Costs of Reproduction: An Evaluation of the Empirical Evidence

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Abstract
Theoretical studies of life history evolution almost unanimously incorporate some form of tradeoff between life history variables, such as reduced parental survival due to increased reproductive effort. These tradeoffs, or costs of reproduction, are fundamental in predicting the optimal life history in any given enviornment. The importance of such costs in theoretical models has motivated researchers to check that they exist by seeking an association between some component of reproductive effort, such as fecundity, and some form of cost, such as parental survival. Four methods have been used for making these evaluations: Phenotypic correlations, where some index of reproductive effort is correlated with a potential cost. Experimental manipulations, where reproduction is directly manipulatedand a cost-related repsonse is measured. Genetic correlations, where formal quantitative genetic designs are used to estimate the genetic correlation between some component of reproductive effort and some component of cost, and correlated responses to selection, where artificial selection causes correlated changes in reproductive effort and some component of costs. Because the theoretical studies are models of evolution a relevant empirical study must deal with genetically based tradeoffs. By this criterion, only the last 2 methods (which represent a small minority of the reviewed studies) properly address the "costs" dealt with by theoreticians: however, this evaluation is not intended as a litmus test for the merits of a given approach to the problem. I discuss both the important contributions of the first 2 methods to our understanding of the costs of reproduction and the limitations of the last 2 methods in assessing "costs".