In springtime some hibernators wake up, warm up and begin eating. But even after the snow has melted, woodchucks can carry on fasting for weeks. Jean-Michel Weber of the University of Ottawa realised that practically nothing was known about the metabolism of non-hibernating animals that go hungry during spring and summer. Teaming up with Shannon Reidy, Weber decided to take a closer look at non-hibernating woodchucks to see how these animals use different metabolic fuels when they stop eating(p. 4525).
Weber explains that when an animal goes hungry, it first burns carbohydrates, then fat, and only uses proteins as a last resort. Most mammals only have small protein reserves locked up in essential structures like heart muscle, so animals need to protect their limited protein stores. When an animal uses protein, nitrogenous waste is produced. Animals excrete this waste in their urine as ammonia, urea and uric acid, and the composition of an animal's urine can reveal how much protein the animal is using up. To find out how woodchucks burn carbohydrates, fats and proteins, Weber and Reidy decided to compare the metabolism and nitrogen excretion of fed and hungry woodchucks in spring and summer.
But first they had to find some willing woodchucks. There are many wild woodchucks around Ottawa, Weber explains, but these aggressive animals have sharp teeth, making them difficult to handle. Luckily, Weber and Reidy found a captive woodchuck colony at Cornell University. The team put these more cooperative creatures into a respirometer to measure the rodents' oxygen consumption and carbon dioxide production. To measure nitrogen excretion rates they collected the woodchucks' urine.
Using the gas exchange and nitrogen excretion measurements, Weber and Reidy calculated how much of the mammals' energy was coming from carbohydrate, fat and protein. They found that summer woodchucks dramatically reduced their need for protein when they went hungry, but fed spring woodchucks already had low reliance on protein and did not decrease their reliance on protein during fasting. So summer woodchucks can drastically reduce their energy expenditure to cope without food, but spring woodchucks already have pretty low resting metabolic rates and cannot depress them any further. This seasonal difference raises the intriguing possibility that, at normal mammalian body temperatures,there might be a minimal metabolic rate below which life cannot be sustained. Equally exciting is the observation that summer woodchucks are capable of extreme protein saving. According to Weber, `the woodchucks' incredibly economical use of proteins provides the key to their fasting success.'
What mechanism might allow woodchucks to reduce their need for protein?Looking at the exact composition of the woodchucks' urine, Weber and Reidy noticed that unfed animals reduced their urea excretion and increased ammonia excretion. `Reading this, one of our paper's reviewers suggested that starving woodchucks might use a process called urea hydrolysis to recycle their nitrogen' Weber recalls. He explains that woodchucks can reabsorb urea through the bladder wall into their bloodstream and transport it to their digestive system, where their gut bacteria convert the urea into ammonia. Since ammonia can be recycled, urea hydrolysis helps the woodchucks make the most of their restricted protein supplies. `The woodchucks' ability to lower reliance on the body's limited protein reserves is unique' says Weber. He concludes that`woodchucks really are fasting champions.'