Sex ratios in high-density populations of the montane vole,Microtus montanus,and the behavior of territorial males

Summary Montane voles (Microtus montanus nanus, Rodentia: Muridae) were studied in unconfined populations and in an exclosure in open fields in Wyoming, USA. Field work involved capture-mark-recapture grids, tagging select individuals with irradiated wires, subcutaneous implantation of dye pellets, and behavioral observations.Territorial males were found in both high and low density populations. They were characterized as having scrotal testes, actively defended territories, residency over time, association with females, few scars, and scent-marking behavioral patterns.Males maintaining territories in high-density populations were nearly the only males in breeding condition. Five high-density populations were trapped in late fall, and age distributions were determined by an eye lens technique. There were two age cohorts of territorial males: those born earlier in the season and those which had overwintered. For each grid, the distribution of males by age was discontinuous at about 95 days. The ratio of older males to older females of comparable ages in these five populations varied between 1:2.1 and 1:3.8. The operational sex ratio was even more unequal.The observations of territoriality at such high densities did not support the hypothesis that territoriality and social rank are two poles of a behavioral continuum related to density.The system of polygyny found, wherein the male has a territory overlapping those of several females, does not neatly fit previous schemata considered favorable for the evolution of polygyny, but is considered adaptive in view of female movements and the labile response of female reproduction to the male.