THE ROLES OF DARKNESS AND LIGHT IN THE PHOTOPERIODIC RESPONSE OF THE TESTES OF WHITE-CROWNED SPARROWS

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Abstract
Male migratory Zonotrichia leucophrys gambelii from the population which winters in the Snake River Canyon of s._e. Washington (65 males in 6 groups) were exposed to different patterns of intermittent light beginning February 14, 1953. Responses at the end of 29-30 days treatment were ascertained by testicular wts. and by histological study of spermatogenesis. Group A, receiving 10 continuous hours of artificial light daily, showed no response. Group B, collected from the wild at the conclusion of the expt. (mean daily photoperiod 12.2 hours), showed early vernal sperma. togenic development and slight testicular wt. increases. Group C, receiving 18 continuous hours of light daily, showed nearly complete spermatogenesis and marked increases in testicular wts. Group D, receiving 10 hours total light divided into 2 periods, 8.5 hours during the day and 1.5 hours in the middle of the night, showed greater spermatogenic response and testicular wts. than wild controls (group B). Group E, receiving 10 hours total light as 9 hours 40 min. of continous light plus 1 min. of light each half-hour for 4 hours before and 4 hours after the continuous period of light, showed markedly greater spermatogenic response and testicular wts. than group D. In group F, which received 10 hours total light as nine 1.11-hour periods equally spaced through the course of 18 hours, spermatogenesis was as completely developed as in group C and testicular wts. were not statistically different from those in group C. Results suggest that the light-sensitive mechanism involves a light-dependent process which responds quickly at the beginning of a period of light and soon produces a sufficient quantity of an essential substance to allow a persistent gonadotropic effect after the cessation of light. The period of gonadotropic effect after cessation of light is referred to as the " carry-over period." Assumption of an active or essential role for the dark period (as has been recently proposed by other investigators) is opined not necessary to rationalize available information.