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

Growth rate, like any other trait, should be under balancing selection in n atural populations, with selection adjusting mean growth rate in a populati on in relation to its site-specific costs and benefits. In the present stud y, we tested for differences in thermal growth optima between a northern an d a southern region of Rana temporaria by rearing tadpoles in three differe nt temperatures in the laboratory (10, 15 and 20 degreesC). Because of the rapid increase in post-melt temperature at high latitudes, spawn and tadpol es from the northern region experience significantly higher minimum, mean a nd maximum temperature throughout the period of pre-metamorph development. Frogs up north also enjoys a shorter breeding season and activity season ov erall. This suggests that growth and development should be maximized at a r elatively higher temperature in the north as a result of directional select ion. In accordance with this prediction we found that tadpoles from the nor thern region grew faster at relatively higher temperature than frogs in the south, whereas the opposite was true at relatively lower temperatures. Nor th tadpoles also had a higher mortality and poorer physiological performanc e than south tadpoles at low temperatures. In summary, our results conclusi vely support the hypothesis that frogs in the north are adapted to relative ly warmer developmental conditions than frogs in the south.