Bisphenol A (BPA) affects animals' behaviour and development because it mimics the hormone oestrogen. Such "synthetic hormones" can affect the males of many species, sometimes "feminising" them and impairing their ability to produce offspring.

But according to Jessica Ward of the University of Minnesota in Saint Paul, the effects go much further than that. She exposed captive blacktail shiners (Cyprinella venusta and red shiners (C. lutrensis) to BPA for 14 days, then compared their courtship behaviour to that of fish that had not encountered the chemical.

Fish exposed to BPA were more likely than the unexposed fish to approach fish of the other species and court them.

Although the study did not include genetic tests to see if the courtship led to interbreeding, Ward says the courtship behaviour is a "good proxy for mate choice in fish". "It would have been harder to tease out some of the more subtle behavioural changes if we had just put three fish in a tank and run genetic parentage tests on the resulting eggs," she says.

Red shiners are an invasive species, often introduced to new areas by fishermen who use them as bait. They often hybridise with native species, and exposure to oestrogen-mimicking chemicals like BPA could make it even more likely that they would do so.

Although it has a lower profile than habitat loss and overhunting, interbreeding is a big threat to biodiversity. "Hybridisation is one of the most common and widespread causes of species loss, especially in fish," Ward says.

The concentrations of BPA that the fish were exposed to are orders of magnitude higher than any that humans experience. Nevertheless, some scientists think BPA harms humans at those lower doses. Earlier this year the US Food and Drug Administration rejected calls to ban BPA from all plastic food containers, but it is now considering banning its use in containers of baby formula, following the examples of the European Union and Canada.

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