A walk through a tropical rainforest can reveal astonishing forms and colors of organisms – from vibrant poison frogs and coral snakes to the vegetative camouflage of stick insects and other cryptic creatures. Perhaps some of the most dramatic displays of variation can occur between the sexes, where males and females can differ so greatly in appearance that they resemble different species. Research in many systems has demonstrated that much of this variation is driven by sexual selection, the force responsible for the evolution of traits that are important for acquiring mates. Individuals may invest as much energy as possible into such sexually selected traits because doing so will give them a competitive advantage for mate acquisition. These traits are therefore considered condition dependent, as their expression is dependent upon the energetic condition of the individual that possesses them. While condition dependence has been the subject of many studies, it is not well known how it may vary between closely related species that share the same traits. If closely related species vary in condition dependence of their shared traits, then this implies that condition dependence could be important for the evolutionary diversity of sexually selected traits.
Together with students from Grinnell College and Reed College, and as part of an OTS (Organization for Tropical Studies) course that I took as an undergraduate at the University of Virginia, we took to the lowland jungles of Costa Rica to answer this question. We studied two anole species from Costa Rica, the slender anole (Anolis limifrons) and the ground anole (Anolis humilis). Specifically, we tested whether several traits that they had in common exhibited condition dependence, including dewlap size, aspects of jaw morphology, and sprinting speed. To test for condition dependence, we first calculated two conventional indices of body condition, the residual index and the scaled mass index, which both take into account an organism’s mass, given its length. We then obtained residuals from the relationship between our variables of interest (dewlap size, jaw width, jaw length, and sprint speed) and snout-vent length (a measure of body length), which allowed us to control for the fact that trait sizes often scale with the overall size of an animal. Finally, we used bivariate linear regressions to test the effect of our indices of body condition on our residual traits of interest, with a significant positive relationship suggesting condition dependence. We found that dewlap size (a trait important for sexual signaling) and jaw width (a trait important for bite force and male combat) exhibited condition dependence in ground anoles, but not in slender anoles. In contrast, neither sprint speed nor jaw length were condition-dependent in either species. Importantly, the presence of condition dependence in one species, but not the other, implies that the condition dependence of shared traits is evolutionarily labile. Additionally, by detecting condition dependence in the dewlap of ground anoles, which have a larger dewlap given their body length when compared to slender anoles, our findings may indicate that the strength of sexual selection differs between these two species. Lastly, our research suggests that variation in condition dependence of the dewlap among species could contribute to the extraordinary diversity of dewlaps in the Anolis genus.
If you would like to read the full paper, published in the Journal of Experimental Zoology Part A, go to:http://onlinelibrary.wiley.com/doi/10.1002/jez.2076/epdf