A QUANTITATIVE EXAMINATION OF TESTICULAR GROWTH IN THE WHITE-CROWNED SPARROW

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Abstract
1. The rate of testicular development in Zonotrichia leucophrys gambelii has been examined quantitatively as functions of day-length, light intensity, and environmental temperature. 2. From these laboratory-established relations a predicted curve for testicular growth under natural conditions was calculated and compared with data obtained from a natural population. The predicted curve indicates the attainment of 100mg. combined testicular weight about ten days earlier than its occurrence in the natural population. Although this relatively small discrepancy may be reasonably explained on a statistical basis, it is not possible to rule out minor effects by environmental variables other than daily photoperiod and temperature. 3. The calculations, and the relatively close agreement with the observations of the natural population, emphasize quantitatively the overwhelming importance of the daily photoperiod as the primary timer in the testicular cycle for this population of Zonotrichia leucophrys gambelii. Other environmental factors, as they operate in the Snake River Canyon, appear to be responsible for fluctuations with a combined maximum possible magnitude of the order of ten days to two weeks.