locate the trail. Species differences in vibrissae Studies raise the question of how detection of hydrodynamic stimuli in these animals is possible given the movement of the vibrissae due to water flow during swimming. Whiskers vibrate with a certain frequency based on swim speed and properties of the whisker. Detection of the water disturbance caused by this vibrissal movement should overshadow any stimulus produced by a distant fish due to its proximity. For seals, one proposal is that they might sense changes in the baseline frequency of vibration to detect hydrodynamic stimuli produced by another source. However, a more recent study shows that the morphology of the seal's vibrissae actually prevents vortices produced by the whiskers from creating excessive water disturbances. In harbor seals, the structure of the vibrissal shaft is undulated (wavy) and flattened. This specialization is also found in most true seals. In contrast, the whiskers of the California sea lion are circular or elliptical in cross-section and are smooth. When seals swim with their vibrissae projected forward, the flattened, undulated structure prevents the vibrissae from bending backward or vibrating to produce water disturbances. Thus, the seal prevents noise from the whiskers by a unique whisker structure. However, sea lions appear to monitor modulations of the characteristic frequency of the whiskers to obtain information about hydrodynamic stimuli. This different mechanism might be responsible for the sea lion's worse performance in tracking an aging hydrodynamic trail. Since the whiskers of the sea lion must recover its characteristic frequency after the frequency is altered by a hydrodynamic stimulus, this could reduce the whisker's temporal resolution. Manatees Similar to the vibrissae of seals and sea lions, Florida manatees also use hairs for detecting tactile and hydrodynamic stimuli. However, manatees are unique since these tactile hairs are located over the whole post-cranial body in addition to the face. These hairs have different densities at different locations of the body, with higher density on the dorsal side and density decreasing ventrally. The effect of this distribution in spatial resolution is unknown. This system, distributed over the whole body, could localize water movements analogous to a lateral line. Research is currently being done to test detection of hydrodynamic stimuli in manatees. While the anatomy of the follicle-sinus complexes of manatees have been well studied, there is much to learn about the neural circuits involved if such detection is possible and the way in which