The upside-down people

Ink wash drawing: our new bat house.

Awhile back, we bought a bat house for our yard. I’d read about these things, here and there, and I think our neighborhood bats deserve the very best digs. They’re excellent neighbors.

They eat bugs who would otherwise be eating me. They spread pollen. Their poop is rich in nitrogen, potassium, and phosphorus. They’re gentle, smart, and fun to watch. And they provide their services — pest control, pollination, fertilization, and entertainment — entirely for free, just because these are the things they like to do; symbiosis without downside.

When we brought our new bat house home from the garden store, I held it up and took a closer look. At first glance it looked familiar; a birdhouse but wider and flatter. About 30cm wide, 30cm tall, and 5-6 cm deep. A sloped roof, plank walls, and a backing board for mounting. So far, it looked like something I’d have built in 7th grade shop class.

But a birdhouse typically has a hole in the front wall to serve as “door.” The bat house, on the other hand, did not. It had no door and… no floor. The bottom was entirely open. The interior space was just a wide slot, about the size of a desktop filing tray. No obvious perches or holds on the walls. The instructions told me to hang it about ten feet above the ground.

I was perplexed. How does this thing work? How would a bat actually use it? I know they sleep hanging upside down, but how would they enter this narrow space from the bottom? Where would they land? It looked like the world’s worst treehouse, a lean-to built by aliens who do not understand Earth’s gravity.

That is: to my human eyes it looked so unintuitive that it was all but incomprehensible.

I started reading up about bats to try and understand. Bats, like us, are warm-blooded, live-birthing, lactating mammals. Some species are closely related to primates. But in so many other ways, bats are the opposite of humans. They are upside-down people; cousins who inhabit a world so different from our own that it’s difficult for us to imagine it.

That’s not quite right, though. Bats live in the same world. They live in my back yard. It’s just that they have a completely different point of view. They don’t walk on the ground, they fly. They don’t lie down to rest, they hang from a tree limb or cave ceiling. They’re awake at night, asleep during the day. Their primary sense is hearing. Bats have incredible ears and many use echolocation to “see” in total darkness.(Note: Sorry, I’m generalizing here. I’m talking about the small bats that live in my region, called Microchiroptera, like the Eastern red bats common in Maryland. Worldwide, the Chiroptera family includes more than 1200 species and is very diverse. Bats are big and small; they have many different diets, including insects, frogs, fruit, and blood; some are nocturnal, some are diurnal; their habitats range from the equatorial tropics to the temperate north.)

For humans, our own adaptive traits inform (and limit!) how we see the world. We are omnivores but also apex predators. We hunt in packs, pursuing large prey over long distances, like wolves. We come from savannah; we stand upright to see over the tall grass. We are mostly diurnal and often depend on our eyesight to navigate and understand our surroundings. We like to be out in the open, under the sun.

These traits determine not only how we hunt and move and live, but how we perceive more complicated things like moral worth. Over time, light has come to represent safety, understanding, and goodness to us. “Light” is virtue; “open” is honest; “upright” is brave and correct.

On the other hand, darkness signifies ignorance, fear, cowardice, isolation, and evil. In the light we feel connected and in the dark we feel alone and afraid. Our eyesight is terrible in low light, and our smell and hearing are pretty mediocre by mammal standards. At the very simplest level, perhaps, we feel this way about the dark because we are so vulnerable there.

For bats, it’s the other way round. They are small, vulnerable creatures. They have no poison, spines, or armor in a world filled with larger predators like foxes, raccoons, snakes, and owls. Safety means avoiding and hiding. Darkness is safe. Tight, inaccessible places are safe. Getting up high, out of reach, is safe. Some species of bats have evolved a tolerance for noxious gases like ammonia, so they can build huge colonies deep inside caves, in gaseous environments that would kill most other animals, including humans.(Note: Zoologists studying bats in the field often have to wear gas masks and PPE to spend any amount of time safely in a cave alongside the colony. Mothers and pups, meanwhile, hang from the ceiling, unprotected, perfectly at home.)

The daytime is blinding, cacophonous, and full of monsters. Swooping hawks, barking dogs. At night a bat can melt into the dark and hunt with their ears. Their echolocation is good enough they can find a single mosquito in total darkness a dozen meters away.

Bats’ biggest talent — at once offense and defense, hunting and escape — is flying. Not just flying, but acrobatic, dextrous, wondrous flying that should make stunt pilots jealous.

Bats are mammals and therefore have heavy, furry bodies. Their wings are webbed membranes stretched between their “arms” (forelimbs) and abdomens. They have solid bones, unlike the lighter hollow bones of birds, and their wing membranes are heavier than feathers. Due to this extra weight, an Eastern red bat has much more inertia than, say, a sparrow.

Given their weight and inertia, a bat should be far less agile. But they are, in many respects, more adept fliers than many birds; they can turn more sharply and perform unique acrobatics. How they do this isn’t well understood, but bats seem to be really good at using their inertia to their advantage. They can “throw” their weight around in a way similar “to the way high divers shift their weight to perform flips and twists, or the way cats reorient themselves to land feet-down when they fall,” as biologists Sharon Schwartz and Kenny Breuer noted in a study they did at Brown University.

Their wing membranes are helpful here, as they offer more flexibility than feathers. A bat can “deflate” or retract their wings just so, to precisely control lift. They can perform “skid” turns by folding one membrane while keeping the other open, pivoting their whole body around their outstretched wing.

And that, it turns out, is how bats can land upside down. Using high-speed cameras, Schwartz and Breuer captured bats landing on the ceiling of an enclosure by retracting one wing slightly while flapping the other, extended wing. This allows them to perform a low-speed half-turn from horizontal flight to their vertical landing position, with their head down and feet up. Bats, as far as we know, are the only flying animal that lands upside down like this.

My bat house, with its “floorless” open bottom, would presumably look as accessible to a bat as a house with an open front door would to me.

So who, exactly, is upside down here? Is it the bat, living in the dark with their feet skyward, mapping their world with reflected sound? Or is it me, living in the sun with my feet on the ground, mapping my world with reflected light?

I still have so many questions. How did bats get this way? What’s it like to rest hanging from your feet like a coat hanger? What’s it like to fly through darkness, “seeing” perfectly?

I have more reading to do.