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Seasonal variation in growth rates has been well documented in some small rodents (Kubik, 1965; Brown, 1973; Iverson and Turner, 1974; Peterborg, 1978; Pistole and Cranford, 1983). During winter juveniles tend to show little or no growth; however, Kubik (1965) found that Cleithronomys glareolus born in late fall went through a two-phase growth pattern. Growing rapidly until winter and then resuming rapid growth the following spring. Additionally, Brown (1973) has reported the same two-phase growth in Microtus pennsylvanicus. Iverson and Turner (1974) demonstrated that Microtus pennsylvanicus adults showed a loss of weight during winter, and Pistole and Cranford (1983) have shown that M. pennsylvanicus subadults continued growth under natural winter conditions but at a significantly slower rate than occurs under summer conditions. Additionally, adults lost weight until winter solstice and then gained weight with increasing photoperiod. This data implies a complex relationship between growth, photoperiod, and ambient temperature.