The ontogenetic shift in thermoregulatory behaviour of newt larvae: testing the 'enemy-free temperatures' hypothesis

Growth and development affect life-history traits, and consequently organismal fitness. The inevitable increase in body size during ontogeny is associated with changes in both resource use and predation risk, which results in the ontogenetic shift in habitat preferences. In this study, we examined whether the shift in preferred body temperatures (T(p)s) of newt larvae Triturus alpestris increases the T(p)s deviation of the most vulnerable stages (after hatching and during metamorphosis) from the T(p)s range of their main predator, dragonfly larvae Aeshna cyanea - the 'predator-free temperatures' hypothesis. Analyses of thermoregulatory behaviour in the laboratory thermal gradient showed that freshly hatched newt larvae maintained lower water temperatures than larvae in later stages, whose T(p)s largely fell into the T(p)s range of dragonfly larvae. With respect to the thermal quality of natural habitat, the anti-predator effectiveness of the T(p)s shift decreased during development. Water temperatures in natural habitats were located largely below the preferred body temperature range of both newt and dragonfly larvae, which limits their potential thermal niche partitioning. We conclude that factors other than predator avoidance drive the ontogenetic T(p)s shift in our model system.