A physiological basis for island tameness

Abstract

Island tameness is a widely documented phenomenon in which island species, particularly those that have been isolated for much of their evolutionary history, show a greatly reduced behavioral response to novel predators. This insufficient escape response has led to widespread population declines among many island species exposed to novel predators, and has become a serious conservation problem. Despite its prevalence, the cause of island tameness is not known. We compared the physiological stress response of the classically tame Galapagos marine iguana (Amblyrhynchus cristatus) when approached by a novel potential predator (human) with that elicited by a native predator (the Galapagos hawk: Buteo galapagoensis). Although iguanas initiated flight from an evolutionarily novel potential predator, they showed no evidence of the cardiovascular stress response that facilitates successful escape. In contrast, when approached by a native predator, marine iguanas displayed the typical highly conserved vertebrate stress response and markedly increased heart rate prior to initiating escape, which is indicative of epinephrine release upon visual contact with a native predator. Our findings suggest that escape behavior in predator-naive species may be constrained by an inappropriate physiological stress response. This altered physiological response could explain why many island species have difficulty adapting to novel predators.