Thesis advisor: Eric Strauss / Thesis advisor: Peter Auger / Northern Diamondback Terrapin (Malaclemys terrapin terrapin) hatchlings from the Great Marsh of Barnstable, Massachusetts on Cape Cod were studied as part of a headstarting program at Boston College. Although headstarting programs are relatively common methods to revive wild turtle populations, concern exists over the ability of headstarted terrapins to exhibit normal behavior when released into the wild. Observations of terrapins in the wild and in the laboratory suggest that terrapins are highly social and that these social interactions are modulated by kinship relations. This study investigated the social behavior of juvenile terrapins while they were basking on a restricted site. Groups of familiar kin and nonkin, and unfamiliar kin and nonkin were tested. Familiar kin were found to bask in larger groups and showed a willingness to share the limited basking site. Unfamiliar nonkin also interacted, but were more aggressive. Unfamiliar kin and familiar nonkin were not distinguishable in their behaviors. These results suggest that terrapins form social groups based on both familiarity and relatedness. Learning how captive headstarted terrapins form social groups and share resources may provide key information for their survival and recovery efforts in Massachusetts. / Thesis (BS) — Boston College, 2007. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Biology. / Discipline: College Honors Program.