Seasonal Reproductive Patterns in Four Coexisting Rodent Species from the Cascade Mountains, Washington

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We studied annual cycles of three sciurids and one cricetid rodent that coexist in a strongly seasonal environment in the Cascade Mountains of Washington. Spermophilus saturatus (Cascades golden-mantled ground squirrel) hibernated for 6–8 months, whereas Eutamias townsendii (Townsend's chipmunk) and E. amoenus (yellow-pine chipmunk) hibernated for only 4–4.5 months. Peromyscus maniculatus (deer mouse) did not hibernate. The three sciurids bred only once a year, with a 1- to 2-week mating period between late March and early May; E. amoenus bred about a month before the other two sciurids. Males emerged from hibernation 1–2 weeks before females, and mating occurred within a week of female emergence. P. maniculatus began breeding in February and continued through October; adult females produced as many as three litters, and the earliest juvenile cohorts reproduced in the year of birth. None of the sciurids became reproductively active until they were yearlings, and some E. townsendii and S. saturatus did not breed until 2 years of age. Average litter size was similar in the four species: S. saturatus, 4.1; E. townsendii, 3.8; E. amoenus, 5.0; and P. maniculatus, 4.6. Because female P. maniculatus bear 2–3 litters per year and juveniles breed in the year of birth, this species has the most flexible timing and greatest annual reproductive potential. The total mass of young produced at the end of gestation and upon emergence from the natal burrow imposes an energetic burden that scales inversely with body mass among the four species. Lipid reserves do not contribute directly to energy requirements of gestation and lactation. Lipid reserves play a major sustaining role during hibernation in S. saturatus (prehibernation body fat >30%), but chipmunks fatten only slightly (body fat < 7%) and rely on food caches during hibernation. P. maniculatus does not fatten substantially. S. saturatus that emerge from hibernation ready to breed contain residual hibernatory fat used for 1–2 weeks as a supplement to ingested food.