Evolutionary Significance of Brain Structure in the Florida Manatee (Trichechus manatus latirostris)

The Florida manatee (Trichechus manatus latirostris) inhabits a unique aquatic niche. Adaptations to this niche include obligate herbivory, low metabolic rate, large body size, and supernumerary teeth. Consequences of these adaptations are, cold intolerance, low population rate of increase, and range restrictions. Manatee life history traits also have influenced brain evolution. In the past, examination of only gross anatomical brain structures in manatee brains have led to the fallacy that manatee brains are poorly developed. In order to answer questions about the internal structure of the Florida manatee brain, cerebral isocortex, caudal to the lateral fissure, of the manatee brain was examined and its cytoarchitecture documented. It was found that the manatee brain is surprisingly complex. Cell counts and cortical thickness were measured in the manatee and other aquatic mammals in order to better understand the manatee’s place in brain evolution. Although manatees possess a small brain size relative to body size and a lissencephalic but unusually thick cerebral cortex, there is a high percentage of cerebral cortex to brain volume, and a highly organized cytoarchitecture which is differentiated into numerous cortical areas. Manatee brain characteristics appear to be on the same evolutionary adaptive level as dolphin brain characteristics. Functions for cortical areas defined in this study and in previous studies are suggested. However, due to the lack of electrophysiological data on manatees and a detailed examination of the manatee thalamus, functional assignments should be considered as preliminary contingent upon further study