Mating system of a population of Apodemus flavicollis inferred by spatial organization and nest sharing

Apodemus flavicollis is the most abundant rodent species in the beech woodlands of eastern Italian Alps. We analyzed two-years of radio-tracking data collected from adult individuals during the breeding season to investigate the sexual differences in the use of space and nesting behaviour, thus deriving information on the mating system. The field trial was based on CMR and radio-tracking (homing-in) techniques. We fitted collars to 64 adult resident mice. From July to October of both 2005 and 2006, we completed 4 radio-tracking sessions of 3 weeks. Fixes were recorded from dusk to dawn; additional daylight fixes allowed to localise burrows. We calculated home ranges as MCP 100% and Kernel utilisation distributions (UD), from which we derived 95% and 50% probability polygons. We performed the home range analysis only on animals which showed an asymptotic curve of number of fixes versus home range size; this was the case for 14 males and 7 females in 2005 and for 11 males and 10 females in 2006. We quantified static overlap between individuals by a range of indexes, including UD overlap and distance between centres of maximum activity. We assessed the effect of sex on the behavioural parameters listed above by GLM or GLMM. We observed that the home ranges of the two sexes extensively overlapped; individual home ranges of males often overlapped with those of several females and individual home ranges of females overlapped with those of several males. In turn, female spatial ranges never overlapped one to each other. Furthermore, male home ranges were larger than female ones. Every mouse used many burrows sequentially or simultaneously; the strict spatial association between sexes was also confirmed by the frequent male-female pairs observed when communal nesting was recorded. We never detected nest sharing among females, whilst we found that individual males could share their burrows with more than one female and viceversa. All these observations, supported the hypothesis that male spatial behaviour in the breeding season was aimed at maximizing the number of potential matings, suggesting a promiscuous mating system, that would be the response to the high costs of monopolizing territorial, asynchronous females