Data from: Evidence of phenotypic plasticity of penis morphology and delayed reproductive maturation in response to male competition in waterfowl

Ducks are an excellent group to study avian genital evolution. Penis
morphology of ducks is diverse, and penis length and elaboration are
positively correlated with levels of male competition resulting from
forced extra-pair copulations, and with female genital elaboration
resulting from sexual conflict. Here we examined whether penis morphology
is affected by social environment. We found experimental evidence that in
a male-biased social environment, consisting of several males and fewer
females, the penis in Lesser Scaup (Aythya affinis) grew longer in 2
separate years, than in males housed in pairs, as predicted if male–male
competition influences penis morphology. In Ruddy Ducks (Oxyura
jamaicensis), males instead showed evidence of reproductive delays that
were explained both by a male's size and his social environment: most
males in social groups exhibited shorter penises, variable onset and
duration of genital maturation, and faster penis growth rate. These 2
species have different levels of post-copulatory competition in nature,
with Ruddy Ducks having more extreme penis sizes and more promiscuity than
Lesser Scaup. The results suggest that waterfowl can exhibit complex,
socially dependent phenotypic plasticity and reproductive maturation that
can generate intraspecific variation in their genitalia.