Melanistic coloration does not influence thermoregulation in the crepuscular gecko Eublepharis macularius

Abstract

Body coloration in ectotherms serves multiple biological functions, including avoiding predators, communicating with conspecific individuals, and involvement in thermoregulation. As ectotherms rely on environmental sources of heat to regulate their internal body temperature, stable melanistic body coloration or color change can be used to increase or decrease heat absorption and heat exchange with the environment. While melanistic coloration for thermoregulation functions to increase solar radiation absorption and consequently heating in many diurnal ectotherms, research on crepuscular and nocturnal ectotherms is lacking. Since crepuscular and nocturnal ectotherms generally absorb heat from the substrate, in these organisms melanistic coloration may have other primary functions beside thermoregulation. As such, in this work we hypothesized that the proportion of dorsal melanistic body coloration would not influence heating and cooling rates in the crepuscular gecko, Eublepharis macularius, and that changes in environmental temperature would not trigger color changes in this species. Temperature measurements of the geckos and of the environment were taken using infrared thermography and temperature loggers. Color data were obtained using objective photography and a newly developed custom software package. We found that body temperature reflected substrate temperatures, and that the proportion of melanistic coloration has no influence on heating or cooling rates or on color changes. These findings support that melanistic coloration in E. macularius may not be used for thermoregulation and strengthen the hypothesis that in animals active in low light conditions, melanistic coloration may be used instead for camouflage or other functions.

Keywords: Color data extraction; Color pattern; Melanism; Reptiles; Thermal physiology.

Fig. 1.

Box plots of the individual body temperatures, temperatures within terrarium and atmospheric temperatures for each block. Boxplots of temperature variations for within the terrarium, atmospheric temperatures, and body temperatures in comparison to the native temperature range of E. macularius for each block. Blocks are as in Table 1. Body and substrate temperatures are based on IR (infrared) images. Cold side, hide, and warm side of the terrarium and atmospheric temperatures are based on temperatures recorded by dataloggers. Native temperatures are based on Global Biodiversity Information Facility. The colors used for each block correspond to the different temperatures as indicated in the figure legend.

Fig. 2.

Influence of melanistic proportion on the change in body temperature. Logarithm of the average stable melanistic proportion of each individual was used for this plot. Heating rates were calculated by taking the average body temperature difference between each block for blocks 5 and 3 per individual. Cooling rates were calculated using the same methods for blocks 1 and 3. Heating and cooling rates were transformed to a log scale. Each dot represents an individual. Blue dots refer to the cooling phase (blocks 1-3), orange dots to the heating phase (blocks 3-5), and black dots refer to the start and end of the experiment (blocks 1-5). Changes in body temperature are similar within each group (cooling, heating, start-end) independently of the melanistic proportion of each individual.

Fig. 3.

Change in average luminance for non-melanistic (A,B,C) and melanistic (D,E,F) areas of the body against change in body temperature. Average luminance was calculated by weighted RGB values taken from images. Heating rates were calculated by taking the average body temperature difference between each block for blocks 5 and 3 per individual. Cooling rates were calculated using the same methods for blocks 1 and 3. Heating and cooling rates were transformed to a log scale. Each dot represents an individual. Blue dots refer to the cooling phase (blocks 1-3, plots C and F), orange dots to the heating phase (blocks 3-5, plots A and D), and black dots refer to the start and end of the experiment (blocks 1-5, plots B and E).

Fig. 4.

Change in average luminance for the non-melanistic (A) and melanistic (B) part of the body against melanistic proportion plotted on a logarithmic scale. Average luminance was calculated by weighted RGB values taken from images. Heating rates were calculated by taking the average body temperature difference between each block for blocks 5 and 3 per individual. Cooling rates were calculated using the same methods for blocks 1 and 3. Each dot represents an individual. Blue dots refer to the cooling phase (blocks 1-3), orange dots to the heating phase (blocks 3-5), and black dots refer to the start and end of the experiment (blocks 1-5). Solid line indicates a significant P-value via a linear model.