Increased sexual dimorphism in dense populations of Olive-backed Sunbirds on small islands: morphological niche contraction in females but not males

Competition takes place not only between species but also within them. Intersexual competition for resources may increase sexual dimorphism in body size to minimise ecological niche overlap. Change in the level of sexual dimorphism in a species is a common feature of island radiations. This is often interpreted as ecological release from interspecific competitors absent from small islands, allowing niche expansion by both sexes of a dimorphic species. The Olive-backed Sunbird (Cinnyris jugularis) is a widespread island-colonising species found throughout the Indo-West Pacific. Here we investigate sexual dimorphism in morphological niche of Olive-backed Sunbird populations in South-east Sulawesi, Indonesia. We found decreased overlap in morphological niche between females and males on the species-depauperate Wakatobi Islands, in comparison to mainland Sulawesi and its larger continental islands, indicating greater sexual dimorphism on the small islands. This change in sexual dimorphism was associated with a decrease in the morphological niche hypervolume of females, but no change in males. Therefore there was no indication of expansion of morphological niche space in the absence of mainland competitors. These morphological differences were associated with the significantly higher population density of Wakatobi Olive-backed Sunbirds. Therefore this increased sexual dimorphism may serve to alleviate intraspecific competition for resources.

竞争不仅发生在物种之间，也存在于物种内部。针对资源的性间竞争可能会增大体型的性二态性（sexual dimorphism），以最小化生态位（ecological niche）重叠。某一物种的性二态性水平变化是岛屿辐射演化（island radiations）的常见特征。这通常被解释为：小型岛屿缺乏种间竞争者，从而带来生态释放（ecological release），使得该性二态物种的两性均能拓展生态位。
橄榄背太阳鸟（Olive-backed Sunbird，学名*Cinnyris jugularis*）是一种广泛分布于印度-西太平洋区域的岛屿拓殖性物种。本研究以印度尼西亚苏拉威西东南部的橄榄背太阳鸟种群为对象，探究其形态生态位的性二态性。
研究发现，相较于苏拉威西大陆及其周边更大的陆联岛，在物种匮乏的瓦卡托比群岛（Wakatobi Islands）上，雌雄个体间的形态生态位重叠度更低，这表明小型岛屿上的性二态性更强。这种性二态性的变化与雌性个体的形态生态位超体积（morphological niche hypervolume）缩减相关，但雄性个体的该指标未发生显著改变。因此，在缺乏大陆竞争者的情况下，并未出现形态生态位空间拓展的迹象。
上述形态差异与瓦卡托比群岛橄榄背太阳鸟显著更高的种群密度相关联。据此推测，这种增强的性二态性或可缓解种内资源竞争。