Lutra 47(2)_Millesi et al_2004

Reproductive strategies of male European sousliks (Spermophilus citellus) at high and low population density

The spring population density of European sousliks (Spermophilus citellus) in a suburban area near Vienna, Austria, changed from 60 to 6 individuals/ha from 1991 to 1998. The decline occurred during the active seasons of 1994 and 1995. In order to investigate effects of population density on reproduction we examined male mating effort and success. Male home-range size, intrasexual aggression, mass loss during the mating period, and individual reproductive success changed with population density. Home-range size increased with density decline, whereas aggression and mass loss decreased. High densities were characterized by more intense malemale competition and a skewed distribution of mates per male. Mate numbers in low-density situations were normally distributed and all males were able to acquire at least one mate. At low density, all yearling males were reproductively active compared to only 13% in the high-density years. Mature yearlings had lower growth rates than non-reproductive ones. Analyses of cortisol secretion indicated high stress levels at low density throughout the season. These changes could be viewed as a facultative response to changes in mate availability and the distribution of reproductive success. On the other hand, the costs of reproduction could be amplified by precociousness associated with population density changes.