Scientists have long suspected high mercury levels in the Everglades might be crimping the breeding efforts of wading birds.
A new study of white ibises by University of Florida researchers suggests the pollutant could have a far more profound impact than imagined: It turned a good chunk of a captive flock gay.
The study, published online Wednesday in a biological journal, documented a number of changes in the mating behaviors and reproductive success of four groups of ibises fed varying levels of mercury over a three-year period.
By far the most surprising effect was on the courtship inclinations of male ibises. In the first year, 55 percent of the males given the highest doses of mercury in their feed hooked up with other males during breeding season.
``They pretty much did everything except lay eggs,'' said Peter Frederick, a UF wildlife ecologist who led the study. ``They built nests, they copulated, they sat in the nests together.''
While the study raises concerns about mercury impacts on wildlife, Frederick flatly dismissed the idea of extrapolating the result to humans, saying that would be a serious misreading.
``Honestly, there is zero relevance for humans,'' he said.
Until a boom in breeding over the last decade, populations of wading birds had sharply declined in the Everglades. Development and declining water levels were primarily to blame, but there was another potential, poorly understood contributor -- mercury, which filtered into the Everglades over the decades, largely from the stacks of medical and municipal waste incinerators.
Though improved management of water levels has played the biggest role in rekindling avian ardor, Frederick and study co-author Nilmini Jayasena wanted to examine mercury's role as well.
Previous studies of mercury and other ``endocrine disrupters'' that affect the flow of hormones had shown a range of reproductive impacts on ducks and other birds, Frederick said. There were a range of impacts in the ibis study, published in the Proceedings of the Royal Society B, with females in the high exposure group producing 35 percent fewer chicks than the mercury-free group. But the starkest change was in sexual orientation.
Homosexual pairing occurred at even the lowest level of exposure, but the higher the mercury, the higher the rate. Some males exhibited bi-sexual interests, changing partners when male-on-male efforts proved fruitless. That's a common response in the wild after failed efforts, Frederick said. Females largely shunned high-mercury males, which didn't display much of the head bobs and bows of normal mating ritual.
Frederick said it's not clear exactly what mercury does to the birds. It may mix up nerve signals or crimp the flow of testosterone, giving males what he called a ``feminized hormonal profile.''