87 Abstract Callimico (Callimico goeldii) is the only species of tamarin or marmoset to regularly produce one infant, possibly representing a derived trait. Studies of adult–infant interactions among callimicos, therefore, offer important insights into the evolution of cooperative infant care in callitrichids. We studied a group of callimicos consisting of one adult female (FH), two adult males (MR and MJ), and one young female (FS), at a Bolivian field site. Opportunistic data on infant care and mating behavior were taken for 3 months following FS’s birth. Then, for 8 months, behavioral data and nearest neighbor distances were collected using focal animal sampling for 1,199 observation hours. Nearest neighbor distances among group members varied significantly (p < 0.001): on average the breeding female was found 2.7 m from her infant, 2.8 m from adult male MJ, and 3.5 m from adult male MR. The frequency of grooming behaviors varied significantly among adults (p < 0.001): FH was groomed most (8.4% of observations) and spent the least time grooming others (3.4%), while MR was groomed least (5.9%) and groomed others most (10.7%). Although FH was observed mating with both males within 2 months after the birth of FS, she did not give birth to additional offspring. All adults shared food with and carried the infant. Adults also shared food with each other. Aggressive behavior among adults was extremely rare (n = 8) and occurred six times in a feeding context and twice in a non-feeding context. These data demonstrate that despite producing singletons, callimicos like many other callitrichids are characterized by a polyandrous mating pattern, spatially cohesive groups, and high levels of social cooperation. Resumen Callimico (Callimico goeldii) es la única especie de tamarinos o mar- mosetas que regularmente produce sólo un infante, posiblemente representando un rasgo derivado. Por tanto, estudios de las interacciones adulto-infante entre los calimicos ofrecen importantes ideas sobre la evolución cooperativa del cuidado del L.M. Porter (*) Department of Anthropology, Northern Illinois University, DeKalb, IL, 60115, USA e-mail: lmporter@niu.edu Chapter 4 Social Behavior of Callimicos: Mating Strategies and Infant Care Leila M. Porter and Paul A. Garber S.M. Ford et al. (eds.), The Smallest Anthropoids, Developments in Primatology: Progress and Prospects, DOI 10.1007/978-1-4419-0293-1_4, © Springer Science+Business Media, LLC 2009