REPRODUCTIVE ENDOCRINOLOGY IN BIRDS 



1141 



Brard (1956) kept young Pekin drakes in 

 total darkness and found that testicular 

 size showed definite cycles which were un- 

 related to temperature or changes in light 

 outside the pens ; however, the cycles of the 

 individual birds parallelled each other. Un- 

 fortunately, no data are available on sper- 

 matogenesis under these conditions. 



Recently, Benoit, Assenmacher and Brard 

 (1959) reported that drakes kept under 

 continuous light after the age of 3 weeks 

 showed maximal testicular size later than 

 birds kept under natural daylight. After 

 maximal size was obtained the testes showed 

 regular cycles that were apparently unre- 

 lated to outside conditions and again the 

 cycles of the individual males paralleled 

 each other. 



Vaugien (1951) showed that ovulation 

 and egg laying occur in budgerigars, Melop- 

 sittacus undulatus, kept in darkness, and 

 Vaugien (1953) contended that males 

 reached full spermatogenesis sooner in dark- 

 ness than when kept under light. Marshall 

 and Serventy (1958) criticized this interpre- 

 tation and stated that spermatogenesis had 

 occurred faster under light. They believed 

 that Vaugien (1953) had misinterpreted 

 the histologic data, which, according to 

 Marshall and Serventy, showed that post 

 nuptial degeneration of the tubules had oc- 

 curred. Marshall and Disney (1957) con- 

 firmed, however, that spermatogenesis would 

 occur in total darkness in Zebra finches, 

 Peophila castanotus. These experiments 

 show that gametogenesis does not require 

 light although light may regulate the cycle. 



An example of the "breaking through" of 

 the inherent rhythm in spite of i)hotoperiods 

 may be found in Australian silver gulls, 

 Larus novne-hollandiae, kept in the Wash- 

 ington Zoo. For two seasons the gulls nested 

 in November, then adapted to the northern 

 spring and summer but later reverted back 

 to nesting during the Australian spring 

 (Davis, 1945). In short-tailed shearwaters, 

 Puffimis tenuirostris, the internal rhythm 

 seems to regulate the onset of the breeding 

 season independentlv of the photoperiod 

 (Marshall and Serveiity, 1959). The differ- 

 ent examples show that in some species, 

 e.g., ducks, light is the main regulatory fac- 

 tor in the initiation of gametogenesis, al- 



though cycles can occur in the absence of 

 light. In other species, e.g., short-tailed 

 shearwaters, the inherent rhythm seems to 

 regulate the onset of gametogenesis, and in 

 species in the tropics, e.g., Quelea quelea, 

 light can affect the initiation of gametogene- 

 sis (Marshall and Disney, 1956) but in their 

 natural habitat, rainfall and the availability 

 of long green grass initiate gametogenesis 

 and determine breeding success (Marshall 

 and Serventy, 1957). 



3. Temperature 



Considerable observational data from field 

 studies (Marshall, 1959) indicate that tem- 

 perature may be an important factor in the 

 regulation of the breeding season. No ex- 

 perimental data seem to be available to 

 demonstrate clearly that temperature is the 

 factor per se and is not affecting the breed- 

 ing season by making the required food 

 available, but investigations have been made 

 to determine the effect of temperature on 

 the light-induced gonadal response. Burger 

 (1949) mentions that the testes of starlings 

 kept at 98 to 100°F. w^ere larger than those 

 kept at 60 to 70°F., but that the number of 

 eggs laid by house wrens was lower when 

 the birds were kept at 77°F. than at 67°F. 

 Farner and Wilson (1957) determined the 

 effect of temperature on the rate constant k 

 in Equation 1 and found that 



K,/K„ = 1 + C(T, - Tj 2 



in which C = constant, Ka = I'ate constant 

 at temperature A, Kjj — rate constant at 

 temperature B, Ta =' temperature A, T^ = 

 temperature B. 



The results showed that C = 0.009 for 

 white crowned sparrows; C = 0.02 for j un- 

 cos (data of Jenner and Engels, 1956) ; C = 

 0.02 for starlings (Burger's data). The con- 

 clusion is that temperature affects the light- 

 induced gonadal response but the effect is 

 rather slight. 



Kosin and his co-workers carried out ex- 

 tensive investigations on the effect of tem- 

 perature on the reproduction of turkeys 

 which, although domesticated, have sea- 

 sonal breeding cycles. Their work showed 

 that pretreating the toms with a tempera- 

 ture of 65°F. during the period January to 

 March, when outside temperatures may be 



