Predator-induced life history changes: Antipredator behavior costs or facultative life history shifts?

We used laboratory experiments to show that the nonlethal presence of pumpkinseed sunfish (Lepomis gibbosus) results in smaller size at emergence, decreased growth and development rates, and lower fecundity of the dipteran Chironomus tentans. Smaller size at metamorphosis is often viewed as a cost of antipredator behavior. However, it may also partly result from a facultative change in life history in which prey increase their development rate to ''escape'' their risky larval habitat. To determine the mechanism responsible for this smaller size at emergence, we compared development rates of chironomid larvae raised in the absence and nonlethal presence of pumpkinseed sunfish. When we statistically controlled for effects of predators on larval growth rate, fish presence had no effect on development rate of female chironomids, but significantly slowed male development. These results clearly indicate that C. tentans does not increase its development rate as a means of escaping a risky habitat, and that smaller size at emergence is best viewed as a cost of larvae behaviorally avoiding fish predators. Observed sex differences in life history responses to predators may have important consequences for fitness.