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Are Both Sexes Flexible?

It is possible to empirically test the hypothesis that both sexes make flexible reproductive decisions, as the SPT predicts. Imagine an experiment in which individuals of both sexes develop in social environments in which they have controlled exposures to future potential mates. Imagine also that the population has been managed so that the w distribution for all individuals of either sex has the same shape (such as might occur in large outbred populations without sex-biased dispersal) and that n is equal. In our thought experiment, investigators move all individuals into same-sex holding arenas just before they reach sexual maturity to guarantee that mating does not occur. Then, they carry out pretouching arena [such as the one pictured in Anderson et al. (2007)] tests, to measure the fraction of potential mates acceptable to focal individuals. Note that the experiments evaluating accept/reject responses must be tightly controlled so that subjects’ responses are not contaminated by intrasexual interactions among the discriminatees or sexual coercion (Kingett et al., 1981). Then investigators manipulate one variable, say, e so that in similar time periods some focal individuals encounter fewer potential mates, and in other trials the same focal individuals encounter more potential mates. This experimental procedure would vary the focal individual’s encounter rate with potential mates and, if carried out, would provide a strong within-subject test of the prediction that focal individuals reject more potential mates as e increases. Importantly, the SPT predicts that if e, s, l, n, and the w distribution are the same for all tested individuals, males and females will show no significant differences in their accept/reject behavior.

The SPT is an Alternative Hypothesis to Anisogamy Theory

The SPT is not only a predictive hypothesis of when individuals should switch their reproductive decisions. It is also a strong alternative hypothesis that can be simultaneously tested along with classic ideas about the evolution of sex roles. Anisogamy theory is silent about ecological and temporal constraints on reproductive decision making. It predicts that in species with gamete size asymmetries, such as D. pseudoobscura and D. melanogaster, the sex with the larger gametes will reject more potential mates (i.e., “be choosier”) than individuals of the sex with the smaller gametes. In species such as Drosophila hydei, with little or no gamete size asymmetries, anisogamy theory predicts that males and females should be similarly “choosy” and similarly “indiscriminate.” In contrast the SPT predicts that for both sexes in all 3 species, individuals will flexibly adjust their decisions to accept or reject particular mates as e, s, l, n, and w distribution vary. These alternatives could be tested with a crucial experiment (Platt, 1964) in which a test of a single prediction—about accepting or



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