Anisogamy is unrelated to the intensity of sexual selection

Males and females often display different behaviours and, in the context
of reproduction, these behaviours are labelled sex roles. The
Darwin–Bateman paradigm argues that the root of these differences is
anisogamy (i.e., differences in size and/or function of gametes between
the sexes) that leads to biased sexual selection, and sex differences in
parental care and body size. This evolutionary cascade, however, is
contentious since some of the underpinning assumptions have been
questioned. Here we investigate the relationships between anisogamy,
sexual size dimorphism, sex difference in parental care and intensity of
sexual selection using phylogenetic comparative analyses of 64 species
from a wide range of animal taxa. The results question the first step of
the Darwin–Bateman paradigm, as the extent of anisogamy does not appear to
predict the intensity of sexual selection. The only significant predictor
of sexual selection is the relative inputs of males and females into the
care of offspring. We propose that ecological factors, life-history and
demography have more substantial impacts on contemporary sex roles than
the differences of gametic investments between the sexes.