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Parasites Found: A Trove of Clues to Evolution

A few hardy biologists view the freeloaders as triggers for changes in behaviors, defenses and anatomy. But the complex mechanisms of hosts and predators are very difficult to unravel.


Say the word parasites and most people want to run. With their itchy, creepy ways--eating their hosts alive from the inside out, for one--it's no wonder parasites have few fans.

Count Janice Moore, though, among their admirers. The Colorado State University biologist is among a growing number of scientists who view the biological freeloaders as rare peepholes into the complicated machinery of evolutionary change.

"These guys have to be on the cutting edge of evolution," said Moore. "They live in a habitat that fights back if it can."

But nothing about Moore's work is easy. For you can't just study a parasite. To understand it, you also have to study the animal it infects. And then, perhaps, the predator that eats that parasite-infected animal. These ecological arms races can spur the evolution of new defenses, behaviors and even anatomy--changes that many biologists struggle to understand.

A few studies, some taking decades, have started to reveal the fantastically complicated relationships between parasites and their hosts. These detailed studies raise serious questions about older parasite research already enshrined in textbooks.

Take the tale of the lancet fluke, a parasitic flatworm that infects ants, but must somehow make its way into its final host, sheep. Sheep don't normally eat ants. But infected ants act strangely. They climb to the tips of grasses and clamp on with their mandibles. The prevailing conclusion: The parasite alters the behavior of the ant so it is more likely to be eaten by a grazing sheep.

While that sounds reasonable, there's no evidence that sheep actually eat these ants. There may be other explanations for their strange ways, said Moore. The ants may be moving into the sun to raise their body temperature and kill off the parasite. "It gets very sticky very fast," she said.

The Messiness of Science

Because these intertwined relationships often are subtle, complicated and difficult to study in the field, they also have become contentious. The debate opens a window into evolution, but also into how messy and inconclusive science can sometimes be.

This summer, scientists at Oxford University published work showing rats infected with the parasite Toxoplasma were less likely than healthy rats to avoid sections of a maze containing cat scent.

Because the rats' sense of smell seemed unaffected, the researchers surmised that the parasite instead was disabling the rats' normal fear response, making them more likely to be eaten by the parasite's final host, the cat.

Intriguing but far from proven. First, there's no evidence that cats are more likely to catch and eat infected rodents. And no research has nailed down how the parasite might affect a rat's complex fear response.

"It's a story that looks really nice on paper and it's a good experiment, but you have to be quite careful" about making conclusions, said Hilary Hurd, a parasitologist at Britain's Keele University.

Moore's work, Hurd said, has provided some of that definitive proof that a parasite alters a host's behavior in ways that benefit the parasite.

Moore spent years in the field and the lab to prove that parasite-infected pill bugs spent more time in the open and on light-colored soil and were more likely to be gobbled up by starlings, which can continue to spread the parasite.

Without such hard-won proof, some of the many descriptions of how parasites alter their hosts become what biologists derisively call "just so stories"--attractive because they make sense, but quite possibly wrong.

"People are always attracted to neat stories; I am too," said Moore. "There's a sociology of science that mitigates against complicated answers. Everyone wants a clear cause-and-effect relationship."

But the parasite world is far from clear. Understanding the relationships between parasites and their hosts requires untangling the complex biology of parasites, hosts and predators--and the subtle interactions among them.

"If you are parasitized, clearly your behavior will change just because you are ill," said Charles Godfray, a leading parasite researcher at Britain's Imperial College at Silwood Park. "It's very easy to say this animal is behaving differently because it's being parasitized and make up a story of how this would benefit the parasite."

The stories can get even more muddied because hosts can fight back. Godfray is studying encapsulation in flies, a process in which cells from a fly's immune system recognize a parasite as foreign, surround it and essentially suffocate it.

Moore also is looking closely at the illnesses caused by parasites. Malaise, fever, loss of appetite--these might be healthy responses in the long run, she thinks, helping hosts kill off their hangers-on.

"The jury's still out on what constitutes sick behavior," said Moore. "Can sick behavior be adaptive? And how can we tell?"

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