• arctic;
  • CMR analysis;
  • population decline;
  • reproduction;
  • sex ratio;
  • survival


In vertebrates with genetic sex determination, large deviations from the 1:1 sex ratio at the population level are rare and demand an explanation. We investigated adult sex-ratio variation of common frogs Rana temporaria in two subarctic breeding ponds over a 6-year (1999–2004) period using capture–mark–recapture (CMR) methods. Using the same data, we also tested for the occurrence of the biannual breeding cycle in female common frogs and sought evidence for population size decline. Sex ratios were highly female biased in both populations: on average, only about 30% of breeding individuals were males. CMR analyses further suggested that the female-biased sex ratio was not explainable by higher adult mortality among males as the survival probabilities over years were similar (c. 70%) for both sexes. Alternative explanations for these highly female-biased sex ratios include sex-specific mortality at earlier life stages and environmental influences on the sex determination system. While further studies are required to differentiate between the alternatives, the observed sex-ratio bias is among the strongest reported in any amphibian population so far, and also in the direction opposite to that usually observed (males>females). Our analyses found no support for the contention that female common frogs in northern populations may reproduce only in every second year. However, both study populations were declining during the study period.