A single cuckoo starts life much bigger than a warbler, and as it grows it gets even bigger. When the warbler looks down at its nest, it sees one big cuckoo mouth, which registers in its brain the same way a lot of little reed warbler mouths would. At the same time the young cuckoo mimics the calls of baby warblers. But rather than mimic the sound of a single warbler, the cuckoo can sing like an entire nestful. So the cuckoo tricks its host not only into feeding it but into bringing it eight warblers’ worth of worms. There may not be much room inside animals for a vertebrate parasite, but an animal’s nest is another matter.
So is a mother’s womb. When a fertilized egg tumbles down into the uterus and tries to implant itself, it encounters an army of macrophages and other immune cells. The new embryo doesn’t have the same proteins on its cells as its mother, which ought to trigger the immune cells to destroy it. The fetus faces the same troubles as a tapeworm or a blood fluke, and it evades its mother’s immune system in much the same way. The first cells that differentiate in a human embryo, known as trophoblasts, form a protective shield around the rest of its body. They fend off attacking immune cells and complement molecules, and they can send out signals that make the surrounding immune system sluggish. Strangely enough, there’s some evidence that these suppressing signals are made in the trophoblasts by some of the viruses that are lodged permanently in our DNA—just as viruses in parasitic wasp genes let them control the immune systems of their hosts.
If you think of parasitism in terms of Dawkins’s definition of genetic interests, then a fetus is a sort of half-parasite. It shares half its genes with its mother, and the rest belong to its father. Both mother and father have an interest, evolutionarily speaking, in seeing the fetus get born and live a healthy life. But some biologists have argued that parents also have strong conflicts on how the fetus grows. As it develops, it builds its placenta and a network of vessels to draw nourishment out of its mother. It knocks out its mother’s control over her blood vessels near the uterus, so that she can’t restrict the flow of blood to the fetus. It even releases chemicals to raise the concentration of sugar in her blood. But if the mother lets her child take too much, it might take a serious toll on her health. She might not be able to take care of her other children, and it might even threaten her ability to have any more. In other words, the fetus threatens her genetic legacy. Research suggests that mothers struggle against their fetus, releasing counteracting chemicals of their own.
While a fetus can take a heavy toll on its mother, how fast it grows will have no effect on its father’s health. It’s in his genetic interest for the fetus to grow as fast as possible. This conflict plays out within the fetus itself. Research on animals has shown that the genes a fetus inherits from its father and mother do different things, particularly in the trophoblasts. The maternal genes try to slow down the growth of the fetus, to control this parasite within her. Meanwhile, the paternal genes clamp down on these maternal genes and silence them, letting the fetus grow faster and draw more energy from its host.
Whenever two lives come into close contact and genetic conflict—even mother and child—parasitism will turn up.
The feeling of being surrounded by a few million parasites is a hard one to put into words. If you put your face close to a jar filled with a graceful ribbon, a tapeworm pulled from a porcupine, you can’t help admire its hundreds of segments, each with its own set of male and female sexual organs, all brimming with life and caught like a photograph in these preserving spirits. Then, just for a second, you start to worry that the whole creature will twitch a little, suddenly flail, and then break out of the glass.
The National Parasite Collection, run by the Agricultural Research Service of the U.S. Department of Agriculture, is one of the three biggest collections of parasites in the world. (Nobody is quite sure whether the American collection is bigger than the national collections of Russia. After you get up to a few million specimens, you tend to lose count.) It sits in a former guinea-pig barn on a farm the Department of Agriculture has been running in Maryland since 1936. In the distance, corporate headquarters push their cool blue-glass heads just over the trees. My guide through the collection was Eric Hoberg, a parasitologist in the shape of a bear. He studies the parasites of the far north, the nematodes that live only in the lungs of musk oxen, the flukes of a walrus. He led me down a flight of gray-striped stairs, past a couple of small labs, past a high stack of card catalogs a woman was slowly keying into a computer—a century of parasites. Then we went through a thick doorway to the collection.
At first I was a bit disappointed. I’ve followed paleontologists past museum displays and slipped through hidden doors into their collections, and we’ve wandered through corridors lined with high, deep cabinents full of whale skulls and dinosaur vertebrae that haven’t been touched since they were dragged out of the ground. You could fit a little diner into the National Parasite Collection, or maybe a shoe repair shop. Hoberg introduced me to a retired science teacher named Donald Poling. Poling sat at a table, wearing hiking boots and a white lab jacket, rescuing slides of nematodes from preserving fluid that had crystallized over the past hundred years into the consistency of brown sugar. “Keeps me out of the bars,” he said, scraping off a cover slip.
The rest of the room was taken up mainly by metal shelves on rollers that glided open with the turn of a three-pronged wheel. When Hoberg and I started walking among the shelves, browsing through the jars and vials, the disappointment disappeared. The collection surrounded me and became my world. We turned sealed jars around to read the labels that had been written in pencil. “Host: Yellowheaded Blackbird.” Tapeworms from Alaskan reindeer. Liver flukes from elks. Frilly monogeneans that held on to the gills of fish from Korea.
At one point, when Hoberg was showing me a nematode—thick as a finger, long as a riding crop, the color of blood—which was still curled up inside a fox’s kidney, I couldn’t help myself. I said, “Gross.” I had actually come to see Hoberg to learn something, not to continue with my horror marathon, but these things have a way of fighting their way out. Now it was Hoberg’s turn for disappointment. “I get irritated by the yuck factor,” he said. “What’s being missed is how incredibly interesting these are. And it’s tended to hurt parasitology as a discipline. Part of it is that people are put off by that,” he nodded to the kidney. “Parasitologists are retiring and not being replaced by new ones.”
We kept looking. We looked at a jar full of Hymenolepis, the tapeworm that uses beetles to get into rats, a great swirl of rice noodles. A piece of pig flesh with Trichinella running through it like a night of shooting stars. We passed closed trays of slides stacked upright like books on the shelves, hundreds of them, each with dozens of slices of parasites mounted on glass. We passed by the twelve thousand slides of specimens Hoberg collected in the Aleutian islands while he was working on his dissertation—twelve thousand slides he doubts he’ll ever find time to write about before he retires. Hoberg brought the slides with him from the University of Washington when he got the job at the collection in 1989. A decade later, he was still coming across surprises. “Crab-eater seal?” he barked at a jar of tapeworms, picking it up and turning it in his hand. He lifted his glasses to his forehead to study the paper label floating in the fluid and said, “This may have been from Byrd’s last expedition to the Antarctic.” We came across a jar of botfly larvae. As horses walk through fields, adult botflies lay eggs on their hair, and when the horses lick themselves clean, they swallow the eggs. The eggs take the warmth of their mouth as a cue to hatch, and they chew their way into the horse’s tongue. From there they drill down to the horse’s stomach, where they anchor themselves and drink its blood. Once they mature, they let go their grip and are carried out of the horse’s digestive tract. They hit the ground and transform into adult flies. In the jar before us, a swatch of horse stomach lay at the bottom, studded with botfly larvae, a cluster of stony little hives. I was fascinated, but Hoberg flinched. “That’s one thing I can do without.” I was glad to see that even a parasitologist has his limits.