Hunger seems like a simple phenomenon: the stomach rumbles until it’s fed, then it’s quiet until it rumbles again. Why, then, does it shape so much behavior that, at least on the surface, has so little to do with food? Part of the answer can be gathered from observations of other animals. For some of them, a little starvation seems to confer a survival advantage. In rodents, hunger appears to heighten sensory perception and speed up mental processing. Last year, researchers in Bordeaux, France, showed that the brains of food-deprived lab mice released endocannabinoids, which act on the same receptors as the psychoactive chemical in marijuana, stimulating the animals’ olfactory cortexes and sharpening their sense of smell. Yale University mice showed similarly enhanced environmental awareness: when injected with ghrelin, the so-called hunger hormone, the mice navigated mazes more quickly than their satiated peers. (The hungry rats of Harvard University, meanwhile, ran further on their in-cage treadmills.) At the University of Texas Southwestern Medical Center, ghrelin was found to reduce depression and anxiety in mice, inuring them to tests for both “social defeat” (bullying by larger mice) and “behavioral despair” (swimming in an inescapable acrylic cylinder).