Absolute versus per unit body length speed of prey as an estimator of vulnerability to predation

To study whether absolute (m/s) or relative (body lengths/s) speed should b e used to compare the vulnerability of differently sized animals, we develo ped a simple computer simulation. Human 'predators' were asked to 'catch' ( mouse-click) prey of different sizes, moving at different speeds across a c omputer screen. Using the simulation, a prey's chances of escaping predatio n depended on its speed (faster prey were more difficult to catch than slow er prey of the same body size), but also on its size (larger prey were easi er to catch than smaller prey at the same speed). Catching time, the time n eeded to catch a prey, also depended on both prey speed and prey size. Rela tive prey speed (body lengths/s or body surface/s) was a better predictor o f catching time than was absolute prey speed (m/s). Our experiment demonstr ates that, in contrast to earlier assertions, per unit body length speed of prey may be more 'ecologically relevant' than absolute speed. (C) 1999 The Association for the Study of Animal Behaviour.