by Riskin, Dan
XI. What’s truly amazing about the duck’s penis inflation system is that it’s not driven by blood pressure, as it is for humans. Instead, that rapid expansion is driven by the lymphatic system—the same system that slowly drains excess water from your swollen tissues. The wonderful biomechanics paper that explains all this is Brennan et al. (2010). It includes some glorious high-speed videos of duck penises doing their thing, sometimes inside see-through glass vaginas. Those videos are also available on YouTube: http://www.youtube.com/watch?v=qwjEeI2SmiU.
XII. Female waterfowl are especially likely to be killed by marauding males in parks, compared with in the wild, because the sex ratio in parks is often even more biased toward males than usual (McKinney and Evarts 1997).
XIII. Yes, even Flipper had a dark side (Connor et al. 1992).
XIV. It’s dark where deep-sea squid live, and it’s hard to tell males from females, so when a male encounters another member of his species, he injects a sperm packet into its skin as quickly as possible. If it’s a female, he’ll probably fertilize some of her eggs. If not, well, no big deal, really. Researchers cleverly named this a “shot in the dark” strategy (Hoving et al. 2011).
XV. One thing I love about these bats is that they can tell the difference between the calls of poisonous and nonpoisonous frogs, so they can selectively attack nonpoisonous frogs and ignore the calls of poisonous ones (Page and Ryan 2005). Unfortunately for the male túngara frog, he is not poisonous.
XVI. The most spectacular male birds you could ever possibly see display are the birds of paradise (Scholes 2008). I’m talking about the actual birds, not the flowers with the same name. If you’re not familiar with birds of paradise, spend five minutes on YouTube to see some of the videos Ed Scholes and Tim Laman have taken. Your eyes will explode out of your head in disbelief. Here’s a start: http://www.youtube.com/watch?v=YTR21os8gTA.
XVII. Those red, yellow, and orange pigments are called carotenoids (Hill 2000).
XVIII. This difference in life history between males and females is part of the reason females can afford to be so big and noticeable, while males need to be small. Their size helps them avoid getting spotted and eaten while they look for females (Vollrath 1998).
XIX. Well, he might get the benefit of having an enjoyable time, but he doesn’t make any babies this way. From his DNA’s perspective it’s a waste of energy and a waste of sperm. To get any real advantage from mating, he needs to mate with a member of his own species.
XX. The link to that video is here: http://www.youtube.com/watch?v=4-94JhLEiN0. Last time I checked, it had more than 80 million hits.
XXI. Shelby’s PhD was mostly about what happens to the quality of water and soil when you convert native Amazonian rainforest to a giant soy field (Riskin et al. 2013).
XXII. The Giardia parasite is one you can also get in North America. You might have heard of it by its nickname, “beaver fever.” It can cause intense (and I mean intense) abdominal pain, diarrhea, blood in the feces, and sometimes even blood in the urine. Humans get Giardia by ingesting water tainted by the feces of animals (beavers, humans, whatever) that are already infected by the parasite. In other words it’s completely preventable so long as people have access to clean water. Despite that preventability, hundreds of millions of people worldwide have that parasite right now.
3
SLOTH
Just Another Day in Parasite Paradise
Sloth, or laziness, seems at first like something that only affects humans. After all, we invented TVs, recliners, and video games. We work at desks, we drive cars, and we take the elevator when it’s only one floor. Obesity is on the rise in almost every country now. By 2008, 1.5 billion adults were overweight, 170 million children were overweight or obese, and those numbers continue to climb.1
Just as you’d expect with all these big people walking around, experts have been coming out of the woodwork with advice on how to lose weight and stay fit. Many of those antidotes have to do with living “naturally.” Our ancestors weren’t fat slobs (so far as we know), so we’re told that our lost connection with nature is what’s made us so lazy. Nature’s a perfect model of hard work, isn’t it? Survival of the fittest, right?
Nice try.
Sure, I’ll concede that a beaver, for example, epitomizes industriousness and hard work as she builds her dam. But beavers aren’t the only animals out there. The belly of that beaver is home to a whole ecosystem of lazy creatures, stealing mouthfuls of her food for themselves, some even feeding on the beaver’s own flesh. Those freeloading parasites thrive inside that beaver’s body, taking full advantage of all her hard work. And they give absolutely nothing in return.
You cannot possibly get any lazier than that.
If you ask, most people will tell you that they love nature, but many of those same people don’t even want to look at a picture of a parasite. Fact is, though, parasites are as natural as the birds and the bees. In fact, birds and bees are covered with parasites of their own, as is pretty much any other animal you can name. In fact, although it may sound counterintuitive, biologists have even argued that parasites are the sign of a healthy ecosystem.2
Like most people, I didn’t love parasites at first. I was curious about them, but I went into my undergrad program to study bats. The only reason I learned anything at all about parasites was to fulfill my degree requirements. Parasites were confusing to me. I can still remember the agony of trying to memorize their Latin names and life cycles and trying to tell them apart in their specimen jars. They honestly all just looked like overcooked spaghetti. I learned what I needed to pass the courses, but the parasites never really engaged me.
I would have laughed if someone had told me then that in just a few years I’d appear on an internationally broadcast Animal Planet TV show about parasites called Monsters Inside Me, or that I’d become a frequent guest on the Late Late Show with Craig Ferguson, talking with him about the very same worms whose names I previously couldn’t even remember. In a few short years, parasites would become a big part of my life, but before all that, I had to somehow realize how incredible they were.
Fortunately, there’s a bat for that.
Within a month of starting my master’s degree, I was sent by my advisor down to Costa Rica to figure out whether a project idea would have any chance of working.I It was my first time in the tropics, and there were so many bats to see. On my second day there, I went with a more senior graduate student named Maarten Vonhof to look for bats in a small cave. That was a very good day—my first encounter with vampires.
Vampire bats drink the blood of other animals, so they are parasites. By definition, a parasite is an animal with a relationship to some other animal, called a host. In that relationship, the parasite gets an overall benefit, and the host pays an overall price. The parasite might get nutrition, protection, or transportation. The host might have some food stolen, or it might get injured, or it might even die. Whatever the exchange, the parasite comes out ahead and the host loses. It doesn’t matter if the parasite is a worm, a fish, or a bat. So long as it’s mooching off a host, it’s a parasite.II
Vampire bats eat blood and nothing else. Although that might seem grotesque, it’s really kind of brilliant. If you think about it, blood is the perfect food. When you eat, your digestive tract pulls nutrients out of your burrito and puts them straight into the bloodstream so that as the blood flows through your body, all your cells can get those nutrients. When a vampire bat drinks a cow’s blood, it taps into a cocktail of molecules that includes everything it needs. That said, getting blood out of a cow that weighs 14,000 times what you do isn’t easy.
Of the more than 1,200 bat species that live in the world, only three drink blood. They don’t turn into well-dressed European men with fangs, but they are called vampire bats nonetheless. All three species live in Central and South America, with two of them feeding on the blood of sleeping birds, by sneaking up on them in trees and biting their toes, and the third, called the com
mon vampire bat, feeding on the blood of mammals. While the common vampire bat usually feeds on cow blood, it can feed on a wide range of animals, and it has been known to feed on sleeping humans.III
The common vampire bat is about the size of a mouse.IV To feed, it sneaks up on a sleeping cow by crawling up to it on the ground. When it gets close enough, it uses heat sensors on its nose to find blood vessels close to the cow’s skin. It’s not looking for a big jugular vein or anything, just some capillaries: think of your cheeks, the top of your head, or your fingers and toes. Places that get rosy from blood just below the surface, and that feel warm to the touch. On a cow, capillary beds like that can be found around a hoof, on the neck, around an ear, or right on the sex organs. That’s where vampires bite them.
Unlike Dracula, the real vampires don’t clamp on with fangs. Instead, the bat neatly shaves fur with its teeth from the area it wants to bite, then makes a shallow slash in the skin with its upper middle two teeth. The resulting divot is only about a quarter inch deep and a quarter inch wide, but it bleeds, just like a man’s cheek does when he cuts the capillaries in his face while shaving. Thanks to compounds in a vampire bat’s saliva, the divot in the cow’s skin will continue slowly dripping blood for as long as the feeding takes.3 The bat will feed for twenty to forty minutes, urinating all the while to rid itself of excess water, until it’s consumed about a tablespoon of blood, increasing its weight by around 50 percent. Then the bat jumps into flight and goes back to its roost to hang out.
It’s an important point that vampire bats approach their hosts by walking up to them, because walking is super weird for a bat. Vampires only walk because, unlike all the other bats, they make their living on blood. Most bats never land on the ground at all, and those that fall accidentally typically jump off the ground as soon as they can. Vampire bats, on the other hand, are perfectly comfortable walking on the ground, and they can launch themselves off the forest floor with amazing dexterity. In fact, there’s a very famous biomechanical study on vampire jumping; it shows that a vampire bat can launch itself in less time than it takes for a human to blink (which, as you already know, is about as fast as a duck unfolds its penis) and can jump upward three feet or more. For an animal the size of a mouse, that’s insane.4
Hiking toward a cave in Costa Rica that I knew might have vampire bats in it made me feel like a twelve-year-old going backstage at a Justin Bieber concert. Maarten, who had been in Costa Rica for several months already, said that the vampires had been in the cave each time he’d checked, but when he saw how excited I was getting, he was quick to mention that he hadn’t been to the cave in several weeks, so there was no guarantee. First, we canoed across a small river, tied the boat to a branch, then set off along a path. As we walked, the forest rose up high above me, and every bump on every branch had a very real possibility of revealing itself to be a monkey or sloth. It was muggy and wet and smelled like mud with a hint of spice. There was a constant buzz of cicadas, and I could see insects all over the forest floor, but there were surprisingly few mosquitoes. Colorful birds zipped across the path ahead of us. I’d always wondered what tropical rainforests were like, and now I was in one.
We stopped about thirty feet from the cave, and Maarten explained the layout so that I’d know what to look for before scaring all the bats away. On the wall near the mouth of the cave I’d find some sac-winged bats. Those are insect-eating bats best known for the way males throw urine, saliva, and semen on females they want to mate with (more about them in the chapter on envy). Beyond those bats, I would need to crouch to enter the main chamber, which was only about four feet high. On the ceiling, I’d see some short-tailed fruit bats—bats that can tell what kinds of nearby fruit species are ripe by smelling the breath of well-fed bats coming into their cave. Past that main chamber, Maarten told me, the cave narrowed to a hole in the back wall at floor level, some eight feet or so from the entrance. That hole had an open shaft above it, almost like a chimney above a fireplace. Maarten told me that if I slid on my back, pushed my head into that hole, and looked up, I might see a few vampire bats three or four feet directly above my face.
I walked to the cave as silently as I could, found the sac-winged bats, and took some photos. Then I crouched to look in the cave. There was movement, but I was still too tall to really see what was going on, so I took off my backpack, turned around, then lay down on my back and slid into the cave. The cave floor was wet and smelled terrible. As I slid through the bat droppings, I felt the sludge slide through my hair, down my neck, and into my shirt. Halfway into the cave, I turned on my headlamp and immediately saw the short-tailed fruit bats, huddled together, looking straight down at me. One of them flew away when my light came on, but then it came right back just a few seconds later and landed right where it had been, next to another two. As they hung, they twisted by their toes, pivoting around, with their faces rotating above me. They were really looking at me, and although I couldn’t hear it, I knew they were observing me with their ultrasonic echolocation calls as well. I’d never experienced anything like this. It was the kind of moment I’d dreamed of having ever since I read my first book on bats in high school. I would have preferred not to have insect-laden guano down my neck, but in spite of that, this was the coolest thing I’d ever done.V
A vampire bat needs to feed pretty much every night or it will starve to death, so if one comes back to the roost without having successfully found a meal, it will beg the other vampire bats to barf up blood for it. The hungry bat does this by going from bat to bat, licking their mouths until one of them pukes a little to help them out. What makes that French-kissing blood-vomit exchange so incredible (besides the fact that it’s a French-kissing blood-vomit exchange) is that bats will even help unrelated individuals this way. In other words, while it wouldn’t be surprising to see a mother feed her baby, or even a brother help a sister, you don’t ever expect animals to help nonfamily members. That would seem to violate the Scrooge-like rules of selfishness that govern all animals. But the food sharing by vampire bats works because the vampire bats are smart enough to remember who has helped in the past and who has not. If a bat keeps on begging but never shares, the group can just stop puking for that one bat. By puking today, a bat ensures that it will get help when it’s down on its luck next time. Other than humans, vampire bats are among the few animals on Earth that will give away food they have already secured just to help out an unrelated member of their own species.5
I turned off my headlamp and slid onward through the dark, until my head went into that hole at the back of the cave. As I slid from a floor covered in pooped-out fruit to a floor pasted with pooped-out blood, the smell got worse. I came to a stop; then I heard the loud screeching noises coming from above me—sounds I’d never heard, but that I could only assume were the voices of vampire bats! I brought my right arm up toward my head to turn my headlamp on, but my knuckles smacked into rock. That’s when I realized that my head was plugging the only exit to the vampire bats’ chamber. It was too dark to really see what was going on.
I tried a different path for my arm, up my chest and against my face, and was finally able to flick on the headlamp.
There were three of them!
I knew those faces instantly: common vampire bats, Desmodus rotundus. In the light they got louder and started moving around on their perch above me. They bared their pointy triangular teeth at me and screeched like miniature dragons. I couldn’t get my camera into the hole with my head, so I had to just lie there and soak it all in. I fully admit that it was uncomfortable—even scary—but I think that was part of what made the whole experience so life-changing. I wasn’t in control. My face was exposed to them. The best way I can think of to describe the vulnerability is to say that it was like swimming in the ocean for the first time. You’re scared, but the thrill and the beauty are enough that you’re willing to be scared.
These three vampire bats were exotic, but what made them even more beautiful to me was the context—a
ll those scientific facts I’d acquired made them familiar, like celebrities. I knew so many things about vampire bats that set them apart from all the other bats. Feeding only on blood, with no variety in diet at all, has caused vampire bats to lose their ability to discern different flavors.VI Because they drink up to half their own body weight in one meal, the stomach of a vampire bat has a side pouch that can fill up with blood quickly and then trickle that meal through the digestive system slowly over the ensuing hours.6 That same out-pocket is what makes it possible for bats to easily puke blood up for one another when they share food. By extension, the food-sharing behavior itself is a by-product of the fact that vampire bats have become parasites.
What started to dawn on me that day was that the vampires are strange and charismatic because they feed on blood. In other words, being parasites is what made vampire bats so wonderful. Lying in that cave was one of the critical moments that set me on the road to becoming a bat biologist, but it was also the first time I really appreciated how incredible parasites could be. That sparked a curiosity in me about whether there might be any other lazy parasites out there worth a second look.
Were. There. Ever.
Parasites live everywhere there is life. The squirrel in your backyard is filled with microscopic creatures; the bird at your feeder is covered in teeny parasitic arachnids. Panda bears have parasites, as do the penguins of Antarctica.7 In fact, to my knowledge, biologists have never found an animal species without parasites. Those lazy parasitic lifestyles are hugely successful. If a full census of life on Earth could ever be completed, biologists have argued, the total number of parasite species would likely be even bigger than the number of nonparasite species.8
Humans can get parasites too. You’ve experienced a mosquito bite, I’m sure. Remember the itch? That happened because when that mosquito bit you, she began by spitting into your skin.VII She hawked that loogie into your blood before she started drinking because her spit prevents your blood from clotting. Your immune system went into action to clean up the spit (too slowly to stop the mosquito, but fast enough to make you itchy for a few days), and the inflammation that resulted was itchy.9
