Water temperature influences the shoaling decisions of guppies, Poecilia reticulata, under predation threat.

In previous experiments, we found antipredator behaviours in female Trinidadian guppies to be influenced by water temperature. To distinguish between hypotheses suggested to explain these observations, we examined the effect of an increase in water temperature on the choice of shoal size of individual female guppies, in the absence and presence of predation threat from a confined cichlid. At 22 degrees C, guppies showed no significant preference for either of two shoal sizes of female guppies, although other responses indicated a reaction to the threat. At 26 degrees C, females chose to shoal with the smaller group when the predator was absent, but, once threatened, switched to a strong preference for the larger group. Variation in preference between individuals was quite high in all treatments, and was somewhat lower for guppies under threat in warmer water, although not significantly so. Our evidence indicates that guppies in warmer water behave as if they are at greater risk of predation, but only when under threat. Temperature-dependent increases in predation risk from ectothermic piscivores could result from less efficient escape responses and from greater vulnerability because of increased foraging activity. We suggest that selection favouring guppies displaying an optimal balance between foraging and antipredator behaviours should act more strongly at higher temperature. Copyright 1999 The Association for the Study of Animal Behaviour.