The Socioecology, Mating System and Behavior of Round-Tailed Ground Squirrels (Xerospermophilus tereticaudus)

Abstract

Social organization of a species may impact behavior, reproductive ecology, mating system, population genetic structure and overall fitness. A spectrum of sociality exists from solitary individuals to aggregations to integrated, highly related groups. A large body of knowledge exists for sociality and life-history characteristics of ground-dwelling sciurids, including several overarching models to explain the evolution of sociality. These models predict round-tailed ground squirrels (Xerospermophilus tereticaudus) to be solitary based on small body size (~125g), relatively long period of activity (January-June) and a short period of adult-juvenile overlap. However, previous behavioral observations suggest round-tailed ground squirrels have a clustered matrilineal structure with a suite of social behaviors, suggesting that they may represent a unique outlier in ground squirrel sociality models. Within the population of round-tailed ground squirrels at the Casa Grande Ruins National Monument in Coolidge, Arizona, USA, rates of amicable and agonistic interactions between adults were low, with no relationship between relatedness of individuals and rates of social interactions. No population substructure was evident with Bayesian analyses, global or pairwise F(ST) values, and average relatedness among females did not differ from males. Contrary to previous behavioral studies, round-tailed ground squirrels did not have high levels of social behavior, nor did they form significant genetic subpopulation structuring. The active season of round-tailed ground squirrels closely followed patterns of precipitation and peak resource availability. Body mass differed between males and females, across years, and within seasons. Males were heavier than females at emergence, prior to mating and pre-hibernation, but not when females began gestation. Emergence of litters and litter size are related to amount and timing of winter rainfall. Foraging and vigilance behaviors compose 64-66% of the activity budget, but differ in that males spend a greater proportion of time foraging, whereas females spend a greater proportion of time vigilant. Round-tailed ground-squirrels have a polygynandrous mating system. Polygyny was evident in 2004, 2005, and 2006, and multiple paternity occurred in the majority of litters with 2.5 sires/litter; litter size was positively correlated with the number of sires. These findings support predictions generated by sociality models for ground-dwelling squirrels.