Effect of absence of light on the breeding season of the ferret

Abstract

Bissonnette's (1932) remarkable discovery that additional illumination would induce œstrus in anœstrous ferrets has naturally led to much speculation as to what controls the onset of the breeding season in the normal ferret. An obvious interpretation of Bissonnette's results, and one apparently favoured by Marshall (1932), suggested that the beginning of the breeding season of the normal female in March-April is influenced by the increasing duration of daylight. Such a hypothesis, however, could not be extended to many mammals. Those breeding in the autumn (sheep, for instance) would require a radical alteration of the hypothesis, while nocturnal and subterranean mammals can hardly be affected directly by the duration of daylight. Even in the ferret the hypothesis scarcely seemed valid, because the initial breeding season changes are evident histologically in December and January (Allanson, 1932). The converse experiment, keeping breeding season animals in daylight of winter duration, was also attempted by Bissonnette (1933), but the results were inconclusive. A somewhat similar experiment was therefore carried out in London, under the very stringent conditions of keeping the animals in total darkness except at the time of feeding. The results offer little support to the theory that length of daylight has any controlling influence on normal sexual periodicity.