• conservation genetics;
  • genetic compensation;
  • genetic diversity;
  • mating system;
  • reproductive success;
  • social plasticity


Effective population size (Ne) is a key determinant of genetic diversity of populations. In amphibians, the ratio effective population size/census size (Ne/N) is often very low, raising concerns for the long-term persistence of genetic diversity in isolated populations. It has been proposed that the phenomenon of ‘genetic compensation’ increases the ratio Ne/N in small populations, but the underlying mechanisms are poorly understood. Polygyny can decrease Ne/N because of the negative relationship between polygyny and Ne. We used genetic information (microsatellites) to evaluate the relationship between census size, polygyny and Ne in populations of the threatened Italian agile frog Rana latastei. We reconstructed parentage in tadpoles from nine populations with eight to 32 breeding females, using a likelihood-based method; we analysed simulated datasets with known properties to confirm the reliability of this approach in reconstructing polygyny. Furthermore, we estimated Ne using approximate Bayesian computation. The level of polygyny differed strongly among populations (average number of mates per breeding male: 2–6.4). Polygyny was greater in populations with larger census sizes. Moreover, variance in male mating success was larger in large populations. Effective population size increased with population size, but was negatively related to polygyny; as polygyny increased in large populations, this was associated with reduced Ne/N. In polygynous species, increasing levels of polygyny in large populations may explain the low Ne/N values, with important implications for the conservation of genetic diversity and for long-term population persistence.