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.鸭类是研究鸟类生殖器演化的极佳类群。鸭类的阴茎形态多样，阴茎长度与复杂程度均与强制婚外交配引发的雄性竞争水平、以及性冲突驱动的雌性生殖器复杂程度呈正相关。本研究旨在探究阴茎形态是否受社会环境影响。我们通过实验发现，在由多只雄性与少量雌性组成的雄性偏倚社会环境中，斑背潜鸭（Aythya affinis）的阴茎在两个独立实验年度中的生长长度均显著高于成对饲养的雄性个体，这一结果符合雄性间竞争影响阴茎形态的预测。而棕硬尾鸭（Oxyura jamaicensis）的雄性则表现出生殖延迟现象，该现象可通过雄性体型与社会环境共同解释：群居环境中的多数雄性阴茎更短，生殖器成熟的起始时间与持续时长存在差异，且阴茎生长速率更快。这两个物种在自然环境中的交配后竞争水平存在差异，棕硬尾鸭的阴茎尺寸更为极端，且交配行为相较于斑背潜鸭更为混杂。研究结果表明，水鸟可展现出复杂的、依赖社会环境的表型可塑性与生殖成熟模式，进而导致其生殖器的种内变异。