28 VERTEBRATE REPRODUCTIVE CYCLES 



w^inter. This result could be achieved either by means of a 

 progressive daily increase of several minutes, or by a 

 sudden and permanent increase of 6 hours. He also 

 investigated the effects of different light intensities, and 

 found that in the conditions of his experiment there was a 

 progressive acceleration in the rate of gonad growth with 

 light of from lo watts to 40 watts, but that with higher 

 intensities no further increase occurred.^? 



Since then many species of birds have been studied 

 with essentially similar results, so that it is now known 

 that an increase of daylength causes growth and matura- 

 tion of the gonads of ducks,^^ mourning doves,'''^* 

 pheasants, quail, ruffed grouse,®^ Japanese white eyes,i*2 

 common sparrows,^i^ golden crowned sparrows^*^ and 

 greenfinches.'^^ Conversely it has also been shown in 

 several of these species that a reduction in daylength 

 causes gonad regression. 



The first study of the reaction of a mammal to light 

 was that of Baker and Ranson,!^ ^^vho found that when 

 the daylength is shortened from 15 hours to 9 hours 

 reproduction almost ceases in the field mouse, Microtus 

 agrestis. At about the same time Bissonnette^* was 

 studying the effect of increasing light on the ferret, which 

 normally breeds from about March to August. Starting 

 in autumn, he increased the daylength of his experi- 

 mental animals by a single increment of 6 hours. From 6 

 to 9 weeks later the females were in full oestrus, but, 

 somewhat surprisingly, the response of the males was 

 not so great and the matings which occurred were 

 sterile. Later Bissonnette found that if the daylength 

 was increased by small daily instalments, instead of in 

 one single step, the male ferrets reacted by a full 

 maturation of their testes and fertile matings followed.^^ 



In England, Hill and Parkes, ^o^* ^^^ also carried out a 

 series of investigations on the ferret, and they corro- 

 borated Bissonnette 's results. However, they failed to 

 prevent breeding in spring by means of a reduction in 

 the daylength to only 30 minutes. The reason for this 



