Sex-biased ranging behaviour of the yellow-necked mouse Apodemus flavicollis at high population density

The yellow-necked mouse Apodemus flavicollis is the most abundant rodent species in woodland of eastern Italian Alps and a relevant reservoir of zoonotic pathogens. We analyzed radio-tracking data collected from adult individuals during the breeding season in a year of high population density and low seed production to determine sexual differences on use of space, nesting behaviour and dispersal. The field trial was based on CMR and radio-tracking (homing-in) techniques. We fitted collars to 20 male and 12 female adult resident individuals. From July to October 2005, we completed 4 radio-tracking sessions of 3 weeks. Fixes were recorded from dusk to dawn; additional daylight fixes allowed to localise burrows. The mean number of nocturnal fixes/ animal/session was 65.7 ± 2.2 and 12.4 ± 0.8 for diurnal fixes. We calculated home ranges as MCP 100% and Kernel utilisation distributions (UD), from which we derived 95% and 50% probability polygons. We could estimate home ranges for 13 males and 7 females. We quantified static overlap between individuals by a range of indexes, including UD overlap and distance between centres of maximum activity. We calculated successive distances at 24 hours interval to assess ranging movements and dispersion. We assessed the effect of sex on the behavioural parameters listed above by GLM or GLMM. All statistical analysis were carried out by R and spatial analyses by ArcGis 9.0. Male home ranges and core areas were larger than those of females, as well as daily ranging movements. Spatial range of individual males overlapped both with that of males and females. In turn, female spatial ranges never overlapped one to each other. Similarly, we never detected more than one female in the same nest. From the end of September to early October, 9 individuals, of both sexes, dispersed to new sites (from 1 km to 2.5 km apart), suddenly and quickly. Our results indicated that spatial behaviour of A.flavicollis is affected by sex. During mating season, males may maximize their probability of meeting sexually active females by covering large areas, while females may compete for food, thus defending core areas from conspecific of the same sex. The food scarcity, exasperated by the high population density, seemed to drive individual dispersal towards habitat spot of better quality. Sex biased ranging behaviour may exert a central role in disease transmission, leading to greater exposure of males to pathogens, thus cumulating to hormonal effects.