K. Tsutsui et al., EFFECTS OF SEASON AND PHOTOPERIOD ON THE PLASMA AND INTRATESTICULAR TESTOSTERONE LEVELS IN INDIAN WEAVER-BIRDS INHABITING THE SUBTROPICAL ZONE, The Journal of experimental zoology, 270(4), 1994, pp. 381-387
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.