Macroevolutionary patterns of sexual size dimorphism among African tree frogs (Family: Hyperoliidae)

Sexual size dimorphism (SSD) is shaped by multiple selective forces that
drive the evolution of sex-specific body size, resulting in male or
female-biased SSD. Stronger selection on one sex can result in an
allometric body-size scaling relationship consistent with Rensch’s rule or
its converse. Anurans (frogs and toads) generally display female-biased
SSD, but there is variation across clades and the mechanisms driving the
evolution of SSD remain poorly understood. We investigated these topics in
a diverse family of African treefrogs (Hyperoliidae). Hyperoliids display
traits considered rare among amphibians, including sexual dichromatism and
protogynous sex change. Using phylogenetic comparative methods, we tested
if adult ecology, sexual dichromatism, and sex change were predictors of
body size or SSD. We also tested whether hyperoliids displayed allometric
interspecific body-size-scaling relationships. We found a majority of
hyperoliid taxa display female-biased SSD, but that adult ecology and
sexual dichromatism are poor predictors of sex-specific body size and SSD.
Regardless of the groupings analyzed (partitioned by clades or traits), we
found support for isometric body-size scaling. However, we found that sex
change is a significant predictor of SSD variation. Species in the
Hyperolius viridiflavus complex, which putatively display this trait, show
a significant reduction in SSD and tend to be sexually monomorphic in
size. Although protogynous sex change needs to be validated for several of
these species, we tentatively propose this trait is a novel mechanism
influencing anuran body size evolution. Beyond this association,
additional factors that shape the evolution of anuran body size and SSD
remain elusive.