Thermal and substrate color-induced melanization in laboratory reared red-eared sliders (Trachemys scripta elegans)

Color and pigmentation patterns of the integument can facilitate crypsis, thermoregulation, and social signaling. According to the “thermal melanism hypothesis”, cold environmental temperature should increase the quantity of melanin that is deposited in the integument thereby facilitating radiative warming. We studied the influences of water temperature (26 °C or 31 °C) and substrate color (black or white) on the degree of melanization in the red-eared slider, Trachemys scripta elegans, under laboratory conditions. Turtles reared on a black substrate, or in 26 °C water, for 120 days were darker than those reared on a white substrate or in 31 °C water. A potential tradeoff between the fitness benefits of crypsis and the benefits of radiative warming through melanism was detected because turtles reared in 26 °C water and on a white substrate were darker than those reared on a white substrate and in 31 °C water. Low temperatures limited metabolic processes because turtles reared in 26 °C water grew more slowly than those reared in 31 °C water. However, histological analyses revealed that melanization was a dynamic process in all treatments confirming that the degree of melanization in the cool water treatment was not influenced by the initial and relatively dark hatchling coloration in individuals that grew relatively slowly.