REPRODUCTIVE ENDOCRINOLOGY IN BIRDS 



1105 



After thyroidectomy sexual maturity is de- 

 layed temporarily and in some cases in- 

 definitely (Benoit and Aron, 1934b; Benoit, 

 1936, 1937a; Greenwood and Chu, 1939; 

 Payne, 1944; Blivaiss, 1947). The develop- 

 ment of the penis of the Pekin drake (Be- 

 noit, 1937b) and of the comb of the rooster 

 (Greenwood and Chu, 1939; Payne, 1944; 

 Blivaiss, 1947) is inhibited after thyroid- 

 ectomy. The lack of development of these 

 organs, which are under androgen control, 

 may be the result either of a lack of andro- 

 gen secretion by the testes of thyroid- 

 ectomized birds or of a diminished tissue 

 responsiveness to androgen in the absence 

 of thyroid hormone. Evidence that the last 

 hypothesis may explain a large part of 

 the smaller combs in thyroidectomized fowl 

 was found in experiments conducted by 

 Morris (1951). In these experiments with 

 intact and thyroidectomized capons, andro- 

 gen elicited a smaller response in the ab- 

 sence of thyroid hormone than in its pres- 

 ence. 



Administration of thyroid hormone caused 

 l)rccocious sexual maturity of the drake 

 (Jaap, 1933; Aron and Benoit, 1934 » and 

 chicken (Kumaran and Turner, 1949d) 

 with an accompanying increase in comb size. 

 Vaugien (1954) reported that administra- 

 tion of thyroxine to house sparrows during 

 the sexual rest period resulted in recru- 

 descence of the testes and spermatogenesis. 

 Thus all the evidence seems to indicate that 

 severe hypothyroidism interferes with tes- 

 ticular development whereas mild hyper- 

 thyroidism promotes spermatogenesis. The 

 work of Woitkewitch, cited by Hohn ( 1950) , 

 showing that thyroidectomy of starlings in 

 the summer prevents the degeneration of 

 the testes in the fall, seems rather sur- 

 prising. Such work should be confirmed be- 

 fore it is accepted, because it is contrary 

 to observations made in all other species of 

 birds on which data are available. 



The effect of mild hypothyroidism in- 

 duced by feeding thiouracil indicated that, 

 at certain stages of development of the tes- 

 tes, hypothyroidism promotes earlier sper- 

 matogenesis (Kumaran and Turner, 1949d). 

 However, the fact that the experimental 

 groups sometimes contained only two birds 

 and the well known great variability of 



testicular size during the stage of rapid 

 testicular development makes it doubtful 

 that the differences were significant. This 

 great variability in testicular size should 

 always be carefully considered in the design 

 of experiments, because it means that large 

 numbers of birds are required to demon- 

 strate significant differences. An illustration 

 of the relative variability of testicular size 

 and stage of spermatogenesis can be found 

 in the following coefficients of variability 

 obtained in two experiments with 6-week- 

 old White Leghorn roosters kept on experi- 

 ment for 10 weeks. Groups were killed at 

 2-week intervals and body and endocrine 

 organ weights recorded. Weights were ex- 

 pressed as milligrams per 100 grams and 

 stage of spermatogenesis as scores 1 through 

 7. The following average coefficients of 

 variability were found for 2 experiments: 

 testicular weight, 80 per cent; comb weight, 

 38 per cent; thyroid weight, 24 per cent; 

 adrenal weight, 31 per cent; body weight, 

 13 per cent; spermatogenesis, 40 per cent 

 (van Tienhoven, Thomas and Dreesen, 

 1956). These coefficients are in excellent 

 agreement with the figures published by 

 Fox (1956) from experiments in which the 

 testes had not yet undergone their great 

 increase in size (the average testes weight 

 was about 10 mg. in one experiment and 

 2.8 mg. in the other). Fox (1956) found 

 that the coefficient of variability for body 

 weight ranged between 4.8 to 14.1 per cent, 

 for comb index between 14.0 to 40.1 per 

 cent, and for testes weight between 27.6 to 

 160.0 per cent. 



The results obtained by feeding of iodi- 

 nated casein to different breeds of chickens 

 at a variety of ages are somewhat contra- 

 dictory. A review of these results has been 

 published recently (Turner, 1959) and its 

 repetition seems unnecessary. 



5. Progesterone 



Progesterone administration to cockerels 

 I'educes testicular size (Fox, 1955; Herrick 

 and Adams, 1955), inhibits spermatogenesis 

 (Herrick and Adams, 1956), and reduces 

 comb size (Libby, Schaible, Meites and 

 Reineke, 1953; Fox, 1955; Herrick and 

 Adams, 1956). These effects are comparable 

 with the inhibition of egg laying observed 



