Photoperiodic Influences on Testicular Development of Deer Mice from Two Different Altitudes

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Abstract
Onset of breeding activity tends to occur later in the year for deer mice from progressively higher elevations. The hypothesis that the critical daylength for development and maintenance of the reproductive system is longer for mice from higher elevations was tested. Offspring of deer mice trapped at elevations of 1219 and 3048 m were raised from birth to 55 days of age in photoperiods that provided 10, 11, 12, 13 or 16 h light/day. Short daylengths (< 13 h light/day) retarded maturation of the reproductive apparatus of juveniles from both elevations. The critical photoperiod for development of the reproductive apparatus was 1 h shorter for mice derived from stock trapped at 3048 m as compared to 1219 m. The gonads of adult mice from each elevation were unresponsive to photoperiod and remained functional in all daylengths tested (10-13 h light/day). Photoperiod does not appear to be a proximate factor underlying the altitudinal gradient in reproduction of male deer mice.