Date of Award
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 are found at the northern extreme of the species range, primarily inhabiting the waters around the Florida peninsula, while the Antillean manatee inhabits the remaining range from the Caribbean islands to eastern Brazil. Only in the range of the Florida manatee does water temperature drop below 20°C during the winter, 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, Florida manatees are larger in absolute body size than Antillean manatees. However, no one has explored differences in body shape independent of size between the two subspecies. This study proposes that Florida manatees have stockier body shapes (smaller SA:V) compared to Antillean manatees due to the selective pressure of the colder water.
Data on Florida manatee morphometrics (n=543) were obtained from the Florida Fish and Wildlife Commission, while Antillean 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 standard morphometric measurements. An ANCOVA on SA:V controlling for total length indicated that the SA:V of Florida manatees was significantly smaller (p=0.004). It appears natural selection has not only increased the total body size of Florida manatees, but also altered the body shape in relation to Antillean manatees. Our results lend further support for Allen's rule and indicate that Florida and Antillean manatees are diverging on separate adaptive trajectories.
Johnson, Juliane, "Is Natural Selection Shaping Florida Manatees? : An Investigation into the Body Shapes Between the Subspecies of the West Indian Manatee" (2019). Honors Theses. 232.
Manatees; West Indian manatee; Natural selection
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