The escape response of the zebra danio ( Brachydanio rerio) I. The stimulus for escape

Abstract

The hypothesis was tested that zebra danios ( Brachydanio rerio) react to an approaching predator when the rate of change of the angle subtended by the predator at the prey's eye (dα/d t) exceeds some threshold level. Danios were presented with real predators (largemouth bass, Micropterus salmoides) and artificial predators of two types: a model predator, whose size and velocity could be varied, and a film of an approaching object. The reactive distance for flight ( RD) could be predicted from the relationship: RD=[( VS/k)−( S 2/4)] 1/2 where V is the predator approach velocity (cm/s), S is the predator front diameter (cm), and k is the threshold rate of change of visual angle (radians/s). The best overall estimate of k was 0·43 rad/s. The reactive field was shown to be circular, demonstrating that the mechanism is not solely a binocular one. Escape velocity, though higher in response to real than to artificial predators, was not correlated with reactive distance.