Non-sexual social selection can underlie the evolution of sexually monomorphic phenotypes. A causal relationship between territorial competition and sexual monomorphism predicts that male and female competitors should employ similar contest behavior and that contest outcome should depend on the same traits in males and females. We test this prediction in a sexually monomorphic cichlid fish of the genus Tropheus, in which males and females defend individual feeding territories. Lineages basal to Tropheus are sexually dimorphic and have non-territorial females, suggesting that a switch to female territoriality and loss of sexual dimorphism occurred in the Tropheus lineage. We compare rates of agonistic behavior and the effects of body size asymmetries on competitive success between male-male and female-female contests in an experimental setup. Body size asymmetry had the same effect in male and female contests, being negatively correlated with contest duration and positively correlated with the probability of winning. Male and female winners employed the same rates of frontal and lateral displays as well as charges against their opponents. Contest duration was longer in females. In tied contests, females displayed more than males. Our data suggest that intraspecific contest competition for territories selects for large body size in both sexes and support a link between the evolution of female territoriality and the loss of sexual size dimorphism in Tropheus.

中文翻译：

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领地竞争和性体型二态性的进化丧失

非性社会选择可能是性单态表型进化的基础。领土竞争与性别单态性之间的因果关系预测，男性和女性竞争者应该采用相似的竞争行为，并且竞争结果应该取决于男性和女性的相同特征。我们在 Tropheus 属的一种性单态慈鲷中测试了这一预测，其中雄性和雌性捍卫各自的摄食领地。Tropheus 的基础谱系是两性二态性的，并且具有非领地性雌性，这表明 Tropheus 谱系中发生了向雌性领地性的转变和性二态性的丧失。我们在实验设置中比较了男性与男性和女性与女性之间的竞争行为率以及体型不对称对竞争成功的影响。体型不对称在男性和女性比赛中具有相同的效果，与比赛持续时间呈负相关，与获胜概率呈正相关。男性和女性获胜者采用相同的正面和侧面展示以及对对手的冲锋率。女性的比赛持续时间更长。在平局比赛中，女性的表现优于男性。我们的数据表明，种内争夺领地的竞争选择了两性的大体型，并支持雌性领地进化与 Tropheus 性别体型二态性丧失之间的联系。