SHORT TERM RECORDS OF MANATEE DIETARY PREFERENCES INTERPRETED FROM CHOLESTEROL d¹³C AND dD VALUES

Florida manatees (Trichechus manatus latirostris) are known to migrate substantial distances each year, moving between marine and freshwater habitats where they consume an array of aquatic plant species. Observations of manatee foraging habits suggest that feeding is non-selective, but movement of manatees between different aquatic ecosystems may impart a seasonal pattern on the type of plants consumed by this species (i.e., seagrass vs. freshwater vegetation). Since the carbon isotope (d¹³C) and hydrogen isotope (dD) values of manatee tissues are controlled by the stable isotope composition of the animal’s diet, compound-specific stable isotope measurement of d¹³C and dD values of cholesterol and other organic molecules that form over a relatively short period of time can track sub-annual changes in foraging habit and supplement information gained from other tissues that reflect multi-year dietary averages (e.g., collagen). Furthermore, stable isotope analysis of cholesterol extracted from ancient bones may shed light on the short-term feeding habitats of manatee populations prior to human contact.

To determine whether the measurement of cholesterol d¹³C and dD values can provide short-term dietary information for manatees, we have begun collecting tissue samples from recently deceased manatees from the Indian River Lagoon along the eastern coast of Florida. From each individual, 3 tissues differing in metabolic turnover rate were selected for analysis including liver (fast turnover), muscle (intermediate turnover), and bone (slow turnover). Cholesterol extracted from each of these tissues will have formed within different periods of time over the course of the year, providing sub-annual records of seasonal dietary patterns based on variations in cholesterol d¹³C and dD values. Comparison of cholesterol isotope values with collagen values from the same individual will then provide a means to quantify the magnitude of seasonal dietary variation relative to long-term dietary preferences. By extending these analyses to recent and fossil material, future work could examine whether the feeding patterns of Florida manatees have changed significantly over time and, if so, whether human or environmental factors have contributed to this change.