Attack and Counterattack: The Never-Ending Story of Hosts and Parasites

After consuming most of the tongue of a spotted rose snapper, a parasitic isopod (a tiny relative of crabs and lobsters) hooks onto the floor of the fish's mouth. There it may take over the tongue's role, helping the fish hold prey--and getting tidbits for itself in the process.

In the Origin of Species, Charles Darwin said little about a particularly powerful evolutionary force that brought him a lot of personal sadness: parasites. His ten children struggled against diseases such as influenza, typhoid, and scarlet fever, and t y the time the Origin came out in 1859 three of them had died. Darwin himself suffered for much of his adult life from fatigue, dizzy spells, vomiting, and heart trouble. He once described his health this way: "Good, when young, bad for the past 33 years." Although no one is sure what made him suffer, some have suggested Chagas' disease, which is caused by a trypanosome, a parasite that in turn is spread by the benchuca, a biting insect of South America. The ways to die of Chagas' are horrible in their variety: your misfiring heart may stop beating, for example, or food may pile up in your colon until you die of blood poisoning. Darwin was bitten by a benchuca as he was traveling around the world on the HMS Beagle and many of his symptoms arose only after he returned to England.

Perhaps parasites caused Darwin too much misery for him to recognize their evolutionary importance. But he ,,vas hardly alone. Only recently have scientists begun thinking seriously that parasites may be just as important to ecosystems as lions and leopards. And only now are they realizing that parasites have been a dominant force, perhaps the dominant force, in the evolution of life.

Organisms of all kinds have evolved means to detect, fight, and avoid parasites. Copadichromis eucinostomus, for example, a small fish that lives in Lake Malawi in southeastern Africa, is one of many animals in which the females employ some sort of parasite test when selecting a mate. To attract females, the males build bowers out of sand on the lake bottom. Some of the bowers are nothing more than a handful of grains sitting on top of a boulder, while others are big cones several inches high. The males build their bowers near one another, creating dense neighborhoods, and each defends his bower against other males. The female fish spend most of their time feeding on their own, but when ready to mate, they go to the bower neighborhood and inspect the males' work. If a female chooses to mate with a particular male, she releases an egg and puts it in her mouth. The male then deposits his sperm there too, and the female swims away with the fertilized egg inside her mouth, where it will develop into a baby fish.

The females apparently use the bowers to find out which males do the best job at fighting parasites. Experiments have shown that they prefer males that build big, smooth-shaped bowers and that these males are also the ones with the fewest tapeworms. Perhaps a fish that's carrying tapeworms has to spend so much time eating that it can't maintain its bower. The bower thus becomes a medical chart and perhaps a sort of genetic profile.

Parasites also have been instrumental in shaping the social structure of some species. Leaf-cutting ants travel from their nests to trees and hack off bits of leaves, which they grip in their mandibles. Then, forming a parade of green confetti on the forest floor, the ants carry the foliage back home, where they use it to grow gardens of fungi--the ants' food. Leaf-cutter colonies are divided into big ants, which tote the leaves home, and little ones. The little ants, known as minims, tend the gardens, but they can also be found riding atop the leaf fragments being carried by their larger nestmates. Entomologists were puzzled for a long time over why the minims would waste their time hitching rides like this. Some suggested that they must collect some other kind of food on the tree, maybe sap, and then ride home on the leaves to save energy. In fact, minims are guards. The parasitic flies that attack leaf cutters have a special approach to their hosts: they land on a leaf fragment carried by an ant and then crawl down to lay their eggs in the gap between the ant's head and thorax. Hitchhiking minims, their mandibles open, patrol the leaf or perch on top of it; when they encounter a fly, they scare it away or even kill it.

No matter what the situation, of course, the best strategy for a potential host is simply not to cross paths with a parasite at all. Consider leaf-rolling caterpillars. These are pretty ordinary insect larvae, with one exception: they fire their droppings like howitzers. As a bit of frass starts to emerge from the caterpillar, it pushes a hinged plate against a ring of blood vessels surrounding the anus. The blood pressure builds up behind the plate, which the caterpillar then releases. The blood slams against the droppings so suddenly that it blasts them three feet a second, in a soaring arc that carries the frass as far as two feet away. What could have driven the evolution of this anal cannon? Parasitic wasps in search of a caterpillar host may be drawn to their prey by the odor of its droppings. High-pressure fecal firing gives the caterpillar a better chance of escaping detection.