The social ecology of the wood mouse, Apodemus sylvaticus (L.)
The purpose of this study was to investigate the ranging behaviour and social structure of a population of wood mice, Apodemus sylvaticus (L.), and to find out if, and by what means, this mouse scent-marks its range. The study was carried out in an 11 ha deciduous wood in Aberdeenshire, Scotland. Miniature radio-transmitters, weighing 2.4g (that is up to 10% of the adult body weight), were attached to adult mice ( >20g) so that their movements could be followed. From field observations, and a laboratory investigation where neutral cage encounters were used to look at aggression and dominance, these collars were not considered to have any deleterious effects on the aspects of movement and activity studied. An automatic recording device was used to monitor the presence or absence of radio-collared mice at their nests in the wild. In the winter each mouse left its nest three to five times each night, each time for an hour or so, following a short-term rhythm. This behaviour pattern gradually changed as the year progressed, the mice spending increasingly longer out of their nests each night, until in summer the mice were absent from their nests all the time from sunset to sunrise. In winter the time spent out of the nest each night was reduced both by a decrease in ambient temperature and by an increase in the intensity of moonlight. A new method of describing home ranges was devised, termed the Restricted Polygon, which was considered more appropriate for use with A. sylvaticus radio-tracking data than existing methods. The ranges of the mice were generally larger than those found by previous workers (who used less accurate methods of collecting home range data). Males had ranges that were on average 3.6 times as big as those of females (5110m2, n = 12, v. 1420m2, n = 12). The male ranges overlapped randomly, whilst breeding females appeared to maintain more or less exclusive ranges; male/female ranges overlapped completely but associations between males and females rarely lasted for more than a few hours. No tendency to patrol ranges was noted for individuals of either sex, although mice usually covered the whole of their ranges in four or five nights. Males moved twice as far between, successive fixes as females. Occasionally mice sallied forth out of their ranges, on one night in five for males, and on one in ten for females. These excursions involved travelling up to 200m beyond their normal range boundaries. In the breeding season adult mice nearly always nested on their own, but in winter they nested in mixed-sex groups of up to at least three. To investigate the scent-marking behaviour of A. sylvaticus, two mice of each sex were injected with 32p and, a few days after their release back into the wild, their home ranges systematically searched for the faeces and urine which had been made radio-active. The spatial distribution of urine/faeces (they could not be told apart in the field) for Doth sexes directly reflected the amount of time spent at each location, with no preferential deposition on range boundaries or on specific landmarks. This does not, however, preclude the possibility that they are still used for range-marking since at least the females were still effectively impregnating their home ranges with these substances. In the laboratory, by training mice to exhibit discrimination between two odours in a 'Y' maze, it was shown that they could distinguish between conspecifics by the odours of both their urine and their faeces. This is an essential requirement for these substances if they are indeed used for scent-marking ranges.