Secondarily solitary: the evolutionary loss of social behavior

Studies of social behaviour frequently assume that evolution proceeds from a solitary state to a social one, and that social lineages give rise to lineages that are also social, excluding parasitic taxa. Recent phylogenetic studies of some bees contradict this assumption, and more examples are known or hypothesized in other animals. Social behavior can be lost to give rise to species that are secondarily solitary. Studies of the conditions that lead to the suppression or loss of social behavior can help to illuminate those factors that lead to its origins and maintenance.