The bottlenose dolphins of Sarasota Bay live in a socially and ecologically complex environment.
For example, female bottlenose dolphins tend to use a relatively small geographic area compared to males who possess a widespread pattern of habitat use.
Differences in the social structure of dolphin societies are likely to influence where and what dolphins consume. Here we call these behavioral tendencies “foraging habits.”
Assessing differences in foraging habits can be problematic. Dolphins often capture and consume prey underwater making observational studies of foraging habits difficult. Alternatively, the stomach contents of deceased stranded bottlenose dolphins identify prey but are only available from a small number of dolphins each year and provide a short-term “snapshot” of what each dolphin consumed, sometimes while they were ill.
Atoms within bottlenose dolphin tissues may yield novel insight into how the foraging habitats of male and female bottlenose dolphins differ. Carbon exists in a “light version” with an atomic mass of 12 (“carbon 12” or 12C) or a “heavy version” with an atomic mass of 13 (“carbon 13” or 13C). These alternate forms of carbon are called isotopes. The abundance of 13C relative to 12C varies in nature and can be expressed as an “isotope value”. Higher isotope values indicate a larger ratio of 13C to 12C. Isotope values become useful to ecology because different plant types have unique carbon isotope values. For instance seagrass incorporates more 13C compared to phytoplankton or algae, thus the carbon isotope value of seagrass is higher than that of phytoplankton. Isotope values are passed up the food web from phytoplankton, for example, to small fish, all the way to bottlenose dolphins. Thus, a dolphin that forages exclusively in seagrass habitat would have a higher carbon isotope value than a dolphin that foraged in open water habitat, where phytoplankton predominates.
Isotope values of bottlenose dolphin skin were obtained from individuals sampled during health assessments. Males and females differ in their use of seagrass habitat. Male bottlenose dolphins possessed a narrow range of carbon isotope values indicating intense utilization of seagrass habitat. Conversely, females demonstrated a wide range of carbon isotope values indicating that some females consumed prey from open water habitat rather than just seagrass habitat. This difference in the pattern of habitat use may be critical to understanding how bottlenose dolphins respond to human disturbance. For instance, males who appear to be highly reliant on seagrass habitat may be more sensitive to the loss of seagrass habitat compared to females. We thank the many interns and volunteers that assisted in sample collection and processing as well as our funding sources, the National Science Foundation and Michigan State University’s Environmental Science and Policy Program.
This article was published on page 22 in the January 2014 Nicks n Notches.