Deep in a wind tunnel at Lund University, Nathusius' pipistrelle bats were flying steady, their bodies doing something that scientists once believed impossible: burning fatty acids to fuel hours of continuous flight. The discovery, made by researchers at the Leibniz Institute for Zoo and Wildlife Research and Helmholtz Munich, upends a fundamental assumption about mammalian metabolism and reveals how these creatures accomplish migrations spanning thousands of kilometers across Europe.
For decades, scientists knew that migratory birds relied on fatty acid oxidation—burning fat for energy—during their long journeys through the sky. Mammals, by contrast, were thought to lack this metabolic superpower. They depend instead on glycogen, a stored carbohydrate that depletes quickly during intense activity, leading to the phenomenon athletes call "hitting the wall." Yet bats, the only flying mammals on Earth, somehow managed to migrate seasonally from northeastern Europe to their winter roosts in the west and south without crashing.
The research team, led by Alesia Walker at Helmholtz Munich alongside Shannon Currie and Christian Voigt of the Leibniz-IZW, cracked the mystery by analyzing blood samples from Nathusius' pipistrelles captured during migration season and non-migration season at ornithological research stations in Latvia, as well as from individuals flying in controlled wind tunnel conditions. Using advanced chromatographic methods, they searched the blood for metabolites—the chemical fingerprints of cellular activity—that would reveal what fuel the bats were burning.
The evidence was striking. When bats flew for extended periods, levels of acyl carnitines—the transport molecules that ferry fatty acids into cells' mitochondria for burning—surged by around 70 percent. Even more telling: bats sampled during their spring and autumn migrations showed significantly elevated levels of phosphatidylethanolamines and phosphatidylcholines, compounds that signal active fat metabolism. Crucially, these effects were most pronounced for unsaturated fatty acids, the longer-chain molecules that pack more energy per unit weight.
What makes this possible for bats but not for humans? The answer lies partly in diet. During migration season, Nathusius' pipistrelles feed intensively on aquatic and riparian insects—mayflies, caddisflies, and other creatures that are naturally rich in long-chain, polyunsaturated fatty acids. By switching their diet strategically, the bats stock their bodies with the precise fuel needed for their journey. Humans cannot synthesize polyunsaturated fatty acids at all; we must obtain them from food. But even with the right diet, our bodies simply lack the physiological machinery to oxidize fat at the rates bats achieve.
"This contradicts the established view that mammals have only limited capacity to transport and oxidize fatty acids during periods of intense activity," Dr. Walker explains in the study, published in the FASEB Journal. The findings suggest that bats have evolved metabolic adaptations strikingly similar to those of birds—a remarkable convergence of biology that solves one of nature's great challenges: how to stay aloft for thousands of kilometers without stopping.
For a small creature weighing just seven to eight grams, this metabolic gift is not a luxury. It is survival. Without the ability to burn fat continuously, these bats could never reach their winter homes. The discovery reminds us that evolution has engineered elegant solutions to impossible-seeming problems, written into the very chemistry of cells.