MEASURING MATE CHOICE USING CORRELATION - THE EFFECT OF FEMALE SAMPLING BEHAVIOR

For intersexual selection to occur, it is necessary that females choos e between males. It is now well appreciated that constraints exist, wh ich preclude females sampling all the available males in a population. These constraints are likely to have caused the evolution of sampling rules (such as the ''best-of-n'' rule) by which females sample males. Here we investigate the impact of female subsampling of the male popu lation, not on the evolution of sampling behaviour, but on the populat ion-level correlation between a male trait and currencies such as repr oductive success. This study is important as it illustrates when popul ation-level correlations can be safely used to infer the presence and strength of sexual selection in the field. We find that the correlatio n between a male trait and a mate choice variable rises steeply as the number of males sampled by each female increases, flattening above se ven to ten males sampled. This shape is found to be remarkably robust, and little affected by, for example, the mate choice variable used, b y noise in assessment, by sampling behaviour depending on female quali ty, or by population size. The only variable found to have a large imp act is male clumping according to their ''quality''. If females are sa mpling about four males, the maximum correlation that can be found at the population level is in the range 0.4-0.6, perhaps as little as 0.1 if males are strongly clumped. A recent review of the literature sugg ests that four is the average number of males that females sample. Thu s, the absence of a strong correlation cannot by itself be used to inf er that sexual selection is weak, as it may be due to females sampling few males.