White ibises have experienced a huge drop in their reproductive rate, and mercury contamination was a prime suspect. But nobody expected pollution to alter the birds' sexual preference, as the male ibises were actually nesting with each other.
This is the first time researchers have observed birds altering their courtship behaviors so drastically due to pollution. As University of Florida ecologist Peter Frederick explains, that doesn't mean it's entirely unexpected:
"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. This suggests that wildlife may be commonly affected."
So what's actually going on? Low to high levels of mercury have contanimated the wildlife of the Florida Everglades since the 1980s, which affected the animals' willingness to engage in courtship behaviors and ultimately give birth to young. The mercury levels were brought under control in the late 1990s, and the ibises regained their interest in nesting, showing levels of enthusiasm seldom before seen. But the actual reproductive rates hadn't risen quite as much, and University of Florida researchers suspected something else was going on.
To test this idea, they took 160 young ibises of both genders into a net-covered aviary and divided them into four groups. Three of the four groups had either low, moderate, or high amounts of mercury in their diets, while the control group enjoyed a mercury-free diet. However, even the high mercury group wasn't consuming more mercury than they would have in nature.
The results were striking. 55% of the high mercury males were nesting together, and the frequency of homosexual pairings increased with the amount of mercury in the diet. This, Frederick says, was totally unexpected:
"Many years ago, I spent thousands of hours in the field documenting pairing behavior in ibises in a place that did not have mercury contamination – and I never once saw males pairing with males."
High mercury females were unlikely to approach males during courtship, in part because the high mercury males had largely lost interest in the ritual head bobs and bows used to signal interest in courtship among ibises. The high mercury females produced 35% fewer fledglings than their peers in the control group.
We really shouldn't have to say this, but it should be stressed that this study has nothing to do with human homosexuality, a point Frederick himself stresses. Mercury contamination in humans has never shown even the slightly effect on sexual preference, and besides the drives of human sexuality are far more complex than of birds and highly unlikely to be affected by a single contaminant. Anyway, the sexual behavior and reproductive physiology of birds is very, very different from that of humans.
What's really interesting about this study is that even low levels of mercury pollution can scramble hormones so severely and wreak so much havoc on the reproductive capabilities of wildlife. Renowned zoologist Lou Guillette praises the work done by Frederick and his team:
"One of the great frustrations the scientific community has had trying to understand environmental contamination are lab studies that do acute, high-dose exposure studies and almost never predict what we see in the wild. So a study like this that looks at environmentally appropriate levels of mercury is probably the most powerful kind of study to tell us what's going on in the real world."