Earlier works of Juha et al. on common frogs (Rana temporaria) have revealed latitudinal variation/adaptations in several life-history traits (i.e. adult body size, testis size, larval developmental rates, survival under different conditions, levels of adaptive plasticity, etc.). The 1500-1600 km long latitudinal gradient (used in this studies) accross Fennoscandia reaching well beyound the Arctic Circle is a striking environmental gradient: the annual activity periods ranges from seven to three months, or if you wish, the hibernation periods ranges from five to nine months.
Energy acquisition and storage are also very important factors in life history variation, especially in species/populations facing long periods without energy income. This is true in particular for capital breeders (relying on stored energy during reproduction) like most temperate zone amphibians. Surprisingly, large-scale geographic comparisons on energy stores within species with populations differing in energetic challenge are sparse (at best) in the literature, and especially in amphibians.We analysed energy storage patterns in eight common frog populations along the gradient, sampling individuals in the spring, right after hibernation and before reproduction.
We analysed lean (eviscerated) body mass, weight of abdominal fat bodies, liver weight and liver fat content. We found pretty clear linear trends: (i) energy stores increased towards north (opposite to what one would intuitively imagine at first place!), (ii) males had larger reserves than females, and (iii) the sexual differences decreased towards the north.
We propose that larger energy stores toward north (after the much longer hibernation period) are important as a buffer against the more unpredictable environmental conditions during the very short activity season, and can aid females in their overlapping vitellogenetic cycles (note that southern females have some ‘rest’ between consequtive cycles). The sexual difference might be a result of the fact that females are already after their main reproductive investments (i.e. the eggs are fully developed, mainly during reproduction) while males are just before that (calling, mate searching, different forms of sexual competition). The latitudinal decrease in sexual difference is hard to interpret, perhaps the harsher conditions of the nordic environments are responsible for the pattern.
Read more about this (in my opinion) unique data:
Jönsson KI, Herczeg G, O’Hara RB, Söderman F, ter Schure AFH, LArson P and Merilä J 2009. Sexual patterns of prebreeding energy reserves in the common frog Rana temporaria along a latitudinal gradient. Ecography, in press.