Tamarin and marmoset mating systems: Unusual flexibility.

Recent studies of wild tamarins and marmosets have shown that at least one species exhibits variable mating patterns, including cooperative polyandry, monogamy and, more rarely, polygyny. Polyandry is thought to occur because the high frequency of twinning and the relatively high weights of infants in these species make the rearing of infants unusually difficult. Nonreproductive helpers (older offspring) and polyandrous males may serve as alternative sources of the extra help needed with infant care. The apparent causes of facultative polyandry in saddle-back tamarins are quite different from those of the cooperative polyandry that has been studied in some bird species.