Random mating for body size despite fitness benefits of size‐assortative mating in a treefrog

Abstract

Size‐assortative mating in frogs and toads should increase fitness, because pairs consisting of partners well matched for size should also have a higher proportion of fertilized eggs. We examined whether the size ratios of mated males and females had an effect on fertilization success in Eastern Gray Treefrogs and tested whether the naturally observed size ratios could be attributed to female preferences or stochastic effects (male availability). We show that in this species, mating with a roughly size‐matched partner (m/f size ratios between 0.9 and 1.1) provides direct benefits by increasing fertilization success. Although the frequency of male advertisement calls would provide females with a cue by which to estimate the body size of a potential partner, the female call frequency preferences are not size‐dependent. Call frequency preferences are therefore unlikely to result in the choice of a well‐matched partner. The size distributions of males and females available at the breeding site appear to result in the distribution of male/female size ratios and the weak pattern of size‐assortative mating we observed in nature, despite apparently random mating with respect to body size.