Regulation of Breeding Density in Microtus pennsylvanicus

(1) This study tests the hypothesis that breeding density in polygamous microtines is determined by breeding females, which compete for space to rear young. Males compete for access to females directly, not for access initially to territories. (2) To test these predictions, we live-trapped three populations from 1978 to 1981: a Control grid; a Female grid, from which all breeding males were continuously removed; and a Male grid, from which all breeding females were continuously removed. (3) In every spring, the number of breeding males on the Male grid declined for approximately 2 months after the onset of the breeding season, the number on the Female grid increased markedly through immigration, and the number on the Control grid remained constant or increased slightly. The number of breeding females remained constant or increased slightly on the Control and Female grids, while the number on the Male grid increased markedly through immigration. (4) Adult males on the Control grid always had higher survival rates and longer expectations of life than did those on the Male grid throughout the breeding season. Adult females on the Control and Female grids had similar survival rates and expectations of life. Adult male survival was strongly related to the density of breeding females. Adult female survival was not related to the density of either sex. (5) Resident breeding males restricted recruitment of other breeding males in the presence of breeding female residents, but in their absence, resident males only partially restricted other breeding males. Resident breeding females restricted recruitment of other breeding females independently of male presence. (6) These results support the hypothesis that breeding female microtines regulate breeding density. We propose that males form dominance hierarchies to gain access to females.