Population Biology of Microtus arvalis. II. Natal and Breeding Dispersal of Females

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Abstract
(1) Using radiotelemetry on individuals marked within 2 days of birth, we documented natal and breeding dispersal in solitary and grouped breeding females of the common vole, Microtus arvalis (Pallas 1779). We tested the hypothesis that breeding dispersal results from nest sites being too low in quality. (2) Breeding dispersal occurred only in solitary breeding females within 24 h prepartum when both grouped and solitary breeding females built a new nest either within the same burrow (residents) or 16-537 m from their burrow (dispersers). Solitary females provided with large artificial nest sites dispersed significantly less frequently than females provided with small artificial nest sites or living at small natural burrows. (3) Natal dispersal occurred on the first day that a vagina could be identified and in eleven females was concomitant with matings leading to a first litter. Littermates settled in the same areas. (4) We conclude that breeding dispersal was a consequence of soil hardness which frustrated the extension of existing burrows and proximately resulted from rises of oestrogen levels at pre-partum which initiates nest building. In contrast, natal dispersal in females was the rule and was related to first mating. (5) The costs of breeding and natal dispersal at high population density in terms of surviving to produce a first litter were negligible. Only one female disappeared in twenty-nine cases of breeding and thirty-one cases of natal dispersal.