RESEARCH ARTICLE Hormonal Changes During the Mating and Conception Seasons of Wild Northern Muriquis (Brachyteles arachnoides hypoxanthus) KAREN B. STRIER 1 *, JESSICA W. LYNCH 1 , and TONI E. ZIEGLER 2 1 Department of Anthropology, University of Wisconsin–Madison, Madison, Wisconsin 2 Wisconsin Primate Research Center, University of Wisconsin–Madison, Madison, Wisconsin We investigated hormonal and behavioral changes in wild male and female northern muriquis (Brachyteles arachnoides hypoxanthus) at the Estac ¸a ˜o Biolo ´gica de Caratinga, Minas Gerais, Brazil, during a 6-mo period that encompassed the onset of the 1998–1999 mating and conception seasons. Individual females resumed mating with the resumption of ovarian cycling, which was not synchronized among them or related to their cortisol levels. Females experienced two to seven cycles prior to conceiving, and the first conception occurred 2 mo after the onset of the group’s mating season. There were no differences in female cortisol levels across their premating, mating, and conception conditions. Cortisol levels were significantly higher in females than in males prior to the conception season, consistent with the prediction that energy reserves may be associated with breeding readiness in females, but not males, in this species. The sustained elevation in male cortisol occurred after the peak in their sexual activity, which resulted in the first conception of the year. Male cortisol levels were positively correlated between years that were similar in rainfall, but differed in the timing of sexual and reproductive events. The timing of cortisol elevations in males appears to be generally regulated by environmental cues, but is responsive to fine- tuning by social and behavioral cues related to the unpredictable timing of reproductive opportunities within their extended mating season. Am. J. Primatol. 61:85–99, 2003. r 2003 Wiley-Liss, Inc. Contract grant sponsor: Liz Claiborne and Art Ortenberg Foundation; Contract grant sponsor: National Geographic Society; Contract grant sponsor: Margot Marsh Biodiversity Foundation; Contract grant sponsor: Graduate School, University of Wisconsin–Madison; Contract grant sponsor: National Center for Research Resources; Contract grant number: 00167, to Wisconsin Primate Research Center. *Correspondence to: Karen B. Strier, Department of Anthropology, University of Wisconsin– Madison, 1180 Observatory Drive, Madison, WI 53706. E-mail: kbstrier@facstaff.wisc.edu Received 10 October 2002; revision accepted 11 August 2003 DOI: 10.1002/ajp.10109 Published online in Wiley InterScience (www.interscience.wiley.com). r 2003 Wiley-Liss, Inc. American Journal of Primatology 61:85–99 (2003)