Experimental alteration of sex ratios in populations of Microtus townsendii, a field vole

Abstract

Sex ratios in two populations of Microtus townsendii were experimentally manipulated toward 80% males and 20% males over a 2-year period. The demography of these manipulated populations was changed very little in comparison with a control population with 50% males. Neither survival, breeding, nor growth were related to the density of voles of the same sex. All populations had restricted immigration and the recruitment of females was more tightly regulated than recruitment of males. The number of young voles recruited per pregnancy is inversely related to female density on all areas, and not related to male density. The survival rate of juveniles was influenced more by the females than by the males.Microtus townsendii seems to fit the model of a species in which spacing behavior is sex specific, so that the partial removal of one sex has virtually no effect on the other sex. We need to determine the factors that restrict the entrance of recruits into a vole population. These experiments suggest that the role of females in the restriction on juvenile survival and female recruitment could be critical for vole population regulation.