Mother Nature Is Trying to Kill You (14 page)

As you read this book, your eyeballs move from side to side because they’re pulled by muscles. The energy burned by those muscles originally came from plants. It’s kind of a shame, when you think about it. All this energy rains down on us from the sun each day, but as animals we can’t tap into any of it. Instead, we let plants do that work, and then we eat the plants. It’s as if instead of bringing lunch money to school, we beat up the kids who bring their own lunch each day, and take theirs.

Whole ecosystems work this way. The energy inside a deer came from solar-powered plants. When a cougar kills a deer, the plant energy is passed on—to the cougar, but also to the cascade of small mammals, birds, insects, fungi, and bacteria that clean up the bits of carcass left behind by the cougar. All those living things (and their parasites, by the way) constantly fight one another to get energy into their bodies. Then the game becomes one of making sure no other animals steal the energy from them. Energy is constantly flowing through nature, and gluttony is the mechanism by which it does so.
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As the nozzle through which energy flows into ecosystems, plants have it pretty rough. There’s an endless parade of animals, from aphids to zebras, constantly trying to take a bite out of them. Plants can’t run and hide, so they have to hunker down and defend themselves. As a result, plants make up some of the most violent, ruthless, and lethal instruments you’ll ever find in nature’s arsenal.

It’s easy to think of plants as harmless, as we look around the produce section of the grocery store, but of course that’s because none of the dangerous plants are there. In the past, foraging for fruits and vegetables meant hunting among hundreds of inedible plant species for something edible. There are more than 250,000 kinds of plants in the world, but we humans get more than 90 percent of our calories from just fifteen of them. That’s around a 160th of 1 percent.
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Most of those plants have been cultivated by humans for thousands of years to make them better for us than nature originally made them, or at least more appealing or easier to eat. Grocery store advertisers may use the word
natural
to describe their foods, but the produce section of the grocery store is a far cry from walking across the African savannah looking for something to eat.

To defend themselves, many plants use thorns and spines. (Ever tried to chew on a rosebush?) Sharp, poking bits can make
it painful to touch a plant—never mind eat one—and plants often make those spikes even more effective by filling them with noxious chemicals. That way, animals that try to eat the plant get blisters and sores to remind them not to do that again. However, by far, the most impressive set of spines on any plant has to belong to the bull’s horn acacia. Working on the principle that sometimes the best defense is a good offense, that plant has found a way to keep weapons inside the thorns that can crawl out to repeatedly sting anyone who comes too close. Even more incredibly, it does this by secreting a chemical that isn’t noxious at all. The acacia plant secretes a nectar.

That nectar is there to feed tiny, vicious, wasplike stinging ants that make their home inside the plant’s hollow branches and thorns. The ants live nowhere else—that’s why they’re called acacia ants. The ants don’t hurt the plant, but they get all their food from it. Acting selfishly, the ants defend that food source against other animals, and that works out very nicely for the plant. If some deer takes a bite of bull’s horn acacia, it ends up with a mouthful of stinging ants.
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And these ants’ stings are particularly painful (a “rare, piercing, elevated sort of pain,” as you will see in the chapter on wrath). Acacia ants can even turn elephants away.
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The ants aren’t parasites of the plant, since the plant gets a benefit. In biological terms, their relationship is called a mutualism: both parties benefit. The plant gets protection, and the ants get room and board. But it’s not all hugs and kisses between them. The two species have been working together so long that the ants no longer have the ability to find food anywhere else, and that gives the plant a lot of control in the relationship. For example, the amount of nectar the plant is willing to produce varies depending on how worried it is about herbivores. Over the course of
the year, and even throughout the day, the plant gives up as little sugar as possible, only secreting more when it needs the ants to provide better protection. The number of ants that can live on the plant’s nectar rises and falls at the whim of the plant. You can think of the plant as a millionaire, paying a fleet of security guards as little as possible to defend its factory, with some dying in the fight against intruders, and others simply dying of starvation when layoffs happen during periods of low crime.

The bull’s horn acacia has turned acacia ants into a living, swarming defense weapon, and it decides when and where that weapon will be ready for use. In essence, the plant has enslaved the ant meat robots to work for its own DNA. From the plant’s perspective, the ants have pretty much become part of its own body.

Although enslaving ants is effective as a defense system, it’s a solution to herbivores that hasn’t evolved in most plants. Instead, the vast majority of plants do something simpler to deal with herbivores: they just make themselves poisonous. It’s estimated that plants have come up with more than 200,000 different chemical compounds, and the effects those chemicals have on the animals that eat them can be wonderfully brutal.
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One challenge with poisons, though, is that the poisons most lethal to animals are often also deadly to plants. That’s bad news for the plant. There’s no point in making poisons to keep animals from eating you if you’re going to kill yourself in the process.

For example, hydrogen cyanide is extremely lethal to animals. The lethal dose for a human is around half a milligram of cyanide per pound, so one hundred milligrams could take out a two-hundred-pound person. (For reference, a toothpick weighs around one hundred milligrams.)
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Hydrogen cyanide kills animals because
it interferes with the molecular machinery used to get energy out of sugars. Since plants also get energy from sugars, hydrogen cyanide kills plants too. But despite that, more than 2,500 species of plants produce cyanide to deter herbivores.
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Somehow, though, the plants don’t succumb to their own poison, and the way they achieve that is brilliant: they basically pack hydrogen cyanide into bombs that will only go off if the plant gets eaten.

Here’s how the bombs work: Instead of making hydrogen cyanide ahead of time, the plant builds a bigger molecule that has hydrogen cyanide inside it. Because the hydrogen cyanide is stuck inside that larger molecule, it can’t perform the chemical reactions that would otherwise make it poisonous. That part is the bomb. Simultaneously, the plant builds an enzyme that can break that hydrogen cyanide free from the rest of that molecule. You can think of that enzyme as the bomb’s detonator. The plant stores the bombs in tiny walled-off sacs throughout its tissues and then surrounds those sacs with clusters of detonators. So long as the plant is uninjured, the chemicals stay separate, and no poison is ever made. But once a herbivore takes a bite of the plant, the sacs are mechanically broken by the predator, the detonators set off the bombs, and the lethal hydrogen cyanide is secreted right into the mouth of the herbivore. It’s a perfect system, and the real beauty of it is that the uninjured parts of the plant don’t get poisoned at all. Only those parts of the plant that are already being chewed on end up getting sacrificed.
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Not surprisingly, hydrogen cyanide creates problems for a lot of herbivores. Gorillas and rhinos, for example, eat plants that defend themselves with hydrogen cyanide, but they limit those plants to a small fraction of all the different foods they eat, presumably so that the poison dose stays small enough not to hurt them.
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Anyone who’s watched a James Bond movie or seen an Agatha Christie play knows that cyanide is deadly to humans, but humans routinely eat plants that produce hydrogen cyanide. Cassava (also called manioc) is a root, sort of like a potato, that is a staple in the diet of roughly 500 million people worldwide. It’s mostly eaten in Africa, the Philippines, and Brazil but also makes its way into kitchens in Europe and North America.
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The roots of that plant are deadly if eaten right out of the ground, which is part of the reason it works well as a crop: few animals can spoil the harvest by eating your crop beforehand. Humans get past the hydrogen cyanide in cassava by presoaking, fermenting, or cooking it, to break down the dangerous chemicals inside it. From time to time, though, people do die as a result of eating unprocessed cassava. It’s a sobering reminder that we survive by exploiting the plants and animals around us. Mother Nature isn’t trying to keep us healthy. We’re just taking what’s available in nature to look after ourselves.

The other strategy plants use to avoid self-poisoning is to make poisons that hurt animals but have no effect on plants. That way, the plant can produce those chemicals to its heart’s content without ever having to worry about getting hurt. Such chemicals usually work by focusing on animal body parts, like nerves, that plants don’t have. One plant, called zonal geranium, puts a drug in the petals of its flowers called quisqualic acid. Beetles that eat the petals feel fine at first but after about thirty minutes start to realize their hind legs don’t work so well, and before long they are totally unable to move. From tests in the lab, scientists know that the drug’s effects last only about twenty-four hours, but out in the woods where this plant grows, a beetle lying defenselessly on the
ground for a day is almost always eaten long before its paralysis is over.
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Another great plant poison comes from the corn lily, which makes a chemical called cyclopamine. Cyclopamine doesn’t hurt the plant at all, but it has a very strange effect on sheep. I’ll give you a second to guess what that is. Your hint is that the name for cyclopamine comes from the one-eyed Cyclops of Greek mythology.

Got your guess? Okay.

Cyclopamine has no effect on an adult sheep, but if a pregnant sheep eats corn lily on the fourteenth day of fetal development (not before or after), the cyclopamine within the plant will block one particular set of genes from doing what it needs to do inside the sheep’s developing fetus.
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That’s it. That’s all this drug can do. One little step on one particular day in the fetal development of a sheep, but it’s a whopper. The fourteenth day of fetal sheep development happens to be the day when cells in the fetus’s head that will one day become eyeballs split into left and right halves. That step is controlled by the gene that is blocked by cyclopamine. What all this means is that if the mother eats corn lily on the fourteenth day of her pregnancy, then four and a half months later she will give birth to a one-eyed monster. As a result, any sheep flocks that try to stay in the corn lily’s area will be unable to reproduce and, within a few short years, will die off from old age and leave the plants alone.
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Did you guess right?

As wonderful as targeted drugs like cyclopamine are, they have the drawback that they might work on just a subset of the herbivores that are trying to eat the plant. Cyclopamine works on sheep, but it might not work on grasshoppers, for example. Many plants that use targeted poisons get around that problem by secreting a whole cocktail of them, with the hopes that something in there will hurt most herbivores somehow. Other plants use a more measured approach. They wait to see what kind of animal is eating them and then secrete the appropriate poison in response. It might seem incredible that a plant can do this, but it’s true. The chewing bite of a caterpillar on a leaf of barrel clover will make the plant produce jasmonic acid, whereas the tiny piercing bite of a spider mite will cause the plant to make salicylic acid instead.
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Plants might look like serene, inanimate objects, but they know how to get by in a world of gluttony.

Remarkably, plants can even communicate with one another about herbivores, so that a plant can start making poisons in anticipation when an herbivore attack has begun nearby. When a sagebrush plant from the southwestern USA, for example, gets chomped on by a herbivore, it releases some chemicals into the air. When those chemicals waft over to a nearby plant, that nearby plant responds by producing antiherbivore toxins in its own leaves so that it will be ready when the herbivores get there.
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Why a plant should send that signal out is a bit of a mystery, since a selfish plant has no reason to help out its neighbors, but there are a few possible explanations. First, the plant may be secreting those chemicals to hurt the herbivores, and then nearby plants may just catch a whiff and react. Or maybe that first plant releases those chemicals as a way of quickly sending a message to all its other branches through the air. In other words, maybe
it’s communicating with itself, and then other plants are merely eavesdropping on the conversation. These are good guesses, but recently researchers showed that plants react more strongly to the smell of their clipped brothers and sisters than they do to more distantly related plants of their own species.
That suggests that plants really may be sending information on purpose, trying to help out closely related individuals that have many of the same DNA strands they do. Future research will tell how it all fits together, but it’s clear that plants have many more tricks up their sleeves to stop animals from eating them than most of us would ever have imagined.
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