As critical as an immune system may be to surviving parasites—particularly an immune system that can evolve rapidly—it’s really a defense of last resort. It fights against invaders that have already crossed the moat and are inside the castle. It would be far better to keep the parasites from getting in at all. Evolution has obliged. Hosts have adapted to fight off parasites with the shapes of their bodies, their behavior, the way they mate, even the shape of their societies—all designed to keep parasites at a distance.
Many insects are shaped expressly to fend off parasites. During their larval youth, some species are covered in spikes and tough coats that discourage wasps from trying to lay their eggs inside. Some have tufts of detachable barbs on their bodies, which entangle a wasp when it tries to land on them. When butterflies form cocoons, they sometimes dangle them from a long thread of silk that makes it impossible for wasps to get enough leverage to stab through their coat.
For some insects, armor is not enough. Thousands of species of ants, for example, are tormented by thousands of corresponding species of parasitic flies. The fly perches above the trail made by the ants from their nest to their food. When a suitable ant passes underneath, the fly dives down onto the ant’s back and wedges its egg-laying tube into the chink between the ant’s head and the rest of its body. Quickly the eggs hatch, and the maggots chew their way into the ant’s interior and then travel to the ant’s head. These larvae are muscle eaters. In a mammal it might make sense for them to make their way into a bicep or a thigh, but in ants the fleshiest place is the head. Unlike our brain-crammed skulls, those of ants hold only a loose tangle of neurons, the rest of the space being dedicated to muscles that power its biting mandibles. A maggot inside an ant’s head chews on the muscles, carefully avoiding the nerves, and grows until it fills the entire space. Finally, one day, the ant meets its awful end: the parasite dissolves the connection between the head and the rest of the body. Like a ripe orange, it drops to the ground. While the headless host stumbles around, the fly begins its next stage, forming its pupa. Other insects have to weave their cocoons exposed to the elements and hungry predators, but the fly develops snug in the tough cradle of an ant’s head.
These flies are so destructive that ants have evolved defensive manuevers against them. Some will run to escape the flies; others stop in their tracks and begin flailing wildly, gnashing their mandibles as soon as they even sense that a fly is overhead. A single parasitic fly can stop a hundred ants in their tracks along six feet of their trail. If the fly lands on the back of one species and gets ready to lay its eggs behind the head, the ant suddenly snaps its head back against its body, crushing the fly in its vise.
Among the leaf-cutting ants, these flies have transformed their entire social structure. Leaf-cutting ants travel from their nests to trees, hack off foliage, and take it back home, forming a parade of green confetti on the forest floor. Leaf-cutters are the dominant herbivores in many forests of Latin America—wildebeest in miniature, although they don’t actually eat the leaves. Instead, they bring them home to their colonies, where they use them to grow gardens of fungi, which then become their meal. If you want to get technical, leaf-cutters aren’t so much herbivores as mushroom farmers.
Leaf-cutter colonies are divided into big ants, which carry the leaves home, and little ants. The little ants (known as minims) tend the gardens, and they can be also found riding atop the leaves being brought home by the big ants. Entomologists have 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 trees, maybe sap, and then go home on the leaves in order to save energy. In fact, minims are parasite guards. The parasitic flies that attack leaf-cutters have a special approach to their hosts: they land on the leaf fragments and crawl down to where the ants grip it in their mandibles. The fly then lays eggs in the gap between the mandible and the ant’s head. The hitchhiking minims patrol the leaves or perch on top, their mandibles open. If they encounter a fly, they scare it away or even kill it.
For bigger animals, the struggle with parasites is just as intense, although it’s not as obvious as an ant wrestling a fly. Mammals are continually assaulted by parasites—by lice, fleas, ticks, botflies, screwworms, and warbleflies—that suck blood or lay their eggs in the skin. In response, mammals have evolved into obsessive groomers. The way a gazelle lazily flicks its tail and nuzzles its flank may look like the picture of peace, but it’s actually in a slow-motion struggle against an army of invaders. The gazelle’s teeth are shaped like rakes, not to help it eat but to scrape away lice and ticks and fleas. If its teeth are blocked, its load of ticks will explode eightfold. Gazelles don’t groom themselves in response to any particular scratch; they clean themselves according to a clocklike schedule because parasites are so relentless. Grooming cuts into the time an animal needs to eat and guard against attacks from predators. The top impala in a herd ends up riddled with ticks—six times more than females—because he is too busy staying vigilant against male challengers.
The shape of an animal’s society may also help cut down on parasites. Animals protect themselves from predators in this way. Fish that stay in schools can pool their vigilance; as soon as any of them senses a predator, they can all swim away. And even if the predator should attack, each member of the school has lower odds of being killed than if it were on its own. It’s time to put the parasite alongside the lion. Increasing the size of a herd not only will lower the odds that each gazelle will be eaten by a lion, but also will lower the odds that each individual will be attacked by a tick or some other blood-sucker. On the other hand, parasites may simultaneously keep herds from getting too big. As animals crowd together in bigger and bigger groups, they make it easier for some parasites to be passed from host to host, whether they are viruses carried on a sneeze, fleas passed on with a nuzzle, or Plasmodium carried by a hungry mosquito.
Parasites may even teach animals manners, according to Katherine Milton, a primatologist at the University of California at Berkeley. Milton studies the howler monkeys of Central America, and she’s been struck by the viciousness of one of their parasites: the primary screwworm. This fly searches for open wounds on mammals; it can even find the hole made by a tick bite. It lays its eggs inside the wound, and the larvae that hatch start devouring their host’s flesh. They do so much damage in the process that they can easily kill a howler monkey.
The screwworm may make howler monkeys leery of fighting with each other over mates or territory. The fight might only be a minor scuffle, but if a monkey gets a scratch, a screwworm could make it the last scuffle it ever has. Screwworms are so efficient at finding wounds, in fact, that evolution may frown on violent howler monkeys. Instead, it may have made them affable creatures, and it may have encouraged them to evolve ways to confront each other without getting hurt, such as howling and slapping rather than biting and scratching. There are many other mammals that also have ways to avoid fights, and it’s possible that they are also trying to avoid parasites.