Effects of in vivo testosterone manipulation on ovarian morphology, follicular development, and follicle yolk testosterone in the homing pigeon.

To date, our understanding of the function of testosterone in female reproductive physiology is only marginal although there are indications that testosterone is involved in modulating follicular recruitment, growth, atresia, and ovulation. Studies elevating testosterone in breeding female birds have, in most instances, found detrimental effects, such as delayed clutch initiation or decreased clutch size. In our previous study, testosterone treatment of female homing pigeons delayed clutch initiation without diminishing fecundity. In this study, we explore whether the observed effect might have been caused by testosterone influencing follicle maturation or ovulation. We implanted mature female pigeons with testosterone prior to pairing, which resulted in constant elevation of circulating testosterone concentrations within the physiological range. We killed females after they had laid the first egg and measured ovarian and follicular development. Ovarian mass and pre-hierarchical yolky follicles were not affected by the treatment; however, testosterone females produced smaller and lighter preovulatory follicles. High plasma testosterone levels at oviposition or a strong temporal increase in testosterone were negatively related to mass and diameter of second follicles. We proposed that sustained elevation of testosterone delays follicular maturation, potentially via negative feedback on the hypothalamo-pituitary-gonadal axis. Furthermore, to gain better insight into the regulation of yolk hormone acquisition, we measured testosterone concentrations in the preovulatory follicles. We found no differences between treatment groups but follicle yolk contained much higher levels of testosterone than yolk of un-incubated eggs, suggesting that hormone measurements performed after oviposition do not correctly reflect maternal allocation.