Color change, background choice, and camouflage in a nocturnal treefrog: Implications for circadian adaptation

Camouflage through background matching is a widespread antipredator strategy across taxa, enhancing survival by reducing the likelihood of detection or recognition by predators. To achieve this, animals often change color and/or choose appropriate backgrounds to overcome the spatial and temporal variation of substrates. However, how these strategies are modulated by diel variation remains unclear. In this study, we investigated color change and background choice in Hainan frilled treefrogs (Kurixalus hainanus), a nocturnal species that exhibits green-brown color patterns. We found that frogs changed brightness, saturation, and hue more rapidly in the dark conditions compared to the light, and exhibited a significantly faster initial response to light during the day than at night. Color change significantly improved background similarity (measured by Euclidean distance) to both green and brown substrates in the light conditions. While frogs did not actively choose backgrounds that matched their coloration when given a choice, they significantly avoided highly contrasting white substrates and were more active during the day. Pattern elements of frogs provided better camouflage than chromatic or achromatic properties when compared with the substrates at their calling sites, as modelled under avian, mammalian, and snake visual systems. Nonetheless, chromatic contrasts varied across predator visual systems, being highest for birds and lowest for mammals. In addition, achromatic contrasts were significantly lower on the observed substrates than on the alternative ones. Our findings provide new insights into the adaptive significance of color change and background choice in nocturnal species, highlighting the importance of diel phase in camouflage strategies.