This week, we will be talking about manatee vibrissae and how different they are from some of the other species we have reported on so far. Much of the information here comes directly from a researcher who studies animal behavior himself, Dr. Gordon Bauer. Dr. Bauer is a psychologist at New College of Florida who studies marine mammal behavior and sensory systems. He is a co-author on many papers about manatee sensory systems, two of which will supplement the information he provided in an interview for this blog post.
Manatees are interesting in that, though they are mammals, they have lost all hair except for their vibrissae, which cover their entire bodies (Bauer et al. 2018). They have three kinds of vibrissae: perioral (around the mouth), bristle-like (on the oral disk), and post-facial (on the rest of the body) (Reep et al. 2011). The perioral vibrissae are prehensile, meaning they are moved and used for grasping food (Reep et al. 2011). This behavior is called oripulation, and it allows manatees to draw food in and spit unwanted items like sand out (GB Bauer, interview, April 26, 2019). Though manatees use their flippers to grasp food and bring it to their mouths, the hairs are needed for maneuvering because eating seagrass with flippers alone would be like “trying to eat M&M’s with mittens on” (GB Bauer, interview, April 26, 2019). The bristle-like vibrissae are very dense and coarse, and they are used for active tactile scanning (Reep et al. 2011). They can discriminate very rapidly and with precision as good as a human’s index finger (GB Bauer, interview, April 26, 2019). The post-facial vibrissae, used for detecting water movement around them, are about 30x less sensitive than the two kinds of facial vibrissae and they are spaced further apart (Reep et al. 2011). The only other animals known to have vibrissae all over their bodies are hyraxes, which are small terrestrial mammals with a thick pelage of regular hair and longer vibrissae sticking out of their coats (GB Bauer, interview, April 26, 2019).
Manatees have very poor visual acuity, though they do have two kinds of cones allowing them to have some blue/green color vision (GB Bauer, interview, April 26, 2019). Like we have discussed in previous blog posts, vibrissae often serve to replace or supplement vision, which appears to be the case for manatees. Manatees live in rather turbid water, where vision would not be all that useful anyway, and they also forage at night in the dark (GB Bauer, interview, April 26, 2019). Manatees have likely not evolved echolocation like many other marine mammals because they tend to live in waters so shallow that returning sounds would not be very accurate or useful (GB Bauer, interview, April 26, 2019). Manatee vibrissae pick up sound best around 150 Hz, which is just below the frequency at which they can hear (GB Bauer, interview, April 26, 2019). A study that used mesh muzzles to cover certain percentages of facial vibrissae found that the greater the number of vibrissae involved, the more sensitive the perception of the vibrations (GB Bauer, interview, April 26, 2019). Vibration studies have also been conducted on the body vibrissae, which found that manatees can distinguish the general direction from which vibrations are coming, and that tweaking a single vibrissae on a flipper can cause the manatee to move its flipper (GB Bauer, interview, April 26, 2019). It is unknown if manatees are able to use their vibrissae to detect objects around based on an object’s distortion of the water and how the bow wave created by the manatee’s movement hits an object (GB Bauer, interview, April 26, 2019). This would be interesting to study, as it could be somewhat similar to how echolocation is used in other species. The manatee post-facial vibrissae have been compared to the lateral line system in fish because it perceives motion in the water at similar frequencies and with a similar sensitivity.
Manatee brains have an enlarged somatosensory area, taking over much of the area where a motor cortex might be, right behind a greatly reduced motor area (GB Bauer, interview, April 26, 2019). This corresponds to the 5300 highly innervated vibrissae covering the manatee body and the minimal need for movement in slow grazing herbivores that have no natural predators (Reep et al. 2011). Manatees have not been found to have barrel regions composed of barrelettes, but they do have Rindenkern in the SI and SII regions of the somatosensory cortex, which appear match up with each individual vibrissae on the body (GB Bauer, interview, April 26, 2019).
Much more needs to be studied about manatee sensory abilities because most of this research comes from a multitude of studies on two particular manatees at Mote Marine Lab in Sarasota, FL – Hugh and Buffet (GB Bauer, interview, April 26, 2019). Dr. Bauer and other researchers have tried to triangulate studies of multiple kinds to support the same results in order to be as sure as possible about their findings for such a small sample size. These include a variety of behavioral and anatomical studies, but more neural work, in particular is needed.
Summary: ~Manatees are different from most other species with whiskers in that their vibrissae cover their entire bodies, and some of these hairs are prehensile and used for moving food into their mouths.
~This system is adapted to the murky environment manatees live in, giving them the ability to sense movement, and possibly even objects, around them quickly and precisely.
~Manatee brains do not have barrels exactly like many other whiskered mammals, but they have an enlarged somatosensory cortex taking over the part of the brain that would typically do motor functions.
~There is much more to study in more manatees and with other methods that look into more of the neural mechanisms for manatee vibrissae sensation.
~Bauer GB, Reep RL, Marshall CD. 2018. The tactile senses of marine mammals. Int J Comp Psych. 2018(31): 1-28.
~Reep RL, Gaspard III JC, Sarko D, Rice FL, Mann DA, Bauer GB. 2011. Manatee vibrissae: evidence for a “lateral line” function. Ann NY Acad Sci. 1225(2011): 101-109.