As winter comes to a close, several species of animals are waking up from hibernation. A recent Science on Tap presentation from UW-Madison focused on not only those hibernating animals, but also the microbes within those animals, how the host and microbes work together to sustain life, and what implications the study of hibernation could have on space travel and human medicine.
Edna Chaing is a microbial Ph.D candidate in microbiology doctoral training at the University of Wisconsin-Madison. In January she made the trip to the Northwoods to present her research as part of the Science on Tap programming. Each month the program hosts a different speaker on various topics. According to Susan Knight from Trout Lake Station, who facilitates the presentations, Science on Tap is an extension of the Wisconsin idea that the borders of the university are the borders of the state. Those who cannot attend the presentations in person can watch them live on the internet at home or the Minocqua library, and may also view each presentation in its entirety once it is uploaded to the website scienceontapminocqua.org.
Chaing's presentation took attendees through the life cycle of the 13-lined ground squirrel and its hibernation. Not only did Chaing speak about the changes through which the squirrel itself goes during hibernation, but also the effects on the microbial community inside of the squirrel.
Hibernation is one strategy animals use to live through times of high energy demand and low resource availability. Some animals migrate, using an enormous amount of energy to move to a place where resources are more abundant. Other animals, such as tree squirrels, spend countless hours during the summer storing food. Other animals, such as the 13-lined ground squirrel, which are native to Wisconsin and the Midwest, build adequate fat stores in the summer months, doubling their body weight going into winter. They will not eat or drink anything for six months while they are hibernating in their sleep-like state, instead relying on those fat stores for energy.
During hibernation, the squirrel goes into what is called a torpor, where the animal goes into a metabolic depression, with its metabolism decreasing by up to 93 percent. Its body temperature also decreases, down to 39 degrees Fahrenheit. The animal then cycles through the torpor stage for about 24 days, then into what is called an interbout arousal stage for 12-24 hours. In this stage, the animal's temperature and metabolism goes back to normal for that short period. Seventy percent of the energy the squirrel uses during hibernation, Chaing said, will be used up in these short periods.
As the squirrel goes through these changes, so, too, do the microbes in its gut. In recent years, Chaing said, humans have become more aware of the importance of microbes in their own guts, often taking probiotics, or prebiotics to maintain a healthy GI tract balance. Animals have the same types of microbes in their guts, as well, to keep them healthy. Those microbes can struggle to survive, decreasing in number and diversity as the host animal cycles through the hibernation phases.
With hibernation, there is nothing moving through the animal's GI tract, which can cause health issues and a weakened immune system. Muscles and bones can suffer as well with muscles atrophying over the course of the winter. The squirrels will emerge in the spring at one-third to one-half of their body weight.
All of this wreaks havoc on the microbial community living within the animal as well. Any food the squirrel consumes while awake, the microbes use for energy. So, with the squirrel not eating during the winter, the microbes need to find another source of food. Many microbes will not make it through the winter, some due to the lack of food for energy and others by simply not being able to withstand the temperature change of the squirrel's body. Those that do survive, will use the microbe carcasses as a food source.
Another food source are sells that are sluffed off of the GI tract of the squirrel. As the microbes consume these cells and use them for fuel for their own sustainability, they also provide a valuable benefit to their host. These microbes recycle nitrogen, which will help keep the host's muscles from atrophying over the long winter months.
Some microbes will create protective spores around themselves and go into a dormant-like state themselves as conditions become less hospitable within the host squirrel. Once conditions improve, the microbes can then emerge and grow again. During periods of interbout arousal, when the squirrel's metabolism returns to a more normal rate and its body temperature increases, microbes fair better. The relationship is complicated, with the lives of the microbe and host being closely intertwined.
Chaing said one of the exciting parts of her research looks at how this can benefit astronauts in space as well as what hibernation or a synthetically-induced torpor could mean for human medicine. In space, she gave the example, if astronauts are ready to come home, but they have a broken part of their space ship, it could take NASA a very lengthy time to get a mission together to get that part to the astronauts. If a synthetically-induced torpor could be created in humans, the astronauts, who may otherwise run out of food, water and oxygen, would be able to be put into a state where these resources would be used at a minimum, if at all. She also said astronauts may be less susceptible to other health problems associated with space travel such as muscle atrophy.
Another example she used was in the medical field, where some patients who have critical injuries are placed into a therapeutic hypothermia. She said a synthetically-induced torpor could be a much safer alternative. The hypothermic state often results in an irregular heartbeat, which can lead to a heart attack.
The biggest difference between the two, Chaing said, is a synthetically-induced torpor works by slowing the metabolism, which in turn brings down the body temperature. A therapeutic hypothermia works the opposite. Body temperature is lowered until the metabolism slows.
Chaing also spoke about the lack of oxygen that could get to the brain of an animal in a torpor. She stated while there may be some concern about brain damage, it has been found the synopsis in the brain that degrade during torpor would reform connections during the interbout arousal phase of hibernation, minimizing brain damage.
While any sort of test involving humans and torpor or a hibernation-type state is years off, she said, if it were to happen at all, she felt the very early signs pointed toward the need for more research. That research, she said, could have implications for both space travel as well as medicine in the future.
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