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I n one set of observations, all young had disappeared from the population in 9 weeks, but this could have been accounted for in part by dispersal. Bourliere (1951) quotes evidence of a similarly short life span for Microtus guentheri from Bodenheimer and for Citellus pygmaeus from Kalabuchov. The only argument contrary to the considerable evidence that a small mammal rarely lives out its potential life span in nature comes from Hamilton (1937b, 1940). He apparently believed that Microtus pennsylvanicus “burns out” physiologically because of (1) attainment of sexual maturity at an uncommonly early age, (2) extreme prolificness, and (3) little cessation of activity in its search for food.

Both of these experiments indicated movement from areas of high to areas of low population density. Stickel (1946) demonstrated similar behavior in Peromyscus leucopus when she marked the mice on a circular, 17-acre area before beginning to remove the mice from a central, 1-acre plot. As the animals were removed from the central plot, others were taken there from increasingly greater distances. Spencer (1941) ran snap-traps on a 5-acre plot for a period of 10 months to measure “drift” in rodent populations.

A t the end of 2 weeks, the population had rebuilt to 25, of which 20 were sexually mature animals that moved from adjacent areas and 2 were young mice. 9-acre area inhabited by 1 pair of deer mice 28 W. PRANK BLAIR vas overpopulated when 22 adult males, 19 adult females, and 4 young females were released there. Trapping during the second week revealed only the resident pair, 2 liberated adult males and 3 females, and 5 juvenile mice. Ten of the mice that disappeared were retaken at distances of 825 to 1815 feet.