More than just environment? Remarkable phenotypic plasticity in developmental traits within a population of the common frog (Amphibia, Ranidae, Rana temporaria)

Background/Question/Methods

Larval development time and size at metamorphosis are two (interlinked) key life history parameters of many organisms with complex life-cycles since they are of crucial importance to an individual’s future performance and fitness. In amphibians size and weight at metamorphosis highly influence juvenile’s mobility, predation risk, and survival during hibernation, as well as age and size at maturity and thus fecundity. Hence larval development may not only affect individuals but also determine dynamics and survival of local amphibian populations. The influence of particular environmental parameters, such as predation or pH, on development time and metamorphic weight are well studied under laboratory conditions. However, information about the impacts of these parameters on developmental traits in natural systems is still scarce. We investigated the influence of environmental parameters on developmental traits within a population of the common frog (Rana temporaria) in southern Germany (Steigerwald, Bavaria; monitored since 2005). From March to October 2007, survival rate, metamorphic weight, and development time of frogs were studied in 18 ponds. For these ponds17 biotic and abiotic parameters as well as predation pressures and larval densities were recorded at three sampling events during larval development using box-sampling.

Results/Conclusions

In 11 out of 18 ponds juveniles successfully finished metamorphosis. Based on the environmental parameters assessed, ponds could not be classified into 'emigration' and 'non-emigration' ponds using cluster analysis and regression trees .However, significant differences in the recorded traits of the juvenile frogs from 'emigration' ponds were observed. Average development time in ponds ranged between 90 and 126 days, the very first juvenile emerged after 79 days, the last one was recorded after 196 days of development. Furthermore, we found huge differences in metamorphic weight: the smallest individual was ten times lighter than the largest metmorph (0.07 and 0.70g, respectively). Average metamorphic weight varied more than factor two between ponds (0.14 to 0.39g). Weight and development time were significantly correlated within ponds, but showed profoundly different patterns. Using information-theoretic model selection, neither predation pressure nor larval density, or other recorded environmental parameters could be accounted for the observed differences. These preliminary results indicate that, even on a very local scale, there might be more than just environmental parameters shaping developmental traits within amphibian populations.