Social behavior is a very complex phenomenon, as we all know. Although we understand a great deal about why many aspects of social behavior evolve, we know surprisingly little about what mechanisms evolve to cause species differences in social behavior. One interesting, and somewhat surprising, aspect of behavioral evolution is that similar types of social behavior can evolve in distantly related species. This convergent evolution is due to the distantly related species experiencing similar environmental/social pressures. For example, maternal care strategies in placental and marsupial mammals have converged, as has web making in Hawaiian spiders and echolocation call structure in bats. My colleague and I studied a potential mechanism to explain behavioral convergence in Caribbean Anolis lizards.
Caribbean anoles display convergent evolution in habitat use, morphology, and behavior. The picture above shows the different “types” of lizards you can find on Jamaica, Hispaniola, Cuba, Puerto Rico, and the Bahamas.
Caribbean anoles are best known for convergent evolution in habitat use and the anatomy associated with locomotion in different parts of trees in the forests in which they live. That is, on different Greater Antillean islands, the same lizard shapes have re-evolved independently based on where they live in the forest. But wait, it gets better! They also display striking convergence in their levels of aggression. Twig anoles are laid-back lizards that rarely display, whereas trunk-ground anoles display at anything that moves! Other species fall somewhere in between. The astute reader may be thinking that differences in aggression may be due to differences in that “male hormone” testosterone, and that’s what we tested. Based on decades of literature that led us to our hypotheses, we tested whether convergent evolution in aggression is due to convergent evolution in testosterone levels, and the results were surprising. Lizard species with high aggression had low testosterone levels (and vice versa) on three of four islands that we studied. Puerto Rican species showed the relationship that we expected: high testosterone = high aggression.
Two extremes from the Bahamas: Anolis angusticeps (left), a twig anole that lives life slowly, and Anolis sagrei (right), a trunk-ground anole that wants you to believe that he can take you out.
Our results suggest that the same social behavior can result from the evolution of different mechanisms. This is a phenomenon that functional morphologists call “many-to-one mapping.” The idea is that the same functional end can be reached by multiple mechanistic means. It seems that being an aggressive anole (or not!) can evolve by changes in different mechanistic pathways. For some testosterone levels have changed in a predictable manner, but in others it is likely due to changes in hormone receptors or some other hormone system completely. We’re currently looking into these possibilities, so stay tuned. For more info, check out our webpage!
Three trunk-ground anoles. Each species evolved the same general body shape (and aggressive behavior) independently from different ancestors.