Social monogamy and its variations in callitrichids: Do these relate to the costs of infant care?

When I studied saddle-back tamarins (Saguinus fuscicollis) by following wild groups of individually marked tamarins around the rainforest in Manu National Park in southeastern Peru, one group particularly intrigued me because of its unusual family structure. Tamarins usually have twin births and infants are carried by adults for their first three months. In this group, when an adult male had tired of carrying the twins and scraped them off his back onto a branch, it was usually not the twins’ mother who finally came to retrieve the screaming young. Instead, it was generally a second adult male who picked up the infants and carried them for a period of time, just as solicitously as the first male had done. When I later compared the infant-carrying contributions of the two males and the female, I found that the males had done roughly equal amounts of carrying, and each had done much more than the female. I also found that both males had mated frequently with the female earlier in the year, before the female became pregnant with the twins, with no clear indication that one was dominant over the other and was more likely to have achieved parentage. None of these observations fits with the traditional view of callitrichids (tamarins and marmosets) as socially and/or genetically monogamous primates. This assumption had been based on the lack of sexual dimorphism in callitrichid species, the fact that only single females bred in captive groups, and the paternal care shown by males (Kleiman, 1977)