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Sex differences in behavior are quite common among nonhuman primates. In sexually monomorphic species, sex differences might be expected to be less evident than in polygynous and highly dimorphic species. Callitrichid primates (marmosets and tamarins) are cooperative breeders that exhibit little sexual size dimorphism. However, several sex differences in the structure and usage of vocalizations have been reported. In one such study, McConnell and Snowdon [Behaviour 97:273-296, 1986] reported that female cotton-top tamarins (Saguinus oedipus) emitted significantly more normal long calls than males during simulated intergroup encounters. In the course of collecting a library of normal long calls, we replicated a portion of that study 20 years later with the same colony and similar methods. To our surprise we found a reversal of sex differences. In the same experimental situation, males gave significantly more normal long calls than females. In a further replication 2 years later, males still called more but the effect was less pronounced. The dramatic change in sex differences within the same species and colony over a 22-year period suggests that behavioral sex differences in callitrichids may be quite labile, and that repeated sampling over several years may be necessary to establish true sex differences.

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