Is growth hormone expression correlated with variation in growth rate along a latitudinal gradient in Rana temporaria ?

In ectotherms, decreasing season length and lower temperature towards higher latitudes often favour higher growth and developmental rates. However, the underlying physiological mechanisms and particularly the hormonal correlates of clinal variation remain unexplored. The growth hormone (GH) plays a crucial role in growth of all vertebrates and high expression of GH is associated with rapid growth in many species. We tested the hypothesis that GH expression is correlated with a latitudinal gradient in growth in Scandinavian Rana temporaria tadpoles. Using quantitative polymerase chain reaction, we measured GH and growth hormone receptor (GHR) expression at two time points from laboratory‐raised tadpoles originating from eight populations collected along the latitudinal gradient. To explore latitudinal differences in stress‐induced changes in GH expression, we also compared GH expression in tadpoles raised with and without predators. In accordance with previous studies we found a clear latitudinal gradient in growth. There were no latitudinal effects, or predator‐induced effects on GH or GHR expression. However, there was some indication for among‐population variation in GH expression. The lack of a latitudinal pattern in GH and GHR expression may be due to that the growth promoting effects of GH is dependent on other factors including insulin‐like growth factor‐I (IGF), IGF‐binding proteins or prolactin. Further studies on these factors may provide insight on the proximal mechanisms of differences in growth in R. temporaria tadpoles.