Data on the ecology, demography, and morphology of 465 golden lion tamarins in 22 wild groups suggest that avoidance of reproduction and weaning of infants during the dry season shaped the pattern of female reproduction. Post-partum estrus may have evolved to allow females to produce and wean two successive litters during the 7-month season of abundant food resources. Support for these hypotheses includes observations that nearly all births, lactation, and weaning of infants occurred during the wet season, a period of relatively abundant trophic resources. Litter sizes in the wild were smaller than for the captive population. Evidence that food limitation may have a greater negative impact on young juveniles than lactating females includes observations that female weight did not decrease with lactation, and that many births occurred during the transition between dry and wet seasons. Juveniles gained less weight during the dry season than during the wet season; however, there was no significant difference between weights of adults born early vs. late in the wet season. For females producing two litters/year, litter size and infant mortality did not differ for litters born early and late in the wet season. Adult males gained weight before and lost weight during the month of highest probability of female estrus, suggesting that competition for mates is energetically expensive for males. Although resources were abundant, adult males did not gain weight during months when most infant carrying took place, suggesting that this activity is also energetically expensive. © 1994 Wiley-Liss, Inc.