by Riskin, Dan
If that weren’t enough, sometimes parasites take things even a step further, changing the behavior of their hosts in surprising ways to improve the parasite DNA’s odds of survival. There are many, many examples of this, but my favorite mind-controlling parasite is a creature called Toxoplasma.
Toxoplasma is a parasite of cats, but it also spends some of its life cycle inside a rat, and the rats are the ones who have their minds controlled. Here’s how the life cycle works. First, a rat accidentally eats Toxoplasma eggs when it eats something contaminated with cat feces. In a few weeks, those eggs turn into cysts all over the inside of the rat’s body. When that infected rat gets eaten by a cat, those Toxoplasma cysts get into the cat’s stomach. Later, the cat poops, and that poop gets onto something the rat eats, and the life cycle repeats. Simple, right? The tricky part of this life cycle (for the parasite) is finding a way to cause the rat to be eaten by a cat. Apparently, that’s much harder for the Toxoplasma parasite than you might expect.29
The problem (for the parasite) is that rats are really good at avoiding cats. For example, if a rat ever comes across the smell of cat urine, nerve signals from the nose go straight to the fear centers in the rat’s brain, and the rat instinctively gets out of there. In other words, cat urine smells terrifying to a rat, and that’s hardwired into the rats by their DNA. Even rats raised in captivity are scared of the smell of cat urine the very first time they smell it. Those rat instincts are a real problem for the parasite, which needs its host rat to be eaten by a cat. Otherwise, the parasite’s DNA will never be passed on.
Here’s Toxoplasma’s solution: When the parasite encysts in the rat, many of those cysts form in the rat’s brain, and some of those form right at the very fear centers normally triggered by the smell of cat urine. Somehow, incredibly, those cysts alter the wiring of the brain so that when a Toxoplasma-infected rat smells cat urine, the nerve signals don’t go to the fear centers at all but instead get rerouted into the sexual pleasure circuitry of the brain. In other words, rats infected with Toxoplasma don’t just lose their fear of cat urine, they get sexually aroused by it.
Not surprisingly, this makes them more likely to spend time near cats than normal rats would, and that improves the odds that the parasites will complete their life cycle.
This is what I’m talking about when I say parasites are just as important to ecosystems as predators and prey. You can watch a cat eat the rat, but if you leave out the fact that the parasite is controlling the rat, you can’t really ever understand what’s going on.
Take a second to imagine that: What if there were some parasite that could alter our own behavior in that fundamental a way? If that idea freaks you out, you may just want to stop reading here. If the thought of being a puppet controlled by a microscopic parasite lodged in your brain is just too creepy for you to handle, maybe this isn’t the book for you.
Still with me?
Here it comes: Sometimes humans accidentally eat cat feces, just like rats do. When that happens, humans can end up with Toxoplasma cysts in their bodies, just like rats do. In fact, that happens all the time. By some estimates, a third of the world’s population is infected. A third! I’m not just talking about developing countries either. The infection rate in the USA is around one in eight people, and in some countries that number is closer to seven out of every ten people. That might be shocking, but it makes sense: cats (and rats) live pretty much everywhere that humans do. Changing the kitty litter every few days for several years, you’re bound to have a little fecal dust go into your mouth at some point.30
Toxoplasma, by the way, is the reason women who are pregnant aren’t supposed to clean up kitty litter: the litter might be filled with Toxoplasma eggs, and although the parasite is relatively harmless to adult humans, becoming infected while you’re pregnant can cause severe damage to your unborn baby.31
At this stage you probably have a lot of questions. Do you have Toxoplasma cysts in your tissues? You very well might. And if so, will the smell of cat urine make you horny?
The answer to the horniness question is probably no. It doesn’t look like the parasite affects people the same way it affects rats. Rewiring does happen, but it doesn’t cause us to become sexually attracted to the smell of cat urine.XI It does other things instead.
For one thing, the reaction time of a human infected with Toxoplasma is around twelve milliseconds slower than normal. That’s a small difference, but it has been assumed to be the reason people with Toxoplasma infections are involved in car accidents more frequently than uninfected people are. Also, when scientists give standardized personality questionnaires to people, those infected with Toxoplasma score differently from people without those infections.32 However, the changes are subtle and hard to interpret. Men seem to disregard rules and act more suspicious and jealous, whereas women seem to be more warmhearted and easygoing. Both sexes show a decreased urge to try new activities. It’s all quite confusing, as though the parasites are tinkering with our minds but haven’t really decided what they want to change about our behavior.
In fact, that’s probably exactly what’s happened. A human brain and a rat brain are similar, but they’re sufficiently different that the parasite can’t use its rat-brain strategy in a human. Instead, Toxoplasma is following its DNA’s instructions, making nonsensical adjustments to its human host’s brain that would have worked in a rat. Since none of the parasites that end up in humans will ever get inside a cat, natural selection hasn’t been able to fine-tune those manipulations into anything meaningful.
Even though those changes to human behavior don’t serve any useful function for the parasites, they certainly make a difference to the humans. For one thing, people have suggested that Toxoplasma could be responsible for many of the cultural differences that exist between populations in different parts of the world. For instance, some might say that “Men disregard rules and women are warmhearted” is more true of Brazil, where the Toxoplasma infection rate in humans is 67 percent, as compared to South Korea, which has a 4.3 percent infection rate. People love to make jokes about how cats manipulate their owners, but when you factor in cat parasites, those jokes are suddenly way less funny.33
When Toxoplasma infects its intended host, the rat, it manipulates that host in a way that will cause the rat to die, because that manipulation will help the parasite live on. But sometimes a parasite’s best strategy is for its host to stay alive. In those cases, a parasite will take care of its host, and my favorite example of this is a story that involves bats. It’s a great story. I told it to my dad more than ten years ago and he still brings it up from time to time.
The parasite in question is a mite that lives on the ear of a moth. That moth uses its ears to listen for the calls of approaching bats. That’s an important thing for a moth to listen for, since bats hunt for moths using echolocation—shouting at high-pitched frequencies in the dark. When the moth hears a bat coming, it can do an evasive maneuver. A moth not listening for bats in the night would be as vulnerable as a gazelle in the daytime wandering blindfolded among lions.
Those ears, by the way, are located on the wings of the moths and are tuned to the high-frequency echolocation calls of bats—sounds so high-pitched that no human could ever hear them. There’s one ear on each wing and, like your ears, they operate independently of each other.34
The problem is that when parasites inhabit a moth’s ear (and there’s nowhere else they can live, by the way), they lay hundreds of eggs—and cause the ear to stop working in the process. That’s bad for moths because it means they can’t hear bats coming to eat them, and that, in turn, is bad for the parasite because once its moth gets eaten, the parasite dies too, along with all her eggs.
Now listen up, because this is the part my dad still can’t get over. A colony of mites will only ever infect one ear of a moth. Sometimes it’s the right and sometimes it’s the left, but once one ear is infected, mites will never infect the other ear. Why? Because that way the parasite get
s the best of both worlds. The moth can still hear bats coming with its one good ear, and the mites get to raise their offspring. The mites have to hurt their host to live, but they take good enough care of that host to keep it alive.35
The parasites are kicking butt out there, lazily stealing from the hard workers, and sometimes even making them do their bidding with mind control. Sloth is alive and well in nature. From vampire bats to Toxoplasma, the parasites are making nature more complex, more interesting, and (I would argue) more beautiful than it would be without them. And like the anglerfish that borrows from the parasite playbook during reproduction, we humans pay homage to parasites early in life as well.
Surely it’s occurred to you that a human fetus acts a little bit like a parasite to its mother, hasn’t it? As a fetus, Sam took oxygen and nutrients right out of Shelby’s bloodstream, and Sam’s fetal body even released chemicals that suppressed Shelby’s immune system so her body wouldn’t reject him. If imitation is the sincerest form of flattery, we humans make overtures to the parasites with every passing generation.
A mother and her fetus are very separate organisms, and sharing a body for nine months is hard on both of them. For example, the tension between a maternal immune system and the fetal immune system can have some surprising consequences having to do with homosexuality. The mother’s immune system develops a stronger and stronger immune response to the male antibodies produced by each successive son. The fact that younger sons get attacked more aggressively by their mom’s immune system is thought to be the reason that those younger sons are more likely to grow up to become homosexual. To be clear, this isn’t the reason homosexuality exists, but the mother’s immune response is probably the reason that the likelihood a boy will be a homosexual increases by 33 percent for each older brother he has. In the first son, it’s around 2.0 percent. In the second it’s 2.7 percent, and so on, to as high as 8.5 percent in the sixth son. This correlation only seems to apply to male homosexuality, not female, and the number of female siblings has no influence.36
And just as the fetus must survive the onslaughts of the maternal immune system, the mother must do that and more. However, despite the morning sickness and the stretch marks, and all the other costs during pregnancy, not to mention the ordeal of childbirth or the considerable work it takes to look after a human being, a child cannot ever be a parasite to his mother. A mother gets such an enormous benefit from her baby’s existence that it outweighs any and all of those costs. It’s the benefit Shelby gets from Sam, and the same benefit I get from Sam, as his father. Through Sam, our respective, separate DNA sequences have been passed on so that they can survive, even after their meat robots die. In the game of life, there’s no greater prize than that.
Parasites complicate the meat robot story, though, because parasites turn the very concept of individuality on its head. When a rat gets sexually aroused by cat urine, is the rat’s body still acting as the rat’s meat robot, or has the rat’s body now become the meat robot of Toxoplasma? When a human spills hundreds of millions of schistosome eggs into a stream through his urine, is that human working for his own DNA or for the DNA of the schistosomes inside him? When a moth survives a bat attack, does it have its own DNA to thank or the DNA of the mite? When Sam wakes up at 3 a.m. for no reason at all, and I run to his room to rock him back to sleep, am I my own DNA’s meat robot, or has Sam’s DNA been pulling my puppet strings? All of a sudden, the DNA strands piloting the meat robots of the world become less discrete. It’s the ultimate in sloth—if you’re too lazy to build your own meat robot, find a way to hijack someone else’s.
* * *
I. The project would ultimately be designed to figure out how bats with these little suction cups on their wrists and ankles use them to hold on to the smooth surfaces of leaves. A couple of years later, I published that paper (Riskin and Fenton 2001) and then followed up with a second paper a decade later, about a similar bat that lives in Madagascar and does the same thing (Riskin and Racey 2010). These two different kinds of bats are a remarkable example of convergent evolution, having evolved their adhesive organs separately. The Costa Rican bat sticks to leaves via suction, but the one in Madagascar uses wet adhesion, like a wet piece of paper sticking to glass. It took me more than twelve years to uncover all that, and it all began with that first trip to Costa Rica.
II. It’s actually possible to drink blood without being a parasite, just so long as you find a way to do it without hurting your host. For example, the spider Evarcha culicivora of Kenya feeds on human blood, but it never harms humans. It gets all the human blood it needs by killing and eating mosquitoes. Mosquitoes that haven’t fed on humans don’t have enough nutrition for the spider—it really needs the blood—but since it takes that blood from humans without ever hurting a human, it’s not a parasite of humans. If anything, since it eats mosquitoes, the spider that drinks our blood is an ally (Cross and Jackson 2012).
III. Common vampire bats almost always feed on cattle, and that presents a wonderful mystery: since vampire bats only live in Central and South America, and since there were no cows in that part of the world before 1492, no one knows what vampire bats fed on before livestock was brought from Europe. Biologists assume that vampire bats originally fed on all kinds of different rainforest mammals, and that they switched over to cattle once the cattle were introduced. Imagine eking out an existence drinking blood from well-hidden, small rainforest mammals that can bite you back, and then suddenly having giant, defenseless cattle brought in. It would have been like getting a McDonald’s drive-through in your neighborhood. Today, it’s almost impossible to find common vampire bats that don’t get all their food from cattle, so we may never know what the original food of vampire bats was.
IV. A non–Gough Island mouse, that is.
V. The book was Just Bats by Brock Fenton (1983). After I read it, I contacted Brock and told him I was interested in learning more about bats. He was very supportive. Years later, he invited me to do my master’s degree with him. He was the one who sent me on this very trip to Costa Rica.
VI. The fact that vampire bats can’t taste anything was uncovered by a guy with whom I did my master’s, John Ratcliffe. The experiment is pretty simple: you feed an animal something with a strong flavor, and then you make it feel sick. Afterward, it won’t want to eat anything that tastes like that. You know how your uncle Bert can’t stand the smell of tequila because of a bad experience he had in Mexico that one time? Same thing. To date, vampire bats are the only animals ever found that don’t learn to avoid a particular taste after repeated experiences of sickness (Ratcliffe et al. 2003).
VII. I say “she” because only female mosquitoes bite humans. Mosquitoes of both sexes eat fruit juices and nectar, but females also feed on blood to get iron and a few extra proteins for laying eggs.
VIII. I say “painlessly” because the show about parasites I work on, Monsters Inside Me, once featured an interview with a guy who got a leech up his nose in Nepal. He didn’t even know it was there until he started getting nosebleeds, and then later periodically saw it stretching out of his nose, waving in front of his face. Throughout his experience he felt no pain, so I assume the animals upon which these leeches typically feed also feel no pain.
IX. In fact, the Greek word schistos, which gives schistosomes their name, translates as “split” or “divided.”
X. It was in the tiny village of Kianjavato, and we were there searching for the elusive Madagascar sucker-footed bats.
XI. If cat urine does turn you on, I’m afraid you’ll need to find a different excuse. Sorry.
4
GLUTTONY
Eat and Be Eaten
One of the classic arguments people use to try to prove that nature’s actually really nice to us is the wealth of foods she provides. You can load up your grocery cart with fruits, vegetables, breads, cheeses, eggs, meats, and all of them came from the natural world. Even the high-fructose corn syrup in your gummy bears came from a plant. It’s
a fair question: If Mother Nature’s so selfish and violent, why does she do such a wonderful job of keeping us alive?
That supposed care she provides is an illusion. She’s not really trying to feed us at all. Instead, we’ve just become masters of working nature over to feed ourselves. Plants and animals weren’t put here to take care of us. They appeared as the result of evolution, just as we did. They are other kinds of meat robots doing their best to survive and reproduce, just like the rest of us. (I suppose it’s a little strange to call a plant a “meat robot,” but I’m going to do so anyway, just to be consistent about the relationship between DNA and the body it makes.)
Plants are selfish, so they go to extraordinary lengths to avoid getting eaten. They produce everything from thorns to poisons to keep animals away. In response, we’ve learned to avoid most of the plants altogether and focus instead on the small minority of plants that we can eat. We actually don’t eat very many kinds of plants at all. We’ve selected a very small number of plants that don’t hurt us and then bred them to make them even more delicious.1 For instance, cabbage, kale, broccoli, cauliflower, brussels sprouts, and kohlrabi all come from the same species of plant, Brassica oleracea. The plants aren’t trying to help us; we’re helping ourselves.
As for the animals we eat, I think it’s a safe bet that they don’t really want to be eaten by you or have their unborn babies made into omelets or have the milk they make for their own babies mixed with your coffee. Fish may be a great source of omega-3 fatty acids, but those molecules aren’t inside that fish for your health. They’re there because they’re good for the fish. When you go for sushi, you’re stealing parts from that fish’s meat robot and incorporating them into your own.
