PARENTAL BEHAVIOR 



1289 



with estradiol pellets developed full vas- 

 cularity within 6 to 11 days, although none 

 of them showed either edema or defeath- 

 erization. Prolactin (luteotrophic hormone, 

 LTH) injected daily into intact or into hy- 

 pophysectomized Inrds had no effect on the 

 \'entral apterium. However, when hyjiophy- 

 sectomized birds wliose ventral apteria had 

 become vascular as a result of treatment 

 with estradiol were treated with prolactin, 

 the apteria became defeathered and edema- 

 tous within 3 to 4 days. When intact birds 

 were treated with estradiol pellets, a normal 

 l)rood patch, vascular and edematous, de- 

 veloped within 9 or 10 days. These data in- 

 dicate not only that the brood patch is 

 formed under the successive influence of 

 estrogen (during egg-laying) and prolactin 

 ( during incubation ) , but that, in these spe- 

 cies at least, estrogenic hormone is capable 

 of stimulating release of prolactin from the 

 pituitary gland. An important point is that 

 the effects of the hormones on the ventral 

 apterium were the same in males and in 

 females, although in these species an incu- 

 bation patch is normally developed only by 

 the female. 



Pituitary hormones and incubation be- 

 havior. (1) The incidence of endogenous 

 prolactin during the reproductive cycle 

 seems to be closely associated with the oc- 

 currence of incubation behavior. Lienhart 

 (1927) found that serum from incubating 

 domestic hens, injected into nonincubating 

 birds, could induce them to sit on eggs. Se- 

 rum from nonincubating females was not 

 capable of inducing incubation in other 

 birds. This was, I think, the earliest demon- 

 stration that the blood of incubating birds 

 contains a factor capable of inducing incu- 

 bation, and that this factor is not present 

 in the blood of nonincubating birds. 



The prolactin content of any tissue or 

 preparation is usually assayed in terms of 

 its effect upon the crop w^all of pigeons or 

 doves. The inactive crop wall of these birds 

 is a thin, almost transparent sheet, consist- 

 ing largely of a thin sheet of muscle with an 

 ^nner epithelium. During incul)ation, the 

 wall of the crop thickens and becomes vas- 

 cular and opaque, largely because of the 

 greatly increased rate of cell division in the 

 epithelium, which becomes many layers 



thick, with growth of blood vessels into the 

 subjacent connective tissue. Toward the end 

 of the incubation period, the superficial 

 layers of the lining epithelium desquamate 

 into the lumen, and the resulting cheesy 

 mass of degenerating epithelial cells forms 

 the food substance which is eventually re- 

 gurgitated to the young (Beams and Meyer, 

 1931). This proliferation of the crop-sac 

 epithelium is induced solely by prolactin, 

 acting locally (Riddle, 1937; Riddle and 

 Bates, 1939). The standard (Riddle's) 

 method of assaying prolactin, which is used 

 in defining the international unit, depends 

 on the increase in the crop-weight of birds 

 of a standard strain, when the material to 

 be assayed is injected over a period of days 

 (weight method). Other methods depend on 

 the fact that, when a small amount of pro- 

 lactin is injected intradermally over the 

 crop, a small area of vascularization de- 

 velops, which can be seen with the naked 

 eye (local response methods) (Reece and 

 Turner, 1936; Lyons, 1937; Riddle and 

 Bates, 1939L 



Burrows and Byerly (1936) removed the 

 pituitary glands from domestic fowl and as- 

 sayed them for prolactin content by Lyons' 

 (local response) method. Considering the 

 amount of prolactin in the pituitary glands 

 of roosters as a unit-standard (1.00), they 

 found that the pituitaries of laying hens 

 contained on the average 1.46 units, and 

 those of incubating hens 4.05 units. Saeki 

 and Tanabe (1955) found that the pituitary 

 glands of laying hens contained, on the 

 average, 0.071 LU. of prolactin, whereas 

 those of incubating hens contained 0.196 

 LU. (Saeki and Tanabe, 1954). Nakajo 

 and Tanaka (1956) found that the pro- 

 lactin content of the caudal lobe of the 

 anterior pituitary gland was 0.8 Reece- 

 Turner units in nonincubating domestic 

 hens, and that it rose in incubating birds 

 to 1.7 to 2.4 Reece-Turner units. They also 

 found that wdien incubation was inter- 

 rupted by continuous lighting of the cages, 

 the level of prolactin in the pituitary 

 gland fell. Breitenbach and Meyer (1959) 

 obtained similar results in the ring-necked 

 ]iheasant: the prolactin content of the pitui- 

 tary gland rose sharply while the birds in- 

 cubated eggs. Byerly and Burrows (1936) 



