A number of studies in recent years have suggested that calorie intake is not the only thing that can alter metabolism to change longevity in lower animals: "Specific odors that represent food or indicate danger are capable of altering an animal's lifespan and physiological profile by activating a small number of highly specialized sensory neurons ... Recent research in model organisms and in humans has shown that sensory experiences can impact a wide range of health-related characteristics including athletic performance, type II diabetes, and aging. Nematode worms and fruit flies that were robbed of their ability to smell or taste, for example, lived substantially longer. However, the specific odors and sensory receptors that control this effect on aging were unknown. Using molecular genetics in combination with behavioral and environmental manipulations, [researchers have identified] carbon dioxide (CO2) as the first well-defined odorant capable of altering physiology and affecting aging. Flies incapable of smelling CO2 live longer than flies with normal olfactory capabilities. They are also resistant to stress and have increased body fat. To many insects, including fruit flies, CO2 represents an ecologically important odor cue that indicates the presence of food (e.g. rotting fruit or animal blood) or neighbors in distress (it has been implicated as a stress pheromone). Indeed, this group of researchers previously showed that merely sensing one's normal food source is capable of reversing the health and longevity benefits that are associated with a low calorie diet. They now establish that CO2 is responsible for this effect."