Parasites Found: A Trove of Clues to Evolution
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?”
Finally, parasites must contend with their own parasites--”hyperparasites,” in the parlance of biologists. “It’s a hard world for everybody,” said Jay Rosenheim of UC Davis, an expert on parasitic wasps.
Changing Behaviors
All this complexity doesn’t mean parasite interactions can’t be sorted out. In addition to Moore’s study, there is work that biologist John Holmes at the University of Alberta and former student William Bethel conducted at a lake near Edmonton, Canada.
The two biologists found that bright orange parasitic worms that infected small crustaceans called amphipods caused them to markedly change their behavior--to the benefit of the worms.
Normally, amphipods avoid the surface of the water. When disturbed, they dive and burrow into the mud, where they are safe from hungry ducks.
But one worm, whose final host is the dabbling mallard, induces amphipods to swim at the surface where mallards feed. These infected amphipods also make themselves more visible by swimming upside down and splashing around--inappropriately using the behaviors normally used in burrowing.
The parasites also induce amphipods to cling to anything they touch, exhibiting behavior, Holmes said, normally only seen during reproduction.
“It’s a normal behavior expressed at an inopportune time,” Holmes said. “The parasite completely alters something in the amphipod nervous system to make it swim up and down.” How it accomplishes this is unknown.
There’s more to the story. Another worm, whose final host is a diving duck, produces very different behavior in amphipods, steering infected amphipods to the middle of the water column where these ducks, lesser scaups, feed.
The study followed through with the final, important step: The scientists proved that parasite infection led to increased predation by ducks. Holmes and Bethel found that all mallards were infected with the parasite, and all of the amphipods found in the birds’ stomachs were infected.
These simple parasitic worms, in their complexity, continue to stun even seasoned biologists. “It’s really quite clever,” said Hurd. “Goodness knows how they’re doing that.”
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Searching for the Perfect Host
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In order to reach their final hosts and continue to spread, many parasites hitch rides with intermediate hosts. Sometimes, they hijack the behavior of those hosts in ways that benefit the parasite and usually lead to an early death for the host. Below is one example.
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Parasite
An acanthocephalan, or thorny-headed worm (Polymorphus paradoxus), is essentially a little bag of reproductive organs attached to a proboscis. It seeks an intermediate host that will help it reach its final host.
Size: Adults 1-9 millimeters
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Intermediate host
An amphipod, such as a small crustacean, eats the eggs of the parasite. The eggs hatch and burrow into the amphi-pod’s body cavity, where they develop.
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Final host
Infested amphipod is eaten by an appropriate vertebrate, such as a mallard duck, muskrat or beaver. There it takes up residence in the small intestine and develops into a sexually mature adult. It then lays eggs that are soon excreted by the host, continuing the cycle.
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* Normal behavior
Amphipods--including crusta-ceans--naturally avoid light and are usually found at the bottom of muddy ponds and lakes. When disturbed, they tend to burrow into the mud.
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* Changing behavior
Parasites, in seeking a final host where they can live as adults, infest amphipods. This induces the amphipods to exhibit behavior countrary to their natural instincts. In this case, they move up in the water toward light.
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* Reaching the goal
In shallower depths or clinging to vegetation near the surface, the amphipods, now intermediate hosts, become prey for surface feeders like mallards--the parasite’s final hosts.
Sources: Scientific American; National Audobon Society Field Guide to Birds
