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 (<i>Cinnyris jugularis</i>) 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.

生物间的竞争不仅存在于不同物种之间，也发生于同一物种内部。针对资源的性间竞争（intersexual competition）可能会提升体型的性二态性（sexual dimorphism）水平，从而最大限度降低两性间的生态位（ecological niche）重叠。物种种群的性二态性水平改变，是岛屿辐射演化的常见特征。此类现象通常被解读为：小型岛屿缺乏种间竞争者，种群由此获得生态释放（ecological release），令具有性二态性的物种的两性均能拓展自身生态位。橄榄背太阳鸟（Olive-backed Sunbird，*Cinnyris jugularis*）是广泛分布于印度-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岛东南部的橄榄背太阳鸟种群为研究对象，对其形态生态位（morphological niche）的性二态性展开探究。结果显示，相较于苏拉威西岛本土及其周边较大的大陆岛，在物种贫乏的瓦卡托比群岛上，该太阳鸟雌雄个体间的形态生态位重叠度更低，表明小型岛屿上的种群性二态性更强。这类性二态性的变化与雌性个体的形态生态位超体积（niche hypervolume）缩减相关，而雄性个体的该指标未出现显著变化。由此可见，在缺乏本土种间竞争者的背景下，并未观测到形态生态位空间出现拓展的迹象。此类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密切相关。因此，这种增强的性二态性或可缓解该物种内部针对资源的种内竞争（intraspecific competition）。