RESUMO
Among defenses against predation, chemical defenses are possibly the most studied. However, when addressing the effectiveness of those chemical defenses, previous studies did not include properties of the chemical substances themselves. Lipophilicity, for instance, may facilitate crossing membranes, and boiling point may define the duration of the substances in the air. Moreover, other variables may also be relevant: the predator taxon; the prey model chosen to conduct experiments; whether the prey is presented grouped or not in experiments; and whether the chemical defense is a mixture of many substances or only one. To understand how those factors influence chemical defenses' effectiveness, we conducted a multilevel meta-analysis with 43 studies (127 effect sizes), accounting for different types of dependence. We used Akaike Information Criterion (AICc) to select the best model. The model with the lowest AICc value included only the boiling point, which defines how quickly a chemical substance volatilizes. This model indicated that the most effective chemical defenses had lower boiling point values, i.e., higher volatility. Moreover, we did not find chemicals with very low boiling points, suggesting there might be an optimum range of volatility. Other models, including the intercept-only model, were also recovered among the best models, therefore further studies are needed to confirm the relationship between volatility and chemical defenses' effectiveness. Our results highlight the value of incorporating physicochemical properties in the ecological and evolutionary study of chemical defense.