Sexual patterns of prebreeding energy reserves in the common frog Rana temporaria along a latitudinal gradient

The ability to store energy is an important life history trait for organisms facing long periods without energy income, and in particular for capital breeders such as temperate zone amphibians, which rely on stored energy during reproduction. However, large scale comparative studies of energy stores in populations with different environmental constraints on energy allocation are scarce. We investigated energy storage patterns in spring (after hibernation and before reproduction) in eight common frog (Rana temporaria) populations exposed to different environmental conditions along a 1600 km latitudinal gradient across Scandinavia (range of annual activity period 3-7 months). Analyses of lean body weight (eviscerated body mass), weight of fat bodies, liver weight, and liver fat content, showed that (i) post-hibernation/pre-breeding energy stores increased with increasing latitude in both sexes, (ii) males generally had larger energy reserves than females and (iii) the difference in energy stores between sexes decreased towards the north. Larger energy reserves towards the north can serve as a buffer against less predictable and/or less benign weather conditions during the short activity period, and may also represent a risk-averse tactic connected with a more pronounced iteroparous life history. In females, the continuous and overlapping vitellogenic activity in the north may also demand more reserves in early spring. The general sexual difference could be a consequence of the fact that, at the time of our sampling, females had already invested their energy into reproduction in the given year (i.e. their eggs were already ovulated), while the males' main reproductive activities (e.g. calling, mate searching, sexual competition) occurred later in the season.