Estimating marine resource use by the American crocodile Crocodylus acutus in southern Florida, USA

Alligators and crocodiles differ in their physiological capacity to live in saline waters. Crocodiles can tolerate high-salinity water, at least for limited timeframes, whereas alligators and their close relatives cannot. Experiments have placed different crocodylians in various water salinities to document physiological responses, but no study has estimated the extent to which nat- ural populations of crocodylids can live independent of fresh water. Here we estimated marine food and perhaps seawater contributions to a population of American crocodile Crocodylus acutus in southernmost Florida, USA. We evaluated the use of carbon, oxygen, and strontium isotopes as tracers of marine versus terrestrial sources. We compared C. acutus isotopic values to those of marine reptiles (marine iguanas and Pacific loggerhead turtles) and to American alligators, which require fresh water. We found that freshwater reptiles can be discriminated from those that drink seawater (or survive on metabolic and prey-included water in saline habitats) based on the mag- nitude of population-level oxygen isotope variation in bioapatite, whereas mean carbon isotope values discriminate between marine versus terrestrial food consumption. We used a 2 end- member (seawater and fresh water) mixing model to calculate percentage of marine resources used by C. acutus. Results indicate that adult C. acutus in southern Florida use marine food about 65% of the time and seawater or water gleaned from marine food about 80% of the time. This sug- gests that behavioral osmoregulatory techniques (i.e. seeking fresh water specifically for drinking, as suggested by other researchers) may not be necessary and that C. acutus is capable of being largely ecologically independent of fresh water.