1146 



SUBMAMMALIAN VERTEBRATES 



l^ortance of daily increments in photoperiod. 

 The correlation between the summation of 

 daily light changes and the age of sexual 

 maturity was 0.96, an unusually high cor- 

 relation in biologic experimentation. 



A question which has been raised in con- 

 nection with the practice of providing arti- 

 ficial light for growing chickens is that of 

 refractoriness. Tomhave (1954) and Shutze, 

 Jensen and IMatson ( 1959 ) found that arti- 

 ficial light provided during the growing pe- 

 riod causes lowered egg production even 

 when the artificial light is supplied only 

 during the period from hatching to 8 weeks 

 of age. Hutt, Goodwin and Urban (1956 » 

 suggested that the failure of ovaries to de- 

 velop in some birds may be the result of 

 artificially long photoperiods during the 

 growing period. Experiments reported for 

 the domestic Pekin drake indicate also that 

 raising young drakes under continuous light 

 delays the onset of maximal testicular size 

 in the first breeding season (Benoit, Assen- 

 macher and Brard, 1959). Further experi- 

 ments are needed to determine whether the 

 observed effect is essentially the same as 

 the refractoriness in seasonally reproducing 

 birds and whether it can be interrui)ted by 

 exposure to short days. 



After egg laying has started, 13 to 14 

 hours of light per 24 hours seem to be opti- 

 mum, provided the photoperiod is given in 

 one dose (Dobie, Carver and Roberts, 1946). 

 AVith interrupted photoperiods the results 

 are similar to those described for seasonally 

 reproducing birds (Fraps, 1959). When hens 

 are exposed to continuous light, egg produc- 

 tion is higher initially than in birds ex- 

 posed to 13- to 14-hour photoperiods, but 

 egg production decreases sooner (Penquite 

 and Thompson, 1933; Greenwood, 1958). 

 This decrease may be the result of refrac- 

 toriness of the hypothalamus or of the pi- 

 tuitary to light (Byerly and Moore, 1941). 

 Greenwood (1958) observed that chickens 

 kept in a constant environment (continuous 

 light?) showed no molting, but there was 

 a steadily decreasing rate of egg production 

 and of hatchability of fertile eggs. After 

 the birds were 3.5 years old, 94 per cent died, 

 all of adenocarcinomas. Although carried 

 out with a limited number of birds, the ex- 

 periment suggests that molting observed in 



l)irds undei- continuous light is precipitated 

 by factors other than light. The high mor- 

 tality may have been the result of the con- 

 tinuous light, although in Greenwood's ex- 

 periment other environmental factors cannot 

 be separated from the light effect. However, 

 data obtained by Benoit, Assenmacher and 

 Brard (1959) with ducks suggest that light 

 may be the most important factor. These in- 

 vestigators found poorer survival by Pekin 

 drakes raised and kept under continuous 

 light than by ducks kept either under day- 

 light conditions or in total darkness. 



Greenwood's data (1958) also suggest that 

 a constant environment is not very de- 

 sirable for optimal reproduction, an observa- 

 tion confirmed in the experiments with tur- 

 keys, mentioned before. Byerly and Moore 

 (1941) demonstrated that an unnatural 

 photoperiod of 14 L + 12 D resulted in con- 

 siderably increased egg production and 

 clutch (number of eggs laid on consecutive 

 days) length. They offered the following ex- 

 l)lanation for this phenomenon. 



1. The long dark period prevents the on- 

 set of refractoriness to light stimulation. 



2. Tli(> limiting effect of the onset of dark- 

 ness is partially removed. As will l)e dis- 

 cussed later, the onset of darkness may de- 

 termine the length of the clutch (Warren 

 and Scott, 1936). No further experimental 

 work to test the hypothesis of Byerly and 

 ]\Ioore (1941) seems to have been reported. 

 The effects of intensivity, wavelength, and 

 interrupted photoperiods have been dis- 

 cussed previously for seasonally reproducing 

 birds. The experimental evidence indicates 

 no fundamental differences between such 

 birds and chickens; thus no further dis- 

 cussion seems to be required. Photoperiod 

 is the main regulator of gonadal activity, 

 but other factors such as nutrition and tem- 

 perature may modify the response to light; 

 some of these factors are discussed later 

 when consideration is given to the several 

 hypotheses offered to explain the timing of 

 the ovulation cycle. 



An understanding of the regulation of 

 the laying cycle requires a short introduc- 

 tion to the events which occur in the forma- 

 tion of the egg after ovulation. Excellent 

 reviews on this subject are available for 

 more details (Romanoff and Romanoff, 



