Unpredictable aggressive defence of the venomous snake, Crotalus ravus, towards predators and humans.

Antipredation behaviour is of high importance for the survival of prey animals, but it is also vital for the predator to understand the antipredator behaviour of potentially dangerous prey. Venomous snakes are particularly dangerous for their predators and humans, as a defensive bite may result in death. Here we studied the behavioural response of the Mexican pigmy rattlesnake Crotalus ravus to the approach of simulated predators (birds and fox) and human, contrasting this to their predatory behaviour. Results showed that C. ravus defensive behaviour depended on the predator and was more aggressive towards humans. Mostly, for each type of behaviour the approach distance at first occurrence, was similar among trials with different predators and reduced from freezing>rattling>escape>bite. However, we did not find clear behavioural patterns. In bird and fox trials, snakes always rattled or escaped before biting, however warning signals were not always displayed before biting and bite frequency was high in human trials, suggesting that this snake is dangerous for humans. Our results demonstrate that these snakes are flexible in their response to potential threats, but that approach distance that elicit specific behaviours are mostly fixed.