The Interface of Population Dynamics, Endocrine-Disrupting Compounds and Enigmatic Disturbance: Understanding Human-Crocodile Conflict in Costa Rica

Abstract

A recent survey indicates a unique male-biased sex ratio in a prized ecotourism species and aquatic sentinel taxon, the American crocodile (Crocodylus acutus), at Palo Verde National Park, an influential conservation area in northeastern Costa Rica. Palo Verde harbors an extensive ephemeral marsh that serves as a critical migratory-bird pathway and is home to multiple species of conservation concern in tropical dry forest habitat. Human conflict with this species heightens as the distributions of humans and crocodiles increasingly overlap in this region and media exposure of crocodile attacks becomes more widespread. Here, I test the presence of a male-biased sex ratio among crocodile age cohorts at Palo Verde and assess sex- specific immigration and emigration within these cohorts. Further, I test two hypotheses in an attempt to explain the male-biased sex ratio. First, I test the presence and effect of regional warming, as a result of global climate change, on sex determination within crocodile eggs, as well as potential compensatory nesting behaviors that may influence crocodile sex determination at the level of nest microhabitat. Second, the efficacy of a synthetic androgen used in local tilapia farming (17α-methyltestosterone, MT) to bias sexual differentiation towards males is tested. Exposure of crocodiles to this synthetic hormone is tested in natural populations of crocodiles and the role of MT as an environmental androgen is assessed in American alligators, a surrogate for the effect of this chemical on crocodiles. Demographic analysis indicates a 2:1 male-biased sex ratio in adults that results from balance migration of females and a net pattern of emigration of sub-adult males from a 4:1 male-biased sex ratio at hatching. Analysis of nest temperatures predicts a female bias in hatchling sex ratio based on nest thermal regimes, a pattern inconsistent with the observed male-biased sex ratio for the hatchling cohort. However, my data show a strong influence of metabolic heating on nest temperatures, a feature previously unknown for American crocodiles. Exposure of American alligator eggs to MT and female-producing temperatures results in hatchlings of both sexes including hermaphroditic individuals. Finally, sampling of the natural population of American crocodiles reveals the presence of MT in egg yolk, hatchling, juvenile and adult blood plasma in all sampling localities. These results indicate that MT is a potent environmental androgen capable of causing the male-biased sex ratio in the Tempisque River basin and other nearby drainages.