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

Abstract

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.