Sexual conflict over copula timing: a mathematical model and a test in the yellow dung fly

Abstract

Sexual conflict over mating occurrence, timing, or duration is common in animals. This explains conspicuous female mate rejection behavior in many species, often involving shaking, fighting, and occasional forced copulations. We present a simple model that generates predictions about whether and when copulation occurs in such conflict situations and how much female rejection behavior should be observed. Predictions depend on 2 underlying parameters affecting female resistance and male persistence. We supply 2 qualitative tests of the model using the yellow dung fly Scathophaga stercoraria (Diptera: Scathophagidae). We manipulated adult age, body size (large and small), and adult food availability (low and high), independently in males and females, staging replicate pairings of all treatment combinations. In agreement with predictions of our model, shaking duration first increased to a maximum at intermediate age, when the average female copulated, and then decreased again. Contrary to expectation, body size did not affect copulation timing, female resistance, or male persistence. As predicted, adult food limitation delayed sexual maturity and hence prolonged female resistance, resulting in later copulations after more shaking. However, although food limitation equally delayed the increase in male persistence with age, copulation also occurred later after more shaking, opposite to the model prediction. We conclude that shaking is driven primarily by female age and male responses to it. Although female shaking can initially successfully deter males in S. stercoraria, this behavior is subtle and has apparently shifted function from an effective means of mate choice to a signal of nonreceptivity, though its importance in nature remains unclear.

Alternative mating tactics in the yellow dung fly: resolving mechanisms of small-male advantage off pasture

“…Previous work in S. stercoraria is consistent with this pattern, finding greater female resistance in non-gravid females, although reports of resistance have described the behaviour as gentle shaking or swaying and suggested that it probably does not function in mate choice [59,60]. In striking contrast to these earlier reports, we observed vigorous resistance, predominantly exhibited by non-gravid females, and provide evidence that such behaviour is nonrandomly directed such that females discriminate against large males.…”

Developmental variation in the forest tent caterpillar: life history consequences of a threshold size for pupation

“…Selection pressures on age and size at maturity differ between males and females (Leimar 1996, Tammaru 1998). Indeed, large size is generally highly advantageous for females due to its strong relationship with increased fecundity (Tammaru et al 2002, Parry et al 2001), whereas rapid development is more important for males in terms of increasing mating success (Blanckenhorn et al 2007). However, selection pressures on larvae are not sex‐based (Esperk et al 2007b), and size optima are presumably the same for both sexes.…”

Evolutionary insight from a humble fly: sperm competition and the yellow dungfly

“…Selection should favour unreceptivity to mating unless the advantages of receptivity outweigh the costs of mating. Females can and do reject copulations: copulations with nongravid females away from dung [13,21] and with females immediately after they have completed oviposition at the dropping [5] are both shortened by the violent side-to-side shaking movements of unreceptive females. However, unreceptivity would subject an ovipositing female to repeated harassment by searching males if not guarded by the last male to mate.…”