Patterns of space use and sexual dimorphism are frequently used to infer mating systems. We examined body size and home range size and intra-and intersexual overlap degree in order to elucidate mating strategy of Akodon azarae males. We studied spacing patterns using 113 and 129 home ranges established by males and females, respectively, in four 0.25 ha enclosures during the breeding season. Significant differences between sexes in home range size and overlap degree were found. Male home ranges were always larger than those of females. We observed exclusive space use among males and among females. Considering only those males that shared their home ranges with females, average intersexual overlap value was about 50%. Males mainly overlap their home ranges with home ranges of two or three females. Significant differences in body size were found between males and females, with males being larger. We concluded that space use and sexual dimorphism in this species is consistent with patterns characteristic of polygynous rodents, and we propose a polygynous system in A. azarae.

Based on the hypothesis that, in Akodon azarae, polygyny operates through female defence, we studied inter-male aggression in order to test the following predictions: during the breeding period (1) resident males are more aggressive than intruder males in the presence of females (FP), and (2) aggressive behaviour is independent of male condition (resident or intruder) in the absence of females (FA). To test our predictions, we used the resident male behavioural response towards an intruder male in relation to FP or FA. We conducted 30 encounters in FP and 27 in FA in 0.79-m 2 round enclosures placed in the Espinal Reservation. Our results support the prediction that, in FP, the intensity of aggressive behaviour exhibited by males varied in relation to resident or intruder condition. Resident males showed high levels of aggression towards intruders, and intruders exhibited the greatest values of submissive behaviours with residents. In FA, the intensity of aggressive behaviour did not vary in relation to resident or intruder condition. Both resident and intruder males exhibited low aggressive behaviour and inter-male encounters resulted mainly in non interactive behaviours. Our results support the hypothesis that, in A. azarae, the polygynous mating system operates through female defence.

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We examined in Akodon azarae if at low availability of females (sex ratio biased toward males, 3:1), the mating tactics of males are determined by the number of receptive females to which they have access, or by the number of male competitors with which they interact. To test these hypotheses, we measured the home range size and overlap degree. At the beginning of the reproductive season, we studied spacing patterns using 57 and 30 home ranges established by reproductive males and females in three enclosure populations. We statistically compared our results with those previously obtained in enclosure populations with a sex ratio of 1:1. Male home range sizes were larger than those recorded in populations with a sex ratio of 1:1. The degree of male intra-sexual overlap increased about 3.5 times with respect to that observed under a sex ratio of 1:1. The increase in home range sizes and intra-sexual overlap degree of reproductive males reflects that A. azarae males may adjust their spacing strategy to increase their mating opportunities at low density of females. Our results suggest that at low breeding female availability, male mating tactics could be determined by the number of receptive females to which they have access.