Sociality, Bateman’s gradients, and the polygynandrous genetic mating system of round-tailed ground squirrels (Xerospermophilus tereticaudus)

Historically, most mammals have been classified as polygynous; although recent molecular evidence suggests that many mammals may be polygynandrous, particularly the ground-dwelling sciurids. We genotyped 351 round-tailed ground squirrels (Xerospermophilus tereticaudus) using seven microsatellite loci to determine paternity in 31 litters from 2004 to 2007. Polygyny was evident in all years except in 2007, when the population size was reduced. Multiple paternity occurred in the majority of litters (55%) with 2.5 ± 0.26 sires/litter (n = 31). Forty-nine percent of resident males (n = 114) sired offspring, and of males that sired offspring (n = 56) 27% sired young in multiple litters in a single breeding season. Litter size was positively correlated with the number of sires. Through an indirect analysis of paternity, we found 21 litters (68%) with an average relatedness of 0.5 or less. Males had a greater opportunity for sexual selection (Is = 1.60) than females (Is = 0.40); Bateman’s gradient was also greater in males (1.07 ± 0.04, n = 56) than females (0.82 ± 0.08, n = 31). The mating system in round-tailed ground squirrels defined through genetic analyses and Bateman’s gradients is polygynandrous compared to the previously suggested polygynous mating system as established by behavioral observations and fits within the predictions of the ground squirrel sociality models. Upon evaluating the predictions of the sociality models among sciurid species, we found a negative relationship between the level of sociality with litter size and the average percentage of multiple paternity within a litter. Thus, recent genetic information and reclassification of mating systems support the predictions of the ground-dwelling squirrel sociality models.