AbstractDuring the reproductive period, intraspecific dyadic encounters were staged within and betweenS. spilosomaandS. mexicanusindividuals 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. mexicannsmales and females appear to be more aggressive than male and femaleS. spilosoma.In both species, encounters between males were more aggressive than between females, which were very tolerant to each other inS. 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 ofS. spilosomaandS. mexicanusis falsified. Some alternative hypotheses are proposed. First, the tolerance ofS. spilosomais an adaptation to the difficult environmental conditions. Thus differences between the two species reflect differences in behavioural evolution in allopatric populations. Second,S. spilosomatolerance is a behavioural response toS. mexicanus' recent arrival.
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