| Summary: | The population dynamics, aggressive behaviour, population genetics and plasma testosterone levels of two populations of bank voles, Clethrionomys glareolus and the population dynamics and plasma testosterone levels of wood mice, Apodemus sylvaticus have been studied in mixed woodland in Castle Eden Dene and Houghali Woods, County Durham. Numbers of Clethrionomys, determined by live trapping, declined to a minimum in late winter and in three cases out of four rose to a maximum during the autumn. At Houghall, numbers of Apodemus were apparently markedly influenced by migration across the woodland edge but this was not so at Castle Eden. A distribution analysis confirmed that restriction of Clethrionomys to areas of suitable cover may be a proximate density-linked factor for this species. The distribution of Apodemus was relatively independent of cover, though dense areas were avoided during the summer. Social interaction between male Clethrionomys studied by neutral cage encounters in the laboratory, though infrequent, was always aggressive. The number of aggressive acts per encounter was maximal during the spring, then declined to a winter minimum. Overwintered adults were dominant over individuals which matured in the year of their birth, and were more aggressive. An electrophoretically detectable esterase polymorphism of Clethrionomys showed significant variation in allele frequencies at Houghall but not at Castle Eden. At the former site the change took place during the exceptionally dry summer of 1975 and may have been a localised temporary response to environmental conditions, similar to those recorded for some other species of rodents. A radioimmunoassay for testosterone was developed for small samples of rodent plasma. In breeding males of both species the mean level was about 5ng/ml. Levels in non-breeding individuals were below the sensitivity of the assay of 0,65ng/ml. The spring increase in testosterone level is coincident with an increase in aggressiveness, and it is suggested that a high level of testosterone acts to initiate behaviours associated with breeding.
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