LESSONS FROM HUGH AND BUFFET: ESTABLISHING STABLE ISOTOPE PROXIES FOR SIRENIAN FORAGING HABITS THROUGH STUDY OF CAPTIVE SPECIMENS

Sirenians (manatees, dugongs) have a long, rich fossil record documenting their transition from land to sea. Whereas the morphological changes associated with this transition are well recorded, information on changes in diet and habitat is less readily available and could benefit from the use of stable isotope tracers retained in fossil material. Since primary producer and surface water isotope values can differ significantly between freshwater and marine ecosystems, the tissues of sirenians foraging in these food webs can be isotopically labeled by the vegetation they consume (δ¹³C) and water they drink (δ¹⁸O), providing a powerful tool for identifying sirenian foraging habits through time.

To aid in the interpretation of isotopic information collected from living and fossil sirenians, blood samples from captive manatees (Hugh and Buffet) housed at Mote Marine Laboratory (MML) in Sarasota, Florida were obtained for stable isotope analysis (SIA). As part of the research program at MML, Hugh and Buffet have been specifically trained for participation in scientific research and are able to handle routine collection of tissue and blood samples. Three blood samples were collected from each manatee between March and December 2006 along with samples of their diet and tank water for SIA. Carbon (δ¹³C) isotope values for red blood cells (-25.5 ± 0.1‰), serum (-25.1 ± 0.3‰) and lipids separated from blood samples (cholesterol = -25.5 ± 0.4‰; total fatty acids = -29.3 ± 0.6‰) varied little over the course of the collection period. These values were then used to calculate discrimination factors (ε¹³C) for each relative to the weighted mean value for diet. Bodywater δ¹⁸O values determined from blood samples (-1.2 ± 0.6‰ VSMOW) did not differ strongly from values measured from tank water (-1.0‰ VSMOW), which suggests that the δ¹⁸O values of sirenian body water and biominerals (e.g., enamel, bone) precipitated in equilibrium with body water should closely track δ¹⁸O values of surface waters inhabited by sirenians. From these results, more accurate ecological interpretations for fossil sirenians are possible, demonstrating the potential benefits available to paleobiologists when working within modern biological systems.