| Summary: | The relationship between male aggressive behaviour and population variables in the bank vole (Clethrionomys glareolus (Schreber, 1730)) was studied using two large (550m2) field enclosures. Known populations were established in each of the enclosures, and followed by live trapping. In an attempt to manipulate the level of aggressive behaviour in one (the experimental) enclosure with respect to the other (the control) a majority of the founding males in the experimental enclosure were castrated. Laboratory arena testing was used to study the behaviour of adult, castrated and immature male voles. It was demonstrated that both castrated and immature male voles were significantly less aggressive than adult males. Adult males were found to fight less with castrated and immature males than with other adults. Castrated males sometimes exhibited retaliatory behaviour when approached by either an adult or immature male, but overt aggressive behaviour rarely resulted. Immature males were very rarely aggressive. Observation of behavioural interactions in the field (at a bait point) showed them to be qualitatively different to those observed in the laboratory arena; voles appeared very wary of each other, and many interactions observed were characterised by mutual avoidance or flight. Oveitaggressive behaviour was rarely seen. It can be inferred that voles do fight in the field, because they commonly exhibit small wounds on rump, tail, and face; these could however, be sustained in the confinement of burrows, where escape and mutual avoidance are less possible than at a bait point, and which may be more closely paralleled by the laboratory arena. Significantly more adult males in the enclosures showed fresh wounds than did castrated or immature males, or adult or immature females. The number of voles in the experimental enclosure increased significantly faster than did the number in the control enclosure; the density in both enclosures was also significantly higher than commonly encountered in the wild. The difference in numbers between the two enclosures was due to a difference in the number of immature animals entering the trappable population; mortality of marked animals in both enclosures was very slight but mortality of infants was significantly higher in the control enclosure than in the experimental. Reproductive inhibition of adult and immature animals of both sexes was observed in both enclosures. At the same time, extremely young animals trapped in the wild populations were found to be sexually mature. The differences in the use of space by different age and sex classes between the two enclosures were also investigated. The study demonstrated that male aggressive behaviour had a significant effect on the population variables of bank voles at the high densities and at the high rates of population growth observed in large enclosures. The findings are discussed in the light of several hypotheses put forward to explain population regulation and cyclic changes in numbers of Microtine rodents.
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