Tropical bird species have less variable body sizes

Ecologists have often predicted that species’ niche breadths should decline toward the equator. Dan Janzen arrived at this prediction based on climatic constraints, while Robert MacArthur argued that a latitudinal gradient in resource specialization drives the pattern. This idea has some support when it comes to thermal niches, but has rarely been explored for other niche dimensions. Body size is linked to niche dimensions related to diet, competition, and environmental tolerance in vertebrates. We identified 68 pairs of tropical and nontropical sister bird species using a comprehensive phylogeny and used the VertNet specimen database to ask whether tropical birds have lower intraspecific body-size variation than their nontropical sister species. Our results show that tropical species have less intraspecific variability in body mass ( = 0.0092; p = 0.009). Variation in body-size variability was poorly explained by both abiotic and biotic drivers, thus the mechanisms underlying the pattern are still unclear. The lower variation in body size of tropical bird species may have evolved in response to more stable climates and resource environments.

生态学家曾普遍预测，物种的生态位（niche）宽度应随向赤道趋近而逐渐减小。丹·詹森（Dan Janzen）基于气候限制条件得出了这一预测，而罗伯特·麦克阿瑟（Robert MacArthur）则提出资源特化的纬度梯度驱动了这一格局。该观点在热生态位领域已获得一定支持，但针对其他生态位维度的相关研究却极为匮乏。在脊椎动物中，体型与饮食、种间竞争及环境耐受相关的生态位维度紧密关联。我们依托一套完整的系统发育（phylogeny）数据集，鉴定出68对热带与非热带鸟类姊妹物种，并借助VertNet标本数据库，探究热带鸟类的种内体型变异是否低于其非热带姊妹物种。研究结果显示，热带物种的体重种内变异性更低（= 0.0092；p = 0.009）。非生物与生物驱动因子均无法较好解释体型变异性的差异，因此该格局背后的具体机制仍不明确。热带鸟类体型变异性更低这一特征，可能是为适应更为稳定的气候与资源环境而演化形成的。