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Science Politely: Why we need bats, and why bats need us

I’m standing bleary-eyed in an Illinois state park picnic shelter in front of an 8 by 40 foot net made of barely-visible threads, staring at yet another little brown bat that’s been ensnared. It’s at that point in the night when I’m my thoughts are more, “Oh, another one,” and less “Great! Another one!” I don’t have a watch on, but songbirds are starting up, so it’s somewhere in the 4 to 5 a.m. range. My feelings are somewhat mixed; I sleep better in my tent if I have nocturnal tree frog croaks for a soundtrack instead of the almost deafening sounds of the diverse array of bird calls, but my data set benefits when my sleep suffers.

I need to collect as much information as I can this year, because in a few years there might not be any more bats at my site. It’s less than 50 miles from a Missouri cave where researchers found bats with white-nose syndrome. I’ve already measured, photographed, and wing-banded 54 bats tonight so I’m content to stop now, but by the time I finish “processing” and releasing this one, I’ve got three more in the net. One of them is banded. I check the number and find that she was one of the pups I banded last year. This is great news: another one of my girls had a successful first-year trek to her winter hibernation cave (that’s a “hibernaculum” if you’re a bat geek) and back to the colony where she was born. I blow at the fur on her chest so I can find her nipples to see if she’s lactating. She squawks at me, and I don’t blame her one bit, but I confirm that she is lactating. This is exactly what I was hoping to find. This is what motivated me to drive three hours to stand alone in the woods from dusk till dawn. For every female that makes it back from hibernation and raises a pup, that’s two more bats that just might be lucky enough to survive the coming plague.

White-nose syndrome is caused by a fungus that infects the bats’ skin while they hibernate. It only affects bats that hibernate in caves and mines, which unfortunately includes the majority of Illinois bats, because it needs the consistent humidity and cold temperatures. We’re still not exactly sure how the fungus kills the bats, but bats that get infected get extremely dehydrated and emaciated before dying. The outlook for the bats is grim: at the average hibernaculum, about three-quarters of them will die. This is a huge problem because of their slow reproductive rates. Bats aren’t like mice that churn out litter after litter all summer long. Most cave-hibernating species have one pup a year at best, so there’s no way they can have enough babies to make up for all the deaths. And we’re completely surrounded — the white-nose fungus has now been detected in every neighboring state except Wisconsin. While 75 percent is the average, affected caves see anywhere from 30–99 percent losses. Though it’s unlikely, I cling to the hope that the bigger Illinois hibernacula will fall closer to the 30 percent mortality range. This leads me to reset my internal clock to bat standard time (a.k.a. “daylight squandering time”) all summer so I can try to figure out all the conditions that make a little brown bat more likely to come back home and make a baby in any given year. Did this year’s super warm spring help? Does drought hurt? Do the conditions of the bat’s house matter, or is it simply “location, location, location?” I’m literally losing sleep wondering.

The reason I care so much is a mix of good sense and the giddy love of a ten-year-old girl for furry animals. I’d always been one of those nature-and-animals kids who wanted a pet whatever. Living in a “No pets!” apartment complex didn’t allow for that. I used to get scolded for feeding peanut butter crackers to the squirrels on our patio out of an apparently desperate need to interact with “nature.” Then I discovered the bats. The attic of the apartment complex evidently had ideal conditions for hundreds of big brown bats to convene every spring to have pups en masse. I’d sit on the patio at dusk all summer and watch them zip around gulping up insects, while the jilted squirrels sat by and longed for the salty taste of Ritz and Jif. Every week or so, a bat would accidentally get trapped in the hallways of the complex, darting all over trying to find that exit. Mom would come back from the laundry room ghost-faced. Then I’d try to put her at ease by getting out my animal kingdom flash cards and explaining that there was nothing to be afraid of:

It says there are only three species of vampire bats, and they all live in central America, so it definitely wasn’t going to suck your blood. Maybe it’s this cute fruit bat on this card … or this other one? No, neither of those live around here … Oh wow, there are over a thousand species of bats! … It says the bats around here all eat insects…

This last bit makes a nice segue into the pragmatic part of my battiness. Insectivorous bats are the main predators of night-time flying insects, including those that destroy crops and may carry disease. A female little brown who is nursing a pup will eat nearly her own body weight in insects every night, and can eat at a rate equivalent to 1,000 mosquito-sized insects per hour. This makes the survival of Illinois bats inherently linked to our food and our economy. I’m assuming that I don’t need to explain that agriculture is important here, but numbers can help drive my point. Researchers estimate that Illinois farmers would need to use at least another $270 million dollars a year in pesticides if we didn’t have any bats. In a post-white-nose world, we could be looking at lower crop yields, more expensive produce, and produce that’s more heavily sprayed with pesticides.

Our Illinois bats have many features that make them such efficient bug-guzzling machines. First, bats are mammals, and that means they need to burn lots of calories to maintain a constant body temperature. Second, the bats around here are very small. The so-called big-brown bat weighs about two-thirds as much as your average mouse. Small mammals have to burn even more calories to maintain their body temperature, partly because small things lose heat faster than big things. Finally, bats fly, which is a very expensive way of getting around. And since they’re mammals, they lack some of the flight-friendly features found in birds. Their bones aren’t as light-weight as birds, and their standard-issue mammalian lungs, composed of dead-end tubes, make breathing much less efficient. For land-loving mammals like ourselves, walking only accounts for a few percent of our total daily energy expenditure, but the energetic cost of flight is immense. A pregnant little brown bat uses about sixty percent of each day’s calories for flight. This leaves less than forty percent for keeping her small body warm, supplying nutrients to the developing pup in her womb, and everything else she needs to keep on being a bat. The solution to this energy-crunch is simple: eat more bugs! (She can also choose to eliminate the cost of flying and maintaining body heat by entering “torpor,” but this has drawbacks.)

In colder climates like ours, bats face an even bigger challenge. There aren’t enough insects to eat in colder months, so the bats go find a nice cool cave in which they can hibernate from about mid-October through mid-April. They survive solely on their stored fat by entering torpor, which involves letting go of this whole “warm-blooded”concept and letting body temperatures drop to match the temperature outside. When a bat’s in deep torpor it’s cold to the touch, and its heart and breathing rates are so slow and shallow, you have to be very patient and perceptive to detect them at all. Since they’re not flying and not generating body heat, their metabolism comes practically to a stand-still. If they ate enough insects to get good and fat before hibernation, they will have enough stored energy to rouse from torpor periodically to go get a drink of water or scratch an itch and still hang in there until the bugs “bloom” in spring.

If I haven’t yet convinced you that Illinois bats push the limits of what’s biologically possible, consider this: bats have to do all this to survive, and somehow still find the energy to make more bats. I’m in awe of basically anyone who can manage the challenges of motherhood, and that certainly goes for bats. Torpor is a great way to get by on very little, but it’s not helpful for getting nutrients to a growing baby. Little brown bat moms have about six months to have a baby, nurse it till it’s nearly full-grown and adept at hunting insects, then pack on enough fat to make it through another winter. The first two month are spent pregnant, and in the last few weeks she gets to that point where she burns sixty percent of her calories just going out to get more food. Her pup will be born at about one-quarter of adult body weight. (Imagine a human mother having a 35 pound baby! Now stop imagining it, because it’s horrifying.) In another month, the pup will already be close to adult size and learning to fly. (I just can’t help but put it in human terms again: what if human babies grew to the size of teenagers four or five months after birth?) This intensely fast growth rate is why nursing little brown bats have to eat nearly their own weight in insects every day. It takes a few more weeks for the pup to get good enough at catching its own food that mom no longer needs to nurse. In a good year, when spring doesn’t come too late and summer doesn’t end too early, this gives the little brown and her pup about a month and a half to go on a bug-binge so they can pack on plenty of fat before heading off to the local bat mating site, and then finally to their hibernaculum.

So these are all interesting little facts about being female and a bat, but what if you don’t care? Maybe you aren’t one of those people who grew up mesmerized by nature shows. I could go on and on about how bats are the second most diverse group of mammals after rodents, with over 1,200 species including fruit bats with six foot wing spans, and pollinators that help cactuses make tequila, and bats that catch fish with their toes, and blah, blah, blah. And maybe if I explain to you that bats aren’t rats with wings, it only convinces you that bats are something else that poops with wings. But here’s the kicker: Illinois’ bats, which are such an unfathomable combination of animal that they must eat non-stop, are virtually the only thing eating the insects flying around at night. So if you don’t like expensive grocery bills, or doubling your pesticide intake, or wondering if that mosquito buzzing around your ear has West Nile Virus, you should learn to care about bats. You don’t have to become a bat nut or anything. But at least just give them the space they need to do what they do best. They need our respect now more than ever before. Who knows, you might even find yourself wanting to help.

Written by Lisa Powers, M.S., Ph.D. Candidate, Program in Ecology, Evolution and Conservation Biology at the University of Illinois.

First three photos by Liz Pritchard. Used with permission. Last photo by Lisa Powers.


Smile Politely is proud to introduce a new running series we are calling “Science Politely” that will feature the work of graduate students at the University of Illinois throughout November and December. Working in collaboration with the students in a graduate course in Integrative Biology, Science Politely is a collaboration aimed at bridging the gap between town and university, between scientist and citizen, and between research and culture.

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