Over the last 10 years, reports of feminized wildlife have fueled chilling headlines. Most of these reports have focused on the many ways that estrogen in sewage effluent can distort normal male development. Now a new study reveals one way that the hormone pollutant can affect females: Too much estrogen causes subtle changes in female fish's courting behavior, which could alter a population's genetic makeup (Environ. Sci. Technol., DOI: 10.1021/es101185b).
Natural and synthetic estrogens find their ways into fish habitats by way of birth control pills or hormone replacement therapy. After women's bodies expel the drugs, the powerful hormones get flushed into sewage systems, where they eventually discharge with treatment plants' effluent.
When this wastewater flows into streams and ponds, the hormone can cause trouble for fish, says ecotoxicologist Charles Tyler of the University of Exeter in the U.K. Previous studies have examined the most-apparent impacts of estrogen on males, such as feminization of testes or reduced number of offspring. But few studies have looked at how the hormone may disrupt female fish reproduction, especially how a short exposure while an animal develops can affect their behavior in adulthood.
So Tyler and colleagues started by briefly exposing zebra fish at early life stages to ethinylestradiol, a synthetic estrogen found in birth control pills, at levels measured in the environment. The researchers then watched for telltale signs of endocrine disruption, such as changes in gonad morphology and levels of reproductive hormones. They found none of these typical changes in either sex. Also adult male fish that had received the brief estrogen dose had the same reproductive success as unexposed males.
But when the female fish reached adulthood, the researchers found that estrogen-exposed females abandoned their normal courting behaviors such as swimming alongside or chasing a suitor, while unexposed fish acted normally. "If a female doesn't give the right signal to a courting male, he won't respond," Tyler explains.
Because dominant males in the breeding colonies took the hint and swam away, estrogen-exposed female fish had fewer offspring with top males when compared to unexposed females. Instead the affected fish bred more with less vital males. Over the long run, genes from these less-dominant males could become more common in an estrogen-exposed population, which could alter the population's ability to adapt to changing environmental conditions, Tyler says.
The study "enlarges the scope of effects that we need to be aware of when studying endocrine disruption," says David Marcogliese, a research scientist at Environment Canada, the nation's environmental agency. Reproductive biologist Glenn VanDerKraak of the University of Guelph in Canada agrees: "People should be looking at subtle effects of estrogen on behavior, and they should be studying effects on females."
Because the results show that brief estrogen exposures early in life can have long-term population effects, Tyler thinks that policymakers should consider regulating estrogen release by sewage plants: "There is now a weight of evidence about environmental estrogens and wide enough concern to think about regulating releases."