Interpopulation comparisons of variation in resource availability and in allocation
patterns along altitudinal and latitudinal gradients allow insights into the mechanisms
shaping the life history of animals. Patterns of between-population differences in
female life history traits have been studied intensively across a wide range of taxa,
but similar investigations in males have remained scarce. To study if testis weight
- ameasure of reproductive investment - varies on a geographical scale in anurans,
we focussed on the variation in relative testis weight (RelTW) and asymmetry in 22
populations of the common frog Rana temporaria along a 1,600-km latitudinal transect
across the Scandinavian peninsula. We found that RelTW decreased towards the north.
Body mass and body length both had independent positive effects on testes mass. We
found evidence for directional asymmetry (DA) in testis weight with the right testis
being larger than the left. The level of DA in testis weight was not related to latitude,
but both body mass and testes mass had independent positive effects on asymmetry.
We discuss the northwards decrease in RelTW in terms of a decreased reproductive investment
as a possible consequence of harsher environmental conditions, and perhaps also, weaker
sexual selection in the north than in the south.