Infant-carrying behavior among callitrichids seems to be a costly activity. Costs have been related to the physical efforts of carrying the weight of very heavy infants and to the resulting reduction in foraging efficiency. However, the costs of carrying in terms of the physical consequences for carriers have not previously been assessed. In this study, we have regarded weight loss in infant carriers as a measure of costs. We studied five families of cotton-top tamarins (Saguinus oedipus) during the first 9 weeks following the birth of infants. Captive-breeding conditions were required so that body weight could be measured frequently. To avoid inflicting undue stress on the subject animals, we used a noninvasive method for weighing the tamarins. Differences in carrying contribution were found amongst fathers and male and female helpers, with female helpers contributing less. We have found that carrying infants in the cotton-top tamarins is an activity that produces a weight loss. Fathers and male helpers go through a maximal body weight loss. While carrying, the tamarins also decrease food intake. However, no relationship was found between contribution to carrying and feeding time or in energetic intake during feeding observations. Thus, it seems that a direct relation doesn't exist between the observations of feeding and weight loss. Fathers increase their contribution to carrying during mothers' periovulatory periods. In this period, male helpers and especially fathers go through a maximal body weight loss. We found body weight losses of up to 11.3% in one subadult male and 9.1% in a father during the fifth week. No changes occurred in food intake in fathers or other male helpers during this period. During periovulatory periods, mothers carried less frequently but did increase their food intake. They gained weight from the second week after birth onward, especially during the periovulatory period. It seems that the infant-carrying behavior of fathers and male helpers may contribute to the improvement of the mothers' physical condition after birth and therefore may support a consecutive pregnancy.

With reference to the recalibrational theory of anger, our results suggest that the sex- and age-dependent associations between fighting ability and physical and nonphysical aggression indicate divergent adaptive skills between sexes, which are driven by intrasexual competition.

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Infant carrying in cotton-top tamarins is a costly activity that results in weight loss by the fathers and helpers. However, to date, measures of carrying costs have been obtained in small indoor enclosures. We studied body weight changes in adult and subadult individuals from eight large groups (n > 5) for 9 weeks after the birth of infants. Four groups were housed in large indoor/outdoor enclosures (42 m(2) x 3.3 m high), and four were housed in small indoor enclosures (12 m(2) x 2.4 m high). All of the individuals were weighed regularly at least three times a week. Reproductive males lost more weight in the big indoor/outdoor enclosures (mean=6.51%) than in the small indoor ones, as did male adult helpers (mean=5.59%) and female adult helpers (mean=4.4%). Still-growing subadult individuals also lost weight in the big indoor/outdoor enclosures (mean=3.17%), although the differences did not reach significance (P=0.07). These results support the hypothesis that cotton-top tamarins living in more natural settings experience higher weight loss than those housed under less-natural conditions.