1296 



HORMONAL REGULATION OF BEHAVIOR 



incubation. We have pointed out that incu- 

 bation behavior may be maintained long 

 past its normal period by the presence of the 

 eggs. What is the physiologic basis of this 

 effect? 



Saeki and Tanabe (1954, 1955) measured 

 the prolactin content of the pituitary glands 

 of domestic hens during different stages of 

 the reproductive cycle under various ex- 

 perimental treatments. They found that the 

 prolactin content of the gland, which is 

 normally much higher during incubation 

 than during laying (see above, page 1289), 

 goes down sharply immediately after the 

 hatching of the eggs. When birds were in- 

 duced to sit for abnormally long times by 

 substituting sterile eggs for their own eggs, 

 the prolactin content of the pituitary gland 

 remained high as long as the birds continued 

 incubating. Further, they induced a number 

 of laying hens to sit upon eggs by means of 

 prolactin injection; some of these birds 

 stopped sitting when the prolactin injection 

 was discontinued, others continued to sit on 

 the eggs. Autopsy data revealed that the 

 pituitary glands of the birds which con- 

 tinued incubation after the end of the pro- 

 lactin administration contained a high level 

 of prolactin, whereas the pituitary glands of 

 the birds which stopped incubating after the 

 end of the prolactin injection had little pro- 

 lactin. It is reasonably clear from these 

 data that the eggs are capable of stimulat- 

 ing prolactin secretion by the pituitary 

 gland of the sitting bird, and that this is 

 probably part of the explanation for the 

 control of incubation behavior by the pres- 

 ence or absence of the eggs. 



In the domestic pigeon Patel (1936) 

 showed that the crops of incubating birds 

 increase in weight as long as the birds are 

 sitting, but regress to the resting condition 

 when the birds are removed from the eggs. 

 The crops of ring doves induced to sit upon 

 eggs by progesterone treatment will grow 

 only if the birds are allowed to continue 

 sitting upon the eggs (Riddle and Lahr, 

 1944; unpublished observations by D. S. 

 Lehrman). In these birds, as in domestic 

 hens, participation in incubation clearly 

 stimulates the secretion of prolactin by the 

 pituitary gland. 



In the case of pigeons and doves, prolactin 



is secreted under the influence of stimuli 

 associated with incubation, even though the 

 bird is not actually sitting on the egg. Patel 

 (1936; Kuroda, 1956) found that the crop 

 of a male pigeon removed from the breeding 

 cage early in incubation, and placed in an 

 adjacent cage from which he could see his 

 mate, would develop as though he were sit- 

 ting upon the egg himself. If a partition was 

 placed between the two adjacent cages so 

 that the male could not see the female sit- 

 ting on the eggs, his crop would regress to 

 the resting state. When the females were re- 

 moved to the adjacent cages, the results 

 were similar, except that some of the males 

 left in the breeding cages abandoned the 

 eggs. In those cases, the crops of the females 

 failed to develop. 



Effect of removal of eggs in midincuba- 

 tion. In many species of birds, even in those 

 species which normally produce only one 

 clutch of eggs per year, removal of the eggs 

 during incubation is followed by the build- 

 ing of a new nest and the laying of a new set 

 of eggs (Salomonsen, 1939; Blanchard, 

 1941; Simmons, 1954; Brackbill, 1958; 

 Grosskopf, 1958; and many others). This 

 clearly suggests that the removal of the eggs 

 permits the secretion of gonadotrophic hor- 

 mones which had been held under inhibition 

 by stimuli coming from the eggs. Although 

 the mechanism of this probable inhibition 

 is not known, we may recall here the anti- 

 gonad action of prolactin. The behavior of 

 the male may be a factor in this renewal of 

 gonadotrophic activity. Miller (1931j re- 

 ports that the male, which is very quiet dur- 

 ing the period when the female is sitting on 

 eggs, shows a burst of renewed singing and 

 courting activity when the nest and eggs are 

 destroyed, possibly as a response to the re- 

 newed activity of the female released from 

 the nest. Thus, in addition to the removal 

 of any inhibiting effect of the eggs on the 

 secretion of gonadotrophins by the pituitary 

 gland of the female, there is a resurgence of 

 the singing and courting activity of the 

 male, which was partially responsible for 

 the original secretion of gonadotrophin (see 

 above, page 1279). 



The time taken for laying new eggs after 

 the destruction or removal of the old clutch 

 varies. Most observers assert that the in- 



