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

Abstract

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.

Change in Body Mass Can Overrule the Effects of Maternal Testosterone on Primary Offspring Sex Ratio of First Eggs in Homing Pigeons

“…The next day females were measured (tarsus 0.1 mm). and oviducts and ovaries were removed to measure follicles for a concurrent study on ovarian dynamics (Goerlich et al 2010). We retrieved tbe implants except from two testosterone-treated females.…”

Gonadal steroid levels in rock pigeon eggs do not represent adequately maternal allocation

“…However, the yolk formation and hormone addition to the yolk is finished by the time of ovulation, as no more yolk can be added to the egg after ovulation. Intriguingly, some studies show that the concentrations of androgens decline significantly already between the time of ovulation and oviposition 5 , 6 and one study showed an increase in progesterone between pre- and post-ovulatory follicles 7 . However, the cause and consequence of this different pattern of change among different classes of steroids are unknown.…”

Maternal adjustment or constraint: differential effects of food availability on maternal deposition of macro‐nutrients, steroids and thyroid hormones in rock pigeon eggs

“…In homing pigeons, as well as in our rock pigeon colony, the second egg is laid on average 44 h after the first egg (Levi ; Goerlich et al. ), and the two chicks usually have 24–36 h difference in hatching time (Johnston and Janiga ; B.‐Y. Hsu, pers.…”

“…Rock Pigeons typically lay two eggs as a clutch and the second egg contains much higher testosterone levels than the first egg when fed ad libitum (Goerlich et al 2009). In homing pigeons, as well as in our rock pigeon colony, the second egg is laid on average 44 h after the first egg (Levi 1998;Goerlich et al 2010), and the two chicks usually have 24-36 h difference in hatching time (Johnston and Janiga 1995;B.-Y. Hsu, pers.…”