P-29: Is Natural Selection Shaping Florida Manatees? An Investigation into the Body Shapes Between the Subspecies of the West Indian Manatee

The West Indian manatee (Trichechus manatus) is currently divided into two subspecies: the Antillean (T. m. manatus) and Florida manatee (T. m. latirostris). Florida manatees (FM) are found at the northern extreme of the species range, primarily inhabiting the waters around the Florida peninsula, while the Antillean manatee (AM) inhabits the remaining range from the Caribbean islands to western Brazil. Water temperature drops below 20ºC during the winter in Florida, and these manatees have to seek warm water refuge for thermal regulation. Allen’s rule predicts that in colder climates animals will adapt to have smaller surface area to volume ratios (SA:V). It has been reported that on average, FM are larger in absolute body size than AM. However, no one has explored differences in body shape independent of size between the two subspecies. This study proposes that FM have stockier body shapes (smaller SA:V) compared to AM. Data on FM morphometrics (n=543) were obtained from the Florida Fish and Wildlife Commission, while AM measurements (n=90) came from live captures in Quintana Roo and Tabasco, Mexico. SA:V was calculated by creating a geometric model of manatee shape using the standard morphometric measurements. SA:V of FM was significantly smaller even when controlling for body length (p=0.004). It appears natural selection has not only increased the body size of FM, but also altered the body shape in relation to AM. Our results lend further support for Allen’s rule and indicate that FM and AM are diverging on separate adaptive trajectories