Mother Nature Is Trying to Kill You: A Lively Tour Through the Dark Side of the Natural World

by Riskin, Dan

  Looking back, though, I can’t help but see the way I experienced that day as textbook animal behavior. Here’s the father meat robot, following orders from his DNA molecules, releasing stress hormones with all the predictable physiological effects, in response to seeing his offspring in danger.

  And I still see myself like that to this day. Even though it feels like magic when Sam does something new, or even when he looks me in the eyes and smiles, it’s all just biology. I know that no matter what it felt like in those first six minutes, the emotions I felt the day he was born were really no more special than the hormones that guide a bird through building her nest.

  Envy is a big part of having a new baby, because you’re constantly comparing notes with other new parents. For Shelby, it was most pronounced when she spoke with other mothers who didn’t have C-sections. In the first few months after Sam was born we talked about it often. It didn’t matter what our doctor had said about the umbilical cord. Shelby couldn’t seem to shake the gut feeling that if she’d just been tougher or more patient, somehow her C-section could have been avoided. For so many of the mothers we’d talked to, it had just worked out, so Shelby’s instincts kept telling her that she’d done something wrong. With a couple of years between her and the experience now, she’s not so sensitive about it as she once was, but I know that it still bugs her.

  Now the comparisons we make with other families tend to be less about childbirth and more focused on child development. And we know that other parents are making comparisons too. Sam woke up four times again last night, but our friend’s kid, Julia, has been sleeping through the night for two months already. Sam’s learned to count to three, but Olivia can count backward from ten. Shelby and I know that the rate of development is different for different kids (and we’ve read several books and scientific papers about development—we’re scientists after all), but it’s still really hard not to have an emotional reaction sometimes when you hear something about another person’s kid. When Sam’s ahead, we feel smug, and when Sam’s behind, we speculate that it’s only because other aspects of his development are so advanced. But I’m embarrassed that we make those comparisons in the first place.

  It’s so stupid, really. Shelby and I know people whose kids have had cancer, for God’s sake. We have friends who have miscarried. You would think that we could just count our blessings and move on. Sam is a healthy, happy kid and he’s doing great. But there’s something about the experience of raising a kid that makes you listen very carefully to see what’s going on with all the kids around you.

  I suppose that from an evolutionary perspective, paying attention to the parenting experiences of other people is a good way to see how well your child is doing. After all, as we discussed in the chapter on greed, your kid is going to have to compete against other kids down the road, so how he compares to those kids is going to matter someday. But worrying about how many words Sam knows at eighteen months compared to the Joneses’ kid is a waste of energy. I really don’t like that I experience envy at all. Envy has been described as “feeling negatively about someone else’s success,” and I don’t want that to be part of how I raise Sam.

  Envy is part of the human experience everywhere in the world, and it can motivate all kinds of crimes, from vandalism to murder. But there’s really no better symptom of envy than theft. When humans experience the desire to have something that another person has, they often just take that object of desire away. Envy in animals is probably much less self-reflective, but theft among animals is everywhere.

  It might seem impossible that animals can steal from one another, since animals don’t have money or property law, but tell that to a cheetah who does all the work to sneak up on, chase, catch, and kill a gazelle, only to have that carcass taken away by hyenas. Since the cheetah has invested time and energy in that carcass, biologists consider it fair to call it theft when it’s taken away.III

  Some animals are more vulnerable to theft than others. Animals that swallow their food immediately, like snakes and frogs, don’t have much of a problem with other creatures intercepting their calories before they’re eaten. But lots of animals need time to ingest their food after it’s captured. Cheetahs are great examples of this, and so are ants.

  Leptothorax ants live in colonies of a few hundred individuals, and for those colonies to function, they need a food transportation system. Food is swallowed by workers, which return to the nest, then regurgitate that food to feed the growing larvae. Some of the larvae grow up to become workers themselves, while others grow up to be soldiers that defend the colony. At the center of it all is a single queen—the only one who can reproduce, and the mother of all new ants born into the colony. She’s the only hope any ant in the colony has of getting its DNA passed on to future generations, so every meat robot in the colony ultimately works for her survival. Because a Leptothorax ant colony is large and complex, it takes time for the colony to process food, and that makes it possible for thieves to work the colony over.

  One such thief is the queen of a different species of ant, called Epimyrma (the Latin name means “above ant”). She sneaks silently into the Leptothorax nest like a cat burglar. When the first Leptothorax guards attack her, she outmaneuvers them like Jason Bourne, then stings them with just enough venom to stun them. She doesn’t kill them because they’ll be useful to her later. Next, she rubs secretions from Leptothorax ants all over her body, to mask any trace of her foreign scent. Once that’s done, she can move through the colony undetected. Because she smells like a Leptothorax ant, the members of the colony have no way of identifying her as an intruder. Cloaked now, the invading female finds her way to the Leptothorax queen’s chamber, approaches the undefended matriarch, and throttles her slowly with her mandibles, until she is dead.1

  With the old queen dead, the Epimyrma queen takes her place as the new leader of the Leptothorax colony. She lays her own eggs, then lets the workers of the colony care for her and feed her offspring, unaware that they’ve been enslaved. In doing this, she’s not just stealing food from the Leptothorax colony. She’s stolen the colony itself.

  This kind of theft is practiced by at least two hundred different kinds of ants. However, not all ants do their stealing the same way. For instance, the ant Polyergus will mount “slave raids” in which roughly 1,500 individuals move together to attack the colony of an ant called Formica.2 The marauding ants will capture as many Formica larvae as they can find, then bring them back to their own colony. When the Formica larvae emerge as adults within the Polyergus colony, they just start working, with the mistaken impression that they are where they belong. The kicker is that Polyergus ants have become so dependent on their slaves that they can’t live without them. Polyergus colonies always have Formica ants in them. Without slaves to do the work, Polyergus ants can’t survive. It’s just another example of an entire species whose survival depends on causing hardships for members of another.

  Unlike ants, spiders are usually solitary, but their habit of storing food before eating it makes them vulnerable to theft too. An orb-weaving spider, for example, processes food relatively slowly. The spider spins a web and sits at its center, waiting for the vibrations that happen when an insect gets tangled up. If it senses those vibrations, the spider heads over to the struggling insect, wraps it in silk, stabs it with venom, and leaves it hanging, in order to continue hunting. As the spider heads back to its perch at the center of the web, the insect lies helplessly bound up, spider venom chemically digesting its internal organs. Later, when the spider wants to get the calories out, it can go back to the now-dead insect and slurp out the liquefied insides like a smoothie.

  The spider Argyrodes could do all that work for food itself too, but it chooses not to. Instead, it hangs out at the periphery of an orb-weaving spider’s web. Orb weavers are impressively large, around two inches long, while Argyrodes spiders are only a quarter to half that size. Sometimes, when a small insect gets tangled in the orb weaver’s web, the Argyrodes spid
er runs over and steals it before the orb weaver gets to it. That’s usually not a big problem for the orb weaver, though, since it most often happens with insects that are so small that the orb weaver probably would have just ignored them anyway.

  But sometimes Argyrodes goes for larger prey. When a nice big, juicy insect hits the web, the orb weaver immobilizes and injects it with venom as usual. The Argyrodes spider can observe the capture by feeling the vibrations in the web. Then it waits for the large spider to get distracted by a subsequent prey item. Once the orb weaver heads off to wrap up that second prey item, the Argyrodes spider scurries in, cuts the original meal out of the web, and runs away with it.3

  To add insult to injury, Argyrodes spiders don’t always eat the food they steal. In some cases, males have even been seen giving stolen food away to females as gifts, to avoid getting eaten during sex.4 We discussed in the chapter on lust how the preferences of females often create life-threatening hurdles for males to jump over before the females will agree to mate with them. A demand by females for gifts that have to be stolen from a predator big enough to eat the male is a perfect example.

  The strategy Argyrodes spiders use to steal from other spiders relies on never getting too close to them, but the spider Curimagua has no reservations about walking right up to a spider thirty times its size and stealing food right out of its mouth. It’s only about a millimeter long, but does its thieving from a one-and-a-half-inch-long spider called Diplura. Those larger spiders make funnel-webs, and when a large prey animal, such as a grasshopper, beetle, or even frog, walks near the opening of the funnel-web, the Diplura launches out, strikes it with its fangs, then drags its prey back to the opening of the funnel to suck its insides out.

  As the big Diplura spider starts feeding, the Curimagua spider walks right over to its mouth and starts eating alongside it.5 In fact, the tiny thief can apparently walk anywhere it wants to—in the funnel-web, or even across the Diplura spider’s eyes. The larger spider doesn’t seem to pay any attention to it all, perhaps because it’s so small that the calories stolen cost Diplura less than the calories it would have to spend catching and killing the intruder.

  It’s a strategy that works for the small spider. So well, in fact, that it has lost the ability to live anywhere else. Unlike other thief spiders, such as Argyrodes, that can make webs and catch their own prey whenever they need to, the Curimagua spider can no longer hunt for itself. So far as I know, it is the only spider species in the world, out of more than 44,000, in which individuals don’t do their own hunting.6

  Another unique spider that commits acts of thievery is a very special jumping spider called Bagheera kiplingi. (That Latin name will stick in your brain if you’ve ever read Rudyard Kipling’s The Jungle Book and you remember Bagheera the black panther.) Bagheera kiplingi is the only known vegetarian spider in the world. Remember those acacia plants that feed nectar to ants in exchange for defense? Well, Bagheera lives on those same acacia plants, eating the nectar that the plant has made for the ants. As you can imagine, this doesn’t thrill the ants, so the spider has to constantly hide from them, jump out of the way when attacked, or else hang from a thread so no ant can reach it. The spider isn’t a strict vegetarian—it dines on larval ants from time to time, so it’s also technically a predator of the ants—but Bagheera gets the vast majority of its food by stealing nectar from ants, rather than by killing them.7

  It’s not just creepy crawlies that deal with theft. It’s a problem for big animals too. Many African predators, for example, supplement the food they kill themselves with food they steal from other predators. Hyenas are famous for stealing from lions, but things aren’t as one-sided as The Lion King might have led you to believe. Lions are aggressive carcass thieves too, and it’s quite often the case that hyenas hunt and kill their own food only to have it stolen by a pride of lions.8

  Theft is more than a nuisance for these animals: it threatens their survival. Kills don’t happen every day, so predators sometimes go several days without food. That’s why having a meal stolen can create big problems, especially right after an animal burns a whole bunch of calories running down and killing its prey. A cheetah, for example, will hide its food as quickly as possible after a kill, but once competing carnivores find it, the cheetah has to move on. This means that even with an abundance of prey to feed on, cheetahs may not be able to use certain habitats because of competition with other predators. In fact, if a cheetah so much as hears the calls of hyenas and lions, it will stop hunting and move to another location, presumably to save itself the ordeal of having food taken away.9

  Similarly, African wild dogs are endangered, with fewer than six thousand individuals left, so nature reserves have been set aside to protect them and other endangered animals where they live. But because hyenas and lions do so well in those same reserves, the African wild dogs can’t seem to get a break. One study in Zimbabwe showed that African wild dogs inside one such park had their food stolen roughly twice as often as they did outside the park. As a result, they chose to spend most of their time outside the very conservation area that had been set aside for them.10

  There’s an appealing concept that nature is naturally balanced and self-regulating, and that the key to making nature thrive is for humans to take themselves out of the picture and let things come to their natural order. But the truth is that except for humans, animals have no concept of order and simply do their best to thrive as individuals. If animals in an ecosystem are left alone for long enough, a balance emerges, but when that balance is perturbed, things often don’t return to the same equilibrium. As a result, if we stopped managing wildlife in the savannahs of Africa, it’s not necessarily true that cheetahs or wild dogs would be able to bounce back. It was humans that brought their numbers down, but even if humans vanished, the problem of theft by other animals, or perhaps something else altogether, might prevent cheetah or wild dog populations from ever recovering.

  Theft by the meat eaters of the African savannahs has special significance to us because we evolved as a species among them. If we could travel far enough back in time, we’d see lions and hyenas stealing food from other predators, but they’d also be battling with a third species for those carcasses, and that third species would be us.

  To early humans looking for meat, chasing some lions away from a wildebeest carcass would have been dangerous, but it would surely have been a much more appealing strategy than the alternative of hunting the wildebeest themselves. For early humans, that second option would entail chasing the animal on foot for eight hours or more in the searing midday sun.

  The fact that humans can run down a large animal that way sounds impossible at first, but persistence hunting, as it’s called, has been documented in traditional societies around the world, including the Kalahari people of southern Africa, the Hadza people of eastern Africa, the Tarahumara and Navajo people of northern Mexico and the American Southwest, and the Aborigines in Australia.11 Humans are built for running. Even people who sit at desks all day but do a little exercise a few days a week can typically finish a 10-kilometer run without extraordinary effort. In fact, each year thousands upon thousands of people complete marathons of more than 26 miles, and some even go farther, completing ultramarathons of 100 kilometers (62.1 miles) or more. The idea that our ancestors could run for eight hours straight isn’t so far-fetched. And although big animals are faster than humans over short distances, even they can’t run as far as a fit human can. Over long enough distances, humans are some of the best runners on the planet.

  When a human chases a big animal under the hot sun, the animal being chased eventually overheats. Humans can cool themselves while running by sweating constantly, but large four-legged animals cool themselves by panting, and since those animals take only one breath with each set of strides, they can’t pant fast enough while running to cool themselves down. Without a chance to stop and pant in the shade, the larger animal eventually overheats, collapses from exhaustion, and is then easily
killed by the humans who pursued it.

  In the 1980s, researchers spent a year living among a group of around fifty Hadza people, who still subsisted off the land in eastern Africa, not yet having been swept up in the agriculture and urbanization around them.12 What the researchers found was that the tribe did use persistence hunting but that members of the tribe always paid close attention to their environment for any signs of a fresh kill by other predators—circling vultures, or the nighttime calls of hyenas or lions. If such a clue was detected, the Hadza men would immediately run in that direction. Leopards and hyenas would run away as soon as the people got there, but lions were stubborn enough that they often ended up becoming part of the human meal. Over that year, 20 percent of all the dead animals brought back to the village had been scavenged from other predators, including meat from elephant, zebra, warthog, giraffe, wildebeest, and impala. Humans living traditionally in other parts of Africa, including Cameroon and Uganda, have also been observed stealing meat from predators. Assuming that those twentieth-century people used hunting methods similar to those used in the same places millennia ago, theft is probably a big reason for our success as a species. All the evidence points to humans being thieves for almost as long as we’ve been able to walk on two legs.