Elsevier

Animal Behaviour

Volume 68, Issue 6, December 2004, Pages 1313-1324
Animal Behaviour

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

https://doi.org/10.1016/j.anbehav.2004.03.004Get rights and content

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.

Section snippets

Study area and species

The wet–dry tropics of northern Australia are hot year-round (mean monthly temperature 27.0°C) but exhibit intensely seasonal precipitation regimes, with 78% of the 1394-mm mean annual precipitation falling during the brief (4-month) ‘wet-season’ (December–March: Brown & Shine 2004). These monsoonal rains cause widespread flooding, especially in relatively flat low-lying floodplains. We worked at Fogg Dam 60 km east of the city of Darwin in the Northern Territory. This site has been used for

Results

We gathered data on 5 nights (26 February to 2 March 2003). Snake numbers were dramatically lower immediately before and after this time, so our study encompassed the main period during which Enhydris were present in large numbers at the dam wall in 2003. Hourly mean water temperatures at foraging sites spanned a narrow range, from 27.5°C (recorded predawn) to 30.0°C (recorded midafternoon). During the time we gathered data (1900–2300 hours), mean hourly water temperatures remained between 28.0

Discussion

Most of the snakes that we approached ignored our presence, continued to forage, and reacted to experimental stimuli with feeding rather than defensive behaviours. This result suggests that experimental studies on foraging in free-ranging snakes will be feasible with other taxa such as North American natricines (Nerodia, Thamnophis, etc.) that forage in similar ways in similar habitats to Enhydris (personal observation). We predict that many (independently evolved) similarities with Enhydris

Acknowledgments

We thank C. Shilton, E. Cox, S. Dusty, M. D'Aquirie and the staff of Beatrice Hill Farm for their assistance. Shine and Elphick wish to record their appreciation for Brown's accuracy and speed with a slingshot, which drove away a 4.5-m saltwater crocodile intent on conducting its own foraging study with us as the subjects. Funding was provided by the Australian Research Council.

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