36 VERTEBRATE REPRODUCTIVE CYCLES 



the general habit of fruit bats which tend to copulate 

 when the days are getting shorter, even though in 

 latitude 7° N. the contraction in day length is only 

 slight. 



9. THE VARIOUS EXTERNAL FACTORS 

 WHICH AFFECT BREEDING 



The suggestion has been made that the environmental 

 control of breeding cycles is a superficial one, and that 

 the widespread response of animals to increasing light 

 in spring has developed merely because it is advantageous 

 for the majority of animals to breed at this time. It is 

 equally possible for an animal to become adapted to 

 respond to decreasing light if for some individual reason 

 this is more expedient. It has also been suggested that 

 tropical animals may react to the quality of light, 

 and particularly to the ultra-violet component. This 

 might explain the two tropical breeding seasons shown 

 by birds (Fig. 2) which can be related to the two seasons 

 of the year when the intensity of the ultra-violet light is 

 at a maximum. It is equally possible that other animals, 

 living in conditions of almost constant daylength, may 

 respond to variations in light intensity as determined 

 by the presence or absence of clouds. 



Further, although not nearly enough observations 

 have yet been made on factors other than daylength, it 

 is already possible to suggest a number of other environ- 

 mental variants which may influence or control the 

 growth and maturation of the gonads. It also seems 

 certain that this number will be increased when more 

 species living in unusual environments are examined. 



One factor which may have considerable importance 

 is the light of the moon. From early times it has been 

 supposed to influence the menstrual cycle and breeding 

 habits of man, and, although this has now been dis- 

 proved, it undoubtedly influences the breeding of some 

 animals. Among invertebrates the remarkable case of 

 the palolo worm has already been mentioned (p. 2), 



