Dual function and associated costs of a highly exaggerated trait in a cichlid fish

Exaggerated secondary sexual characteristics are apparently costly and seem to defy natural selection. This conundrum prompted Charles Darwin to propose the theory of sexual selection. Accordingly, exaggerated secondary sexual characteristics might be ornaments on which female choice is based and/or armaments used during male-male competition. Males of many cichlid fish species, including the adaptive radiation of Nicaraguan Midas cichlids, develop a highly exaggerated nuchal hump, which is thought to be a sexually selected trait. To test this hypothesis, we conducted a series of behavioral assays in F2 hybrids obtained from crossing a species with a relatively small hump and one with an exaggerated hump. Mate-choice experiments showed a clear female preference for males with large humps. In an open-choice experiment with limited territories, couples including large humped males were more successful in acquiring these territories. Therefore, nuchal humps appear to serve dual functions a...夸张的第二性征显然需付出高昂代价，且似乎违背自然选择法则。这一难题促使查尔斯·达尔文提出了性选择理论。据此，夸张的第二性征既可能作为雌性择偶所依据的装饰物，也可能是雄性间竞争时使用的武器。诸多慈鲷鱼（cichlid fish）物种的雄性——包括适应性辐射演化的尼加拉瓜金母慈鲷——会长出极为夸张的颈背隆起，该结构被认为是一种性选择性状。为验证这一假说，我们利用由颈背隆起较小的物种与颈背隆起夸张的物种杂交得到的F2杂交后代（F2 hybrids）开展了一系列行为实验。择偶实验结果显示，雌性明显偏好拥有较大颈背隆起的雄性。在带有有限领地的开放式选择实验中，搭配有大颈背隆起雄性的配对更成功获得领地。因此，颈背隆起似乎兼具双重功能……