D3026: Fitness consequences of thermal melanism

The thermal melanism hypothesis is often invoked to explain the adaptive value of melanism in cold environments, stating that because darker colors absorb more solar radiation, more melanized individuals can maintain higher body temperatures. However, direct empirical tests of this hypothesis are inconsistent and the functional consequences of thermal melanism, such as effects on growth and development, are often overlooked. We tested predictions of the thermal melanism hypothesis in Hyles lineata, where degree of melanism is plastic in response to temperature and photoperiod. We exposed individuals that vary in their melanism to radiant heat and measured heating up rate and maximum body temperature with a thermal imaging camera. Additionally, we raised larvae under a radiant heat source and measured growth and development. We found that melanism influences heating up rate and maximum body temperature only in high radiant heat treatments, while in low radiant heat treatments they are most influenced by weight. This suggests that the adaptive thermal benefits of melanism are context dependent and that other factors may swamp its effects. While there was no relationship between growth rate or development time and melanism, individuals with an intermediate degree of melanism reached a larger weight, suggesting there is an optimal degree of melanism. Because melanism can increase body temperature under certain conditions, darker individuals may have increased activity, allowing them to reach a greater body size. However, melanism may only be advantageous to a point, after which investment into this trait is costly.