Treatment of zebra finch chicks (Taeniopygia guttata) with the female hormone estrogen masculinizes female brains. As adults, such females are capable of song, whereas normally female finch cannot sing. Oral exposure of chicks to estrogen also impairs later reproductive performance by increasing the incidence of eggshell breakage and, in males, by decreasing fertility. Vulnerability to early exposure to estrogen may be related to their relatively immature state of development at hatch/birth (altriciality). Manmade substances in the environment (xenoestrogens) that chemically resemble animal estrogens and substances produced naturally by plants (phytoestrogens), collectively termed "environmental estrogens," may similarly alter normal development of songbirds. The objective of this project is to characterize the consequences of early environmental estrogen exposure on adult reproductive performance. The specific aims are: a) to determine how drought increases the estrogenic potency of seeds consumed by granivorous songbirds; b) to determine the potency of xeno- and phytoestrogens to induce oviduct growth in female chicks and to induce biosynthesis of an estrogen-sensitive protein, vitellogenin, in male and female chicks (both are estrogen bioassays); and c) to determine dose-response relationships between phytoestrogen and xenoestrogen exposure and an array of reproductive performance measures known or postulated to be altered by early estrogen exposure (e.g., number and fertility of eggs, eggshell structure, alteration of male copulatory behavior). The results of this research may reveal novel mechanisms of plant-animal interactions, wherein drought-stressed plants may increase phytoestrogen production to combat predation by avian predators. The results may also have regulatory relevance, with respect to the development of endocrine disruptor screening programs to evaluate the estrogenicity of thousands of drugs and chemicals. This research could dramatize the need to include an ecologically- and phylogenetically-justified altricial avian species in endocrine disruptor screening programs.
