by Riskin, Dan
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.19 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.20
As a twist on the theme of plants talking to one another, the tobacco plant may have my favorite antiherbivore strategy. When it gets fed upon by the tobacco budworm caterpillar, it releases chemicals into the air. But in this case, the tobacco plant isn’t crying out to another plant. It’s sending out a signal for help, almost like Commissioner Gordon sending out the bat signal on a rough night in Gotham. The chemicals waft out into the air, and in response, parasitoid red-tailed wasps come swooping in to the rescue. They quickly lay eggs inside the caterpillars, dooming them to the torture of being eaten alive from the inside out by wasp larvae.21 The wasps and plants gang up on the caterpillars, and Gotham is saved . . . until next time.
It’s a constant battle, back and forth, between the plants that make sugars and the animals that want those sugars. For the most part, plants tend to get the upper hand, but there’s one herbivorous animal out there that has really outdone itself. It’s an inch-long animal that lives in the warm waters of the Atlantic Ocean. It resembles a snail with no shell, and it’s called an emerald sea slug.
There are many kinds of sea slugs, and they’re honestly some of the most breathtakingly beautiful, colorful things you’ll ever see, but the emerald sea slug is even more fascinating for its diet than it is beautiful. It eats photosynthetic algae (sugar-producing organisms that are kind of like plants, but single-celled). Here’s where things get crazy, though. When the emerald sea slug eats algae, it doesn’t completely break the algae down to get the sugars out of them. Instead, it steals the molecular machinery from within the algae and moves that machinery close to the surface of its own transparent skin. Because the algae are green, that makes the emerald sea slug turn green. Then, astoundingly, the machinery continues cranking away, converting sunlight into sugars, inside the sea slug. The sea slug still has to eat algae from time to time, since the machinery wears down after a few months of use, but the result is an animal that performs photosynthesis—the near-impossible trick that Mr. Jani claims to have mastered.22
Even more incredibly, that solar-powered machinery isn’t the only thing the emerald sea slug has stolen from the algae. Somehow it has also managed to copy a chunk of DNA out of the alga’s genome and paste it into its own genome. That sequence of DNA is responsible for building some of the molecular machinery needed for photosynthesis. The sea slug still has to get most of the machinery by eating algae, but it makes a few bits of the machinery itself.VI
The relationship between the emerald sea slug and the algae it feeds on adds a whole new layer of complexity to the story of DNA and meat robots. Suddenly, DNA sequences are jumping between meat robots.VII It’s a reminder that DNA strands are the real players in the game of life, and that meat robots are just the biomolecular machinery.
The emerald sea slug isn’t the only animal that gets a little energy hit from sunlight. There’s an aphid that is orange when it grows up in normal conditions but grows up to be green if it’s cold outside. That green color occurs because under cold conditions, the aphid produces photosynthetic machinery. The blueprints for that machinery come from a DNA sequence that looks to have been stolen from plants.23 However, neither the emerald sea slugs nor the green aphids make enough energy themselves to survive completely without food.
To the plants, animals are often annoying predators that just make life harder, but that’s not always the case. Some of the most successful plants have found ways to make animals work for them. I’ve already mentioned the bull’s horn acacia, which enslaves ants to work as security guards, but that’s just the tip of the iceberg. It’s far more common for plants to enslave animals for something far better . . . and that’s to satisfy their sexual urges.
Yes, plants have sex. Plants have boy parts and girl parts, and they use those to make babies. Pine trees, for example, have sex by releasing their pollen (sperm, basically) into the wind from male pine cones, and hoping some of the pollen happens to land on the ovaries that lie between the cracks of another tree’s female pine cones.VIII Plants with flowers have sex by getting animals, like bees, to visit those flowers in exchange for nectar. In the process, the animals accidentally pick up pollen from one flower and drop it off at the next. Bees have basically become flying penises, meat robots under the influence of plant DNA.
Sex is the only reason flowers exist (which is why I sometimes smirk when I see people sticking their noses right into a bouquet of flowers: those are plant sex organs, after all). Many insects besides bees pollinate flowers, as do several birds, most notably hummingbirds. But what a lot of people don’t realize is that some bats feed on nectar and pollinate flowers too. Those bats get their food by sticking their faces down into flowers while they hover, just like hummingbirds do, getting covered with pollen in the process. In fact, there are quite a few flowers in the world that don’t use birds or insects at all but rely solely on bats to help them have sex.
One particular species of bat from the cloud forests of Ecuador, called the tube-lipped nectar bat, gets all its food from flowers. Because it’s become so dependent on the plants, it has fallen under the influence of the plant’s DNA, and this has made the bat into one of the strangest animals in the world.
Extend one of your arms out in front of you. Now stick your tongue out, and imagine your tongue could reach all the way to the tips of your fingers. The tube-lipped nectar bat can extend its tongue three times that far—one and a half times the distance from its head to its toe.
The plant that these bats feed upon is called Centropogon nigricans. At first, these bats probably enjoyed getting nectar from flowers much like those in your garden, but once those bats started relying on Centropogon nigricans for food, the bats’ bodies began changing in response to the selfish demands of the plants. It turns out that the deeper a bat sticks its face into a flower, the more pollen it will transfer. Plants with slightly deeper flowers forced bats to stick their heads deeper inside than plants with shallow flowers, and that gave deep-flowered plants a reproductive advantage. Over time, flowers got deeper and deeper. At the same time, only bats with long noses and tongues could keep up, so bats changed, locked in step with the flowers. Over evolutionary time, the bats grew longer and longer snouts, and longer and longer tongues. Today, as a result of a plant’s trying to pass on as much pollen as possible with each bat visit, the tube-lipped nectar bat has the longest tongue, as a percentage of body size, of any mammal.24
That bat’s tongue is a great example of the way plants have bent the bodies and behaviors of animals through evolution, for no ot
her reason than to serve the selfish needs of the plants themselves. Since plants can make the sugars animals need, plants have the power to make animals do pretty much anything they want them to. In essence, animals are slaves to their stomachs. They have to eat: gluttony is the only way to survive.
Besides gaining security or a way to have sex, plants get animals to do their bidding for one other task as well—helping plants move around—and this is where humans get as manipulated as any other creature out there.
The main problem with being a plant is that you can’t walk, so if you try to reproduce by dropping seeds on the ground, they’ll just end up trying to grow right next to you. That’s bad for the plant, since it puts it in direct competition with its own offspring for things like water and light and nutrients.
So instead of just dropping its seeds on the ground, many plants build a succulent, sugar-filled ball of deliciousness and hide their seeds in the middle of that. Animals come to take the food and end up carrying the seeds away too. That’s what fruits are, and although it might seem like nature created avocados and apples and oranges and bananas to make humans happy, those plants really just wanted to get their seeds dispersed. Feeding animals such as humans was a convenient way to get that done.
In some cases an animal will carry a fruit away, eat it somewhere else, and drop seeds in the process. In other cases it will eat the seeds along with the fruit and then poop them out later, somewhere far from the host tree. In fact, many seeds germinate better if they’ve been through the digestive tract of an animal than if they haven’t.25
There are thousands of different kinds of animals used by plants to disperse seeds. Many of them are things you’d expect, like toucans, parrots, monkeys, and fruit bats. But there are also more than two hundred different kinds of fish that eat fruits. That might sound impossible, but when you consider that the yearly flooding in the Amazon rainforest can raise the water level more than sixty-five feet, it’s not so hard to picture a fish swimming up to a branch and biting off a fruit. Some fish can carry seeds more than three miles before pooping them out. To the plant, that’s just as good as having a flying animal do the job.IX
For animals to do that work for the plants, the fruits need to be as attractive as possible, and that is why fruits are so damned delicious. No hydrogen cyanide, no one-eyed mutant babies, no paralysis. Just sweet fruity goodness. Papaya, watermelon, mango, cherries . . . the list goes on. In some cases we humans have just taken the fruits pretty much as they are from nature (mangoes, for example), but in other cases we’ve used selective breeding to make the fruits even more delicious than nature made them.
This is true of apples and oranges and many other fruits. Wild bananas, for example, have numerous large, stonelike seeds within them. Humans, over the millennia, have cultivated some kinds of bananas so that the seeds have been reduced to nonfunctional black specs. That’s taken away the banana plant’s ability to pass on seeds through its fruits, but since humans grow banana plants all over the world just to eat the fruits, you could argue that their fruits are still doing their job of ensuring that banana plants survive and reproduce.26
Plants make their own food, but gluttony’s still been an important factor in their evolution because it’s allowed them to extend their influence to the animal meat robots around them. But the importance of gluttony to the natural world is even more striking when the gluttons eat other gluttons.
Some of nature’s most charismatic animals are predators. You can tell because predators make for cool tattoos: polar bears, Bengal tigers, great white sharks, tarantulas, barn owls, saltwater crocodiles, rattlesnakes, orcas, wolves, giant squid, praying mantises . . . There are literally thousands of kinds of predators, living in all kinds of habitats, killing other animals every single day in the name of gluttony.
From an energetic perspective, eating meat isn’t really very different from eating a plant: the molecular machinery of one living thing gets broken apart, then incorporated into the molecular machinery of another living thing. But as humans we see a very important difference between carnivores and herbivores. So far as we know, plants don’t experience pain or fear, but when you kill and eat another animal, your lifestyle imposes suffering on other living things.
That distinction means a lot to us. We empathize with animals. Meat-processing companies spend millions to reduce the stress experienced by the animals they slaughter, and many people choose not to eat meat at all, just to limit animal suffering. But the overwhelming evidence from nature shows that other animals don’t care very much about that kind of thing (at least when it’s not happening to them). It’s impossible to know, but I have a hard time imagining that a spider eating a fly gives it any more thought than a sheep gives the blades of grass it chews on.
Carnivores have to eat meat, so they kill animals all the time, but there’s something strangely creepy about the fact that herbivores do this from time to time as well. For example, orangutans mostly eat fruit, but back in the 1980s, people started noticing orangutans killing and eating small (and heartbreakingly cute) primates called slow lorises.27 In one well-described case, the orangutan smacked a slow loris out of a tree, crawled down to pick it up, bit into its skull, sucked out the brain and eyeballs, then ate the palms of the hands, the genitals, and finally the internal organs and skin. For some reason, that’s creepy to me. An eagle does that and I don’t blink an eye, but when an orangutan shows its dark side, I get all freaked out. The fact is, though, meat’s a great source of food. I don’t know why I should hold orangutans to a different standard than anyone else. Just because orangutans are usually vegetarian doesn’t mean they’ve chosen that lifestyle to avoid hurting animals.
Eating meat has a lot of advantages over eating plants. For one thing, muscle tissue is packed with energy, so you get a bigger bang for your buck than you do eating leaves. Also, whereas plant-eating animals often need to eat many different kinds of plants to get all the nutrients they need (and avoid getting too much of any one plant-produced poison), a predator can find just about everything it needs in just about any animal. A mountain lion doesn’t need to eat deer one time and then rabbit the next. It just needs a certain amount of meat. That allows it to prey on whatever animals happen to be abundant, and that gives it flexibility in an unpredictable world.
If we’re going to talk about gluttony in predators, an obvious starting point is the wolverine, since its scientific name, Gulo, literally means “glutton” in Latin. A wolverine is badass. It weighs somewhere between twenty and forty pounds but can take down an eight-hundred-pound moose by jumping on its back and severing the tendons in its neck. You can think of wolverines as ninja mini-bears with semiretractable claws. (Seriously, that’s not far off.) In one instance, a single wolverine was recorded to have killed ten reindeer in one day. (Sometimes wolverines stash the food they don’t eat so they can come back to it, but that must surely have been overkill.)
As great as they are at hunting, wolverines are even better at finding carcasses of animals that have already died and eating those. In fact, wolverines have been known to track predators like wolves and lynx, wait until those predators make their kill, and then steal the carcass for themselves.
Using a combination of killing and scavenging, wolverines do very well for themselves. An impressive number of different animal species have been found in the stomachs of wolverines: moose, elk, caribou, deer, foxes, lynx, hares, marmots, ground squirrels, porcupines, beavers, voles, lemmings, shrews, magpies, hawks, ptarmigans, fish . . . even seals, walrus, and whales.28 That’s the flexibility you get by eating meat. Wolverines can eat whatever they get their claws on, dead or alive. But for all the glorious gluttony of Gulo, it’s far from being the most gluttonous predator out there. That title goes to an animal that at first might seem much less dangerous, the shrew.
Shrews make their living eating insects, worms, and the carcasses of larger dead animals. They’re small, but don’t underestimate them. Shrews are arguably the big
gest gluttons in the animal kingdom. Sure, a 3,750-pound elephant eats far more food than a 0.1-ounce shrew does, but when you take body size into account, the tables turn. The roughly 225 pounds eaten each day by that elephant make up around 6 percent of its body weight. Compare that to a recorded maximum 384 percent of body weight consumed by a shrew in one day and there’s no contest.29
Shrews and elephants are part of a more general trend: as you look across mammals of different sizes, the smaller they are, the more energy they use per unit mass. Since shrews are the smallest mammals, they’re the most gluttonous. Incidentally, if you gathered enough shrews to equal the weight of an elephant, those half a million individuals would collectively eat sixty-four times more food than the elephant would. Pound for pound, nothing’s so gluttonous as a shrew. 30
But there’s another way to measure gluttony in a carnivore, and that’s to ask what animal kills the most other animals to survive. Something like a wolverine might only make one kill a week, and the shrew might get by on just a few earthworms a day. Who does the most killing in the name of gluttony? For this one, we go to the other end of the size spectrum, to the largest animal in the world.
Blue whales aren’t just the biggest animals on Earth, they’re the biggest animals to have ever lived on Earth. They’re bigger than dinosaurs. A single animal can weigh more than 360,000 pounds, yet it feeds on shrimplike creatures called krill that weigh less than an ounce.31 As a result of that size mismatch, blue whales have to kill a lot of prey animals to survive. Fortunately for the whale, though, krill routinely swim around in dense clouds of individuals. That makes getting enough krill as easy as swimming to the right place and opening your mouth. To get that krill without drinking too much seawater, though, the whale does something analogous to the way you separate your spaghetti noodles from the water you cooked them in. The whale swims into a cloud of krill, closes its mouth around that cloud, then pushes water out of its closed mouth with its tongue, forcing the water through hairlike baleen it has around the seal of the mouth (where you’d find teeth in most mammals). With the water expelled from the mouth through the strainer-like baleen, the whale swallows the krill, then sets out preparing for the next mouthful.
