The best strategy for a host is simply not to cross paths with a parasite at all. Some of the adaptations hosts make to avoid the notice of parasites are so grotesque, so outrageous, that it’s hard to tell at first that they actually are designed for parasites at all. Consider leaf-rolling caterpillars. They’re 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 back against a ring of blood vessels surrounding its anus. The blood pressure builds up behind the plate, which the caterpillar then releases. The pressure of the blood slams against the droppings so suddenly that it blasts them three feet a second, in a soaring arc that carries them up to two feet away.
What on Earth could have driven the evolution of an anal cannon? Parasites could. When parasitic wasps home in on a larva such as the leaf-roller caterpillar, one of the best clues is the odor of their host’s droppings. Since caterpillars are sedentary, not racing from branch to branch, their droppings will normally accumulate close by them. The intense pressure put on leaf-roller caterpillars by wasps has pushed the evolution of high-pressure fecal firing. By getting their droppings away from them, the caterpillars have a better chance of not being found by wasps.
Vertebrates, like insects, will also go out of their way to avoid parasites. Cow manure fertilizes the grass around it, making it grow lush and tall, but the cows generally stay away. They keep their distance because the manure often carries the eggs of parasites such as lungworms, and the parasites that hatch from them crawl up the neighboring blades of grass in the hope of being eaten by a cow. Some researchers have suggested that mammals that make long migrations, such as caribou and wildebeest, plot their course in part to avoid parasite-thick spots along the way. Swallows will fly back to their old nests and reuse them, unless they discover that their nests have been infested with worms and fleas and other parasites, in which case they’ll build a new one. If baboons discover that the area where they sleep has been overrun with nematodes, they’ll go away and won’t return until the parasites have died away. Purple martins go so far as to line their nests with plants like wild carrot and fleabane that contain natural parasite-killers. Owls sometimes catch blind snakes, but rather than tear them apart to feed their chicks, they drop them into their nests. There the snakes act as maids, slinking into the nooks of the nest and eating the parasites they find there.
Even if your mother was an excellent judge of fish bowers, even if you perfected your fly-killing head-snap, even if you can blast your frass into the neighboring meadow, you may end up with a parasite inside you. Your immune system will do its level best to stave off the invasion; it’s an exquisitely precise system of defense brought about thanks to the evolutionary pressure of parasites. But hosts have evolved other kinds of warfare. They can enlist other species to help them; they can medicate themselves; they can even reprogram their unborn offspring to prepare for a parasite-ridden world.
When a plant is attacked by a parasite, it defends itself with its own version of an immune system by creating poisonous chemicals that the parasite eats as it chews on the plant. But it also fights by sending out cries for help. When a caterpillar bites a leaf, the plant can sense it—a feeling not carried by nerves but felt nevertheless. And in response, the plant makes a particular kind of molecule that wafts into the air. The odor is like perfume for parasitic wasps; as they fly around searching for a host they are powerfully lured by the plant’s smell. They follow it to the wounded leaf and find the caterpillar there, and they inject it with eggs. These conversations between plants and wasps are not only timely but precise. Somehow the plant can sense exactly which species of caterpillar is dining on it and spray the appropriate molecule into the air. A parasitic wasp will respond only if the plant lets it know that its own species of host sits on a leaf.
Animals will sometimes defend themselves against parasites with a change of diet. Some will just stop eating—if a sheep is hit by a bad dose of intestinal worms, for instance, it may graze only a third of its normal intake. Such a change clearly can’t benefit the parasite, which wants the sheep to eat a lot so that it can eat a lot and make a lot of eggs. Researchers suspect that eating less may somehow boost the host’s immune system, making it better able to fight the parasite. On the other hand, the animals may not be simply fasting but may be being choosier about what they eat, choosing food that has the right nutrients to help them fight the infection.
Sometimes animals under attack by parasites will start eating foods they almost never eat. Some species of woolly bears, for example, normally eat lupine. They sometimes get attacked by parasitic flies that lay eggs in their bodies. Unlike the flies that attack ants or other insects, though, these parasites don’t always kill their hosts when they emerge from their bodies. And the woolly bears improve their own odds of survival by switching from a diet of lupine to one of poison hemlock. The parasitic flies still crawl out of their bodies, but some chemical in the hemlock helps the woolly bears stay alive and grow to adulthood. The woolly bears, in other words, have evolved a simple kind of medicine. Medicine may be pretty widespread among animals—there are plenty of records of animals sometimes eating plants that can kill parasites or expel them out of their gut. But researchers are still trying to prove that they actually eat those foods when they get sick.
When things get truly bleak—when there’s little hope a host can kill a parasite inside it—it cuts its losses. It has to accept that its life is doomed. Evolution has given hosts ways to make the best of the time they have left. When some species of snails are infected with flukes, there’s only a month or so before the parasites castrate them and turn them into nothing more than food-gathering slaves. That still gives the snails a month to produce the last of their offspring. They take full advantage, producing a final burst of eggs. If a fluke gets into a snail that’s still sexually immature, it will respond by developing its gonads much faster than if it were healthy. If they’re lucky, the snails can squeeze out a few eggs before the parasites cut them off.
When the fruit flies of the Sonoran desert are attacked by parasites, their response is to get horny. They feed on the rotting flesh of the saguaro cactus, and sometimes they encounter mites there. The mites leap onto the flies and jab their needlelike mouths into their bodies, sucking out their internal fluids. The consequences can be grave—a heavy infestation of mites can kill a fly in a few days. Biologists have found a big difference between the sexual activities of healthy and mite-infested male fruit flies. The parasites trigger the males to spend more time courting females, and the more parasites a male has, the more time he spends doing so, in some cases tripling his efforts.
At first this might seem like another display of puppetmastery, as a parasite speeds up its own transmission by putting infected flies in contact with healthy ones. In fact, the mites seem to get on flies only when they feed on cactus. They never hop from one mate to another. It appears that parasites have essentially driven flies to evolve a habit of mating more when death—and no more matings—seems imminent.
Why don’t the flies make the fast-and-furious lovemaking style a permanent one? The answer, probably, is that the mites aren’t always assaulting the flies. Some cactuses are covered with them; others are mite-free. As with bees, sex puts a lot of demands on fruit flies, making them an easy target for predators. Better to be flexible, mating at a slower speed normally and speeding up in the face of parasites.