Tactile Hairs on the Postcranial Body in Florida Manatees: A Mammalian Lateral Line?

Abstract

Previous reports have suggested that the sparsely distributed hairs found on the entire postcranial body of sirenians are all sinus type tactile hairs. This would represent a unique arrangement because no other mammal has been reported to possess tactile hairs except on restricted regions of the body, primarily the face. In order to investigate this issue further, hair counts were made systematically in three Florida manatees (Trichechus manatus latirostris), and hair follicle microanatomy was studied in 110 specimens gathered from 9 animals. We found that the postcranial body possesses approximately 1500 hairs per side, and hair density decreases from dorsal to ventral. External hair length ranged from 2–9 mm, and most hairs were separated from their nearest neighbor by 20–40 mm, resulting in an independent domain of movement for each hair. All hairs exhibited the anatomical characteristics of follicle-sinus complexes typical of tactile hairs, including a dense connective tissue capsule containing an elongated circumferential blood sinus and innervation by 20–50 axons which ascend the mesenchymal sheath. We conclude that this represents a unique distributed underwater tactile system capable of conveying detailed and significant external information concerning approaching animals, water currents and possibly the presence of large stationary features of the environment. Such a system would be analogous to the lateral line in fish, and would be particularly useful in the turbid habitat frequented by Florida manatees.

Audiogram and auditory critical ratios of two Florida manatees (Trichechus manatus latirostris)

“…Manatee tactile hairs are unique among marine mammals in that they are found all over the body, have heightened blood supply and innervation, and appear to have specialized cortical representations like barrels seen in terrestrial mammals (Reep et al, 2002). The lack of response to 200Hz in our study may be due to differences in the setup between the two studies.…”

Bat wing sensors support flight control

“…In mammals, tactile hairs are usually located on the head in the form of sinus hairs (e.g., whiskers). Sinus hairs on other body parts have been described only for the manatee (14). The authors suggested that the sparsely distributed hairs on the manatee's body could form a lateral line system analogous to that of fish and amphibians, where it serves to detect water currents surrounding the animal's body.…”

Microstructure and innervation of the mystacial vibrissal follicle‐sinus complex in bearded seals, Erignathus barbatus (Pinnipedia: Phocidae)

“…Both longitudinal sections and cross-sections were used for microstructural morphometrics, and these sections were chosen based on the orientation of the cut and location of the section (longitudinal: near midline of the axis; cross-sections: midway through the LCS). Sections were stained with hematoxylin and eosin, a modified Masson's trichrome stain, a modified Bodian silver stain, or toluidine blue (Armed Forces Institute of Pathology, 1968Pathology, , 1994Reep et al, 2001Reep et al, , 2002. Traditional histocytochemical stains were used because it was not possible to collect samples from remote locations in a manner that would have allowed immunolabeling techniques (e.g., Rice et al, 1997;Ebara et al, 2002).…”