HYPERACTIVE fish, stupid frogs, fearless mice and seagulls that fall over. Animals around the world are increasingly behaving in bizarre ways, and the cause is environmental pollution.

The chemicals to blame are known as endocrine disrupters, and range from heavy metals such as lead to polychlorinated biphenyls (PCBs) and additives such as bisphenol A.

For decades, biologists have known that these chemicals can alter the behaviour of wild animals. And in recent years it has become clear that pollutants can cause gender-bending effects by altering animals’ physiology, particularly their sexual organs.

But now two major reviews have revealed that the chemicals are having a much greater impact on animal behaviour than anyone suspected. Low concentrations of these pollutants are changing both the social and mating behaviours of a raft of species. This potentially poses a far greater threat to survival than, for example, falling sperm counts caused by higher chemical concentrations.

The two research teams have independently collected evidence revealing the effects on egrets and gulls, snails, quails, rats and macaques, minnows, mosquito fish, falcons and frogs. Behaviours altered include mating and parenting, nest building, learning, predator avoidance, foraging, activity levels and even balance.

In one study, for instance, male starlings exposed to dicrotophos insecticide decreased their singing, displaying, flying and foraging activities by 50 per cent. And newts exposed to low levels of the pesticide endosulfan found it harder to sniff out the attractive pheromones of potential mates.

Researchers have also shown that an increasing numbers of male western gulls hatched from eggs exposed to DDT attempt to mate with each other. In recent years, scientists have also found that lead affects the balance of gulls, while atrazine makes goldfish hyperactive and the chemical TCDD makes the play behaviour in macaques rougher.

Despite this wealth of evidence, these effects have gone largely unnoticed by toxicologists, says Ethan Clotfelter of Amherst College in Massachusetts, lead author of one of the reviews, published in August 2004 in Animal Behaviour.

Not only are we failing to acknowledge the scale of the problem caused by endocrine disrupters, but toxicologists may be missing a trick: changes in animal behaviour could be an early warning that certain chemicals are harmful. “You might see behavioural effects long before you see a population crash,” Clotfelter says.

Other behavioural biologists back the authors’ call for biologists and toxicologists to work more closely to determine the scale of the problem. “It’s been decades since the first evidence appeared that chemicals in the environment can influence behaviour,” says John McCarty of the University of Nebraska in Omaha, who researches the impact of pollutants on birds. — Samina Iqbal