![]() These molecules are lodged in intestinal cell membranes and shepherd urea from the blood into the gut where the microbes that contain urease are found. During hibernation, their gut cells increase production of proteins called urea transporters. We also found that the ground squirrels contribute to this remarkable symbiosis. Hibernators’ liver and muscle tissue incorporated the most urea nitrogen during late winter – that is, the longer they’d been hibernating and without food. Our observations confirmed that this process was indeed dependent on the gut microbes’ ability to break down urea and liberate its nitrogen in the hibernators’ guts. ![]() But squirrels with depleted microbiomes displayed minimal urea nitrogen salvage. In squirrels with normal microbiomes, we saw evidence of urea nitrogen salvage at each step of the process that we tested. We treated some animals with antibiotics to reduce gut microbes at three times of the year: summer early winter, when they were one month into fasting and hibernation and late winter, whwithen they were four months into fasting and hibernation. To confirm that the microbes were doing the nitrogen recycling, we compared squirrels that had normal gut microbiomes to squirrels that didn’t. Wherever we saw higher levels of the heavier form of nitrogen, we knew that urea was the source of the nitrogen, and therefore gut microbes had to be responsible for getting the urea nitrogen back into the animals’ bodies. We were able to follow these heavier nitrogen atoms as the injected urea moved from the blood into the gut, then as microbial urease broke down the urea into its component parts, and finally into the squirrels’ tissue metabolites and proteins. ![]() Using the 13-lined ground squirrel, we designed experiments to investigate key steps in urea nitrogen salvage.įirst, we injected into the squirrel’s bloodstream urea molecules in which the two nitrogen atoms were replaced by a heavier form of nitrogen that naturally occurs only in tiny amounts in the body. This was our challenge as scientists: Could we demonstrate urea nitrogen recycling in hibernators and show that it is particularly helpful to them the longer they fast? Our experimental game plan But for other animals – like hibernators and us – it was less clear whether and how the urea nitrogen could make its way into the animals’ bodies to support protein synthesis. Peculiarities of the ruminant digestive system allow those animals to benefit greatly from this process. Microbes then absorb ammonia and use it to make new protein for themselves. They make the urease enzyme and use it to chemically split urea molecules, freeing up the nitrogen, which becomes part of ammonia molecules. It turns out certain microbes that are normal residents of animals’ guts can do just that. LAGUNA DESIGN/Science Photo Library via Getty Images Our research team tackled this question by investigating how hibernating animals might be getting a major assist from the microbes that live in their guts.Ī model of the urea molecule, with two nitrogen atoms (in blue) along with a carbon (gray), an oxygen (red) and four hydrogen (white) atoms. How do hibernators pull this off? It’s been a real head-scratcher for hibernation biologists for decades. Despite as much as six to nine months of inactivity and no protein intake, they preserve muscle mass and performance remarkably well – a very handy adaptation that helps ensure a successful breeding season come spring. But muscle wasting is minimal in hibernating animals. In people, long periods of inactivity, like prolonged bed rest, lead to muscle wasting. ![]() This is a particular problem for muscles. That long fast comes with a downside: no new input of protein, which is crucial to maintain the body’s tissues and organs. Torpor greatly reduces how much energy the animal needs to stay alive until springtime. Their metabolism drops to as low as just 1% of summer levels and their body temperature can plummet to close to freezing. While hibernating, ground squirrels enter a state called torpor. They need to store a lot of energy as fat, which becomes their primary fuel source underground in their hibernation burrows all winter long. Ground squirrels spend the end of summer gorging on food, preparing for hibernation.
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