Data from: Size doesn't matter, sex does: a test for boldness in sister species of Brachyrhaphis fishes

The effect of divergent natural selection on the evolution of behavioral traits has long been a focus of behavioral ecologists. Predation, due to its ubiquity in nature and strength as a selective agent, has been considered an important environmental driver of behavior. Predation is often confounded with other environmental factors that could also play a role in behavioral evolution. For example, environments that contain predators are often more ecologically complex and “risky” (i.e., exposed and dangerous). Previous work shows that individuals from risky environments are often more bold, active, and explorative than those from low-risk environments. To date, most comparative studies of environmentally driven behavioral divergence are limited to comparisons among populations within species that occur in divergent selective environments but neglect comparisons between species following speciation. This limits our understanding of how behavior evolves post-speciation. The Central American live-bearing fish genus Brachyrhaphis provides an ideal system for examining the relationship between selective environments and behavior, within and between species. Here, we test for differences in boldness between sister species B. roseni and B. terrabensis that occur in streams with and without piscivorous predators, respectively. We found that species do differ in boldness, with species that occur with predators being bolder than those that do not. Within each species, we found that sexes differed in boldness, with males being bolder than females. We also tested for a relationship between size (a surrogate for metabolic rate) and boldness, but found no size effects. Therefore, sex, not size, affects boldness. These results are consistent with the hypothesis that complex and risky environments favor individuals with more bold behavioral traits, but they are not consistent with the hypothesis that size (and therefore metabolic rate) drives divergence in boldness. Finally, our results provide evidence that behavioral trait divergence continues even after speciation is complete.

趋异自然选择（divergent natural selection）对行为性状（behavioral traits）演化的影响，长期以来都是行为生态学家的核心研究议题。捕食作用（predation）因在自然界中广泛分布，且作为选择动因的效力显著，被视为推动行为演化的重要环境驱动因子。但捕食作用往往与其他同样可能影响行为演化的环境因子相互混淆，难以单独剥离其效应。例如，存在捕食者的生境往往在生态上更为复杂，且具有更高的“风险”（即暴露度更高、环境更凶险）。既往研究表明，来自高风险生境的个体，通常比低风险生境的个体表现出更强烈的大胆性、活跃性与探索性。
迄今为止，绝大多数针对环境驱动型行为趋异的比较研究，仅局限于同物种内处于趋异选择环境下的不同种群间的比较，却忽略了物种形成完成后跨物种间的对比分析，这极大限制了我们对物种形成后行为演化路径的认知。
中美洲胎生鱼类短刺鳚属（Brachyrhaphis）为探究选择环境与行为在物种内部及跨物种间的关联，提供了绝佳的研究系统。
本研究针对栖息于分别带有与不带食鱼性捕食者（piscivorous predators）的溪流中的姊妹物种（sister species）罗氏短刺鳚（B. roseni）与特拉短刺鳚（B. terrabensis），检验二者大胆性的差异。
研究结果显示，两个物种的大胆性确实存在显著差异：栖息于带有捕食者生境的物种，其大胆性显著高于无捕食者生境的物种。
在每个物种内部，不同性别个体的大胆性亦存在差异：雄性个体的大胆性显著高于雌性。
本研究同时检验了体型（作为代谢率的替代指标）与大胆性间的关联，但未发现体型对大胆性存在显著影响。
由此可见，影响大胆性的因素为性别，而非体型。
上述结果支持“复杂且高风险的生境更青睐具有更强大胆性行为性状的个体”这一假说，但并不支持“体型（进而代谢率）驱动大胆性趋异”的假说。
最后，本研究结果证实：即便在物种形成完成后，行为性状的趋异演化仍会持续进行。