Social organization in cyclic subarctic populations of the voles Clethrionomys rufocanus (Sund.) and Microtus agrestis (L.)

Abstract

Fluctuating populations of C. rufocanus and M. agrestis were studied by capture-marking- recapture trapping and snap-trapping at Kilpisjärvi, Finnish Lapland during several years. All categories of C. rufocanus were nearly twice as trappable as those of M. agrestis, but differences in social status caused differences in trappability between individuals of the same category. Trappability was therefore used as a measure of the social status of the individual. In both species the population had a group structure; each group consisted of many mature females with territories and some semi-territorial or non-territorial mature males. The immatures were non-territorial. In M. agrestis these groups, increased by immigrants, changed during the summer to harem-like structures defended by highly aggressive territorial males. The numbers of reproducing females were controlled by territorial behaviour. In C. rufocanus maturation ceased when all habitable space was occupied, whereas in M. agrestis the young females emigrated to independent home ranges shortly before the birth of their first or second litter. In males of both species maturation was controlled by the aggressive behaviour of the highly mobile mature males. These formed a dominance hierarchy, and as a result some of the males that matured at a later stage were forced to emigrate. During a population decline the oldest dominant age classes survived best, whereas during a population increase the young age groups survived somewhat better. In suboptimal habitats, changes in age structure always resembled those of a declining population. In competition M. agrestis is superior to C. rufocanus, but the difference is slight, as indicated by the impact of M. agrestis upon the age structure of C. rufocanus. Neither the early cessation of breeding observed in some years nor the population declines were directly correlated with population density, but other factors must be involved, possibly nutrition.