Female's courtship threshold allows intruding males to mate with reduced effort.

Female decision rules can influence the nature and intensity of sexual selection on males, but empirical demonstrations of rules underlying choice are rare. We hypothesized that female choice is largely based on a courtship duration threshold in the Australian redback spider (Latrodectus hasselti) because females kill males before copulation is complete (premature cannibalism) and reduce their paternity if courtship is brief. We used published data to infer that the female's threshold is approximately 100 min of courtship. We support this hypothesis by showing that premature cannibalism is common when the male's courtship duration is below this threshold, but is infrequent and unrelated to duration once courtship exceeds the threshold. We then ask whether females discriminate the source of courtship when rival males compete, as this will determine the effect of the threshold on male competitive tactics. We staged competitions where 'resident' males initially courted females in the absence of competition, exceeding the courtship threshold before 'intruding' males were introduced. Intruding males mated rapidly but were not prematurely cannibalized by females, in contrast to cases where competition starts before the threshold is surpassed. This suggests females do not distinguish which male satisfies the threshold, allowing intruders to parasitize the courtship efforts of residents. To our knowledge, such exploitation of mating efforts by rival males mediated by a female choice threshold has not been demonstrated elsewhere. Ironically, this female choice threshold and the attendant possibility of courtship parasitism may lead to selection for lower-quality males to recognize and seek out (rather than avoid) webs in which competitors are already present.

A change in competitive context reverses sexual selection on male size.

In studies of sexual selection, larger size is often argued to increase male fitness, and relatively smaller males are explained by genetic and/or environmental variation. We demonstrate that a size-development life-history trade-off could underlie the maintenance of a broad, unimodal distribution of size in male redback spiders (Latrodectus hasselti). Larger males are superior in direct competition, but redback males mature rapidly at small size in the presence of females. In field enclosures, we simulated two competitive contexts favouring development of divergent male sizes. Relatively smaller males lost when competing directly, but had 10 times higher fitness than relatively larger males when given the temporal advantage of rapid development. Linear selection gradients confirmed the reversal of selection on size, showing that it is critical to consider life-history decisions underlying the development of traits related to fitness.

Extreme repeated mating as a counter-adaptation to sexual conflict?

The Australian scaly cricket, Ornebius aperta, can copulate over 50 times with the same partner; the benefits of such extreme repeated copulation are unclear. We support the hypothesis that repeated copulation increases insemination success, as the number of sperm transferred increases with each spermatophore. This probably increases paternity for males, as on average a female mates with over 40 males. Despite intense sperm competition each ejaculate has only a few hundred sperm, orders of magnitude less than in related crickets. We show that all sperm are transferred from each spermatophore in the few seconds before a female removes and eats it. Repeated copulation increases effective copulation duration while a small ejaculate ensures that this strategy is not excessively costly. Thus repeated copulation in these crickets may have arisen as a counter-adaptation to female-imposed limits on copulation.