Territoriality and male reproductive success in arctic ground squirrels

Although territorial defense is a common form of reproductive competition among male vertebrates, the exact reproductive consequences of this behavior are often poorly understood. To explore relationships between territoriality and reproductive success in a nongroup-living mammal, we quantified patterns of space use, mating success, and fertilization success for males in a free-living population of arctic ground squirrels (Spermophilus parryii plesius). Because litters of this species are sired almost exclusively by a female’s first mate, we predicted that territory ownership would be associated with first access to estrous females. During the 2-week period when mating occurred, each male in the study population attempted to defend a distinct portion of the habitat, although the success of this defense varied among individuals. Twenty-six of 28 females monitored mated with the male on whose territory they resided. However, the majority of females observed throughout estrus (65%; n 20) also mated with at least one other male, indicating that territory ownership was not associated with exclusive access to females. In contrast, territory ownership was significantly associated with first access to estrous females; 20 (71.4%) of 28 females mated first with the male on whose territory they resided. In this regard, the behavior of S. parryii plesius parallels that of socially monogamous birds in which territorial defense by males functions to deter extrapair copulations by females. Although territorial defense represents an important component of male reproductive success in arctic ground squirrels, other aspects of male behavior (e.g., the ability to dominate agonistic interactions on the day of a female’s estrus) are also critical. We suggest that future studies of vertebrate mating systems will benefit by viewing such defense as only one of multiple axes along which conspecific males compete for access to females.