Environmental variation connected with sea- sonality is likely to affect the evolution of life-history strategies in ectotherms, but there is no consensus as to how important life-history traits like body size are influ- enced by environmental variation along seasonal gradients. We compared adult body size, skeletal growth, mean age, age at first reproduction and longevity among 11 common frog (Rana temporaria) populations sampled along a 1,600-km-long latitudinal gradient across Scandinavia. Mean age, age at first reproduction and longevity increased linearly with decreasing growth season length. Lifetime activity (i.e. the estimated number of active days during life-time) was highest at mid-latitudes and females had on average more active days throughout their lives than males. Variation in body size was due to differences in lifetime activity among populations—individuals (especially females) were largest where they had the longest cumula- tive activity period—as well as to differences between populations in skeletal growth rate as determined by skel- etochronological analyses. Especially, males grew faster at intermediate latitudes. While life-history trait variation was strongly associated with latitude, the direction and shape of these relationships were sex- and trait-specific. These context-dependent relationships may be the result of life- history trade-offs enforced by differences in future reproductive opportunities and time constraints among the populations. Thus, seasonality appears to be an important environmental factor shaping life-history trait variation in common frogs.

Field collected data on adult common frogs from 11 locations along a 1600 km latitudinal gradient from southern Sweden to northern Finland. Ages of collected individuals were estimated with skeletochronological methods in laboratory. Available are also e.g. body sizes (snout-vent lenghts) and weighs of the individuals.

For methods of age estimation, consult the published paper.