This year had some fascinating scientific articles published and we’re here to tell you all about the squirrely news. Keep reading to find out how crazy a ground squirrel’s microbiome is during hibernation, whether ground squirrels really go through puberty during torpor, and if urban red squirrels are sugar addicts!
Ground squirrel gut microbes do WHAT with Nitrogen?
The questions of hibernation:
Many animals survive long winters by using torpor, or periods of rest with very low metabolism, sometimes less than 1% of usual. In spite of not moving or eating for months, the squirrels don’t lose muscle mass. How is this possible? Well, Matthew Regan and his team hypothesized that squirrels might rely on ureolytic gut microbes. Because these gut microbes recycle urea, they might help hibernating squirrels maintain their protein balance!
What they did and why:
The researchers gathered thirteen-lined ground squirrels and divided them into two groups, one group with an intact gut microbiome and the other group with a gut microbiome depleted through antibiotics. They tracked the ground squirrels through summer, early winter, and late winter. Next, the scientists determined whether the squirrels’ gut microbes were salvaging nitrogen by returning any would-be excreted urea nitrogen into the squirrel’s protein pool. The team also compared the number and diversity of microbes across seasons in both treatment groups.
What they found:
The team found that the squirrel group with intact gut microbiomes had higher levels of nitrogen in their livers and muscle than the group with depleted microbiomes, demonstrating that urea nitrogen recycling was microbe-dependent. They also found that the late winter squirrels had 2-3x the amount of nitrogen compared to the summer squirrels, suggesting that this process is most beneficial during this season.
Regan and his research team also found evidence that during hibernation, ammonia was more often recycled into glutamine and not urea. Glutamine is used by the body to produce proteins which suggests this mechanism helps squirrels conserve protein during negative energy production. This kind of nitrogen recycling could also increase water-retention because of low urine production during torpor.
Regan’s research could someday be used to help treat muscle wasting in people. It turns out that humans carry the necessary microbes and genes for urea-nitrogen recycling, too. Beyond being a cool, new, physiological mechanism to explain how squirrels sleep through winter, this research could potentially be applied for helping the lives of others!
Citation: Regan, M. D., Chiang, E., Liu, Y., Tonelli, M., Verdoorn, K. M., Gugel, S. R., Suen, G., Carey, H. V., & Assadi-Porter, F. M. (2022). Nitrogen recycling via gut symbionts increases in ground squirrels over the hibernation season. Science, 375(6579), 460–463. https://doi.org/10.1126/science.abh2950
Hibernation arousals might be more important than we thought!
More questions about hibernation:
Why do hibernating animals spend the energy to wake up periodically throughout the winter season? If it is so energetically costly, why not just stay in torpor for the entire season? Dr. Dai Pra and his research team believe they know why the thirteen-lined ground squirrel, and possibly other animals, experience interbout arousal: sexual maturation.
What they did and why:
The team noticed that juvenile male ground squirrels are able to reproduce and produce viable offspring shortly after their first hibernation season. Pups usually enter hibernation in September as sexually immature juveniles, so something must be happening during hibernation to let them reproduce when they emerge. But this isn’t intuitive, right? Sexual maturation is energetically expensive, and mammals typically only mature sexually when enough energy is available. The team allowed some juvenile, male, ground squirrels to hibernate for 167 days. They monitored the squirrels’ body mass, hormones, and gonadal growth and found something crazy!
What they found:
Juvenile ground squirrels DID reach sexual maturity during hibernation! As body and fat mass decreased, luteinizing hormone and follicle stimulating hormone increased. Testicular growth also increased, showing that the reproductive axis was active even when the squirrels were fast asleep, maintaining an average body temperature around 4°C for weeks on end. This is where those arousal bouts came into play; almost all of the hormone production and testicular growth happened during these short periods of wakefulness and elevated body temperatures. This result suggested that periodic arousals play an important role in allowing juvenile ground squirrels to achieve sexual maturity while hibernating.
Dr. Dai Pra and his team believe this process is adaptive because it shortens the time between hibernation arousal and mating. This would give more time for reproduction and pup development. More research needs to be done to determine if female ground squirrels experience the same phenomenon.
For more information, check out this informative video produced by the research team!
This research could provide an insight into why other hibernating animals experience interbout arousal. Knowing more about animal development can help inform conservation and management plans.
Citation: Dai Pra, R., Mohr, S. M., Merriman, D. K., Bagriantsev, S. N., & Gracheva, E. O. (2022). Ground squirrels initiate sexual maturation during hibernation. Current Biology, 32(8). https://doi.org/10.1016/j.cub.2022.02.032
The sugar addicts of the urban world (no, not just humans!)
Many cities have built parks to support native wildlife, but how well are these green spaces keeping animals healthy? Supplementary food provided by humans might help an animal momentarily, but over long periods, these foods can be harmful. This is exactly what PhD student Bianca Wist and her team found in their study of how urbanization affects food choice and diet of the Eurasian red squirrel (Sciurus vulgaris).
What they did and why:
In their home country of Germany, Wist and her team captured squirrels from two small parks and a wilderness site. They weighed each captured squirrel, then placed each animal in a controlled solitary environment and presented it with food. The squirrels were given a buffet of foods that the team had seen squirrels eating prior to capture, including human foods like biscuits or “natural” foods like pine cones or grasshoppers. At the end of each day, any leftovers were collected and weighed to determine which foods the squirrels had noshed on the most. Every day for 2 months, the researchers repeated this process, recording the animals' food choices, nutritional condition, and body mass.
What they found:
Urban squirrels consumed more energy per day than the forest squirrels, although both populations selected fat over protein in the buffet. There was also a huge difference in the amount of sugar consumed between the two groups: urban squirrels ate about 50% more sugar than their country cousins each day. Urban squirrels actively sought out foods with high amounts of sugar while forest squirrels avoided these foods.
Although the urban squirrels ate more on average than the forest squirrels, they had lower body masses than forest squirrels, suggesting that although urban squirrels may have access to more food overall, they aren’t getting adequate nutrients to sustain their body mass. And, much like us, given the choice of more nutritious and energy-rich foods, urban squirrels still chose foods with fewer nutrients and more sugar!
This research provided insight into the negative side effects of supplementing urban wildlife diets with inappropriate food items. This finding may help scientists to develop better strategies to conserve urban wildlife, as well as more effective public education campaigns about littering and unhealthy food consumption.
Citation: Wist, B., Stolter, C., & Dausmann, K. H. (2022). Sugar addicted in the city: Impact of urbanisation on food choice and diet composition of the Eurasian red squirrel (sciurus vulgaris). Journal of Urban Ecology, 8(1). https://doi.org/10.1093/jue/juac012