Endangered Florida manatees have a number of unusual anatomical and behavioral adaptations related to their unique role as mammalian aquatic herbivores. Chief among these is the presence of sensory hairs over the entire body, whereas in most mammals these are only present as facial whiskers. The present research uses two captive manatees at Mote Marine Laboratory to assess the ability of this system of specialized hairs to detect water-borne vibrations associated with other moving animals, river currents, and tidal flows, all of which are cues likely to be used in navigation. This research relates not only to the normal behavior of manatees, but also to their ability to detect and avoid approaching boats. These two animals have been trained on a number of related tasks and are able to report behaviorally whether a stimulus has been detected. It will be possible to determine the range of frequencies and amplitudes to which the animals are capable of responding, and their degree of directional localization ability. It will also be possible to discover whether certain portions of the body are more sensitive than others. Anatomical experiments on postmortem brains will be performed at the University of Florida to map the neural connections that mediate these abilities. This work utilizes special fluorescent tracers that travel down nerve fibers by diffusion over a period of several months. In addition, a significant portion of this project includes educational interactions with students and teachers at Mote, New College, the University of South Florida, and the University of Florida.
The manatee brain exhibits many unusual traits compared to other mammalian brains, and some of these features may be related to the unusual lifestyle of manatees as mammalian aquatic herbivores. One of the most Intriguing features of manatees is the presence of sensory hairs all over the body. Our research has found that the facial hairs are used primarily for investigation of objects by active touch, whereas the postfacial hairs are used to detect water movements in the frequency range of 10-150 Hz. In order to better understand the organization of neural pathways in the manatee brain that may underlie these abilities, we performed axonal tracing in nine fresh postmortem manatee brains. In slabs of postmortem manatee brains, a crystal of the fluorescent neural tracer Fast DiI was placed in either the auditory pathway or somatosensory pathway. After about 6 months the tracer diffused along the nerve fibers to its synaptic targets in the thalamus region of the brain. The slabs were cut into thin sections that were mounted on glass slides and viewed on a fluorescent microscope. In this way we have been able to define the auditory and somatosensory regions of the thalamus. Based on behavioral findings we previously hypothesized that there is functional overlap between the auditory and somatosensory systems in manatees. In hearing studies, manatees are trained to station at an underwater bar, then respond if they hear a tone that has been played. At frequencies between 400 HZ and 25 kHz, manatees are able to station and report heard tones by leaving the bar and touching a response paddle. However, at low frequencies (below 400 Hz) manatees twist to change the orientation of their bodies while stationed at the bar, presumably to allow them to better detect these vibrations using the sensory hairs on the postfacial part of the body. This behavior suggests that they may be shifting from an auditory to a vibrotactile (somatosensory) mode of detection. Therefore, these neuroanatomical findings define the area in which this intermediate information processing may occur.
