Melanophore metachrosis response in amphibian tadpoles: effect of background colour, light and temperature

The developmental and biochemical mechanisms of colour change through chromatophore metachrosis in amphibian tadpoles are relatively well studied, but the environmental factors driving colour change remain unclear. A cryptic response to background colour in order to reduce predation is an intuitively valid explanation, however, other hypotheses need to be explored. In this study, we aimed to investigate the environmental factors driving the melanophore metachrosis process in Alytes dickhilleni tadpoles. First, we tested the response to two backgrounds with clearly distinct reflectance: black and white. The proportion of dark tadpoles became significantly higher when they were located on the black background, and pale tadpole proportion was dominant on the white background, as expected from the crypsis hypothesis. Secondly, we added two new factors, temperature and photoperiod, maintaining the background variation. Our results suggest that lower temperatures, and short photoperiods were significantly driving a change to dark colouration in tadpoles, possibly allowing a more efficient thermoregulation, and in consequence, development and growth. Next we tested whether dark and pale tadpoles selected backgrounds that matched their colouration (black and white background), and found no evidence for behavioral selection. The apparent response in colour change to background appears to be mediated by the background reflectance of light, that there does not seem to be behavioral selection of matching background by the tadpoles, and therefore it suggests that color change is more likely to be a physiological response with thermoregulatory implications.