in its structure and function from summer feeding to winter fasting.
This salvage operation depends on the chemical breakdown of urea molecules to release their nitrogen. But here's the kicker: Chemical breakdown of urea requires urease, an enzyme that animals do not produce. So how does a cow, for instance, get that nitrogen out of urea? First, 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.
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, when they were four months into fasting and hibernation. This assist means that what little urea the animal makes during hibernation has an easier route to the gut.