The overall goal of the proposed research is to explain the behavioral development of kin recognition abilities that result from social interactions during early ontogeny. Kin recognition, inferred when differential treatment of individuals correlates with genetic relatedness to the actor, will be investigated in a comparative study of Belding's ground squirrels, a relatively social rodent, and golden-mantled ground squirrels, a relatively nonsocial rodent. For Belding's ground squirrels, questions to be answered are: (1) when do weaned juveniles first recognize siblings? (2) how do social interactions with littermates and nonlittermates affect sibling recognition? and (3) how do juveniles retain the ability to recognize siblings after litters dissolve? For golden-mantled ground squirrels, effects of kinship (related vs. unrelated) and rearing association (reared together vs. reared apart) on the development of dam-offspring and sibling recognition will be investigated. Predictions about inter-specific differences in kin recognition will be tested by recording behavioral interactions between animals that inhabit large, outdoor enclosures for up to 14 days. Recognition abilities will be inferred by examining differences in rates of behavioral interactions (e.g., play) between animal groups that vary in kinship and early social experience. Kin recognition is central to inclusive fitness theory, which predicts that preferental treatment of kin (nepotism) will often enhance an individual's evolutionary (reproductive) success. Here, ground squirrels will be studied in a comparative framework to learn how social experience during early development produces generalized "rules of thumb" that allow kin to be recognized in many species. The proposed research will enhance our understanding of the developmental and evolutionary basis of social discrimination and nepotism, phenomena that are crucial to human social relations.