1278 



HORMONAL REGULATION OF BEHAVIOR 



laying hens, and cocks (Fraps, Hooker and 

 Forbes, 1949), but not in that of capons. In 

 addition, progesterone induces the formation 

 of secretion products by the albumen-secret- 

 ing glands in the oviduct, the growth of 

 which has been accomplished under the 

 previous influence of estrogen (Brant and 

 Nalbandov, 1956j. It thus seems probable 

 that there is a short episode of progestin 

 secretion by the ovary, just preceding ovu- 

 lation, and that this follows a period of 

 estrogen secretion. (See chapter by van 

 Tienhoven.) 



The actual laying of the egg appears to 

 involve some posterior pituitary activity. 

 Injection of posterior pituitary preparation 

 induces the laying, within 2 to 25 minutes, 

 of eggs already ovulated, but which would 

 not normally have been laid for up to 20 

 hours (Burrows and Byerly, 1940, 1942). 

 When the neurohypophysis is removed no 

 oviposition takes place until after there has 

 been time for the regeneration of the nerve 

 connection between the hypothalamus and 

 the pituitary gland (Shirley and Nalban- 

 dov, 1956a, b). Rothchild and Fraps (1944) 

 removed the ruptured follicles and all the 

 rapidly growing preovulatory follicles in 

 hens, so that an ovulated egg was in the 

 oviduct, but no more eggs could be ovu- 

 lated. They then placed some of these hens 

 in normally lighted rooms, others in rooms 

 on reversed light cycles. The majority of 

 eggs in both groups were laid during the 

 daylight hours. They therefore concluded 

 that a light-sensitive, nonovarian process 

 was involved in the laying process, in addi- 

 tion to those factors controlling ovulation 

 itself. (See chapter by van Tienhoven.) 



3. Stimulation of Ovulation 



Our interest in the nature of the condi- 

 tions stimulating ovulation derives from the 

 fact already pointed out, that nest-building 

 activity is in part based on physiologic con- 

 ditions induced by hormones coming from 

 the developing egg follicle; and from the 

 further fact that the physiologic events 

 associated with ovulation somehow set the 

 stage for the occurrence of incubation be- 

 havior, which normally follows egg-laying. 



Neural stimulation of ovulation. It has 

 become abundantly clear in recent vears 



that the activity of the pituitary gland is 

 controlled and influenced in considerable 

 detail by the hypothalamus (Harris, 1955; 

 see chapters by Greep, Everett, and van 

 Tienhoven). The physiologic and anatomic 

 details of the relationship between the hy- 

 pothalamus and the pituitary gland are ade- 

 cjuately discussed in these other chapters, 

 and do not concern us here. We may, how- 

 ever, describe a striking example of the 

 evidence for neural control of pituitary ac- 

 tivity. Huston and Nalbandov (1953) sewed 

 a loop of thread into the magnum of the 

 oviducts of a group of domestic hens, and 

 tied it into place, so that it provided a con- 

 stant mechanical stimulation of the oviduct 

 wall. Domestic hens normally ovulate 30 

 to 60 minutes after the laying of the previ- 

 ous egg. During the 25 days following the 

 operation, however, 58 to 75 per cent of the 

 operated birds laid no eggs. Among the op- 

 erated birds which did lay eggs, the mean 

 number of eggs laid was 1.5 per bird; in 

 sham-operated birds wdth no loop sewed into 

 the oviducts, the number of eggs laid was 

 5.5 ])er bird. LH or progesterone injection 

 could induce ovulation at any time in those 

 birds which were not laying because of the 

 ]:)resence of the thread. The ova of the ex- 

 perimental birds did not degenerate, and 

 their oviducts and combs remained normal. 

 These data suggest that the mechanical 

 stimulation of the oviduct wall inhibits LH 

 secretion by the pituitary gland, without 

 substantially interfering with the secretion 

 of FSH. Huston and Nalbandov suggest 

 that the jiresence of an egg in the oviduct 

 acts in this way to prevent the ovulation of 

 the succeeding egg until after the previous 

 egg has been laid. 



External stimuli and ovulation. Since the 

 secretion of gonadotrophic hormones can be 

 influenced and controlled by the hypothala- 

 mus, and by stimuli arising in the body 

 outside of the central nervous system, it is 

 reasonable to expect that external stimuli 

 representing various environmental situa- 

 tions and events may have an influence, 

 through this neurohypophyseal link, on the 

 activity of the ovary. 



(a) Light has long been known to influ- 

 ence gonadal activity. In seasonally repro- 

 ducing birds, the increasing length of the 



