Plasticity of age at maturity between two neighbouring populations of the common frog (Rana temporaria L.)

Maturation and growth were studied in two neighbouring lowland populations of Rana temporaria exhibiting a great difference in body size. This study took place in the southeast of France from 1986 to 1989. In this region, a local harvesting technique ensuring the release of tadpoles after intensive rearing results in regular genetic mixing between populations. Hence, only environmental influence on the growth rate could explain a difference in body size at maturation. The skeletochronological method was used to determine age. Most of the frogs in the two populations lived no longer than 3 years. The difference in body size between populations was reflected by a difference in age at maturity, which varied from 3 years at site 1 (where the frogs were larger) to 2 years at site 2. Newly metamorphosed froglets were larger at site 1 than at site 2. Back-calculated sizes of 1-year-old individuals did not differ between sites. Size was always greater for age-classes 2, 3, and 4 at site 1. We conclude that, relative to size at metamorphosis, faster growth at site 2 enables maturation to occur earlier and at a smaller size than at site 1. The significance of closer lines in bone cross sections as an indication of age at first reproduction is questioned.