Syntopic, congeneric species often deploy ecological strategies to avoid interspecific competition for any resource. Here we explore if the monthly and seasonal biosynthesis of sexual steroids involved in the Δ4 pathway, is one of such strategies for syntopicPeromyscusmelanotis and Peromyscusdifficultiesfelipensis, since a differential pattern could affect, in turn, the temporality of their respective annual reproduction patterns. We found, indeed, an ecophysiological relationship between these two Peromyscus inhabiting at a midlatitude, temperate forest, since each had a monthly and seasonal species-specific pattern for sexual steroid production. We discuss how such differences are likely to account for the interspecific temporal decoupling of their respective reproductive patterns.