1140 



SUBMAMMALIAN VERTEBRATES 



species, the birds will go into the progressive 

 phase. During this phase the rate of gonadal 

 response depends on the summation of 

 photoperiods. In the spring the accumulated 

 effects of the photoperiods become effective 

 in stimulating the gonads, and, if ecologic 

 conditions are satisfactory, breeding can 

 start. 



For birds migrating to the equatorial zone 

 or remaining in the temperate zone, the same 

 current of events occurs except that a cycle 

 of fat deposition is added to the gonadal 

 cycle. According to Wolfson's hypothesis, 

 the rate of response is determined by the 

 summation of photoperiods so that the 

 response can occur near the equator even 

 though no change in photoperiod occurs. For 

 transequatorial migrants, the exposure to 

 daylight would increase in October and 

 November and decrease after December. 

 Again summation of photoperiod would be 

 the deciding factor, provided that the ex- 

 posure to enough short days has occurred to 

 break up the refractory period. 



Marshall (1959) has criticized AVolf son's 

 hypothesis mainly on the basis of field 

 studies showing that adult rooks, mallards, 

 and starlings show sexual displays in the fall 

 at the same time that in starlings the bills 

 become yellow and 13.6 per cent of the 

 rooks show spermatogenesis. This, accord- 

 ing to Marshall, argues against a need for an 

 exposure to short days to obtain a response, 

 because the response is obtained before the 

 shortest day. As Farner (1959) stated, these 

 phenomena can be explained by assuming 

 that the refractory period ends relatively 

 early in the late summer, when the photo- 

 period is still long enough to cause stimula- 

 tion. Not all individuals show these cycles 

 because certain other ecologic factors in- 

 terfere with the response. Evidence that a 

 refractory period may be short or even ab- 

 sent is found in the experiments of Kirk- 

 patrick ( 1959) with bobwhite quail. 



Marshall and Disney (1956) subjected 

 the "summation of photoperiod" hypothesis 

 to an experimental test with the tropical 

 nonmigratory Quelea quelea and observed 

 no response when the daily photoperiod was 

 increased 5 minutes over the natural photo- 

 period, although the summation of the pho- 

 toperiods was an amount which the birds 



under natural conditions would have ex- 

 perienced only after a period of 27 years. 

 The test proves that the summation of 

 photoperiods does not hold true for this 

 species, but it does not eliminate the possi- 

 bility that it may hold true for temperate 

 zone birds of the Northern hemisphere. In 

 any controversy of this kind it would seem 

 desirable that experiments be undertaken 

 with the same species. Any comparison be- 

 tween Quelea quelea and Zonotrichia al- 

 bicollis should take into account the dif- 

 ferent ecologic factors which may play a 

 role in the determination of gonadal and 

 migratory cycles. For some species living 

 in the temperate zone of the Northern hemi- 

 sphere, light may be the most important 

 single stimulating factor; for another spe- 

 cies, for instance Melopsittaciis undulatus 

 in a different but temperate zone environ- 

 ment, light may not be a factor (Vaugien, 

 1951,1953). 



Experiments in which short })hoto])eriods 

 of one to several minutes interrupt long 

 dark periods should be mentioned here. 

 Farner (1959), in a well conducted series 

 of experiments, determined the rate constant 

 of Equation 1 for various lengths of photo- 

 periods with light given in different doses. 

 He established that 6 hours of light in 

 equally spaced 50-minute doses resulted in 

 k value similar to that for 12 hours of light 

 given in one dose. The effectiveness of the 

 short photoperiod was dependent on the 

 intervening dark periods. Farner proposed 

 the following hypothesis to explain this 

 effect: a substance generated during the 

 photoperiod decays gradually during the 

 dark period, but remains able to stimulate 

 the hypophysis for a certain length of time. 

 It has been estimated that it takes about 

 1 minute to generate the substance in equi- 

 librium amounts, whereas it has been esti- 

 mated that the decay of the substance once 

 generated, takes at least a few hours (Far- 

 ner, 1959). This hypothesis does not assign 

 any i^ositive function to the dark period 

 as suggested by Jenner and Engels (1952) 

 and Kirkpatrick and Leopold (1952). 



Although light plays a powerful role in 

 inducing spermatogenesis in drakes, cycles 

 of testicular activity can occur in the ab- 

 sence of light. Benoit, Assenmacher and 



