Josh Wickham / Univ. of Florida / IFAS file
Researchers found that a high-mercury diet had an effect on the mating behavior of white ibises confined in a net-covered aviary at the University of Florida. They said the degree of homosexual pairing increased along with the birds' mercury exposure.
A years-long study at the University of Florida suggests that mercury pollution can alter the hormones of white ibises to make males more likely to mate with other males.
"We knew that mercury can disrupt hormones -- what is most disturbing about this study is the low levels of mercury at which we saw effects on hormones and mating behavior," Peter Frederick, a wildlife ecology and conservation professor who led the study, said in a news release this week. "This suggests that wildlife may be commonly affected."
The study was published online on Wednesday in the Proceedings of the Royal Society B.
Frederick and his co-author, Nilmini Jayasena, were hasty to warn against drawing any inferences about the roots of human homosexuality. They didn't set up the experiment to find out what makes birds gay -- rather, they were trying to figure out why ibises in the Everglades went through a stretch of poor breeding in the early 1990s, followed by a baby boom in the late 1990s.
Scientists knew that improvements in the birds' watery habitat was one factor behind the increased breeding, but they suspected that mercury concentrations played a role as well. During the downswing in breeding, low-level mercury contamination made its way increasingly into the Everglades via municipal and medical waste incineration -- but that waste became more regulated at about the same time as the start of the baby boom.
To find out if there was a connection between mercury contamination and a low birth rate, the university set up a 13,000-square foot net-covered aviary in 2005. They brought in 160 ibises, and divided the birds into four groups with equal numbers of males and females. Each group was fed a different diet -- low, medium or high mercury, or no mercury at all. The highest level of mercury was no higher than what the birds would have consumed if they had been in the wild during the early 1990s.
In 2006, about 55 percent of the high-mercury-diet males were nesting with other males. Frederick said the degree of male-male pairing was proportional to the degree of mercury in the diet. That played a role in breeding differences, as well: In comparison with the control group, high-mercury males were less likely to be approached by females during courtship. All of the mercury-consuming males were less prone to perform the ritual head bows and bobs that are part of the ibises' mating ritual.
Frederick and Jayasena, who was Frederick's doctoral student and is now based at the University of Peradeniya in Sri Lanka, reported that the high-mercury-diet females produced 35 percent fewer fledglings than the females in the control group.
The study lasted for three breeding seasons. After the experiment, the birds were put on a cleansing, mercury-free diet for several months and then released back into the wild.
Does all this mean homosexuality is linked to mercury pollution? For the ibises, maybe. For humans, almost certainly not. Frederick pointed out that there have been a number of long-term studies on the effects of mercury on humans, and none of those studies has noted a change in sexual behavior. Speaking more generally, the researchers noted that sexual preference is a much more complex phenomenon for humans than it is for birds.
In a report on Nature's website, a German expert on animal physiology cautioned that the results might not even be applicable to other bird species. Heinz Köhler of the University of Tübingen told Nature that this might be something that's just between ibises. "Their behavior may be less fragile and more robust to methylmercury," he said.
More perspectives on the research:
- Nature: Mercury causes homosexuality in male ibises
- BBC: Mercury 'turns' wetland birds homosexual
- New Scientist: Mercury poisoning makes male birds homosexual
- Miami Herald: Mercury makes ibises gay, study says