“The rarity of size-assortative mating in animals: Assessing the evidence with anuran amphibians”
David M. Green (Feb 2019)
The rarity of size-assortative mating in animals: assessing the evidence with anuran amphibians
Assortative mating is rarer than you think
One of the foundations of population genetics and evolutionary theory is the idea that individuals mate at random because if mating is not random, there are consequences. Assortative mating is a particular form of non-random mating in which animals select their mates according to a shared trait, which results in mated individuals resembling each other phenotypically more than expected by chance. This mating selectivity may affect genetic co-variance and gene flow, sexual co-evolution and mate choice, and potentially even lead to reproductive isolation sufficient to pull populations apart to form new species. Consequently, evidence for assortative mating has been widely sought, and found, to such an extent that it looks to be quite common. Is it?
In this study, Green exhaustively surveys the scientific literature on size-assortative mating among frogs and toads and presents a comprehensive meta-analysis of the evidence in light of the behavioral ecology of these animals. Frogs and toads are particularly well studied in this regard. Mated pairs are easy to find and measure, large size is reproductively advantageous, and the calling behavior of male “sitters” allows for female choice, which is unlikely among male “scramblers” that instead fight for possession of mates. Green finds considerable publication bias against non-significant results and analytical bias resulting from the inappropriate pooling of samples, which significantly inflates the apparent occurrence and strength of size-assortative mating. Taking these biases into account, Green finds little, if any, credible evidence for size-assortative mating behavior in any anurans. Instead, large-male advantage among scramblers leads to a different pattern, termed “disproportionate” mating, which should have relatively little evolutionary impact. Green’s work redefines the meaning of assortative mating, reassesses the evidence for its occurrence, and resets the terms for studying this evolutionary important principle.
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 resemble each other phenotypically 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, therefore, should 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 non-significant 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 2° pattern of concomitant non-random 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.