Studies of Natural Populations of White-Footed Mice: Reduction of Reproduction at Varying Densities

Abstract

A population of white-footed mice ( Peromyscus leucopus noveboracensis ) on an 11-ha area was studied monthly during 1983–1989 with 600 live-traps and with 254 wooden nest boxes attached to trees. I hypothesized that regulation of population growth would occur each year in the same area at markedly different numerical levels and that the characteristics of reproductive curtailment would be similar between animals in populations of widely different densities. More than 90% of the population known to be alive were captured each month; <20% of suckling young recorded in nest boxes subsequently were captured in traps. The number of adults in the population varied from 22.9/ha in March 1983 to 0.3/ha in November 1984. Significantly more adult males (56.7%) than adult females (43.3%) were captured during the study and a smaller percentage of males than females was reproductively mature in 58 of 72 monthly comparisons. A yearly bimodal curve of reproduction was noted over 7 years with significantly lower proportions of males and females in reproductive condition during May, June, and July than during February–April and August–October. Correlation coefficients between the number of adults of each sex and the percentage in reproductive condition from April 1984 through November 1989 were negative for females at P ≤ 0.01 and for males at P ≤ 0.1. Yearly correlations between the same measurements generally were negative for both sexes, but at widely different densities. No consistent pattern of change in body weight in males and nonpregnant females was noted. Weights of testes, seminal vesicles, ovaries, and uteri generally were significantly smaller in May, June, and July than in March, April, August, September, and October. Factors producing this significant retardation of reproduction at widely different density levels are unknown and are not related to density per se.