The role of tadpole coloration against visually oriented predators

An animal's vulnerability to predators can be influenced by its behavior, morphology, body size, coloration, habitat preferences, and palatability. We tested whether the coloration of Bokermannohyla saxicola and Scinax machadoi tadpoles affects their survival when exposed to local visually oriented predators at a site in southeastern Brazil. We tested three aquatic invertebrates (Aeshnidae, Belostoma sp., Lethocerus sp.) and birds as tadpole predators. We predicted that predation rates would differ depending on the substrate where the tadpoles positioned themselves (light or dark), hypothesizing that each tadpole would use preferentially a background that conferred camouflage and that predation levels would be lower on such backgrounds compared to others. B. saxicola had higher survivorship than S. machadoi on light backgrounds at some instances, in accordance with its crypsis hypothesis. However, B. saxicola tadpoles did not use light backgrounds more often than dark ones. S. machadoi coloration looked disruptive on both light and dark backgrounds, and tadpoles showed no preference or differences in survival rates between these backgrounds. Predation rates did not differ between the two species in a way that could confirm a previous hypothesis of aposematic/mimetic coloration for S. machadoi tadpoles. Our results show that colorations that appear to function to impair visual detection may play this role at some circumstances but not others. Tadpole colorations may have evolved in another context, in which avoiding visual detection by predators was a stronger selective pressure. In a context with lower predation pressure from visually oriented predators, the expected background choice behavior for increased camouflage may not be strongly selected for.