Photoperiodic influences on testicular development of deer mice from two different altitudes

Onset of breeding activity tends to occur later in the year for deer mice from progressively higher elevations. We tested the hypothesis that the critical day length for development and maintenance of the reproductive system is longer for mice from higher elevations. Offspring of deer mice trapped at elevations of 1219 and 3048 m were raised from birth to 55 days of age in photo-periods that provided 10, 11, 12, 13 or 16 h light/day. Short day lengths (less than 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 day lengths 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.