, manatees with a greater body length have increased surface area for their feeding apparatus.  Similarly, “as the oral disk increases in size, distance between U2 bristle fields and the gape also increase” (Marshall et al., 2000).  These conditions account for why there are differences in the timing of feeding mechanisms between adult and juvenile manatees, as there appears to be an inverse relationship between manatee chewing rates and body length. 

            In spite of this, it appears that surface area of mastication area is not solely responsible for the variability found in the consumption rates of different aquatic plants (See Table 1 below).  The mean grasps/s of the perioral bristles do not differ that much between each of the plant species, except for Thalassia, but this may result from the mere structure of the plant.  As a result, it is suggested that introduction of food into the mouth via perioral bristles and mastication function independently to affect the rate of consumption (Marshall et al., 2000).  For instance, Hydrilla, Myriophyllum, and Syringodium all possess tubular stems, which are easier for manatees to grasp using their perioral bristles (Marshall et al., 2000).  Since manatees spend so much time foraging and reap relatively low energy benefits from their diet, the implications from this study are very important.  Energetically speaking, it would be better for a manatee to spend its time consuming more Hydrilla versus Thalassia, as they would be able to consume much more of it within a time period.  At this point in time, however, the nutritional benefits from each of these species of plants are unknown, as are the energetic benefits (Marshall et al., 2000). 

Table 1. Adapted from Marshall et al., 2000