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- From the Folger
Assistant Professor of Biology and Neuroscience Ethan Clotfelter checks one of the 200 Siamese fighting fish (Betta splendens) that he is using to study the behavioral effects of endocrine-disrupting chemicals.
Animals on the fritz
Given recent social developments, it’s probably just as well that certain politicians never heard about the extraordinary number of female pairs among gulls nesting on some islands off the coast of California in the 1970s. Despite appearances, the gulls’ unusual behavior was not the harbinger of constitutional revisionism, but the leading edge of a hidden global environmental problem. For a number of years, scientists have recorded incidents of peculiar animal behavior—hysterical mallards, emasculated alligators, flaccid guppies, stupid rats and tipsy egrets, among others—and they have linked that behavior to the presence of endocrine-disrupting chemicals, or EDCs, in the environment. (Some of the California gulls were males altered by the feminizing effects of an estrogen-mimic EDC.) Those reports stood in isolation, however, so no one realized that the problem was global or that it affected a very large number of species.
But in a recent paper published in the journal Animal Behavior, Assistant Professor of Biology and Neuroscience Ethan Clotfelter and colleagues from the University of Glasgow and Texas Tech University presented the case for just such a conclusion. They conducted an extensive review of published research on toxicology and animal behavior and found numerous examples of links between EDC exposure and aberrant behavior. Their paper gathered these findings into a single comprehensive picture. “We were trying to show how this one group of chemicals can have such wildly varying effects on almost every aspect of what animals do,” Clotfelter says, “and across really different kinds of animals, from little planktonic critters to primates and everything in between. We were able to pull together the literature in a way that hasn’t been done before, to give readers a sense of ‘Here’s all we know about what’s happening in animals.’ We strongly believed that people needed to be able to sit down and look at it all together.”
Endocrine disrupters are a group of more than 100 chemicals that profoundly affect reproductive, pituitary and thyroid systems in most vertebrate species. The product of industrial, agricultural and municipal processes, EDCs are extremely persistent in the environment and have spread so widely that there is virtually no part of the planet that is free of them. (Polar bears in the Arctic all have high levels of EDCs.) Many EDCs are fat soluble and collect in ever-greater concentrations as they go up the food chain, becoming particularly lethal in top predators. This effect was seen most dramatically in the near-extinction of raptors during the ’60s and ’70s because of DDT, which, along with PCBs and lead, is an endocrine disrupter. One of the most peculiar aspects of EDCs is the fact that their effects and their dosage do not tend to have a linear relationship. In some cases, the chemicals are more potent at low dosages than at high ones, and in other cases, the dose-effect curve is U-shaped, with greatest effects seen at greater and lower doses and least effects at medium doses. To make matters more complicated, a given EDC may have a very different effect on a given animal if it is combined with other EDCs.
That variation is one of the reasons that the correlation with animal behavior was not immediately recognized. Tissue samples might show high levels of EDCs in an animal that was behaving normally and low levels in an animal that was behaving in an unnatural way. As the name suggests, EDCs have virtually the same physiological effect on all species—they disrupt the endocrine system—and that disruption tends to produce distorted behavioral effects centered on sex and the various ways animals have of eating and being eaten. In short, the chemicals undermine the behaviors that ensure species’ continued existence. But, Clotfelter says, while the general effects may be the same, the behaviors themselves are very specific to each animal. “That’s what keeps one frog species, for example, from mating with another,” he says. “Animal behavior can be quite stereotyped. Bird courtship or fish courtship is going to be fairly fixed. Evolution has shaped it so that male fish do it the same way every time because that’s what female fish are looking for. So when you start to see deviations from that sequence, you know something is wrong.”
One of Clotfelter’s main goals in publishing the paper was to demonstrate that behavioral anomalies could be used as a sensitive indicator of environmental pollution and an early warning of more serious ecological consequences. “I wanted to hammer a few points,” he says. “I wanted to tell people that behavior is a useful measure because it’s an outward expression of everything that goes on inside the animal. You might see changes in behavior long before you see changes in other things. You’re not going to see gross physical abnormalities right off the bat. You’re going to see changes in behavior that could then be followed by gross physical abnormalities. It’s the canary in the coal mine.”
Clotfelter also wanted to promote communication among laboratory researchers and field researchers. Lab researchers had collected extensive data on the physiological effects and behavioral consequences of EDCs. But field scientists don’t necessarily travel in the same circles or see the lab researchers’ information, so a given biologist may not have recognized the significance of a peculiar behavior observed in the wild. And lab biologists tend to use only a handful of species that are conducive to research, making it harder to see cross-species patterns. “There are a lot of people who know very well how a chemical affects the rate of production of a particular hormone in a particular strain of laboratory rats,” Clotfelter says, “and that’s useful information, but it’s a piece of a big puzzle, and it’s often hard to see the entire puzzle.”
The puzzle, in this case, involves not just strange animal behavior, but also the implications and consequences of that behavior. Because EDCs tend to affect reproductive orsurvival behaviors, and because the chemicals are so widespread, it would be logical to assume that at some point they must threaten the viability of entire populations, or even entire species and ecosystems. But at this point, Clotfelter says, there simply isn’t enough data on such large-scale population trends to know the extent to which this may be happening.
The other unknown piece of the puzzle is what EDCs may be doing to humans. “In spite of all our behavioral complexities,” Clotfelter says, “the physiological underpinnings of that behavior are still pretty similar to what they are in birds, frogs and fish. There’s no reason to think that all these things we see in animals are not going on in people. There’s a lack of real causal data, but some people have suggested that the current epidemic of everything from autism to ADHD to adolescent violence could be attributed, at least partially, to exposure to these chemicals. Changes in aggressive behavior are certainly something that we see in mice as a result of exposure to these chemicals. Changes in cognitive behavior are something we certainly see in primates. So there is reason to extrapolate from these animals to humans. I think it’s the kind of research that will probably become very, very hot in the next five or 10 years.”
Photo: Frank Ward