Island tameness: an altered cardiovascular stress response in Galápagos marine iguanas

Island tameness is a widely documented phenomenon in which island species, particularly those that have evolved with no or few natural predators, show a greatly reduced behavioral response when faced with unfamiliar predators. This insufficient anti-predator 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 underlying physiology of island tameness is not known. Here we report that although Galápagos marine iguanas (Amblyrhynchus cristatus) initiated flight from an evolutionarily recent and unfamiliar potential predator (humans), they failed to show the cardiovascular stress response that facilitates successful escape, even after a prior capture experience. In contrast, when approached by a native predator (the Galápagos hawk; Buteo galapagoensis), marine iguanas show markedly increased heart rate independent of initiating escape movement. The secretion of catecholamines appears to be central to the initiation of escape behavior: naïve animals remotely injected with epinephrine immediately increased flight initiation distance, whereas those injected with corticosterone did not. Our results provide the first evidence that muted escape behavior in predator-naïve species is indicative of both a cognitive deficit in recognizing potential predators and a catecholamine deficit in response. Understanding how the response to predators differs in predator-naïve species could enable the design of maximally effective techniques for inducing an anti-predator response in these vulnerable species.