I walked with seven biologists, led by one scientist named Daniel Brooks. He was about as far from my picture of the intrepid jungle biologist as he could get: heavy frame, a drooping mustache, and big aviator glasses, dressed in a red-and-black jogging suit and sneakers. But as the rest of us passed the time on the walk by talking about how to photograph birds or how to tell the difference between a poisonous coral snake and a harmless mimic, Brooks kept ahead, listening to the peeps and croaks that surrounded us. He stopped suddenly at the side of the path, waving his right hand back and low to shut us up. He moved toward a broad ditch filling with the night’s rain and lifted his net slowly. He put one sneaker into the water and then suddenly brought the net down on the far bank. Its pointed end started dancing and punching, and he grabbed the net midway before raising it. With his other hand he took a plastic bag from me and blew it full of air. He transferred a big beige-striped leopard frog into the bag, where it jumped frantically. He knotted the open end of the bag, still fat with air, and wedged the knot under the drawstring of his sweatpants. He started walking down the path again with his bulging frog bag, a transparent sack of gold.
Frogs and toads were everywhere that night. Brooks caught a second leopard frog not far down the path. Tungara frogs drifted in the water, in powerful choruses. Marine toads, some as big as cats, waited until we were close by before taking a single big lazy hop to keep their distance. We walked past blobs of foam as firm as bubble bath, out of which hundreds of tadpoles squirmed into the nearby water. We caught blunt-faced microhylid frogs, with tiny stupid eyes crowded up just over their nostrils and fat low bodies shaped like dollops of chocolate pudding.
For some zoologists, the hunt for their animals would be over at this point. But Brooks wasn’t sure yet what he had actually found. He brought the frogs back to the headquarters of the Area de Conservacion de Guanacaste. He left the frogs in their bags overnight, with some water to keep them damp and alive. In the morning, after a breakfast of rice and beans and pineapple juice, he and I went to his lab. The lab consisted of a shed with chicken-wire walls on two sides.
“The assistants here call it the jaula,” said Brooks. There was a table in the middle of the shed that held dissecting microscopes, and woolly bears and beetles crawled across its concrete floor. A mud wasp nest hung from the light cord. Outside, beyond the vines that surrounded the shed, a howler monkey roared in the trees. Jaula means “jail” in Spanish. “They say that we have to stay in here or we’d kill all their animals.”
Brooks took out a leopard frog from the bag and dispatched it with a sharp thwack on the edge of the sink. It was dead in an instant. He laid it on the table and began snipping its belly open. He used tweezers to pull the guts delicately free of the frog’s trunk. He put the organs into a broad petri dish and put the husk of the frog under a microsope. During the previous three summers, Brooks had looked into the insides of eighty species of reptiles, birds, and fish at Guanacaste. He had started making a list of every parasite species that lives in the reserve. There are so many different kinds of parasites within the animals and plants of the world that no one had ever dared such a thing in a place the size of Guanacaste. He adjusted the lights on their long black stalks, two curious snakes looking at the dead frog. “Ah,” he said, “here we go.”
He had me look: a filarial worm—a relative of guinea worms in humans—had come wandering out of its home in one of the veins in the frog’s back. “It’s probably transmitted by mosquitoes that feed on the frogs,” Brooks explained. He pulled it out intact and dropped it in a dish of water. By the time he had gotten a dish of acetic acid (industrial-strength vinegar) to fix it in, the parasite had exploded into a white froth. But Brooks was able to get another one out untorn and into the acid unexploded, where it straightened out, ready to be preserved for decades.
That was the first of many parasites we looked at. A string of flukes came out, like a writhing necklace, from another vein. The kidneys carried another species that only mature when the frog is eaten by a predator like a heron or a coati. The lungs of this frog were clear, although often the frogs here will have parasites in their lungs as well. They get several malarias in their blood, even get flukes in their esophagus and ears. “Frogs are parasite hotels,” Brooks said. He worked apart the intestines, slitting them carefully so that he wouldn’t snip any parasites inside. He found another species of fluke, a tiny fleck that swam across the microscope’s view. “If you didn’t know what to look for, you’d think it was garbage. It goes from a snail to a fly, which is then eaten by a frog.” The fluke has to share this particular set of intestines with a trichostrongylid worm that takes a more direct route to get there, burrowing straight into the frog’s gut.
Brooks pushed the dish out from under the microscope. “That was real disappointing, guys,” he said. I think he was addressing the parasites. I was pretty overwhelmed by all the creatures I’d just seen in one animal, but Brooks knew that a single frog species may have a dozen species inside it, and he wanted me to see as many as I could. He spoke to the frog: “Let’s hope your compadre has more.”
He reached into the bag for the second leopard frog. This one had two toes missing from its front left foot. “That means he escaped from a predator that wasn’t as successful as me,” Brooks said, and dispatched it with another swift thwack. When he got its open belly under the microscope, he said “Oh!” with a sudden brightness. “This is nice. Sorry. Relatively speaking, this is nice.” He had me look through the eyepieces. Another fluke, this one called a gorgoderid, for its resemblance to the writhing snakes on Medusa’s head, was twisting out of the frog’s bladder. “They live in freshwater clams. This tells me this frog has been somewhere where there are clams, which need a guaranteed water supply, sandy bottom, calcium-rich soil. And its second host is a crayfish, so the habitat has to support clams, crayfish, and frogs, and do it year round. Where we caught him yesterday is not where he comes from.” He moved on to its intestines. “Here’s a nice little vignette”—nematodes alongside flukes that form cysts on the frog’s skin. When the frog sheds its skin, it eats it, thereby infecting itself. The flukes were acrobatic sacs of eggs.
Cheered up now, Brooks moved on to a blobby microhyalid frog. “Oh my, you’ve brought me luck,” he says, looking inside it. “This thing must have a thousand pinworms. Holy cow, this guy is crawling.” In the pinworm soup there were squirming iridescent protozoa, single-celled giants that were almost as big as the multicellular worms.
A few of the parasites we saw already have names, but most are new to science. For now, Brooks went to his computer and typed in vague descriptors—nematode, tapeworm—that would be honed down by himself or some other parasitologist who would come up with a Latin name. The computer carried in it the records of other parasites Brooks had recorded over the years, including some of the ones I had watched dissected over the course of the previous few days. There were the iguanas with their tapeworms, the turtle with an ocean of pinworms. Just before my arrival, Brooks and his assistants had opened up a deer and found a dozen species living in or on it, including nematodes that live only in the deer’s Achilles tendon and flies that lay their eggs in the deer’s nose. (Brooks calls these last ones the snot bots.)
Even within this one reserve, Brooks was probably not going to be able to count every parasite. Brooks is an expert on the parasites of vertebrates as parasites are traditionally defined—in other words, excluding the bacteria and viruses and fungi. When I visited him, he had identified about three hundred of these parasites, but he estimated there would be eleven thousand in total. Brooks doesn’t study the thousands of species of parasitic wasps and flies that live in the forest, devouring insects from within and keeping them alive till the last moment of their feast. He doesn’t study the plants that parasitize other plants, stealing the water their hosts pump from the ground and the food they make out of air and sun. He doesn’t study fungi, which can invade animals, plants, or even other fungi. He can only hope that other parasitologists will join him. They are spread thin over their subjects. Every living thing has at least one parasite that lives inside it or on it. Many, like leopard frogs and humans, have many more. There’s a parrot in Mexico with thirty different species of mites on its feathers alone. And the parasites themselves have parasites, and some of those parasites have parasites of their own. Scientists such as Brooks have no idea just how many species of parasites there are, but they do know one dazzling thing: parasites make up the majority of species on Earth. According to one estimate, parasites may outnumber free-living species four to one. In other words, the study of life is, for the most part, parasitology.