CONICET | Buscador de Institutos y Recursos Humanos

C. musculinus females are territorial and typically mate with more than one male during each estrous period. At low number of adult females, breeding males decrease their home range size and intra-sexual home range overlap, presumably as a consequence of intra-sexual competition for receptive females. We tested the hypothesis that at male-biased sex ratio C. musculinus males exhibit more intrasexual aggression than when there is not male-bias. The study was carried out in four 0.25 ha enclosures (two control and two experimental) during breeding season in 2004. Using field dyadic arena test, we carried out 17 male-male trials in control and 18 in experimental enclosures. Mean sex ratio (male/female) values ranged from 1 to 1.1 in control (CE), and from 2 to 2.3 in experimental enclosures (EE). Dyadic encounters were conducted between non-familiar and sexually mature males of similar weight and age, which established home ranges in the same enclosure at the same trapping session. Encounter was performed in the home range of one of the opponents (owner), with the other opponent acting as an intruder. We estimated 34 and 36 male home ranges in CE and EE respectively. In CE males overlapped their home ranges with 3 or 4 males, but in EE the intermale overlap occurred only with 1 or 2 males and did not exceed the 10 % of overlapping. Aggressiveness, submissiveness and amicableness of owner and intruder varied in relation to treatment (one-tailed Mann-Whitney U tests, P <0.0031). Males were more aggressive in EE than in CE. Intruder males were more submissive than owners in EE. Both owners and intruders never exhibited amicable behaviour in EE. Male behaviours varied in relation to sex ratio and home range ownership status. Our results support the hypothesis that in C. musculinus intermale aggression is related to female availability.

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