REPRODUCTIVE ENDOCRINOLOGY IN BIRDS 



1149 



in the ovulation sequence compared to lag 

 in the oviposition secjuence showed that the 

 lag for the second place in the ovulation 

 sequence remained relatively large, whereas 

 in the oviposition sequence it became rapidly 

 smaller as the number of eggs per clutch 

 increased. Thus, the time that the second 

 egg remains in the oviduct decreases more 

 rapidly as clutch length increases. Another 

 important characteristic of the ovulation 

 cycle is the interval between terminal ovula- 

 tion of one clutch, C,, , and ovulation of the 

 first follicle, Ci', of the next clutch, an in- 

 terval w^hich is of the order of 44 hours 

 when }i = 2 and 40 hours when n = 6. The 

 difference between this value and the mean 

 interval between ovulations in a clutch gives 

 an approximation of the "period of lapse." 

 This period of lapse approximates the ad- 

 ditional time elapsing between C„ and Ci 

 follicles over what might be expected, had 

 ovulation occurred on the day of the missed 

 ovulation. 



In order to explain these conclusions of 

 Fraps, consideration will now be given to 

 the effect of environmental factors on the 

 ovulation sequence. A fundamental study 

 by Warren and Scott (1936) demonstrated: 

 (1) under normal daylight, the onset of 

 darkness seems to determine the termination 

 of the clutch and more than 90 per cent of 

 the eggs are laid during the day; (2) when 

 lighting is continuous and entirely artificial, 

 time of laying is equally distributed over the 

 24 hours; (3) when light and dark periods 

 were reversed, the hens would lay in the dark 

 for a few days, demonstrating that ovi- 

 position can occur in the dark; the hens 

 would shift gradually to laying in the light 

 period; (4) hens laid 90 per cent of the eggs 

 during the day when daylight was supple- 

 mented with continuous artificial light; this 

 was attributed to a psychologic factor. 



McNally (1947) found that the period of 

 feeding determined the time of oviposition 

 for chickens kept under continuous light, and 

 Fraps, Neher and Rothchild (1947) investi- 

 gated both the effect of feeding and time of 

 light with respect to time of lay. Birds were 

 first exposed to 14 hours light; they laid dur- 

 ing the light period. After the ovijiosition 

 pattern was established, lights were turned 

 on continuously, and eggs were still laid dur- 



ing the daytime. By changing the period at 

 which the birds were fed and tended, tlie 

 investigators were able to shift the time of 

 lay to the period in which feed was availa- 

 ble. An incidental observation was that the 

 time of minimal body temperature coincided 

 wiih the time of release of gonadotrophin 

 to cause ovulation. We have tried to estab- 

 lish experimentally whether lowering of 

 body temperature would result in prema- 

 ture ovulations. Hens were placed in i-e- 

 frigerators (with good ventilation), with 

 and without lights. This was done during 

 the period of the cycle (day of lay of the 

 last egg of the clutch) when progesterone 

 injections cause premature ovulations. Body 

 temperature dropped 1° to 3°F., but in no 

 case were we able to induce premature ovu- 

 lation. This suggests that the observation 

 made by Fraps, Neher and Rothchild ( 1947) 

 may be the result of coincidence. 



Bastian and Zarrow (1955) recorded ac- 

 tivity of hens during light and dark periods 

 and found that practically all activity oc- 

 curred during the light hours. As release of 

 the gonadotrophin for ovulation occurs dur- 

 ing the dark period of a 14 L + 10 D day, 

 the investigators determined the effect of 

 light and enforced activity on ovulation of 

 the Ci follicle. Enforced activity on the 

 evening (9:00 p.m.) of oviposition of the 

 terminal egg of a clutch delayed ovula- 

 tion 26 minutes. A combination of light and 

 enforced activity caused a delay of 2.16 to 

 4.5 hours. Light plus Nembutal anesthesia 

 (to prevent increased activity) did not af- 

 fect the time of ovulation. They concluded 

 that the daily fluctuations in light and ac- 

 tivity of the hen prevented the release of 

 the hormone for ovulation on the day of lay 

 of the last egg of the clutch. 



A brief discussion of the hormonal re- 

 lationships in the induction of ovulation 

 is required before the hypotheses offered to 

 explain the laying cycle of chickens can be 

 considered. 



For the present it will be assumed that 

 LH and the ovulation-inducing hormone are 

 the same, a point not proven but made very 

 acceptable by the observation that as little 

 as 1 /xg. LH from chicken pituitaries can 

 induce ovulation in 50 per cent of expei'i- 

 mental bird^ (Fra'^s, Fevold and Neher. 



