Yes, manatees have hair, a feature that distinguishes them from many other fully aquatic mammals and is a remnant of their terrestrial ancestry. These unique marine herbivores possess hair across their entire bodies, though it is often sparse and may not be immediately visible, leading to the common misconception that they are hairless. The hair does not provide warmth, but serves a highly specialized sensory function vital for the animal’s survival. This subtle covering helps the manatee navigate and forage in the often-murky coastal and river waters where they reside.
The Unique Types of Manatee Hair
Manatees possess two distinct populations of hair, both categorized as vibrissae, or sensory hairs. The most noticeable are the thick, stiff bristles concentrated on the muzzle and the broad, prehensile upper lip. These facial vibrissae are stout, numbering approximately 2,000, and are organized into nine distinct regions on the face, an arrangement unique among mammals.
The second type consists of thin, fine hairs sparsely distributed over the rest of the manatee’s body, known as postfacial vibrissae. There are an estimated 3,000 to 3,300 of these body hairs. Although smaller and less densely innervated than those on the face, they function as specialized sensory structures. Their presence gives the manatee’s skin a texture different from the smooth skin of dolphins and whales, which possess almost no hair.
The structure beneath each hair is a Follicle-Sinus Complex (FSC), which includes a dense connective tissue capsule and a blood sinus. This complex arrangement confirms that all manatee hairs are true vibrissae. The density of innervation in the facial vibrissae is significant, with some bristles connected to 70 to 225 nerve axons each. This contrasts sharply with the simple hairs on a human arm, which are typically connected to only about five neurons.
The Sensory Role of Vibrissae
The manatee’s vibrissae function as a highly specialized mechano-sensory system, compensating for the animal’s relatively poor eyesight, especially in dark or turbid water. The facial bristles are actively used in tactile investigation and foraging, acting like sensitive fingers to explore objects and manipulate food. The manatee’s highly muscular upper lip is split, allowing it to move independently on each side to grasp aquatic vegetation, with the stiff vibrissae assisting this process.
The vibrissae on both the face and the body are extremely sensitive to minute changes in water movement and pressure. This high sensitivity allows the manatee to detect hydrodynamic stimuli, aiding navigation. Studies suggest the postfacial body hairs may function like a mammalian version of the lateral line system found in fish, which detects water vibrations.
The body hairs can detect vibrations as low as one millionth of a meter. This incredible sensitivity allows manatees to perceive the subtle wakes created by other animals or objects in the water, aiding in orientation and obstacle avoidance. The sensory information from the vibrissae is processed by a large proportion of the manatee’s brain, which has specialized neuron clusters dedicated to the sense of touch.
Why Marine Mammals Retain Hair
The retention of hair in manatees is linked to their evolutionary history as part of the Sirenia order, whose closest living terrestrial relatives are elephants and hyraxes. Unlike most fully aquatic mammals, such as whales and dolphins, manatees have not completely lost their hair during the transition to an aquatic lifestyle. This suggests the trait was repurposed rather than discarded.
The sparse body hair of manatees does not serve a thermoregulatory purpose. The manatee’s body size and blubber layer are the primary source of insulation in the relatively warm waters they inhabit. Instead, the hair has been adapted entirely for its sensory function, making the manatee a unique case among marine mammals.
The sensory function of the body vibrissae is so refined that it is considered an active adaptation, not merely a vestigial trait. The entire body surface acts as a vast, hydrodynamic receptor. Manatees retained their hair because it is an effective tool for navigating the often-murky, low-visibility waters of their habitat.