Data from: Reproductive skew drives patterns of sexual dimorphism in sponge-dwelling snapping shrimps

Sexual dimorphism is typically a result of strong sexual selection on male
traits used in male–male competition and subsequent female choice.
However, in social species where reproduction is monopolized by one or a
few individuals in a group, selection on secondary sexual characteristics
may be strong in both sexes. Indeed, sexual dimorphism is reduced in many
cooperatively breeding vertebrates and eusocial insects with totipotent
workers, presumably because of increased selection on female traits. Here,
we examined the relationship between sexual dimorphism and sociality in
eight species of Synalpheus snapping shrimps that vary in social structure
and degree of reproductive skew. In species where reproduction was shared
more equitably, most members of both sexes were physiologically capable of
breeding. However, in species where reproduction was monopolized by a
single individual, a large proportion of females—but not males—were
reproductively inactive, suggesting stronger reproductive suppression and
conflict among females. Moreover, as skew increased across species,
proportional size of the major chela—the primary antagonistic weapon in
snapping shrimps—increased among females and sexual dimorphism in major
chela size declined. Thus, as reproductive skew increases among
Synalpheus, female–female competition over reproduction appears to
increase, resulting in decreased sexual dimorphism in weapon size.