by Dike Blair

The librarian of Murry Bergtraum High School in Manhattan noticed that one of the library's books was being repeatedly stolen. In little over a year, five copies of Parasite Rex: Inside the Bizarre World of Nature's Most Dangerous Creatures had gone missing. In a show of educational enlightenment, the school answered the thefts by inviting the book's author, Carl Zimmer, to lecture at the school. Parasite Rex's popularity with high schoolers isn't that surprising as it's about the kingdom of parasites, including the kinds of scary creatures whose behaviors are often mined for horror movies. In fact, Zimmer lends a few pages of the book to our collective parasite phobia as it's reflected by the entertainment industry.

Zimmer grew up on a 25 acre farm near Flemington, New Jersey where his parents raised sheep, chickens, and cows--"All splendid hosts for parasites," he now says. He majored in writing at Yale, but he also took science courses. When he graduated in the late 80s, he tried his hand at writing fiction before getting a job as a fact-checker with Discovery. Something clicked; he spent ten years at the popular science magazine, becoming a senior editor from 1994 until he departed in 1999. Zimmer has authored numerous articles and three books, and has garnered prestigious awards for his writing, which is usually about some facet of evolution. His first book At the Water's Edge was about scientists' exploration of how life first moved from sea to land, and how later in history whales reversed that path. His last book, Evolution, is an ambitious companion book to the PBS TV series of the same name.

Like the kids at Bergtraum High, it was Parasite Rex, first published in 2000, which really grabbed me. The book is a remarkably compressed account of the evolutionary strategies of a range of parasitic species, from microscopic blood flukes to 60-foot tapeworms. Along with some wonderfully lurid accounts of parasitic behavior, Parasite Rex provides a great overview of how recent findings about parasitic life are affecting all the life sciences. It also chronicles the tremendous and devastating impact of parasites on world health. I spoke to Zimmer in a coffeehouse not far from the NYU library where he is researching a new book on the beginnings of Neurology.

Dike Blair: It seems as if contemporary science writing is offering theory in the public arena. Are scientists presenting their theories to the public and their peers simultaneously?

Carl Zimmer: It's a funny thing. Popular science writing has grown pretty steadily from the 1970s; it's become a substantial portion of what people read. It does offer an opportunity for public scientific intellectuals, like an E. O. Wilson or Stephen Jay Gould, to voice their theories on this and that. It's a tricky thing because a lot of times the theory that's being offered to the public hasn't been run along the` parallel track, which includes doing basic research, publishing in scientific journals--all the unsexy, underpaid work that makes any scientific idea fly, or not.

Dike Blair: Does persuasive writing sometimes cloud the scientific discipline?

Carl Zimmer: I know that there's probably an element of jealousy, but it certainly exasperates scientists who have done decades of unglamorous work in their given fields when people like Jared Diamond or Stephen Jay Gould come along and use their pulpit, so to speak, to express differing views.

Dike Blair: Is there any notion that the science writer functions as a curator to weave together disparate areas of research?

Carl Zimmer: I would agree with that. It's something of the way I see my role. It may seem as if I'm putting across an idiosyncratic idea, but that's not true. I'm just putting together what the scientific community has already come to realize. The ideas have been kicked around in small papers. The sad thing, to me, is that people do wonderful research on some strange animal and they publish in some journal that, maybe, only 500 people in the world read. It's nice to be able to bring their work to a public-at-large that is interested in this stuff.

Dike Blair: In Parasite Rex you say, "When it comes to the tapestry of life, parasites are the hand on the loom." That's a great line, can you explain what you mean?

Carl Zimmer: Parasites are not just along for the ride. The heart of evolution is that some individuals survive and get to have offspring, and some don't. That's how you get all life forms. Parasites are very good at killing off their host, or castrating them, or otherwise making it hard for them to have offspring. They represent a huge pressure on the rest of life to evolve, and they keep driving evolution because they can evolve as well. There's a continuous race between host and parasite. But that's not the only aspect of it. When the human genome project came out, I was drawn to the fact that half the DNA in the genome does not make genes in our conventional sense of the word. What those DNA sequences do is to get themselves replicated and reinserted in the genome. They're sort of like built-in viruses. Some of them have lost the ability to replicate themselves but some find ways to insert themselves back into the genome. We're carrying around all these parasites inside our own DNA and, as they hop around our genome, they can disrupt the genes that build our bodies and change the way they work; and that's another way that evolution happens.

Dike Blair: How do you define "parasite?"

Carl Zimmer: Well, a parasite is not just a tapeworm sitting in someone's intestines. It's any group of genes that are using other groups of genes for their advantage, to get themselves replicated. That's an idea that Richard Dawkins came up with and it's a really good one because you don't get hung up trying to talk about some particular group of species. Any time that you try to create a more restrictive definition like, "An organism that lives inside another organism," you run into problems. For example, what about the parasitic wasp? It lives inside a caterpillar as a larva, but then comes out and lives the rest of its life in the air. Is that a parasite? What about a cuckoo that lays its eggs in the nest of another bird? It manipulates a host bird, using strategies of more classical parasites. It's not living inside the host yet the genes of the cuckoo are exploiting the genes of the host bird for their own benefit.

Dike Blair: What is the proportion of host to parasite species?

Carl Zimmer: There are more species unknown to us than there are known, so estimations of how many species exist on earth are fraught with uncertainty. You can go into a rainforest and shake a tree and dozens of new species of insect fall out, and the tree itself might be a new species. But if you do a statistical study of what we know, there would be about four parasites for every host. There are some individual species that may host between twenty and one hundred parasitic species. If a scientist cuts open an animal in a rainforest, they will invariably find a parasite they've never seen before. Definitely the vast majority of life is parasites.

Dike Blair: So parasites are a more important force in evolution than previously thought?

Carl Zimmer: It's kind of understandable that people wouldn't want to give much credit to parasites in the history of life. To us it doesn't reflect well on life itself that parasites be the dominant form of life. So we've seen a scientific repulsion toward parasites for decades and decades. You have to remember that before the mid-1800s, nobody knew how parasites worked--before then people didn't know that bacteria caused disease. But I think scientists are pretty much recognizing the significance of parasites and evolution.

Dike Blair: Could you give an example of male display and how parasites might affect selection?

Carl Zimmer: Peacocks all have big tails. You can actually see the link between the size of the tails, the number of eyes on the tail, and how well their offspring do. This is what's called an honest signal. If the peacock is beset by challenges, like parasites, they can't allocate energy to building a tail. It sounds weird that an animal can allocate resources like a budget, but that is what happens. The average peacock's tail has around 150 eyes. If you clip 20 of the eyes, the peacock won't be able to get any mates. The females just aren't interested.

Dike Blair: You seem to suggest that parasites may actually function positively?

Carl Zimmer: I guess scientists are beginning to realize that parasites are not simple things. There can be cases in which a parasite actually has some positive side effects. And sometimes the interests of the host and the parasite overlap. There's a parasitic crustacean that attacks a fish and, after it eats the fish's tongue, it sticks its legs into the gill bars, hunkers down, and acts like a replacement tongue for the fish. The fish uses the hard surface of the crustacean to crush its food. The fish can still feed and a little bit goes to the crustacean.

Dike Blair: I remember that incredible image from the book. Do you have a favorite parasite?

Carl Zimmer: I like these parasites that engage in mind control. A fluke that can get an ant to crawl up a blade of grass and wait there all night to be eaten by a cow or sheep that will be its next host. If the ant doesn't get eaten, the fluke will make the ant go down to the ground during the day--the fluke doesn't want to bake in the sun--but it sends the ant back up a blade of grass the following evening. What's so neat about those kinds of parasite is that they seem to possess a diabolical intelligence. They must know what they're doing but, in fact, these things don't even have brains. There's an intelligence to nature that's not like ours and where brilliant tricks come into being because of evolution. These tricks can result in something beautiful like a butterfly or a peacock's tail, or in something fiendish like a parasite. I guess I'm partial to the latter.

Dike Blair: You mention that there's some evidence that Toxoplasma alters human behavior. How was that discovered?

Carl Zimmer: You can readily test people for the presence of Toxoplasma. Those tests show that about 50% of the worlds' population have Toxoplasma. The reason for this is the huge cat population that we humans support. Typically Toxoplasma moves between cats and their prey--such as rats. Experiments have shown that rats infected with Toxoplasma have less fear than those not infected. Fear in a rat is a good thing--it keeps them away from cats. A rat infected with Toxoplasma however, smells a cat and may even display curiosity rather than fear. So, scientists suspect that infected rats are more likely to get eaten by a cat and Toxoplasma moves along in its life cycle.

Some scientists in the Czech Republic wondered if people might display any behavioral changes due to Toxoplasma. So they ran batteries of psychological tests and found differences between those people infected with Toxoplasma and those who weren't, and their results have held up under scrutiny. Most of the differences were subtle, but people who were infected tended to either be more outgoing, or distrustful of authority. However subtle, both of these behaviors can be risky to a host.

Dike Blair: You speak with a cautionary voice when it comes to evolutionary psychology.

Carl Zimmer: You get very elaborate but, often, thin ideas in evolutionary psychology. For example, say there's an idea that creativity in performance is a sexual display, and that females use that to make some kind of choice. You actually have people that study how close to the stage women will sit at a concert, as if that's some kind of validation of the idea. It's just seems feeble to me, that sort of research. There's certainly nothing wrong with the study of how our behavior might have evolved, but it's such a vast subject--we're an organism that has 100 billion neurons, we've been evolving for 5 million years, and our society is much different than hominid life has been for most of that time. Then you get into the whole question of genetics. You can't just say there's a gene for putting on a good concert and that gene became more widespread because females were attracted to it. Most people who do evolutionary psychology know all of this, but I think straight lines get drawn too easily between behavior today and how it may have evolved. Also cultural difference is rarely accounted for. You can't just go survey 20 kids at a college in Indiana and say you've discovered something about a universal human condition. A friend who studies sexual selection--she specializes in crickets and their songs--says she can barely grasp the complexity of that behavior. And a cricket is a comparatively simple organism.

Dike Blair: In your book parasitologist Eric Hoberg says that he's part of a dying field. Is that true?

Carl Zimmer: The funny this is, the study of parasites is fragmented, and many that study parasites wouldn't call themselves parasitologists. A person who studies viruses, for example, calls himself a virologist. One who studies salmonellae and other bacteria is a microbiologist. The term parasitology should apply to all those things but it is usually reserved for people who study things like tapeworms and certain amoebas. It's a shame that these people aren't talking to each other more because all these different life forms employ the same great strategies to determine how much harm they can cause to their host, or how they get from one host to the next. The reason that parasitology is a dying field probably has to do with the study of diversity in general. The number of taxonomists, the people who study diversity of species, is shrinking. Unfortunately there are all these species that nobody knows about and in a century, half of them might be gone.

Dike Blair: What are you working on now?

Carl Zimmer: A book that's about the discovery of how the brain works during the 1600s. Basically it's about the dawn of neurology. In those days there were a lot of interesting people figuring out the basics of how the brain works. They set aside all these medieval ideas about humours, demonic possession, or what have you. They're fascinating people as is their work and the context it came out of. A lot of what they were doing came out of alchemy. When the scientific revolution came about, it wasn't very seemly to talk about alchemy, yet alchemy gave rise to neurology. It's a bit of a departure for me from evolutionary themed things, but the brain is the most amazing product of evolution.

Zimmer, Carl. 2000.Parasite Rex: Inside the Bizarre World of Nature's Most Dangerous Creatures. New York: The Free Press.