Last winter, I drove from my home in Oakland to Bishop, California to meet up with a back-country cowboy ecologist who would accompany me in my search for snoozing bats. Along the border between California and Nevada, a large population of Townsend’s big-eared bats (Corynorhinus townsendii) use the cool, humid mine shafts within the White-Inyo Mountains as safe havens for overwintering. These mines, left over from the region’s history, are appealing locations for hibernating bats. They maintain relatively cool, but above-freezing, temperatures and high humidity, which support lower metabolic rates and reduce evaporative water loss in the bats that roost there. While this species remains active at lower elevations during much of the year to feed on plentiful insects and find mates, only females migrate to elevations ranging from 4,500 to 10,000 feet to hibernate throughout the harsh winter.
Many temperate bat species—including Townsend’s—have unusual patterns of annual reproduction compared to other small mammals. They mate in the autumn months when males and females spend time foraging for food and roosting in the same spaces, but female bats do not immediately become pregnant. Instead, their reproduction is delayed until spring. Female reproductive physiology is adapted to store sperm and delay ovulation while hibernating. When optimal environmental conditions cycle back in the spring, females impressively manage the energetic demands of flight in addition to pregnancy and lactation—hello baby weight! Because they become pregnant almost immediately upon arousing from hibernation, I wondered whether these “future” reproductive costs may affect how female bats manage energy reserves during hibernation, or influence where they decide to cozy-up for the winter.
Digging into this question relied on one very elusive thing: finding hibernating bats. In need of help, I somewhat hopelessly emailed a listserv of chiroptologists (people who study bats) and was amazed by the number of replies that filled my inbox. All were pessimistic, except for one. Dr. Michael Morrison, professor and chair within the Department of Wildlife and Fisheries Sciences at Texas A&M, wrote, “I know where a lot of Townsend’s hibernate.”
And so, one year later, I found myself behind the steering wheel of a 4WD rental car filled with field gear and with the windows cracked open just enough to avoid CO2 asphyxiation from sublimating dry ice. I was making my way to the White Mountain Research Center situated in the Owens Valley just a few miles beyond the Main Street strip of Bishop. There is something about being in the presence of enormous mountains that makes me emotional, and this particular drive down U.S. Route 395 filled me equal parts nervous energy and sheer joy. Arriving well after dark, I spent the evening counting cotton balls, sterile needles, and small cryovials—a quiet meditation before spending time in the field where everything is a lot less tidy.
Over the next week, I made myself a large thermos of coffee every morning before throwing my backpack and boxes into Morrison’s pick-up. We spent each day driving through rocky mountain passes and down gravel paths that I hesitate to call roads. In order to understand how female bats manage their stored energy over winter and the ways that their chosen hibernation location impacts their physiology, we needed to record field measures of body condition and collect samples that I could bring back to the lab at UC Berkeley.
As I wiggled, quite literally, through a small hole in a mountain cranny, I immediately understood why so few people know where bats routinely hibernate and tried to focus on my gratitude for being there, rather than the fact a small earthquake could leave me trapped underground. Once inside, I relied solely on my headlamp to shed light on the jagged rock around me. Confirming the space was just large enough for me to crouch in, I began moving mindfully through the mine using my lamp like a search light sweeping for something small and fuzzy stuck to the rock. Unlike many biologists who capture bats, we don’t need fancy nets or detectors. Our study animals cling quietly to rocks with their delicate toes, their bodies oddly cold to the touch when we find them.
As I recorded mass and body temperature measurements, I found it hard to believe 11-gram bats could be alive with a body temperatures hovering around 59 degrees Fahrenheit (over 30 degrees less than active body temperature). With black leather driving gloves under a pair of nitrile—the biologist’s secret to never getting a bite—I carried each bat out of the mine to work on the bed of the truck. There I let each bat slowly wake up, uncurling its big ears from over its eyes like a sleeping mask and increasing its metabolic rate and body temperature. Once active, it was safe to take a tiny sample of blood, gently wash out some cells from the reproductive tract, and prepare slides. Later, I used the slides to look for blood parasites and check for the presence of sperm that would indicate a female had mated. A small sample of fat, about the size of a grain of rice, was carefully removed and frozen over dry ice and a snippet of soft fur taken from its back and saved in a paper coin envelope. Together, these samples provide information about diet, immune capacity, mating history, and the biochemical molecules stored for energy during hibernation.
The project will highlight how individual animals cope with physiological and energetic challenges, but will also help us learn more about entire bat populations. Bats are an essential component of ecosystems around the globe, as they help control agricultural pests and play important roles in seed dispersal and pollination. Unfortunately, many bat populations have been decimated due to white nose syndrome, a fungus which predominantly affects hibernating bat species by activating their immune response during a time when energy stores are low. Through my work, we will be able to identify various factors that impact energy use and immune system capacity during hibernation—such as reproductive status or microclimate in specific hibernation locations—and learn about strategies that permit some individuals to fare better than others. Ultimately, this may inform conservation efforts regarding population susceptibility as well as individual animal health.
After checking off the final boxes for sample collection in my field notebook, I walked the bat back to the portal of the mine from which it came. My fingers loosened their grip around its small body, opening my hand enough for the strong female to rest in the palm of my hand. I raised my arm up high to elevate her, giving her room to drop and catch lift. A few delicate steps to the tips of my fingers, and then I felt her let go, only catching a quick glimpse of wide wings gracefully flapping before disappearing into the dark.
Mattina Alonge is a graduate student in integrative biology.
Image Design: Shannon O’Brien