Commensals: Theme and Variations

Humans, who make up .01 percent of the biomass of the earth, have destroyed 83 percent of the wild mammals in it, and half of all the plants.

—Yinon M. Bar-On, Rob Phillips, and Ron Milo, “The Biomass Distribution on Earth,” Proceedings of the National Academy of Sciences of the United States of America, May 2018


We didn’t want to kill them. 

We ’d come to feel we understood them: they had not just their rituals but their recreations. One night they ate seventy-five dried hot peppers I ’d stored in the basement. They had the sense to leave the hottest ones, the chiles de árbol, but they downed the guajillos, pasillas, anchos, goats’ horns, and left just-ripped packages and some seeds. The rats had been in our house three months when this happened, and we understood this eating as the kind of punctuation mark you stick in your life after too much repetition. An interior burn, to beat ennui. Boredom can call for a little suffering, even a little heat.

The rats lived in our walls and the crawlspace above our bedroom, so at night—when rats are most active—we lived in something like a surround-sound carnival, the cheap kind, with small rides squealing and groaning and loud but unintelligible barkers. The rat man we called said rats are too smart for catch-and-release. They’ll remember anywhere warm with food, he said. And they’re burrowers, they love walls: warm, sheltered, the right amount of freedom. 

They arrived with the equinox, just before my birthday. These rats, we came to think, had a sense of occasion. It was like them to act on transitional days, ceremonial days.

The rat man, Tod, was young, probably twenty, with a tan uniform that struck me as fur-like (though I might have been biased that way) and slicked-back hair. He told us we had twelve rats, an oddly precise figure given that he did not see them all, but we accepted it on the grounds that these rats would live in the context of a meaningful number. One day Tod leaned into the crawlspace, then came down and held a dead rat in our faces, caught in some kind of exterminator pincers. It lay on its side, lids locked and tail slack, mouth open in the shape of a sigh. “It’s not a roof rat,” he said, as I screamed a little. “It’s a Norway rat.” We had assumed: under roofs, roof rats. These rats really do come from Norway, he said, and I wondered why they couldn’t find their way back to that place.

Rats are one of a handful of species that thrive along with humans and have proliferated beyond any natural order. They crest along on our surf, if you can imagine surf as homes and garbage. We give them food, warmth, breeding space, and lots of stuff to shred for their nests. They belong to a lucky group called commensal species, those whom human lives accidentally favor. The root of the word means to share a table or dine together, which we did with our rats that scrabbled through the wall alongside our dining table. Meals became, at times, almost cozy, our dinnertime clanking and passing, their scritching and swooping, as if we played at this survival game together. And in fact (I learned), rats live in family groups and rarely interact with rats beyond those, as many modern humans can hardly name their neighbors. Our kinship is true.

Other commensals are deer, coyotes, raccoons, and some bacteria.

Few wild species have the fortune of commensals. Overall, we’ve scalped this planet of its biomass, flattening it in the wild to about a sixth of what it was. I read about this in an article published in the Proceedings of the National Academy of Sciences, by authors who complain that science does too little weighing. Many people believe humans will have to colonize other planets—maybe leave this one altogether—to survive. This is science fiction that’s rewritten itself in our dreamings, as science-fact, or perhaps, science-hope. We have a problem, and this feels like an answer. We are 7.6 billion and growing, and the number’s impossible, especially with warming weather—our climate each year becomes subject to more disasters and fewer crops. The Arctic’s melting, and city towers will scrape the bellies of ships. To stay in place in these cities, we’ll need to be afloat, wood-cupped.

The fact is, we can’t survive without the biomass we’ve destroyed and are destroying. People from Tesla’s Elon Musk to physicist Stephen Hawking have pleaded for colonizing, and Hawking before his death became obsessed with the idea. First he said we had one thousand years to flee the planet, but then as warming accelerated he changed the number to one hundred. As Hawking phrased it, “To stay risks being annihilated.” Likely we’ll start with Mars or the moon. Pioneers will build enclosed dwellings, great bubbles, to hold our atmosphere. Each bubble pumped to life with gasses, like carnival balloons. It won’t be easy.


The number of rats our symbiosis creates has become out of control, a “ratpocalypse,” which the New Republic claims we are on the verge of. People in some cities, such as Chicago, spread rat birth control powder on their streets, tapping it into their apartments on their heels. Rats thrive on disruption. If you could walk through an untouched forest, you would not see rats. They ’d need landslides, flooding—something to toss things around, create burrow space and dead organic matter. Humans represent continuous disruption: we are always plowing and reaping, tearing down and rebuilding, even blowing up. Two healthy rats left alone to breed (no pause, no predators) can in three years lead to a rat population of half a billion: twelve years breeding, and every human on the planet would be twinned to his or her or their own rat. All sprung from one rodent Adam and Eve. Then which species would overwhelm the other? Rats begin breeding at four weeks, gestate for three weeks, and deliver six to twelve pups, thumb-sized and blind, misshapen as tubers, and a hairless and intestinal pink—which in a month will breed, and so on, and so on. I ’d have to say: advantage, rat.

Wild rats aren’t our only symbionts—we’ve bred others, lab rats. My university has science, so I must walk past labs where these white rats crouch in daytime, then chitter and pace at night: spectral ovals. I don’t know where they’re kept, because my school doesn’t want anyone to know; from time to time people in ski masks break into these labs, pulling up the little wire doors. The ovals pour down from the cages, drops of cream hustling toward the exits. The ski masks know these rats, infused with disease, lumpy and sick, can’t survive. It’s a gesture, as it is when you look in a mirror and curl your hair with a finger, suck cheeks in through your teeth, to look good to an imagined person you desire, but there is only you.

Most medical rats are Wistars, a bio-engineered rat. Rats resemble humans physiologically, they have malleable genes, and they live only a few years, so you can follow disease progression to the end. Here is the marketing language for the basic Wistar: “Ideal For general multipurpose model, infectious disease research, safety and efficacy testing, aging, surgical model” [error and awkwardness theirs]. It is also cancerous, developing spontaneous tumors. There are hybrids of hybrids: the Wistar Furth, bred to get leukemia, will also get renal failure and grand mal seizures. Wistar Kyoto is hypertensive. And so on. 

Wistars can cost anywhere from fifteen dollars to more than one hundred dollars per rat, though they do go on sale. You buy them from an online store, exactly as you ’d buy from Amazon; you click on the rats you’ve selected and put them in your cart, and when you have enough rats, you proceed to checkout. Live rats are shipped using a method called “Sew Easy,” and I admit I haven’t looked into this, enjoying my mental pictures too much: an apron with pockets swollen with rat, a patchwork quilt (maybe the popular Double Wedding Ring, the interlocked circles), each square thick, and moving.

Wistar rats were initially developed for anatomy instruction by Dr. Caspar Wistar, who in 1892 founded his self-named institute in Philadelphia. When the Wistar Institute opened, it housed dead humans: dried or skeletal or preserved with injections of wax. Wistar had no idea the Institute would become a rat-womb, though probably he did realize his name contains the word “rats,” spelled backwards. (And contains the word “star,” which in its own way is relevant.) Milton Greenman, who helped create Wistars, called them a “living analogue to the pure chemicals” used in experiments to get accurate, reproducible results. Identical drips and systems, expressed in small body after small body. Other lab rats exist, but most descend from Wistars. The rat types are called strains or models. They’re bred to be docile, and specially handled when young in what the sales material calls a “gentling process,” so they won’t bite.


Lab mice were the first creatures given human brain cells, a type of cell called glial (like happiness, like a song), extracted from fetal tissue and injected into pups. In the mouse brains the glial cells reproduced beyond any imagining—three hundred thousand glial cells became twelve million, formed into a type of glials called astrocytes. These are beautiful, and look like thready and vulnerable stars. Astrocytes drape their tendrils over neurons and speed up and intensify signals between them. As the pups grew, their brains became encased in an astrocyte net, like each mind had a Milky Way sunk over it. “We could see the human cells taking over the whole space,” said a researcher named Steve Goldman, one of the first to do this, who sounded a bit stunned. “It seemed like the mouse [cells] were fleeing to the margins.”

After mice, rats were infused with human brain cells, on the idea that rats were already more intelligent than mice, so let’s see what happens. It’s a kind of reasoning that feels very human, chasing bad logic into acts for no purpose, like admiring your own face through eyes you can only pretend are not your own. These mixed-brain creatures are called brain surrogates or chimeras, generally chimeras, the ancient word for creatures stitched together out of different parts. Yes, these lab mice and lab rats are far smarter than normal rodents. They learn much faster, and they remember, and become little whizzes at running mazes and the like. Chimeric mice memories, reports one researcher who tested them with electric shocks, exceed normal mice memories by four times.

In another experiment mice and rats had miniature human brains—each brain the size of a lentil, grown in a lab from stem cells—placed in their craniums, at the side of their own brains, hooking in, like parasites. Researchers did this to see how long such organoids could survive in the brain, but with the side interest of seeing how these cells, too, might stimulate intelligence. Each implant had bored above it a transparent, lentil-sized window. These lentils intermeshed with rodent neurons, but did not make the creatures any smarter. The human parasite died in a few months.

We have no idea if the chimeras have human self-awareness, understand their cage as cage, their fate the same as the mouse next door who got his brain opened and body shredded last Tuesday. It is possible, according a piece by seventeen ethicists in the science journal Nature, that the chimeras will become sentient, will develop “advanced cognitive capacities.” The Nature authors suggest it may be most humane to keep the chimeras in a functional coma throughout their lab lives, so they don’t have a chance to think. Or perhaps just destroy the enhanced mice faster than the normal ones, or even not destroy them but rehome them after testing, if alive. Researchers should act according to their diverse sensitivities, in the authors’ words. My scientist friend, Thor, recalls that one chimera researcher promised to immediately kill any mouse who began displaying humanlike awareness. Thor wrote to me: “I had this image of injected mice in a little doll house being watched by white-coated humans and as soon as one of the mice picked up a little toy tool and started to use it, he would immediately be snatched up and murdered.”


Like the rats in our walls, colonizers will have to create new recreations and rituals. Special days like arrival days, days to acknowledge that at one time a place burned too hot. Animals will escape and go feral, as they do, and plants will go weedy, as hybrids do. (You can’t trust their seed, which reverts to the less desirable parent genes.) And humans who are born and come of age in colonies will forget we humans made it all. As Bar-On, et al., point out, they’ll take 83 percent of what they have and waste it away.

A scientist named Adam Ozimek writing in Forbes argues against planet colonizing, calling it absurdly hard, and not necessary. Finally, I think as I start reading, someone believes we can make it here. But no—his thought is building biodomes for us under the oceans to flee to. Those ship bellies our moons. Marslings and moonlings and oceanlings, as we are Earthlings. Coyly we use the diminutive.

I imagine all the creatures we might want to bring with us. For good outcomes we ’d hybridize like mad, maybe a bouncing chicken adjusted for new gravity. We ’d pack to leave and place the strains of cows and goats and poultry into our little carts. And what if we’re someday able to expand throughout the galaxy? Would we lose 83 percent of that, Milky Way erased to a little smear?


Instead of worrying as I should, I think of words. The mass of biomass, or body mass, comes from an Anglo-Saxon word meaning lump. The word commensal has a root meaning meal or, more accurately, table. And though priests will tell you Mass also comes from this word for table (Mass as communion, food), it does not: it comes from Latin missus, parent also to missive and message, meaning to send away. 


If we leave this planet I imagine many of the mice and rats in labs will make their own escape. Knocking down and/or clawing open their cages. Rats practice dominance, so I expect they’ll have it out: the strength of the darker ones, the intelligence—but docility—of lighter. Or they’ll cross-breed. They’ll have their desires and their habits, and they’ll have imagination. Fat, rich: the new apex predator.

We might bring some of the Wistars with us. (We might consider that rats don’t forget.) We might make a plan to come back to our Earth-home when it cools. We might then learn our planet is something other than what we thought: we misunderstood, or our understanding has been all this time tenuous and limited.

We imagined Earth was ours. Our children will have grown up with this story, and our children’s children: the dome-less air, the houses you could build anywhere as long as you shaved the land free. But our blank homes won’t count, just the walls will, new cities of many brains, lovingly crocheted with consciousness. The rats making their own real little tools. We have pushed the rats out of our home’s borders, and now they will push us out of theirs. I doubt we will be their commensals. How many billions of them then? How many homes, empty to our eyes, but in-between, full and seething? Oh, we will dine with them again, sealed off, off from those living edges. 

for Bruce


Susanne Paola Antonetta’s Make Me a Mother, ranked a Top Ten Book of the Year by Image Journal, was published by W. W. Norton (2014). She is also the author of Curious Atoms: A History with Physics (Essay Press, 2016), and five poetry collections, most recently Stolen Moments (Shebooks, 2013). Her works have been New York Times Notable Books and an Oprah Bookshelf pick, and she has earned honors that include an American Book Award and a Pushcart Prize. She lives in Bellingham, Washington, and is the editor-in-chief of the Bellingham Review.