Initiation of breeding by northern Peromyscus in relation to temperature

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Abstract
Initiation of breeding by Peromyscus leucopus noveboracensis (Fischer) near London, Ontario, P. maniculatus maniculatus (Wagner) near Pinawa, Manitoba, and P. m. borealis (Mearns) near Hay River, N.W.T. was examined in relation to seasonal changes in daily minimum temperatures. Spring breeding commenced in all three populations before minimum temperatures reached 0 °C. A comparison of mean breeding temperatures between populations indicated that first litters of P. m. borealis were conceived at significantly higher temperatures (−1.7 °C) than those of P. m. maniculatus (−5.8 °C). During the first 15 days of the nesting period, minimum ambient temperatures and the percentage of nights below freezing were higher and lower, respectively, for P. m. borealis (6.0 °C; 4%) than for P. m. maniculatus (3.2 °C; 30%) or P. leucopus (4.3 °C; 22%). We suggest that P. m. maniculatus and P. leucopus breed early in order to maximize the number of descendants during the summer. Earlier breeding by P. m. borealis would not enhance the number of descendants per season.