Copulatory plugs of round-tailed ground squirrels (Xerospermophilus tereticaudus)

Sperm competition, where sperm from more than one male competes to fertilize eggs within the reproductive tract of a female, is prevalent in mammals. In rodents, this competition often has led to copulatory plugs, a passive defense against further sperm competition. Copulatory plugs have been reported in many species of sciurids; however, not in round-tailed ground squirrels (Xerospermophilus tereticaudus). Here we report two observations of copulatory plugs in round-tailed ground squirrels and suggest they represent an indirect form of mate guarding in affording males a chance to seek additional mating opportunities as seen in other species of rodents.