Males in seemingly female-like plumage do not mimic females: UV reflectance reveals temporal cryptic dimorphism in a manakin species exhibiting delayed plumage maturation

Manakins (Pipridae) are neotropical birds that usually exhibit delayed plumage maturation (DPM). Thus, while plumage of most adult male manakins is brightly conspicuous, subadult males and females are basically dull-olive green. Although sexual dichromatism in some bird species may be evident only through UV reflectance, this phenomenon, known as hidden sexual dichromatism, has not been previously studied in manakins to compare subadult males and females. Within this framework, we carried out spectrophotometric analyses in searching for hidden sexual dichromatism in the white-bearded manakin Manacus manacus, through comparison of UV spectra in females and subadult males in green plumage. Our results revealed UV reflectance in both sexes in green plumage. Moreover, we found UV spectral differences in homologous color patches between sexes, particularly at belly. Since the observed differences may allow intraspecific sex recognition of individuals in green plumage, our results do not support the female-mimicry hypothesis to explain delayed plumage maturation in the white-bearded manakin. Although our findings dismiss the female mimicry hypothesis, we cannot state whether these results support the non-mutually exclusive cryptic and status signaling hypotheses. We propose then, that dull coloration of subadult males may serve both as a cryptic trait and to limit the energetic costs of acquiring the adult plumage before sexual maturity. Meanwhile, differential UV color traits between sexes in green plumage may allow adult males to avoid unnecessary energy expenditures in courtship displays in the presence of males near leks, and to selectively focus their the courtship displays on females. In accordance with the signaling status hypothesis, subadult males can be recognized both as males and subordinates and consequently may practice courtship displays without suffering aggressions by adult males. Our results highlight the importance to include a wider range of spectrophotometric information analyses for testing hypotheses regarding delayed plumage maturation.

娇鹟科（Pipridae）的娇鹟是新热带界鸟类，普遍表现出羽色延迟成熟（delayed plumage maturation, DPM）现象。因此，尽管多数成年雄鸟的羽色鲜艳醒目，但亚成体雄鸟与雌鸟的羽色基本呈暗沉橄榄绿色。部分鸟类的性二态性（sexual dichromatism）仅可通过紫外反射得以体现，这一现象被称为隐性性二态性（hidden sexual dichromatism），但此前尚未有研究针对娇鹟的亚成体雄鸟与雌鸟开展此类对比研究。基于此研究框架，我们针对白须娇鹟（Manacus manacus）开展了分光光度分析，通过对比处于绿色羽衣阶段的雌鸟与亚成体雄鸟的紫外光谱，探寻其隐性性二态性。研究结果显示，两类个体的绿色羽衣均存在紫外反射现象。此外，我们发现两性间同源色斑的紫外光谱存在差异，尤以腹部区域最为显著。鉴于上述差异可帮助个体在种内实现对绿色羽衣个体的性别识别，本研究结果不支持用以解释白须娇鹟羽色延迟成熟的雌性拟态假说。尽管本研究结果否定了雌性拟态假说，但我们尚无法判定其是否支持互不排斥的隐蔽性假说与地位信号假说。据此我们提出，亚成体雄鸟的暗沉羽色可能兼具两项功能：一是作为隐蔽性特征，二是可降低性成熟前获取成体羽衣所需的能量成本。与此同时，两性绿色羽衣间的紫外色彩特征差异，可帮助成年雄鸟在求偶场中遇到其他雄鸟时避免不必要的求偶炫耀能量消耗，并选择性地将求偶炫耀指向雌鸟。依据地位信号假说，亚成体雄鸟可同时被识别为雄性与从属个体，因此它们可以开展求偶炫耀而不会遭受成年雄鸟的攻击。本研究结果凸显了纳入更广泛的分光光度信息分析，以检验羽色延迟成熟相关假说的重要性。