The branch of ecology that focuses on the evolutionary causes of variation in behavior among populations and species. Thus it is concerned with the adaptiveness of behavior, the ultimate questions of why animals behave as they do, rather than the proximate questions of how they behave. The principles of natural selection are applied to behavior with the underlying assumption that, within the constraints of their evolutionary histories, animals behave optimally by maximizing their genetic contribution to future generations. For example, animals must maintain their internal physiological conditions within certain limits in order to function properly, and often they do this by behavior. Small organisms may avoid desiccation by living under logs or by burrowing. Many insects must raise body temperatures to 86–95°F (30–35°C) for effective flight, and achieve this by muscular activity such as the shivering of butterflies in the early morning or by orienting to the Sun. Other adaptive behaviors that are studied may fall in the categories of habitat selection, foraging, territoriality, and reproduction.