Chronobiologic study in the domestic duck. I. Quantitative and temporal study of the action of the white light, factor releasing the gonad-stimulation.

I. White light is an essential and specific external factor in the development and growth of the duck gonads. The effects of light vary according to: a. Its intensity, up to a certain maximum, beyond which the effect on the gonads becomes reduced. b. Its daily duration. c. Its duration throughout the year. Too prolonged daily exposure to light throughout the year, as in temperate climates, produces a refractory phase due to a sort of temporary fatigue of the gonad-stimulating mechanism and not to weakness or fall in the testicular reaction. d. Its distribution in time. Dividing of the exposure to light for a given time, favors growth of the testis. II. Light is not essential for testicular development in ghe duck. This development occurs in total darkness but is slower, smaller and presents fluctuations which are less marked and shorter than in natural light. When several ducks are placed in darkness in the same surroundings, the fluctuations of various individuals present certain synchronization which can only be explained by the intervention of social factors which are probably tactile. III. Submitted to constant, continuous and prolonged light for years, male ducks present, with a few differences, a behavior similar to that of subjects submitted to total darkness. Finally, light is not essential for testicular growth, but it is necessary in nature to trigger off and stimulate testicular development and ensure its annual cyclic character. Social fractors, and other external factors, also play and important role to ensure synchronization of the development of the gonads of subjects in the same group.