Heads or tails? - Variation in tadpole body proportions in response to temperature and food stress

Given sufficient time and genetic variation, populations of ectothermic animals living in different thermal environments are expected to diverge in their thermal optima for growth. Several recent studies have used common frog (Rana temporaria) tadpoles as models to study animal adaptation to geographic variation in ambient temperatures, but little attention has been paid to how the growth of different body parts responds to variation in temperature. We examined the variation in early growth in body. tail and relative tail length of common frog tadpoles in a common garden experiment that subjected individuals from six populations situated along a 1600-km latitudinal gradient to different temperature and food treatments. We found a high degree of latitudinally unordered inter-populational differentiation in relative tail length, and that increasing levels of environmental stress (low temperature and restricted food) lead to increased mean relative tail length. This occurred mainly because of changes in body growth, not because of changes in tail growth per se. This suggests that growth of the tail itself is less sensitive to environmental stress than body growth, but at the same time the relative length of tail can be a good indicator of growth conditions experienced by individuals. In general, the results do not support the earlier proposition that common frog tadpoles in the south have relatively longer tails due to increased risk of predation, but indirect evidence was found to support the contention that the populations along the latitudinal gradient have different thermal optima for growth as indicated by latitudinally ordered differences in how relative tail length changed in response to temperature treatments.