SOCIAL AND SEASONAL INFLUENCES ON REPRODUCTIVE-BIOLOGY IN MALE MOUSTACHED TAMARINS (SAGUINUS-MYSTAX)

In several primate taxa there is evidence that the social and physical environment can exert a significant effect on reproductive behavior a nd biology. In this paper we examine social and physiological factors influencing group composition and reproduction in free-ranging moustac hed tamarin monkeys (Saguinus mystax mystax). This species is characte rized by cooperative care of the young and a breeding system that incl udes both polyandrous and polygyandrous matings. Body measurements col lected on adult males residing in multimale groups indicate marked wit hin-group differences in testes volume. In 12 of 17 groups examined, t estes volume of at least two resident adult males differed by 21-174%. Among these males, testes volume was not correlated with either body weight or adult age class. We also examine whether factors such as tim e of year had an effect on reproductive condition. An analysis of body measurements of 128 adult male and 127 adult female moustached tamari ns, wild-trapped and released in northeastern Peru, indicates cyclic c hanges in genital size. For males, mean monthly testes volume in July (712 mm(3)) was twice that recorded in June (351 mm(3)). Females exhib ited a similar pattern. Although endocrine information on intra- and i ntersexual social effects on fertility are unavailable for S. mystax, given the high degree of social cooperation and lack of overt aggressi on among adult male group members, we offer the possibility that resid ent male moustached tamarins compete for access to the groups' lone br eeding female through socially induced reproductive suppression and sp erm competition. (C) 1996 Wiley-Liss, Inc.