Sometimes scientists need to make their research subjects' lives harder. No matter how much affection they may feel for those flatworms or fish or pigeons, there are certain things they can only learn by forcing the animals to use more energy. But for animals living in the wild, this can be tricky. Now scientists studying rodents in Eastern Europe say they've found a convenient way to do it: just give the animals a quick shave. Paulina Szafrańska, at the Mammal Research Institute of the Polish Academy of Sciences, is interested in energy budgets. The energy animals get from food, she says, is like money to be spent on their daily activities: scurrying around, hunting, keeping their bodies warm, and so on. "When we make a hole in the budget" by increasing the cost of one of those activities, she says, the animal has to cut back somewhere else. "We call it [a] trade-off." How it chooses to spend the balance of its energy budget shows scientists what's most important to the animal. Does it kick some eggs out of the nest? Eat its young? Invest in its immune system? To tweak the energy budget of birds, for example, scientists can clip their feathers and make flight more difficult. In the lab, they can give animals less food. But for wild mammals, there are fewer options. Here's where the shaving comes in. "When you shave an animal you know that it probably faces such a trade-off choice," Szafrańska says. In the wintertime, a mammal missing its fur should have to work harder to keep itself warm. Previous efforts to test this in the wild hadn't succeeded, but they'd used small samples of only a few animals. And lab studies using "pelt-covered, internally heated metal casts of animals" had shown that fur-free skin ought to lose heat faster. So Szafrańska and her colleagues gathered 240 wild root voles (Microtus oeconomus) and gave half of them haircuts. While one person held each rodent (smaller than a rat but bigger than a mouse), another used a beard trimmer with no tip attached to shave about 50 percent of the animal's back down to the skin. To measure the voles' metabolisms, the scientists used a method called doubly labeled water. They injected the animals with water that contained isotopes of oxygen and hydrogen. Then they released the voles, both shaved and unshaved, within a fenced field in winter. After letting the voles do their thing for a day or two, the researchers used traps to recapture them (they caught about a quarter). Then they drew a little blood and compared the amount of oxygen isotopes remaining in each animal to the amount of hydrogen isotopes. This revealed how quickly each animal was making carbon dioxide—in other words, how fast its metabolism was running. Shaved voles burned more energy than unshaved ones
, the researchers saw. In their brief stay in the chilly outdoors, they also lost more weight than voles that had all their fur. Their bare skin apparently made them lose heat faster, and when they cranked up their metabolisms to warm themselves, their energy budgets went into the red. Since the experiment was a success, Szafrańska thinks other scientists who study energy budgets should try shaving their animals too. It's an easy way to make mammals burn more energy in the wild. However the animal responds to the unbalancing of its energy budget will tell scientists how it prioritizes its needs—at least until researchers start providing animals with a sweater budget too.
Images: Top, Tomasz Samojlik. ("It's a mole-let!" said my husband, in what would have been an A+ pun if the drawing were of a different rodent.) Bottom, Karol Zub.
Szafrańska PA, Zub K, Wieczorek M, Książek A, Speakman JR, & Konarzewski M (2014). Shaving increases daily energy expenditures in free living root voles. The Journal of experimental biology PMID: 25278468