And then there are cases in which invasive species have provided food or habitat for threatened natives. The gall wasps, for example, that spread into England with turkey oaks were lifesavers for the native birds of Britain (the young of the birds, affected by climate change, had started hatching before the caterpillars they eat emerged from their pupae). In Puerto Rico, the once-endangered coquí frog has rebounded in new forest habitat made up of nonnative trees.
There’s no doubt that these immigrants change our environment, sometimes drastically. As human movement has stirred the worldwide biological stew, aggressive species have wiped out humble ones that had previously survived in some secluded corner. But nature thrives on change. Disturbance and mixing spur evolution, and scientists are finding many new species and hybrids arising with migration. On balance, it seems, the result of ecological immigration isn’t gray uniformity, but just the opposite: Naturalizing species have given us richer biodiversity.
House Pets
A month after our pecan sandies adventure, the temperature dropped and the rain began to fall. Argentine ants generally stop foraging when the temperature drops below 41°F, so it was no surprise that they began to forage in the one place where it was still dry and warm—our kitchen. This gave me plenty of time to observe their behavior and look for clues to my mystery (why weren’t they interested in pecan sandies?). The ants came streaming in, hunting out forgotten crumbs in the cupboard, infesting the trash can under the sink, and gorging on the sugar high up on a shelf. Some of my research proved useful during these invasions: Instead sweeping away the ants immediately, I carefully traced their trail to its destination and removed the food source. Then I’d wipe the ants away with a sponge. I felt a little ridiculous as I washed hundreds of crushed bodies off my hands when I recalled that I’d experienced a moral tremor upon drowning a single ant from the camellia.
On several subsequent days I noticed that the ants were nosing around a narrow crack between the butcher-block counter and the cupboard. The crack was just slightly wider than the body of an ant, and I wondered if they might be scouting it out as a new home. Then one night I saw that the countertop was thick with ants, all hurrying toward this crack. Looking closer, I saw that one of the ants was twice the size of the rest. I deftly captured it. Some species of ants produce workers of varying sizes that belong to different castes and can more efficiently perform certain tasks, such as defending the colony, but a quick Internet search confirmed that Argentine ants have only two types: workers and queens. The colony visiting my house was attempting to start a new stronghold inside my kitchen, and I had caught the queen. I sealed up the crack in my counter.
In the weeks that followed I caught half a dozen more queens. I kept one in a jar on my desk in the hope that I might see it lay eggs. But raising tiny pets can be difficult: Beth put that particular ant farm in the dishwasher without noticing there was anything inside.
The ants were implacable. No matter how careful we were to clean up our messes and lock away everything they might like, they always were able to locate some new food source. When we left the house for a week, I hoped we’d come back to discover the discouraged ants had left. Instead, they’d found their way inside a box of spice cookies I’d enclosed in not one, but two plastic bags. At that point, we broke down and bought traps containing ant food spiked with insecticide.
The ants ignored these traps in the same way they had ignored the pecan sandies. For all my research, I still hadn’t figured out why my Argentine ants hadn’t been interested in the cookies. But as I watched them in my kitchen, I saw that the ants would frequently become fixated on one particular food and walk past all others. It was like the movement of a crowd: If a great mass is pushing in one direction, it’s very hard for one member of the crowd to stop it or change its course. The colony’s interest in any particular goal grew organically, I speculated, from an initial discovery at just the right time, when there weren’t other options competing for attention. Putting out the traps was a little like trying to get a convoy of people headed for a concert to take a detour to a baseball game instead.
When I asked Spicer Rice about my theory, she confirmed that I was finally on the right track. A colony will often focus on one thing at a time, she said. If ants are gathering water, they will keep going until a forager tries to put her water in the mouth of an ant in the nest but finds that that ant is already tanked up. The same is true with food. “They go out and get protein until they are full, and then switch and get sugar,” she said.
We kept the kitchen cleaner than we’ve ever managed to before and eventually the ants disappeared. Either they found the traps, or they simply moved back outside after the rains stopped and the ground dried out.
After the Argentine ants disappeared, Beth mentioned that she’d seen another species in our kitchen. I was incredulous. Beth shrugged, “Well, maybe they were queens,” she said. “But they did look different.”
A few days later I spotted one of these ants. It was a bit darker than the Argentines, and its abdomen was big and round; it looked like it had an enormous butt. I nabbed it and dropped it into my specimen jar. But I was busy with other things and forgot about it.
A couple of months later, when I called Spicer Rice to ask about something completely different, she happened to mention that winter ants can live in tandem with Argentine ants. As soon as I hung up the phone, I fished my quarry out of the jar and bent over it with my lens. Again it took me a while to find the correct angle and focus, but when I did, it was clear that my ant had the distinctive hourglass thorax and swollen posterior of a winter ant.
Spicer Rice calls winter ants the white rabbits of the ant world, because they disappear down their holes into another world. Their tunnels may go twelve feet underground, the human equivalent of digging a mile deep. In the spring they disappear down these shafts and wait out the warm season underground. In the temperate Mediterranean climate where I live, Argentine ants dominate for most of the year, but they slow down in the coldest part of winter. Then the winter ants emerge and forage madly for food. As they eat, those already significant posteriors expand like balloons until they are stretched so thin that light passes from one side to the other. Then, their gasters full of fat, they return to the depths and seal the entrance for another year.
HIVE MIND
In his book Adventures among Ants, the ant scientist Mark Moffett suggests four ways of looking at an ant: as an individual, a selfless suffering drudge (the satirical newspaper The Onion once characterized ant farms as “the fun way to teach your kids to accept their miserable fate stoically”); as a society admirable in its organization; as an organism, a single body of many pieces; and, finally, as a mind, with intelligence arising from a crowd of simple choices.
This last metaphor, the colony as a mind, struck me when I read about research revealing how ants lay down chemical markers at a fork in a path to point their sisters toward the most fruitful direction. Each worker pauses at this decision point to make her sign pointing one way or the other. The following ants reinforce or contradict this sign, so the traffic on the network nimbly adjusts as conditions change. If there is a need for workers to haul a grasshopper home, the signals will point them in that direction. If invaders are approaching, the signals will direct workers more urgently to the battlefront. This fluid processing of complex information with a simple series of either-or decisions is similar to the binary system that drives computer science. And those forking paths aren’t so different from neural pathways in the brain. Humans are more sophisticated thinkers than ant colonies. Then again, sometimes it feels like we would be better off with less sophistication. What is life but a series of trail bifurcations anyway? Perhaps the decisions we agonize over are, in fact, predetermined—the sums of a million individual cellular choices. Our conscious minds assume that we are in control, but often the role of consciousness is simply to justify and explain decisions over which it has no control. Are consciousness and reason just thing
s evolution trumped up to keep us from going insane, a Matrix-style fantasy world that keeps us from recognizing the horrific reality that we have no agency and all the perseverating we do over choices is really just rationalization to convince ourselves that we have free will? Or, to flip the comparison around, could an ant colony develop consciousness? Feelings? Spirituality? Crumble some pecan sandies on a note-card with your daughter and eventually you end up grappling with the basic tenets of philosophy. These are the questions that arise if you spend enough time staring at ants.
CROW
From the beginning, I knew that I wouldn’t necessarily succeed in imposing my enthusiasm for neighborhood nature on my daughter, and that my efforts might backfire. So, when Josephine became fascinated with the color pink, told me that my squirrel-spying walks were a little boring, and wanted to talk most of the time about what it meant to be a girl, I wasn’t too disappointed.
“When I grow up I’m going to be a fairy, princess, ballerina,” she would tell me.
I couldn’t resist subtly slipping in “scientist” at the end of that litany, but I never pushed it any further than that.
By the time I had decided I needed to take a closer look at crows, Josephine was in full-throttle fairy-princess-ballerina mode and totally uninterested in the birds. That was all right, I decided. I never meant to force her to share my interests, I had just wanted to demonstrate to her, and to myself, that my nonhuman neighbors are important by paying attention to them.
It was the raptors that led me to crows. The story of the return of the raptors is fairly well known at this point: Ospreys and peregrine falcons were killed off in large numbers by organochlorine pesticides like DDT, but after use of these chemicals was restricted, they bounced back. These birds aren’t invisible; some have adoring fans and webcams trained on their nests. Whenever I see the characteristic shape of a bird of prey, I look up eagerly. And it was in one of these moments that I began to see crows.
At first I assumed that both of the birds I was watching sail across the sky must be raptors. They crisscrossed my field of vision, disappeared behind the houses, then circled back. One was smaller and faster; the other was a red-tailed hawk, flapping ponderously. On the next pass there were two of the smaller birds. They were low enough now that I could see that they were black. The smaller birds weren’t falcons or hawks, they were crows, and they were molesting the bigger bird mercilessly.
In San Francisco we lived at the foot of a tall, grassy hill—a good place, if you are a raptor, to find voles and other small rodents. I often saw hawks hunting there. I also saw crows ganging up on larger birds of prey. The crows made the bigger birds look positively clumsy. They’d swoop and turn in a fraction of the time it took the hawks to flap their wings. This behavior is called mobbing, but initially I could find no satisfactory explanation for why they do it.
Were the crows trying to keep the predator away from their nests? That seemed to make sense until I realized that I’d seen crows chasing hawks in the winter, when crows have no nests. The opposite could be true: The crows might be after the raptor’s eggs. But why choose such dangerous food when there are pigeon eggs everywhere? That’s like searching for tyrannosaurus eggs while surrounded by chickens. The crows, I learned, occasionally get just a bit too close to the birds of prey, which quickly kill them.
The crows may have been trying to steal something the bigger birds had killed. They do this sort of thing all the time, employing a two-crow con. Naturalists have observed one crow walking toward an eating eagle with feigned disinterest, just drifting closer nonchalantly. Then another crow will dash in from behind and pull the eagle’s tail, causing it to leave its meal and give pursuit. The first crow darts in and snatches a piece of the kill, which it later shares with its accomplice.
None of these explanations, however, accounted for the fact that the crows I saw often harassed raptors for half an hour, maybe more. I would lose interest, carry on with my errands, then look skyward as I stepped out of a shop to see them still at it. You don’t need that kind of time to steal a mouse.
I began to think of this differently after I read Gifts of the Crow by John Marzluff and writer and illustrator Tony Angell. Marzluff researches corvids—a family that includes crows and ravens—at the University of Washington, and he’s written several books with Angell. They note that there’s a direct evolutionary benefit to mobbing when it keeps predators from pillaging nests, but suggest that there’s something more at work here. Mobbing looks a lot like a product of a culture, they write. “As a social exercise it provides a forum within which individuals can display their flight skills and aggressive tendencies. Not unlike the human recognition for a demonstration of valor, these attributes may win a daring crow a mate, or a higher rank in its avian hierarchy.”
Among the Maasai, an adolescent boy used to have to kill a lion to earn his place in the patriarchy, and it’s not so crazy to think that crows might earn status by challenging a larger predator.
Or maybe instead of earning status, they are just having fun: their version of extreme sports. Perhaps they do this aerial dance with death for the same reasons that humans do ostensibly pointless things like big-wave surfing or mountaineering. I like to imagine that crows are doing it out of sheer delight, for the joy of besting a force that could easily crush them.
That’s not as fanciful as it sounds. Ravens have been observed “surfing” the wind by holding flat pieces of bark in their claws and riding mountain updrafts. They use bits of plastic to sled down snowy roofs, ride rotating sprinklers, and slide on their breasts down the onion-domed cupolas of Russian Orthodox churches (helpfully polishing them in the process). Angell and Marzluff once spotted an airborne group of crows playing with a ball of paper above a University of Washington football game. One crow would carry the ball a few wing lengths and then drop it, at which point the others would dive in, the fastest one snatching it from the air. They repeated rounds of this corvid quidditch over and over again, causing attention in the stands to stray from the earthbound athletes. And at the University of Montana, a crow learned to gather up small packs of dogs by whistling and calling what for all the world sounded like “Here, boy!” The bird would lead the dogs on frenzied chases across campus for no apparent reason.
To say that animals play strikes some as dangerously speculative. If you think of an animal as a Cartesian machine—a complex wetware robot that responds mechanically to a stimulus—it’s hard to imagine why it would play. How could evolution tolerate frivolous risk and use of energy? But there’s a clear scientific consensus: Animals do play, and crows are among the most playful.
“Many birds play,” Angell and Marzluff wrote. “But no group of birds has been reported to play as frequently, as variably, or with as much complexity as the corvids.”
This doesn’t disprove the Cartesian argument, it only proves it’s wrong in drawing a distinction between animals and humans. The more valid and interesting question is this: If we are all just complex machines designed to reproduce and pass on our DNA, why does anyone play?
People used to think that natural selection favors play when it provides valuable training; in crows, for example, mobbing might teach agility and build muscles. This makes intuitive sense, but scientists have found it doesn’t work that way. Researchers have tested this out with mice, coyotes, kittens, squirrels, bears, and rats. They found that the individuals who played were no better at hunting or fighting than those who didn’t. The behavioral ecologist Lynda Sharpe spent years watching meerkats tussle; to tell them apart during the rollicking melees, she had to sneak up to them as they napped and draw on them: “I’d crawl around the group on my stomach, clutching a fistful of marker pens and surreptitiously scribbling coloured rings on everyone’s tails,” she wrote. But after analyzing her data, Sharpe found that participating in this “training” had no relationship to success in real fights.
And yet, the play does do something: Research shows that animals that play are mo
re likely to survive, and become better parents. Rats—one of the most playful species—that are deprived of play react to minor conflicts by flying into a rage or quaking in a corner.
“There’s something about play that increases overall fitness, but it’s not about hunting or fighting,” said Kaeli Swift, a researcher who studies crows with Marzluff.
The definivite reasons for play are still mysterious. It’s clear that play is simply fun; both crows and humans experience a flush of opioids in the brain when they play. But we don’t know how this proximate cause of playfulness—the thrill of experiencing joy—leads to its ultimate cause, which is increasing reproductive fitness.
It’s also clear that there’s a social reason for playfulness. Play is a way of making meaning of the world. It provides an opportunity to act out aspirations (as children do in playing make-believe), reaffirm tribal unity (as fans do every day in stadiums around the world), and pass on a moral education for managing conflict. Perhaps mobbing is a dangerous ritual affirming the culture and solidarity of a crow clan.
THE CULTURAL MEMORY OF CROWS
Marzluff is probably most famous for showing that crows can recognize individual people. In 2002, he trapped a crow and slipped three plastic rings over its ankles—two light blue and one dark blue—to help him identify the bird. It turned out that this crow was Marzluff’s neighbor, and whenever the bird saw him, it would caw wildly, castigating him and warning others that there was an evil crow grabber on the loose, one capable of inflicting his dubious fashion judgment upon his victims’ ankles. This went on for more than seven years. Marzluff noticed that he always got this abuse no matter whether he was wearing a parka or a short-sleeved shirt, no matter whether he was alone or with a friend, no matter whether he was walking or driving his truck.
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