Multiple responses to different photoperiods occur in the mouse, Peromyscus leucopus

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Abstract
Peromyscus leucopus exhibits multiple responses to different daylengths. The winter molt and gonadal regression are complete in populations chronically exposed to 12, 11, 10, or 9 h of light per day while hypertrophy of brown fat and occurrence of daily torpor were greater in mice on 9 or 10 h of light when compared to animals on 11 or 12 h photocycles. A precise “critical” daylength is not observed in this species for gonadal regression, occurrence of daily torpor, or hypertrophy of brown fat. Certain mice never exhibit a short-day response following 12 weeks exposure to 12, 11, 10, or 9 h of light per day.