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Low levels of mercury in the diet of male white ibises cause the birds to mate with each other rather than with females. As a result many of the females can't breed, and fewer chicks are produced.
It's the first time a pollutant has been found to change an animal's sexual preference. Many chemicals can "feminise" males or reduce fertility, but males affected in these ways still prefer females.
Mercury is extremely toxic, particularly in the form of methylmercury, which reduces breeding in wild birds by disrupting their parenting behaviours. To find out if it also affected mating, Peter Frederick of the University of Florida in Gainesville and Nilmini Jayasena of the University of Peradeniya, Sri Lanka, captured 160 young white ibises from south Florida. They gave them food laced with methylmercury and monitored them closely.
The birds were split into four groups. One group ate food with 0.3 parts per million methylmercury, which most US states would regard as too high for human consumption. A second group got 0.1 ppm, and the third 0.05 ppm, a low dose that wild birds would be exposed to frequently. The fourth group received none.