Today’s most elderly bats aren’t supposed to exist. Ounce for ounce and pound for pound, they are categorically teeny mammals; according to the evolutionary rules that hold across species, they should be short-lived, like other small-bodied creatures.
And yet, many of Earth’s winged mammals buck this trend, sometimes blowing decades past their anticipated expiration date. One species, Brandt’s bat, which weighs just four to nine grams as an adult—all the heft of a quarter—has been recorded surviving to the age of 41 in the wild, almost as long as a standard four-ton Asian elephant, and nearly 10 times as long as its body dimensions might otherwise predict. “That’s just amazingly long-lived for their size,” says Jerry Wilkinson, a biologist at the University of Maryland. “Longer than any other mammal.”
No single factor can explain the astounding longevity of bats. They are clever and collaborative, and their superpowered immune systems help them tolerate viruses that make other animals disastrously sick—traits that undoubtedly help them survive. But one of their anti-aging tricks, among the most biologically elusive in the world, is to simply put off getting older for months out of every year.
As fall dips into winter, the little mammals huddle into caves, trees, and mines, folding up their wings and hanging feet over head. Their body temperature plunges, sometimes approaching freezing; their heart rate slows to a handful of beats per minute; they barely take any breaths at all. “They basically shut down their entire body, drastically reducing all the functions that we typically associate with life,” says Aline Ingelson-Filpula, a biologist at Carleton University. Stretches of hibernation like these have long been understood as almost suspended animation, used to conserve the body’s resources in times of great need. For bats, Wilkinson and his colleagues have found that it may also drastically extend their tenure on Earth.
Time, per se, isn’t really what kills us; it’s how we spend it that does us in. For most creatures, the calendar of days and months progresses in lockstep with the internal process of aging. But bats, and likely other hibernators as well, are effectively able to uncouple those clocks, advancing their biological age only when they’re active and awake—even as their chronological timepiece ticks on. “Think of hibernators as just being turned off,” says Hanane Hadj-Moussa, a biologist at the Babraham Institute. “They don’t get as damaged as an organism that has to just deal with life.”
Many scientists think of aging as what happens when the body accumulates life’s wear and tear—the costs of metabolizing food and burning through daily energy demands, the gunky buildup of cellular waste. Hibernation brings those burdensome processes to a near halt. Animals that manage it are “barely doing anything metabolically, and they’re very cold,” says Jenny Tung, a biologist at the Max Planck Institute for Evolutionary Anthropology. It’s caloric restriction and cryopreservation rolled into one, a slowdown that preserves physiological battery life, like toggling an iPhone into low-power mode.
Scientists were first clued in to the notion that hibernation might be a way to temporarily delay death in the early 1980s, when a team of medical researchers at Harvard found that Turkish hamsters that spent an especially long time in seasonal pseudo-slumber perished later than their peers. In the years that followed, researchers quickly identified several other creatures that belonged to the Wake Less, Live More Club. Among them were ground squirrels, bats, marmots, and lemurs—all of which outlast similar species that don’t hibernate, clear hints that the hibernators were somehow “cheating the game,” says Gabriela Pinho, a biologist at the Ecological Research Institute in São Paulo, Brazil.
But definitively nailing hibernation as an anti-aging ploy is tough. If animals are holed up in dens for months of the year, they’re also usually better hidden from predators and more sheltered from the elements. To confirm that these stints of dormancy were actually, on a molecular level, hitting the “Pause” button on animals’ inevitable march toward death, Tung told me, scientists needed a way to “start asking what’s going on within the cells themselves.”
This year, two groups of researchers, led by Pinho and Wilkinson, respectively, published some of the most convincing data on that front to date, on yellow-bellied marmots and big brown bats. Both studies scoured the genomes of the little mammals, looking for epigenetic modifications—molecular punctuation marks that annotate stretches of DNA, making them more or less easy to read. These marks get shuffled and more scattered as we age, and researchers have studied them closely enough to read their patterns, almost like tree rings, and determine how far our tissues have progressed along the path to old age. When researchers then compare that with the actual number of years an animal has lived, they can get a sense of whether a creature is, molecularly speaking, particularly spry for its chronological age, says Danielle Adams, a biologist at Towson University who worked with Wilkinson on the big brown bats.
By inspecting marmot and bat genomes at different times of the year, Pinho, Wilkinson, Adams, and their colleagues were able to show that in the winter, the animals’ biological aging effectively stalled, even as they accumulated months of chronological time—then rapidly picked back up in the spring as they roused. The differences in DNA modifications were stark enough between the seasons that they were visible “within six months in the same individual,” says Isabel Sullivan, who was part of Wilkinson and Adams’s team.
Hibernation, to be clear, didn’t manifest just to fill nature with geriatric marmots and bats. Its primary purpose is to rescue animals from almost-certain death during resource-poor and often chilly times of year. “It’s a mechanism for survival, just making it to the next stage,” says Liliana Dávalos, a biologist at Stony Brook University. Maybe it was a happy accident that these freeze-frames also doubled as a fountain of youth.
Humans have other ways of making it through rough winters—we’ve never needed hibernation to survive. But the prospect of mimicking the act still tugs at our brains. It could buy time for emergency surgeries. It could enable far-reaching space travel, keeping astronauts alive ’til they reach their destinations, while largely eliminating the need to pack tons of food. If longevity is a perk as well, some people would likely line up.
Still, “I’d be cautious about saying if we hibernate, we could just double our life span,” Wilkinson said. Plenty of species hibernate and still die at about the age their body size would predict. And as cushy as hibernation might sound, it threatens to exact a tax. While inactive, animals’ brain function fizzles, their weight plummets, and their digestive tract shrivels. They cease nearly all movement, and their reflexes slow, making them easy prey for predators that chance upon them, and raising the risk of their muscles atrophying and their bones demineralizing. The immune system’s potency also ramps way down, making bodies super-susceptible to infection. (That’s one huge reason that droves of bats have, in recent years, been felled by white-nose syndrome, a lethal fungal disease that hits hibernators hard.) Creatures that have evolved to hibernate have also cooked up many strategies to counteract its costs, allowing them to bounce back each spring. Humans, however, have not—which means the toll would be that much greater on us.
Even the prep for hibernation is arduous. In the fall, pre-hibernation squirrels and bears have to eat themselves into a diabetic coma to stockpile several months’ worth of fat. Yellow-bellied marmots, which can hibernate for up to eight months of the year, have just “four to five months to basically double their weight and reproduce,” Pinho told me, condensing their most important tasks into the brief stretches during which they’re awake. (Their offspring, too, must frantically chow down shortly after they’re born, or risk dying in their first winter underground.)
Hibernation can’t guarantee restful slumber, either. Most mammals must rouse themselves—usually once every couple of weeks or so—to eliminate waste, perhaps sip a bit of water, and, ironically, sleep. These wake-ups are massively expensive: “Every single arousal that a squirrel does takes about 5 percent of the energy that it uses over the entire hibernation season,” Ingelson-Filpula, of Carleton University, told me. The etiquette of torpor is also … different. Some male bats will rouse themselves in the dead of winter to have sex with still-dormant females, which may wake weeks later to find themselves toting around a stranger’s sperm.
And then there’s all the FOMO. Hibernation “would be a way to see the world at a future time, and that’s kind of appealing to think about,” Wilkinson told me. “But then you lose the opportunity to see things now.” Tung, too, wouldn’t want to forgo any chance “to watch my parents age or my kids grow.” Hibernation might be thought of as getting as close to death as possible without fully succumbing to it. If that’s indeed the price bats and marmots must repeatedly pay to prolong their years, maybe they aren’t really living that much longer at all.