Population divergence of developmental thermal optima in Swedish common frogs, Rana temporaria

Growth rate, like any other trait, should be under balancing selection in natural populations, with selection adjusting mean growth rate in a population in relation to its site-specific costs and benefits. In the present study, we tested for differences in thermal growth optima between a northern and a southern region of Rana temporaria by rearing tadpoles in three different temperatures in the laboratory (10, 15 and 20 °C). Because of the rapid increase in post-melt temperature at high latitudes, spawn and tadpoles from the northern region experience significantly higher minimum, mean and maximum temperature throughout the period of pre-metamorph development. Frogs up north also enjoys a shorter breeding season and activity season overall. This suggests that growth and development should be maximized at a relatively higher temperature in the north as a result of directional selection. In accordance with this prediction we found that tadpoles from the northern region grew faster at relatively higher temperature than frogs in the south, whereas the opposite was true at relatively lower temperatures. North tadpoles also had a higher mortality and poorer physiological performance than south tadpoles at low temperatures. In summary, our results conclusively support the hypothesis that frogs in the north are adapted to relatively warmer developmental conditions than frogs in the south.