Large‐scale variation of size‐ and age‐related life‐history traits in the common frog: a sensitive test case for macroecological rules

Abstract

AbstractThe potential causes underlying geographical variation of 12 age‐ and size‐related life‐history traits were studied in the common frog Rana temporaria. The meta‐analysis was based on own field data obtained in the Austrian Alps and additionally on published studies on 35 populations covering the whole range of latitudinal, longitudinal and altitudinal distribution. This extensive dataset allowed for co‐variance analyses evaluating the statistical support for five hypotheses on large‐scale trait variation derived from general macroecological rules. Contrary to Bergmann's rule, adult frogs' body size variation was unrelated to latitude (and altitude), but co‐varied with age as expected for a species that grows indeterminately. Unlike the prediction of Rensch's rule, the slight female‐biased sexual size dimorphism (SSD) did not differ significantly among populations with small and large average female size. SSD increased with altitude, but did not co‐vary with latitude, longitude and age, disagreeing with the predictions of the differential‐plasticity hypothesis. Common frogs at higher altitudes and latitudes reached reproductive maturity at significantly older ages and enjoyed an increased longevity as well. However, potential reproductive lifespan increased significantly only with altitude, compensating for the smaller clutches relative to body size in alpine populations. In conclusion, body size variation of common frogs does not actually mirror geographical gradients, but differences in age, a factor unconsidered in Bergmann's and Rensch's rules. Large‐scale environmental gradients are therefore good predictors of age‐related traits. Combining the local co‐variation of the single traits age, body size, clutch size and reproductive lifespan to calculate the resulting female lifetime fecundity (FLF), surprisingly little variation of the derived trait FLF is observed across the geographical range of the common frog. The ultimate cause of variation in age‐ and size‐related life‐history traits seems to be the selection for an optimal FLF.