Field experiments on foraging in free-ranging water snakes Enhydris polylepis (Homalopsinae)

Laboratory-based experimental studies of snake foraging have revealed complex and flexible reliance upon multiple cues for prey location and capture: thus, major features of foraging biology depend upon the detailed context of predator-prey encounters and prey antipredator tactics. Accordingly, we need field-based studies to evaluate and extend conclusions from laboratory studies, by (1) recording locations and behaviours of both predators and prey, and (2) experimentally manipulating prey stimuli to identify causal determinants of snake feeding responses. In the Australian wet-dry tropics, the homalopsine snake Enhydris polylepis forages at night in shallow water for sleeping fish. Such fish allow close approach, but flip upwards (and thus at least briefly, out of the water) when touched. Experiments showed that these snakes locate their prey primarily by visual cues, especially movement, rather than by scent or waterborne vibrations. Strikes are elicited by tactile cues, especially the splash of water as a fish leaps upwards. Demonstrating the value of field-based study, data on the location of prey (very shallow water) and their antipredator behaviour (flipping rather than swimming) thus enables us to identify the cue (splashing) most important for eliciting the snake's strike. A conventional laboratory study would not have yielded this insight. (C) 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.