During the reproductive period, intraspecific dyadic encounters were staged within and between S. spilosoma and S. mexicanus individuals under controlled conditions. Video recordings were used to describe and quantify behaviour of males and females of these two sympatric ground squirrels. To compare dyads within and between species two indices (an Agonistic index and a Tolerance index) were designed from measures of behavioural frequencies. During intrasexual encounters, S. mexicanns males and females appear to be more aggressive than male and female S. spilosoma. In both species, encounters between males were more aggressive than between females, which were very tolerant to each other in S. spilosoma. In most cases encounters between male and female conspecifics exhibit an intermediate level of agonistic behaviour and tolerance.
The null hypothesis of similarity of the social behaviour of S. spilosoma and S. mexicanus is falsified. Some alternative hypotheses are proposed. First, the tolerance of S. spilosoma is an adaptation to the difficult environmental conditions. Thus differences between the two species reflect differences in behavioural evolution in allopatric populations. Second, S. spilosoma tolerance is a behavioural response to S. mexicanus' recent arrival.