Department of Neurobiology and Behavior, Cornell University, 215 Tower Road, Ithaca, NY 14853, USA; Cornell Lab of Ornithology, 159 Sapsucker Woods Road, Ithaca, NY 14850, USA; Smithsonian Tropical Research Institute, Balboa, Ancón, Republic of Panama.
Department of Neurobiology and Behavior, Cornell University, 215 Tower Road, Ithaca, NY 14853, USA; Cornell Lab of Ornithology, 159 Sapsucker Woods Road, Ithaca, NY 14850, USA.
Curr Biol. 2021 Oct 11;31(19):4381-4387.e6. doi: 10.1016/j.cub.2021.07.043. Epub 2021 Aug 26.
Ornamentation is typically observed in sexually mature adults, is often dimorphic in expression, and is most apparent during breeding, supporting a role for sexual selection in its evolution. Yet, increasing evidence suggests that nonsexual social selection may also have a role in the evolution of ornamentation, especially in females. Distinguishing between these alternatives remains challenging because sexual and nonsexual factors may both play important and overlapping roles in trait evolution. Here, we show that female ornamentation in a dichromatic hummingbird, the white-necked jacobin (Florisuga mellivora), cannot be explained by sexual selection. Although all males are ornamented, nearly 30% of females have male-like plumage. Remarkably, all juveniles of both sexes express ornamented plumage similar to adult males (androchromatism), but 80% of females acquire non-ornamented plumage (heterochromatism) as they age. This unique ontogeny excludes competition for mates as an explanation for female ornamentation because non-reproductive juveniles are more likely to be ornamented than adults. Instead, avoidance of social harassment appears to underlie this female-limited polymorphism, as heterochrome taxidermy mounts received more aggressive and sexual attention than androchrome mounts from this and other hummingbird species. Monitoring electronically tagged birds at data-logging feeders showed that androchrome females accessed feeders more than heterochrome females, presumably because of reduced harassment. Our findings demonstrate that ornamentation can arise purely through nonsexual social selection, and this hypothesis must be considered in the evolution of not only female-limited polymorphism but also the spectacular ornamentation often assumed to result from sexual selection.
装饰通常在性成熟的成年个体中观察到，在表达上常常存在二态性，并且在繁殖期间最为明显，支持了性选择在其进化中的作用。然而，越来越多的证据表明，非性社会选择也可能在装饰的进化中发挥作用，尤其是在雌性中。区分这些替代方案仍然具有挑战性，因为性和非性因素可能在特征进化中都发挥着重要且重叠的作用。在这里，我们表明，双色蜂鸟（Florisuga mellivora）的雌性装饰不能用性选择来解释。虽然所有雄性都有装饰，但近 30%的雌性具有雄性般的羽毛。值得注意的是，所有两性的幼鸟都表达出类似于成年雄性的有装饰的羽毛（androchromatism），但随着年龄的增长，80%的雌性获得了无装饰的羽毛（heterochromatism）。这种独特的个体发生排除了竞争配偶作为雌性装饰的解释，因为非生殖幼鸟比成年个体更有可能具有装饰性。相反，避免社会骚扰似乎是这种雌性限性多态性的基础，因为与雌雄同体的标本相比，异态标本的标本更容易受到攻击性和性关注。通过电子标记的鸟类在数据记录喂食器上的监测显示，雌雄同体的雌性比异态的雌性更频繁地访问喂食器，大概是因为骚扰减少了。我们的发现表明，装饰可以纯粹通过非性社会选择而产生，因此，这种假设不仅必须在雌性限性多态性的进化中考虑，而且必须在通常假定是性选择结果的华丽装饰的进化中考虑。
