Movement is energetically costly, yet many animals travel vast distances as part of their normal life history. From a Darwinian evolutionary perspective, this would be unexpected unless it provided some fitness advantage for survival or reproductive success. Movement is often necessary to find food, shelter, or mates. The natural reward circuit is defined as a collection of interconnected brain regions involved in the perception of pleasure. In the 1950s, Olds and Milner conducted experiments with rats placed in operant boxes in which a lever could be pressed by the rat on a voluntary basis that would deliver mild electric shocks to various parts of the brain through differential placement of the electrode. Dopamine has historically been considered the substrate for reward, and there is abundant evidence in the literature supporting this idea. For example, dopamine levels increase in extracellular spaces in the nucleus accumbens in response to inherently rewarding stimuli such as sweet flavor orgasms and drugs of abuse.