Photoperiodicity in the female ferret.

Previous work has shown (Bissonnette, 1932) that added electric light after dusk has a pronounced stimulatory influence on the anterior pituitary (Hill & Parkes, 1933), causing the onset of oestrus in the ferret during the non-breeding season of the year from September to March. Marshall (1940) also showed that the time of the onset of the breeding season caused by exposure to light varied with the intensity of the light. The fact that light promotes the onset of the oestrus in those mammals which have a summer breeding season has hitherto been assumed to be due to the effect of the light itself as the primary activating influence, and that darkness was merely an alternative to light. Thus the term ' light ration' has been frequently used. Any possibility that darkness as such has an influence on the function of the anterior pituitary and thus affects the oestrous cycle has been left uninvestigated. In recent work with chrysanthemums Borthwick (1947) has shown that what are termed ' short-day' plants can be made to flower after treating them with a short light period and a long dark period during the 24 hr., whereas 'long-day' plants placed under the same treatment did not flower. This effect was due to the fact that in the 'short-day* plants the dark period in each 24 hr. was long, and not because the light period was short. This raises the question whether the stimulating agent in summer breeding animals is the total amount of light per 24 hr. or whether it is the length of the dark period. To test this it was decided to set up experiments, using ferrets, in which the length of the dark period in relation to the length of the light period could be varied by using a different sequence of light and darkness over the 24 hr. period.