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.

竞争不仅存在于物种之间，亦发生于物种内部。为争夺资源而发生的两性间竞争，可能会加剧体型的性二态性（sexual dimorphism），进而最大限度降低生态位重叠。某一物种的性二态性水平发生改变，是岛屿辐射演化的常见特征。该现象通常被解读为：小型岛屿缺失种间竞争者，使得种群获得生态释放，进而允许具二态性的物种的两性均拓展生态位。橄榄背花蜜鸟（Olive-backed Sunbird，Cinnyris jugularis）是广泛分布于印度-西太平洋区域的岛屿拓殖物种。本研究以印度尼西亚苏拉威西岛东南部的橄榄背花蜜鸟种群为研究对象，探究其形态生态位的性二态性特征。研究发现，相较于苏拉威西岛本土及更大的陆联岛屿，在物种匮乏的瓦卡托比群岛，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为显著。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，而雄性个体的形态生态位超体积未发生显著变化。因此，在本土竞争者缺失的情况下，并未观察到形态生态位空间出现拓展的迹象。上述形态差异与瓦卡托比群岛橄榄背花蜜鸟显著更高的种群密度密切相关。据此推测，这种增强的性二态性或可缓解种内资源竞争。