Rarity of Size-Assortative Mating in Animals: Assessing the Evidence with Anuran Amphibians

Assortative mating in animals can have substantial evolutionary impact. Numerous reports also make it appear to be pervasive in occurrence. In assortative mating, defined here in behavioral terms, animals select their mates according to a particular shared trait such that mated individuals phenotypically resemble each other more than expected by chance. Body size is a widely studied assortment trait. This is especially relevant for anuran amphibians (frogs and toads), among whom reproductive advantages may accrue to large individuals of both sexes. Anurans also exhibit discrete forms of male mating behavior. Sedentary calling behavior of "sitters" allows for female choice, whereas fighting for possession of mates by "scramblers" precludes female choice. Size-assortative mating in anurans should therefore be a property of sitters, not scramblers. I used meta-analysis to assess the occurrence of true size-assortative mating in relation to mating behavior and other variables in 282 studies of 68 species of anurans. I found publication bias against reporting nonsignificant results and analytical bias resulting from pooling of samples collected at different times or places (Simpson's paradox). Pooled samples significantly inflated the apparent occurrence and strength of size-assortative mating. Controlling for such biases left little credible evidence for size-assortative mating behavior in any anurans. Instead, large-male advantage among scramblers was associated with a secondary pattern of concomitant nonrandom mating. In this disproportionate mating, neither sex behaves according to mate choice rules that could lead to consistently strong assortment. It should thus have relatively little evolutionary impact compared to true assortative mating.