Saguinus fuscicollis (Callitrichidae) were studied in the Cocha Cashu Biological Station, Manu National Park, Peru. Availability of both fruits and insects was substantially lower during the annual 4-month dry season (May-September) than at other times of the year. Individual tamarins lost an average of 5% of their weight over this period. Three-quarters of births occurred between November and February; none occurred between mid-March and mid-August. Tamarin births at Cocha Cashu are probably timed such that lactation and weaning occur when food is abundant, because during the period of low food availability, there would be insufficient food to meet the demands of lactation and to serve as easily obtainable weaning foods. -from Authors