REPRODUCTIVE ENDOCRINOLOGY IN BIRDS 



1129 



cubation in species which have an incubation 

 patch, whereas in ring doves {Streptopelia 

 risoria), which do not have an incubation 

 patch, incubation is not correlated with an 

 increase in prolactin secretion (Lehrman, 

 1958, and his chapter in this book) . 



Gonadotrophin assays of pituitaries from 

 the time of hatching into adulthood when 

 reproductive activity is cyclic seem to have 

 been made only on chickens. The results ob- 

 tained by two groups of workers are sum- 

 marized in Table 18.7. Their results are 

 expressed in the common standard of chick 

 units as defined by Breneman (1955). The 

 results obtained by Riley and Fraps 

 (1942a, b) are in essential agreement with 

 those in the table with respect to the ratio 

 of gonadotrophic potency of pituitaries from 

 roosters and laying and nonlaying hens. 

 They were not included because of the diffi- 

 culty in converting mouse uterine units into 

 chick units. 



As a part of the survey of the data con- 

 tained in Table 18.7, it should be noted that 

 the amount of gonadotrophin in the pitui- 

 taries of young birds is closely correlated 

 with ovarian weight (r — 0.898). This 

 correlation provides an argument for the 

 concept that the secretion of gonadotrophic 

 hormone can be estimated from assays of 

 the pituitary, in immature pullets as well as 

 in adult hens. Just before ovulation begins 

 in young hens pituitary gonadotrophin po- 

 tency (and secretion?) reaches its peak. It 

 is at a much lower level in older laying hens. 

 This decreased gonadotrophic potency (and 

 decreased gonadotrophin secretion?) is 

 probably due to estrogens which are se- 

 creted in large amounts by the ovary. The 

 lower gonadotrophin secretion would be suf- 

 ficient to maintain follicles already present 

 and to stimulate new ones to grow to ovula- 

 tory size. This concept that less gonado- 

 trophin is required for maintenance and 

 stimulation of follicles already present than 

 is required for stimulation of an immature 

 or an inactive ovary finds support in the 

 following experimental evidence: 



1. Vaugien (1957) stated on the basis of a 

 rather small number of experiments that in 

 song birds the ovary is more sensitive to 

 exogenous gonadotrophin wdien one or more 

 medium-sized follicles are present than 



TABLE 18.7 



Gonadotrophic potency of pullet 

 pituitaries in the domestic 



and hen 

 fowl 



* Chick unit is eciuivalent to 35 per cent in- 

 crease over control assay. 



t Calculated (AvT) from data of Saeki et al., 

 1956. 



when the ovary contains only small folli- 

 cles. 



2. In order to obtain follicles of about 

 17-mm. diameter Taber, Clay tor. Knight, 

 Gambrell, Flowers and Ayers (1958) had 

 to inject 18 to 20 broiler pituitaries (equiva- 

 lent to 270 to 300 I.U.) per day, whereas 

 when laying hens were injected with 80 to 

 160 rat units of PMS enormous stimulation 

 of the follicles was obtained in a few days 

 (Fraps, 1955b). to be sure, the ratios be- 

 tween these levels of exogenous gonado- 



