A laboratory study of social behaviour of pairs of females during the reproductive season in Spermophilus spilosoma and Spermophilus mexicanus.

Spermophilus spilosoma and Spermophilus mexicanus are closely related species with a low degree of sociality. They are sympatric in the Bolson de Mapimi (Chihuahua Desert, Mexico), where they occupy similar ecological niches. In Sciurid rodents, advanced social structures may have evolved through the formation of sedentary female groups. The purpose of this study is to compare the ability of S. spilosoma and S. mexicanus females to cohabit with another conspecific female under space constraints during the mating period. The stability of the female relationships was tested by introducing a male of the same species. In both species, females managed to cohabit under space constraints with a low level of agonism. Nevertheless, an increase of defensive behaviour and a decrease of amicable behaviour were observed during female cohabitation in S. spilosoma. Moreover, female S. mexicanus had a higher tolerance index than S. spilosoma during this same period. The introduction of a male induced an increase of the level of agonism of S. mexicanus females while no reaction was detected in S. spilosoma. These differences were discussed in terms of mechanisms which could preclude the formation of sedentary social groups of females.