Determinants of multiple paternity in a fluctuating population of ground squirrels

摘要

Multiple paternity is common in vertebrates that produce several offspring in the same reproductive bout, but the rate often varies among and within populations. Three primary explanations for this variation have been advanced: null models based on encounter rate of mates, socioecological models dependent on the ability of males to monopolize females, and age-or condition-dependent models of female choice. We used 18 years of genetic and demographic data to examine the mating system and patterns of multiple paternity in a free-living population of golden-mantled ground squirrels (Callospermophilus lateralis). The mating system was polygynandrous, but opportunity for sexual selection was lower for females than for males. Annual reproductive success of males was low for yearlings and new immigrants and increased with breeding tenure in the population. Multiple paternity was evident in 62% of litters. In accordance with the socioecological model of male monopolization, rates of multiple paternity decreased with female spatial clustering, unless male-male competition, as indicated by male density, was also high. From Bateman gradients, we found no direct fitness benefit of multiple paternity for females. Though not statistically significant, multiple paternity appeared to decrease with maternal age and peri-oestrous mass, in possible support of the female choice model. Together, our results suggest that variation in the rate of multiple paternity in golden-mantled ground squirrels was determined by density and the active strategies of males and females.