Smithsonian Migratory Bird Center, Smithsonian Conservation Biology Institute, National Zoological Park, Washington, DC 20008, USA.
Biol Lett. 2013 Apr 24;9(3):20130118. doi: 10.1098/rsbl.2013.0118. Print 2013 Jun 23.
Sexual size dimorphism is expected to be more pronounced in vertebrates on islands, particularly in trophic characters, as a response to decreased interspecific competition for food. We found (based on measurements of 1423 museum specimens) that bill size dimorphism was greater in island than mainland populations of song sparrows. However, dimorphism varied among islands and was positively correlated with high summer temperature and island size. Island song sparrow bills follow the overall positive temperature bill size relationship for California song sparrows, which includes larger bills on large, warmer islands. Large bills dissipate more heat and may be an adaptation to summer heat stress. Dimorphism increases because the slope for males is greater than females. Thus, the greater magnitude of bill dimorphism on islands with warmer summers may result from males experiencing greater thermal stress during territorial activity, creating different thermal optima. In contrast, bill dimorphism was unrelated to climate on the mainland. We hypothesize that reduced interspecific competition releases island populations from a constraint so that sex-specific physiological optima can be achieved, whereas mainland birds are constrained.
性二型在岛屿上的脊椎动物中更为明显，尤其是在营养特征上，这是对食物种间竞争减少的一种反应。我们发现（基于对 1423 个博物馆标本的测量），岛屿上的歌声雀的喙大小二型比大陆上的歌声雀更大。然而，二型性在岛屿之间存在差异，并与夏季高温和岛屿大小呈正相关。岛屿歌声雀的喙遵循加利福尼亚歌声雀的总体正温度喙大小关系，其中包括较大的喙在较大、较温暖的岛屿上。较大的喙可以散发更多的热量，可能是对夏季热应激的一种适应。二型性增加是因为雄性的斜率大于雌性。因此，夏季温暖的岛屿上喙二型性的幅度更大可能是由于雄性在领地活动期间经历更大的热应激，从而产生不同的热最佳状态。相比之下，喙二型性与大陆上的气候无关。我们假设，种间竞争的减少使岛屿种群摆脱了一种限制，从而可以实现性别特异性的生理最佳状态，而大陆鸟类则受到限制。
