It is cool to be clear: Transparency induces a thermal cost in clearwing butterflies

In butterflies, wing transparency raises the question of potential costs for vital functions, such as thermoregulation. The thermal melanism hypothesis posits that darker colorations enable higher absorption of solar radiation, leading to the prediction that such colorations should be favoured in colder environments. This prediction extends to the near‐infrared range, a large component of solar radiation. Here, we assess the implications of wing transparency for light absorption and thermal properties in 42 species of clearwing butterflies (Ithomiini) and examine the distribution of such optical and thermal properties in relation to climatic conditions across different elevations. As expected, we find that transparent wings are less efficient than opaque wings to absorb light across ultraviolet, visible and near‐infrared ranges, and are poorer at collecting heat. However, the proportion of transparent wing areas and the proportion of transparent species increase with elevation and decrease with temperature, going strongly against the predictions of the thermal melanism hypothesis in terms of species distribution with regard to climatic conditions. Our results suggest that factors other than adaptation to climate, such as predation pressure, physiology or behaviour, may have driven the evolution of wing patterns in Ithomiini. Read the free Plain Language Summary for this article on the Journal blog.