Thermal biology of a colour-dimorphic snake, Elaphe quadrivirgata, in a montane forest: do melanistic snakes enjoy thermal advantages?

Abstract

The adaptive significance of colour polymorphisms in animals has received extensive scientific attention. In snakes, a generally accepted hypothesis is that melanistic individuals enjoy thermal advantages compared to normal coloured individuals. Elaphe quadrivirgata on Yakushima Island exhibits a distinct melanistic/striped colour dimorphism. To test this hypothesis, the thermal biology of free-ranging E. quadrivirgata was investigated using temperature-sensitive radio transmitters. The thermal quality of habitats was also evaluated using physical models of the snake. In addition, the species' set-point range (Tset) was estimated using a laboratory experiment. In July, thermal environments appear to be benign because snakes were able to maintain their body temperature (Tb) within Tset from the midday to evening by using average thermal habitats. By contrast, later months of the year were severe in thermoregulation, and snakes had difficulty maintaining their Tb within Tset by using average thermal habitats. There were no significant intermorph differences in thermoregulation indices in any months, whereas slight differences were detected in hourly comparisons. Most of these comparisons indicated active and precise thermoregulation (with respect to Tset) in striped individuals by using thermally favourable but rare microhabitats such as forest gap. Thus, the obtained values do not support the prediction that melanistic individuals are precise thermoregulators. Yet, melanistic individuals do modify their thermoregulation strategy with respect to the available thermal environments in contrast to striped individuals. Together with the fact that body heating is slower in striped individuals than in melanistic individuals under experimental conditions, it is concluded that melanistic individuals have the potential to enjoy thermal advantages but that this might be of no practical use in terms of Tb in the wild because of the greater thermoregulatory efforts of striped individuals, and because melanistic individuals may use forest gap rarely due to conspicuousness to visually orientated predators under the exposed habitat.