Abstract
Assortative mating is a common pattern in sexually reproducing species, but the mechanisms leading to assortment remain poorly understood. By using the European common frog (Rana temporaria) as a model, we aim to understand the mechanisms leading to size-assortative mating in amphibians. With data from natural populations collected over several years, we first show a consistent pattern of size-assortative mating across our 2 study populations. We subsequently ask if assortative mating may be explained by mate availability due to temporal segregation of migrating individuals with specific sizes. With additional experiments, we finally assess whether size-assortative mating is adaptive, i.e. influenced by mating competition among males, or by reduced fertilization in size-mismatched pairs. We find that size-assortative mating is in accordance with differences in mate availability during migration, where larger individuals of both sexes reach breeding ponds earlier than smaller individuals. We observe an indiscriminate mate choice behavior of small males and an advantage of larger males pairing with females during scramble competition. The tactic of small males, to be faster and less discriminative than large males, may increase their chances to get access to females. Experimental tests indicate that the fertilization success is not affected by size assortment. However, since female fecundity is highly correlated with body size, males preferring larger females should maximize their number of offspring. Therefore, we conclude that in this frog species mate choice is more complex than formerly believed.
Highlights
IntroductionReproductive success is the most important aspect of individual fitness
Male SVL Female SVL observed body size differences between paired and nonpaired individuals. Such a size-assortative pattern is typically interpreted as a consequence of male mate choice
The complex mating system of R. temporaria is embedded in a multicausal framework
Summary
Reproductive success is the most important aspect of individual fitness. Random mating would mean that all individuals of a given population would mate with the same probabilities, but due to natural and sexual selection, physical constraints, and stratification of populations, nonrandom mating is the rule in taxa with sexual reproduction One common pattern of nonrandom mating is assortative mating, defined by the correlation of traits (phenotypic or genotypic) across mated pairs. The strength of assortment differs between taxa and traits, the direction of assortment is usually positive, i.e. individuals with similar traits are more likely to mate (Thiessen and Gregg 1980; Crespi 1989; Acord et al 2013; Jiang et al 2013). Assortment can be incidental, due to spatial or temporal segregation
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