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.