hide
Free keywords:
index of association; mating competition; mating system; mean crowding; sex roles; sexual selection
Abstract:
The operational sex ratio (ratio of sexually receptive males to females) has been extensively studied in behavioral ecology,
whereas other demographic factors such as the effect of density on mating behavior have received less empirical attention. We
manipulated mating competition by establishing breeding populations of the sex-role reversed broad-nosed pipefish (Syngnathus
typhle) at 2 sex ratios (male biased or female biased) and 2 densities. We used mean crowding (m*) and the index of association
(X) to measure spatial distributions within and between the sexes, respectively, and investigated how these measures reflect the
predicted strength of mating competition. In general, female m* increased as fewer males were available for mating, which suggests
increased intrasexual competition in the most competitive sex. However, male m* also increased as the operational sex ratio
became more female biased, suggesting that m* did not reflect mating competition for males. Association between the sexes (X)
was higher under male bias than female bias, probably because males were still available for mating under male bias. In addition,
X decreased in the female-biased treatment as the operational sex ratio became even more female biased. Higher density
increased m* in both sex ratios and sexes, although for both sexes in the female-biased high-density treatment the operational sex
ratio did not influence m*, probably because female–female competition inhibits further crowding in this treatment. In this study,
we show that the use of m* and X can be a useful tool in behavioral studies but their interpretation requires detailed information
about the mating system. Therefore, we recommend caution with their broadscale application.
