Effects of variations in copulatory behavior on pregnancy in two species of Peromyscus

Stimulation requirements for the initiation of pregnancy were studied in cotton mice, Peromyscus gossypinus, and white-footed mice, P. leucopus. Mating to satiety rather than for one ejaculatory series significantly increased the per cent of P. gossypinus females pregnant (63% vs 0%). Among females mated to satiety, those which became pregnant received significantly more copulations than those which did not. While no P. leucopus female mated for one ejaculatory series became pregnant, the increase in the probability of pregnancy following tests continued to satiety (or satiety plus overnight) was not significant, as just 20% of females in the satiety groups became pregnant. The multiple ejaculation pattern in these species appears to be an integral part of pregnancy initiation.