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）因其在自然界中广泛分布且作为选择因子（selective agent）的作用强度，被视为推动行为演化的重要环境驱动因素。然而捕食作用常与其他同样可能影响行为演化的环境因素相互混淆。例如，存在捕食者的环境通常生态复杂度更高且具有更高的“风险”（即暴露度高且存在危险）。既往研究表明，来自高风险环境的个体通常比低风险环境的个体更大胆、更活跃且探索性更强。截至目前，大多数针对环境驱动的行为趋异的比较研究，仅局限于对处于不同选择环境下的物种种内种群进行比较，却忽略了物种形成（speciation）后物种类群间的比较，这限制了我们对物种形成后行为如何演化的认知。中美洲胎生鱼类（live-bearing fish）短体鳉属（Brachyrhaphis）为探究物种内外的选择环境与行为之间的关联提供了理想的研究体系。本研究针对分别栖息于有食鱼捕食者（piscivorous predators）和无食鱼捕食者溪流中的姐妹种（sister species）罗森短体鳉（B. roseni）与特拉本西斯短体鳉（B. terrabensis）的大胆性差异开展检验。研究结果显示，两个物种的大胆性确实存在差异：栖息于有捕食者环境的物种比无捕食者环境的物种更大胆。在每个物种内部，不同性别间的大胆性也存在差异，雄性较雌性更大胆。本研究同时检验了体型（作为代谢率（metabolic rate）的替代指标）与大胆性之间的关联，但未发现体型产生的显著效应。因此，影响大胆性的因素是性别而非体型。上述结果与“高复杂度与高风险环境更青睐具有更大胆行为性状的个体”这一假说相符，但与“体型（进而代谢率）驱动大胆性趋异”的假说相悖。最后，本研究结果证实，即使在物种形成完成后，行为性状的趋异依然会持续发生。