1102 



SUBMAMMALIAN VERTEBRATES 



and steatogenesis of the tubules (Riddle and 

 Bates, 1939; Breneman, 1942; Nalbandov, 

 1945; Lofts and Marshall, 1956). It also 

 prevents the response of the testes to light 

 in the white-crowned sparrow, Zonotrichia 

 leucophrys pugetensis (Bailey, 1950). Pro- 

 lactin does not affect testicular activity in 

 mammals (Riddle and Bates, 1939; Lofts 

 and Marshall, 1956) . The action of prolactin 

 on avian testes is probably caused by an 

 inhibition of pituitary gonadotrophin se- 

 cretion because small doses of FSH, given 

 simultaneously with prolactin, prevent the 

 decrease in testicular activity (Bates, Rid- 

 dle and Lahr, 1937; Nalbandov, 1945). 

 Breneman (1942) also observed that pro- 

 lactin causes a decrease in the number of 

 basophil cells which are implicated in the 

 secretion of the gonadotrophic hormone. 



Prolactin also is apparently required for 

 the formation of the incubation patch, be- 

 cause in hypophysectomized passerines the 

 incubation patch can only be formed after 

 combined estrogen-prolactin treatment 

 (Bailey, 1952). Further, the prolactin con- 

 tent of the California gull pituitary was cor- 

 related with the presence of the incubation 

 patch. The occurrence of the incubation 

 patch in both sexes is well correlated with 

 the incubation behavior of the two sexes. If 

 it is present in a species, it occurs in the sex 

 or sexes which incubate (Bailey, 1952). 



Prolactin injection causes molting in the 

 fowl. This is probably the result of a direct 

 effect on the feather follicle, because the 

 hormone is equally as effective in roosters 

 as in capons ( Juhn and Harris, 1958) . 



Finally, prolactin is probably concerned 

 with incubation behavior in the fowl (Saeki 

 and Tanabe, 1955), although it seems that 

 progesterone regulates incubation behavior 

 in the ringdove, Streptopelia risoria (Lehr- 

 man, 1958, and his chapter in this book). 

 The fact that hypophysectomy of the pigeon 

 may lead to progesterone secretion by the 

 testes (Lofts and Marshall, 1959) makes 

 such an animal an excellent tool for estab- 

 blishing whether or not progesterone acts 

 directly or by way of prolactin secretion in 

 the pigeon. 



The following hypothetical series of 

 events may take place in the regulation of 

 the male's breeding cycle of birds in which 



both sexes incubate the eggs. In the spring 

 under the influence of increasing daylight 

 and provided that other ecologic factors are 

 favorable, the testes of the male reach full 

 activity before the ovaries of the females are 

 fully developed (Benoit, 1956). The court- 

 ship activities and vocalizations may stimu- 

 late the females' ovaries by way of the 

 nervous-pituitary system so that the fol- 

 licles mature and ovulation and oviposition 

 can occur. At the time when about half of 

 the clutch has been laid, estrogen secretion 

 is at its peak and the first signs of vascu- 

 larization of the incubation patch become 

 visible (Bailey, 1952). Visual stimuli from 

 his incubating mate may cause secretion of 

 prolactin by the pituitary of the male, in 

 pigeons at least (Patel, 1936). This release 

 of prolactin could serve a triple purpose of 

 inducing the formation of the incubation 

 patch (Bailey, 1952), precipitating incuba- 

 tion behavior, and causing the functional 

 collapse of the testes. This collapse through 

 withdrawal of androgen might then end sex- 

 ual activities which would divert the male 

 from incubating. 



Evidence exists that the mechanisms pro- 

 posed above operate in birds as a group, but, 

 as far as the author is aware, this sequence 

 of events has not been proven for any single 

 species. One problem in need of investiga- 

 tion in the male is the source of the estrogen 

 required for the formation of the incubation 

 patch. This sequence of events would also 

 account for the progesterone secretion by 

 the testes of the pigeon. (The assumption is 

 made that the changes in the testes are the 

 same after prolactin secretion as after hy- 

 pophysectomy. Such an assumption seems 

 to be warranted on the basis of the similari- 

 ties in the histology of the testes after these 

 treatments.) The progesterone, in turn, 

 would cause incubation behavior. More re- 

 search is required to determine which secre- 

 tion starts first in the pigeon, prolactin or 

 progesterone. Seeing his mate incubate and 

 finish incubation may release prolactin and 

 so explain the eclipse molt of the mallard 

 drake, which occurs about 3 weeks sooner in 

 mated than in unmated drakes (Hochbaum, 

 1944). 



In species in which the male is polygamous 

 and not concerned with incubation and care 



