Citation: de Jong, K., E. Forsgren, H. Sandvik, and T. Amundsen (2012) Measuring mating competition correctly: available evidence supports operational sex ratio theory. Behavioral Ecology, 23, 1170–1177.
Key words: Courtship, frequency measure, OSR, propensity measure, sex role, sexual selection.
Abstract: Central to sexual selection theory is the question of when individuals should compete for mates. Theory predicts that the sex ratio of ready-to-mate individuals (operational sex ratio; OSR) affects male and female mating competition. In accordance with this, the strength of mating competition, measured by agonistic behaviors and courtship displays, has been found to co-vary with the OSR in field populations of several species. However, laboratory experiments have often produced results that seemingly contradict OSR theory, especially for courtship behavior. We argue that this may be because experiments typically measure frequencies of competitive behaviors. Frequencies of courtship and agonistic behavior are not only affected by the level of mating competition, but also by the number of potential mates or competitors encountered. In contrast, the propensity to behave competitively at a given encounter represents a behavioral response, and thus directly reflects mating competition. We show in 2 simple models that 1) courtship frequency can be expected to respond differently from courtship propensity to changes in OSR and 2) an increase in frequency of agonistic behaviors could occur even if propensity is not affected by the OSR, even though the frequency responded qualitatively similarly to the propensity. In a meta-analysis of studies on courtship competition, we show that frequency measures produced largely opposite results to propensity measures, as predicted by our model. Moreover, courtship propensity increased when the OSR became more biased toward competitors. This presents strong evidence that the OSR affects competition, in the form of courtship, as predicted by OSR theory.
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Lay summary: In most sexually reproducing animals, males compete for females. However, it is not always that simple: in some animals, females compete for males or both sexes compete. The classical theory to explain the intensity of mating competition has been that the more abundant a sex is, the more competitive. This is expected simply because the other sex will then become a scarce resource. Surprisingly, this theoretical expectation is often not confirmed by experiments. Mating competition may be expressed through contests within a sex to establish dominance or courtship to convince the other sex of one’s superiority. For courtship behavior, many experimenters find that animals court more when the opposite sex is abundant, which – seemingly – contradicts theory. We argue that this apparent clash between theory and experimental outcomes may be caused by the way behavior is measured in experiments. Often, the number of courtship displays per animal is counted under different sex ratios. We show that one cannot properly test theory this way. The reason is simple: when the other sex is abundant, an animal will more often meet a potential mate and will have more opportunities to court. This can lead to an increase in courtship displays even if the animal is less eager to court. Instead, we need to measure the propensity to court – how likely it is that an animal courts when it meets a potential mate. We show that studies measuring courtship this way support theory: when mates are limited, animals are more eager to court.
Supplementary material: An overview of studies reporting the effect of variation in OSR on courtship behavior is available here (as an Excel spreadsheet, 49 kB).