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

Abstract

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 natural 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 magnitude of population-level oxygen isotope variation in bioapatite, whereas mean carbon isotope values discriminate between marine versus terrestrial food consumption. We used a 2 endmember (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 suggests 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.

Isotope incorporation in broad-snouted caimans (crocodilians)

“…To successfully conserve crocodilians and their aquatic ecosystems, it is important to understand and predict changes in their diet and habitat use, as well as to characterize their capacity to contend with environmental modification. To date, only three studies have used stable isotope analysis to investigate crocodilian trophic ecology (Rosenblatt and Heithaus, 2011; Radloff et al, 2012; Wheatley et al, 2012). We studied rapidly growing caiman juveniles maintained under constant optimal conditions, so our measured rates of incorporation probably represent higher rates for this species than those seen in wild caimans enduring episodic nutritional constraints and growing more slowly (e.g.…”

“…sea turtles (Ceriani et al, 2012)]. In this context, crocodilian taxa are surprisingly understudied; only three recent studies of crocodilian trophic ecology employed stable isotopes (Rosenblatt and Heithaus, 2011; Radloff et al, 2012; Wheatley et al, 2012). Given these species' complex habitats and nocturnal foraging behavior, stable isotopes may present a considerable advantage over traditional methods, such as stomach content analysis and feeding observations, when analyzing crocodilian diets.…”

Insights into the habitat of Middle Permian pareiasaurs (Parareptilia) from preliminary isotopic analyses

“…Cetacea and Carnivora, Clementz & Koch ; Anapsida and Diapsida, Wheatley et al . ). Habitat information is provided by the mean and variance of oxygen isotope values, and variance tends to be more informative (e.g.…”

“…; Wheatley et al . ). Hence, a Hippopotamus ecomorph for pareiasaurs would be evident as lower δ 18 O enamel values compared with dinocephalians and therocephalians.…”

“…We have found a single report for the oxygen isotopic composition of carbonate in turtle bone (Wheatley et al . ): 1.3 ± 0.6‰ PDB (i.e. 32.26‰ SMOW), loggerhead turtle ( Caretta caretta ).…”