Seasonal changes of testicular weight, sperm production, serum testosterone, and in vitro testosterone release in Korean ring-necked pheasants (Phasianus colchicus karpowi)

To study the biology of reproduction of male Korean ring-necked pheasants k ept under natural conditions of temperature and photoperiod, testicular wei ght, serum testosterone concentrations, testosterone release from the lutei nizing hormone (LH)-stimulated testis in vitro and sperm production were me asured. Significant changes associated with seasonal cycles were found. Tes tis weight decreased dramatically in August, remained low until February, r apidly increased from March to high levels to June, and decreased subsequen tly. Serum testosterone concentrations remained little from August until Fe bruary, but increased sharply in March to reach the highest levels in April . Thereafter, the concentrations decreased significantly from June. The tes tosterone release was low from August to February, increased abruptly in Ma rch to reach the highest levels in May, and showed rapid decrease thereafte r. Sperm production decreased to nondetectable levels from August to Februa ry, increased markedly in March, reached a peak in May, and sharply decreas ed thereafter. Thus, the pheasants breed from late March to late June. Thes e results indicate that the Korean ring-necked pheasant, under natural cond itions, exhibits characteristics of a seasonal cycle in reproduction.