Maternal Behavior and Maternal Stress Are Associated with Infant Behavioral Development in Macaques Massimo Bardi Department of Ethology Ecology, Evolution, University of Pisa Pisa, Italy E-mail: mbardi@richmond.edu Michael A. Huffman Section of Ecology Primate Research Institute Kyoto University Inuyama, Japan ABSTRACT: The simultaneous effects of naturally occurring individual differ- ences in maternal care and maternal peripartum stress on infant development have been sparsely reported in nonhuman primates. In this work, we used a comparative approach to assess how changes in peripartum maternal excreted cortisol levels and the quality of mother–infant interactions correlate with infant behavioral development in group-living rhesus and Japanese macaques. We tested the hypothesis that peripartum maternal stress was associated with infant behavioral characteristics during development. Due to the difference in mothering style between the two species, we provided separated analyses for two groups. A sample of mother-infant pairs (Japanese macaques, N ¼ 14; rhesus macaques, N ¼ 10) was observed during the first 3 months of the infant’s life. Follow-up observations (at 5, 7, and 9 months of age) were collected for the infants. Maternal cortisol levels were measured during the peripartum period. We found preliminary evidence that maternal peripartum stress and differences in key components of maternal behavior are associated with infant behavior throughout the developmental phase. We also provided a working hypothesis regarding maternal behavior and maternal stress as factors playing unique roles in different components of infant behavioral deve- lopment. ß 2005 Wiley Periodicals, Inc. Dev Psychobiol 48: 1–9, 2006. Keywords: Japanese macaque; rhesus macaque; maternal behavior; maternal responsiveness; cortisol; HPA activity; infant behavior; infant development Research suggests that profound, and sometimes even subtle, differences in maternal care can have long-term consequences on a variety of somatic, reproductive, and behavioral systems in a wide array of mammals. The separation of the infant from the mother is the extreme form of altered maternal influence. Infant separation studies have shown that both short- and long-term re- sponses occur after removal from the mother (Gilmer & McKinney, 2003; McEwen, 2003; Rilling et al., 2001). Acute and chronic physiological effects of maternal separation have been reported in both rodent and primate models (Pryce, Ruedi-Bettschen, Dettling, & Feldon, 2002). Serotonin transporter gene variation affects limbic- hypothalamic-pituitary-adrenal (LHPA) axis activity and the influence of the serotonin transporter gene promoter variation (rh5-HTTLPR) on hormonal responses during stress is modulated by early experience (Barr et al., 2004). Rhesus monkeys (Macaca mulatta) reared in peer groups could be differentiated by the serotonin transporter genotype, indicating a genotype-dependent effect of en- vironmental challenge on serotonin function (Bennett et al., 2002). Prenatal stress has also received significant attention recently. In rodents, even short periods of prenatal stress can result in offspring with abnormal behavior and physiological responses to fearful stimuli (Weinstock, 1997), and adults with increased corticoster- one and defensive withdrawal behaviors (Ward, Johnson, Salm, & Birkle, 2000). Received 15 December 2004; Accepted 7 July 2005 Correspondence to: M. Bardi Contract grant sponsor: Japanese Ministry of Education, Science, and Culture Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/dev.20111 ß 2005 Wiley Periodicals, Inc.