THE EFFECT OF TEMPERATURE ON TESTICULAR RECRUDESCENCE IN JUNCOS AT DIFFERENT PHOTOPERIODS

¹ Department of Zoology, University of North Carolina, Chapel Hill, N. C.

1. The problem of the environmental control of the initiation of seasonal gonad development in birds was studied experimentally by giving attention to the factors of temperature and day-length at levels near those occurring in nature at the time of normal recrudescence.

2. Forty-seven male Juncos, collected at Chapel Hill, N. C., in January and February, were placed, in groups of seven to nine, on a 10-hr. photoperiod in warm cages from mid-February to mid-April. They were then placed for six weeks on photoperiods of either 12 hours or 11 hours, and in one case continued at 10 hours. One of the 12-hr. groups and one of the 11-hr. groups were transferred to a cold-room at 4°-8° C. for this period. Another 12-hr. group was placed in the cold-room for the dark period only, removed during the daily light period. One 12-hr. group, one 11-hr. group and the 10-hr. group were continued in the warm cages, where temperatures ranged from 24° to 29° C.

3. After six weeks, testes of birds in the 12-hr. warm group were about four times as large as testes of birds in either of the 12-hr. cold groups. Testes of birds in the 11-hr. warm group were nearly as large as those in the 12-hr. cold groups and were larger than those in the 11-hr. cold group. Testes of birds in the 11-hr. cold group were significantly larger than those in the 10-hr. warm group.

4. Thirteen of twenty-three birds on a 12-hr. photoperiod (56%) developed spermatozoa; one of seventeen on an 11-hr. photoperiod (6%) and none of seven on a 10-hr. photoperiod (zero %) did so.

5. Four additional males, in warm (20°-29° C.) cages with two females, were exposed to an 11-hr. photoperiod beginning at the winter solstice; after five months only one had developed spermatozoa. Range of testes development in this group was comparable to that in the 11-hr. warm group above.

6. In another experiment fifteen males were placed on an 11-hr. photoperiod beginning at the winter solstice; after six weeks none had reached spermatogenic stage IV. However, in the 10-hr. group above, one reached stage IV, one stage V.

7. From these data, and from consideration of the pertinent literature, it is concluded a) that low temperatures retard the rate of testicular recrudescence in Juncos, higher temperatures accelerating this rate; but b) that day-length is in some way an integral part of the mechanism of recrudescence, being either permissive or prohibitive of sperm development above or below approximately 11 hours, temperature having here only a modifying role; and c) that although the existence of a critical, minimum length of day is not conclusively demonstrated, it is not contra-indicated, at least as regards the critical stages of completion of maturation divisions.