Effects of corticosterone on the proportion of breeding females, reproductive output and yolk precursor levels.

In this study we investigated the role of corticosterone (B) in regulating the proportion of laying females, timing of breeding, reproductive output (egg size and number), and yolk precursor levels in chronically B-treated female zebra finches (Taeniopygia guttata). Corticosterone treatment via silastic implant elevated plasma B to high physiological (stress-induced) levels (24.1 +/- 5.3 ng/ml at 7-days post-implantation). B-treated females had high plasma levels of very-low density lipoprotein (VLDL) but low levels of plasma vitellogenin 7-days post-implantation, suggesting that corticosterone inhibited yolk precusor production and perhaps shifted lipid metabolism away from production of yolk VLDL and towards production of generic (non-yolk) VLDL. Only 56% of B-treated females (n = 32) initiated laying, compared with 100% of sham-implanted females (n = 18). In females that did breed, corticosterone administration delayed the onset of egg laying: B-treated females initiated laying on average 14.5 +/- 0.5 days after pairing compared to 6.4 +/- 0.5 days in sham-implanted females. B-treated females that laid eggs had significantly higher plasma B levels at the 1st-egg stage (45.9+/-9.0 ng/ml) than did sham-implanted females (7.9+/-6.8 ng/ml). Despite this there was no difference in mean egg mass, clutch size, or egg composition in B-treated and sham-implanted females. These results are consistent with the idea that elevated corticosterone levels inhibit reproduction, but contrast with studies of other oviparous vertebrates (e.g., lizards) in relation to the role of corticosterone in regulating egg and clutch size.