REPRODUCTIVE ENDOCRINOLOGY IN BIRDS 



151 



10 



p.m. 



6 

 a.m. 



10 

 p.m. 



Fig. 18.11. Relationship between diurnal rhythm of neural thre.shold (E) and excitation 

 hormone concentration (C) 7-day cycle. Shaded areas are times when release of ovulation- 

 inducing hormone occurs according to Fraps (1954). (From R. M. Fraps, Froc. Nat. Acad. Sc, 

 40, 348-356, 1954.) 



interval before expected ovulation the Ci 

 follicle is more sensitive than the other fol- 

 licles. Fraps' hypothesis can be explained 

 best by reference to Figure 18.11. According 

 to him, diurnal variations in threshold oc- 

 cur, as represented by the curve Ei , E2 . . . 

 Eg . Ci , C2 . . . Ce , C'l indicate the excita- 

 tion hormone levels. Only when these hor- 

 mone levels reach the threshold values, do 

 ovulations take place. Follicle C'l would not 

 be ovulated as follicle C- of the previous 

 clutch because the threshold value of E7 is 

 too high and cannot be reached. When the 

 threshold values go down, the hormone level 

 reaches its threshold value and C'l ovulates. 

 This hypothesis needs experimental testing 

 to demonstrate the fluctuations in the ex- 

 citation hormone and the diurnal rhythm in 

 thresholds. 



Nalbandov (1959b) proposed a mecha- 

 nism based on a phenomenon discovered by 

 Huston and Nalbandov (1953) . When a loop 

 of thread was placed lengthwise in the ovi- 

 duct of laying hens, ovulation was inter- 

 rupted and the ovary remained fully func- 

 tional, but no follicular maturation occurred. 

 Evidence was obtained that this effect was 

 probably neural in nature. Secondly, not 

 more than one ovulation per hen was ob- 

 tained. However, daily progesterone in- 

 jections induced ovulations in the same hen 

 fit different days. The experiments thus 



showed: (1) the thread inhibited gonado- 

 trophin release, but enough gonadotrophin 

 was secreted to prevent atresia of the fol- 

 licles and regression of the comb (Huston 

 and Nalbandov 1953) ; (2) according to van 

 Tienhoven (1959) and Nalbandov (1959c), 

 the results of this experiment indicate that 

 gonadotrophin is secreted or released as a 

 complex, for when progesterone causes re- 

 lease of hormone for ovulation it also causes 

 maturation of the next largest follicle. 



Further investigations showed that a 

 thread placed in the isthmus prevented ovu- 

 lation for a longer time and in a greater per- 

 centage of birds than did a thread placed in 

 the funnel (infundibulum) or in the upper, 

 middle, or lower magnum (van Tienhoven, 

 1953). Sykes (1953) subsequently showed 

 that a thread in the shell gland caused pre- 

 mature oviposition but no change in ovula- 

 tions. Nalbandov (1959c) proposed that, 

 after ovulation, the egg in the oviduct in- 

 hibits gonadotrophic secretion until the egg 

 passes from isthmus to shell gland. The pi- 

 tuitary then requires time to recover and 

 produce enough gonadotrophin to induce the 

 next ovulation. Clutch length thus would be- 

 come a function of recovery rate of the pi- 

 tuitary. This recovery rate would be af- 

 fected by light only insofar as light causes 

 production of gonadotrophins. The hypothe- 

 sis is attractive because it provides for an 



