Measuring the cost of reproduction

Gravid and barren Daphnia pulex were exposed to a variety of predators in laboratory aquaria. Small fish (guppies, sticklebacks and shiner fry) consistently preferred the gravid females, establishing the existence of a behavioural cost of reproduction. However, no such cost was associated with predation by more efficient visual predators (sunfish) or by nonvisual predators (hydras), and the excess of gravid females eaten by backswimmers was found to be attributable to their distribution in the water column. Moreover, the cost associated with predation by small fish was observed only when the Daphnia were presented against a light background, and was abolished when a dark background was substituted. In a further series of experiments with guppies we attempted to show that each egg added to the brood caused a decrease in survival; in two such experiments survival rate was related to body size but not to fecundity, while in a third the effect of body size did not appear, and a negative correlation between survival and fecundity could be demonstrated. Although these experiments unambiguously demonstrate a cost of reproduction they also illustrate the elusiveness of the phenomenom and emphasize the need to develop theories which specify the type and magnitude of costs generated by different ways of life.