CONTROL OF ANNUAL GONADAL CYCLES 723 



indicates a conformance with equation ( 1 ) and further that /: as a 

 function of p is similar to that illustrated in Fig. 2 except that the 

 slope may be slightly steeper and is displaced somewhat to the left. 



PHOTOPERIODICALLY INDUCED TESTICULAR DEVELOPMENT 

 AS A FUNCTION OF LIGHT INTENSITY 



In his early investigation of photoperiodism in Sturnus vulgaris, 

 Bissonnette (1931b) found that the rate of testicular development 

 was a positive function of light intensity (white light from incandes- 

 cent lamps) for intensities up to about 180 lux (16.5 ft-c). For the 

 same species Burger (1939) demonstrated nicely that increasing in- 

 tensity from about 50 to about 2000 lux (5 to 190 ft-c) is non- 

 stimulatory if the length of the photoperiod is too short. In other 

 words, the requirement of a day length above a certain minimum can- 

 not be replaced simply by increasing intensity of periods of subliminal 

 duration. Bartholomew (1949) has studied carefully with Passer 

 domesticiis the role of intensity (white light from fluorescent lamps) 

 in the rate of testicular development. In winter an intensity of 0.4 lux 

 (0.04 ft-c) was very slightly effective. A greater rate of response was 

 obtained at 110 lux (10.3 ft-c) than at 7.5 lux (0.7 ft-c) whereas the 

 rate was not increased further by increasing intensity to 2600 lux 

 (244 ft-c). Kirkpatrick (1955) found that the rate of photoperiodic 

 testicular response in Colinus virginianus (17-hr daily photoperiods 

 with incandescent lamps) was probably maximum at intensities as low 

 as 1 lux (0.1 ft-c). However, the time required by females to produce 

 the first egg appears to be an inverse function of intensity up to at 

 least 3200 lux (300 ft-c). In our laboratory the rate of testicular de- 

 velopment in Zonotrichia leucophrys gambelii has been studied as a 

 function of light intensity (white light from incandescent lamps). In 

 winter it was found that the rate constant k of equation ( 1 ) was 

 greater at 31 lux (3.0 ft-c) than at 11 lux (1.0 ft-c) but that there 

 was no appreciable increase in k for intensities up to 375 lux (35 

 ft-c). Intensities lower than 11 lux could not be used since the birds 

 could not be trained to feed in such dim light. The functional relation- 

 ship between k and intensity for low intensities must therefore be 

 explored by means of another approach. 



