Hannah Carey has always been interested in what makes animals tick. As a professor of comparative biosciences in the School of Veterinary Medicine at the University of Wisconsin in Madison she's investigating whether the microbes that live in the guts of hibernating animals might play a role in helping their hosts accomplish their amazing adaptation.
Carey, a AAAS Fellow, studies the 13-lined ground squirrel, a smallish, omnivorous rodent of the North American prairies that hibernates in underground burrows from late September to May. While it typically requires an animal to go without food for several months, hibernation is fairly common, she said, a neat solution to the problem of how to survive that annual catastrophe called winter, when temperatures drop and plant-food sources disappear.
Just teasing apart hibernation's mysteries over the course of her career has been "so exciting." However, Carey's ultimate goal is to apply some of hibernating animals' strategies for surviving extreme conditions and events to human medical care.
"How do they keep their tissue and their organs healthy? There's some change that occurs as they get ready for hibernation that protects these animals from physiological extremes," she said.
The key to hibernation is torpor, the process by which animals reduce their metabolic functions to a tiny fraction of what they are in summer. Some birds and mammals use a variation, daily torpor, suspending activity and lowering their body temperature for several hours, perhaps at night. But many animals, including at least some tropical lemurs among the primates, throw in the towel for the whole sparse season, and hibernate.
"The fact that there is hibernation in the primate lineage makes any discoveries with biomedical potential that much more applicable to human beings," Carey said.
Torpor doesn't persist all winter in hibernating animals, though, and that's one of its most intriguing mysteries. Hibernating animals experience an "arousal" for half a day or so every so often, their body temperatures returning to normal for several hours before sinking back into torpor. Typically, the animals don't eat or drink anything during these episodes, and they actually sleep through much of the event.
Understanding torpor is "one of the biggest holy grails" of zoology, Carey said.
She and her team suspect the bacteria that live in hibernators' gastrointestinal tracts might play a symbiotic role in maintaining the systems that allow the whole little ecosystem to survive the winter. After all, with no nutrients making their way to the gut, those microbes have to adapt to lean times, too. Bacteria types certainly fluctuate under different conditions as Carey's rodents move through their "circannual" rhythms—summertime feast to famine and back.
"We think it's likely that some of the changes squirrels undergo are due to microbiome changes," Carey said.
Researchers have learned in other contexts that metabolites in an organism can call for a release of serotonin, which is found in the gut lining as well as the brain. Carey's lab is exploring whether the varyingly dominant bacteria in the squirrel's gut might direct cells to turn on certain hormones at crucial times in the hibernation process.
Scientists have only recently begun to realize the importance of gut bacteria to an animal's overall health.
"The early views on any microorganisms were very negative, as in, ‘How do we get rid of them?’ But the ones that live with us have been with us since the beginning; they've evolved with us, and we have to give them their due," Carey said.
She is delighted with the collaborative nature of the work her lab is doing, with UW specialists in metabolomics—notably Fariba Assadi-Porter and metagenomics and microbiology authority Garret Suen—pitching in with knowledge and resources. "I could never do all this myself," she said.
Carey’s desire to be a scientist was "innate." She originally wanted to be a veterinarian, but said she should have known better when, even as a little girl spending time with friends on the Long Island Sound, she caught killifish to "play veterinarian" and promptly cut them open to see what was inside
At the State University of New York at Binghamton, at a time when it was uncommon for undergraduate students to conduct research of their own, Carey was "enthralled" to hear her science teachers talk in class about their research with animals. Just those conversations pushed her to get her PhD in zoology from the University of California at Davis.
Carey studied adaptations among marmots in the White Mountains of California in a post-doctoral study she loved, in part because it was "in the natural world," at high altitudes, where food can be scarce. She then signed on as a basic scientist in the UW Veterinary School. She also has an appointment in pediatrics in the UW Medical School for her study of the digestive systems of baby pigs, which she said is very informative about human babies.
One of Carey's favorite responsibilities is her role as the director of the Biotron, "global biology in a box." The facility, which was built on the UW campus 50 years ago by the National Science Foundation, can simulate all kinds of climates in dozens of environmentally controlled rooms—the tropics, the arctic, the prairie, the desert.
"I couldn't do my work without it," she said. As the longest-term user, at more than two decades, she figured it made sense for her to oversee the facility. "I always wanted to be the director of a field station. So now I am, but the field is on the inside of a building."
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