EFFECTS OF SEASON AND PHOTOPERIOD ON THE PLASMA AND INTRATESTICULAR TESTOSTERONE LEVELS IN INDIAN WEAVER-BIRDS INHABITING THE SUBTROPICAL ZONE

Previous studies showed marked changes in the plasma gonadotropin conc entration in response to fluctuations in natural and artificial enviro nmental conditions in Indian weaver birds. Therefore, we studied the e ffects of season and photoperiod on the plasma and intratesticular tes tosterone levels in this subtropical bird. Although the plasma testost erone concentration changed markedly during the annual breeding cycle, the level did not show a parallel change with testicular weight. The testicular weight was maximal in the breeding phase (June) and minimal in the nonbreeding phase (December), while the plasma testosterone co ncentration showed two large peaks, one in the breeding and the other in nonbreeding phases. No significant difference in the plasma testost erone level was detected between these two phases. A similar change in plasma testosterone levels was evident in female birds. The intratest icular testosterone content per unit weight was unexpectedly higher in the nonbreeding phase than in the breeding phase. Therefore, calculat ed total testosterone content per testis showed no statistically signi ficant difference between the two phases despite the small testicular mass in the nonbreeding phase. On the other hand, exposure to short-da y (SD) photoperiods for 10 weeks during the breeding phase, which caus ed a marked decrease in the testicular weight, tended to decrease the plasma testosterone concentration. Similarly, exposure to long-day (LD ) photoperiods for 10 weeks during the nonbreeding phase tended to ind uce an increase in the circulating testosterone level in addition to a marked increase in the testicular weight. However, such an effect of artificial photoperiod on testosterone levels was statistically not si gnificant due to a large variance. These results suggest that in India n weaver birds testosterone actively circulates not only in the breedi ng phase but also in the nonbreeding phase. Not only photoperiod but a lso other environmental factors may be involved in the regulation of c irculating testosterone in male birds. (C) 1994 Wiley-Liss, Inc.