Anim. Behav., 1981,29, 822-829 SEXUAL DIMORPHISM IN RESPONSES TO UNFAMILIAR INTRUDERS IN THE TAMARIN, SAGUINUS OEDIPUS By JEFFREY A. FRENCH & CHARLES T. SNOWDON Department of Psychology, University of Wisconsin, Madison, Wisconsin 53706 Abstract. Aspects of social structure in group-living species can be inferred by observing the responses of individuals to unfamiliar animals. This study examined the responses of mated pairs of cotton-top tamarins (Saguinus oedipus oedipus) to presentation of either unfamiliar conspecifics or members of a related tamarin species, the saddle-back tamarin (Saguinus fuseicollis fuscicollis). Male and female responses to intruders differed: resident males threatened, displayed piloerection, approached, and attacked intruders, especially males, while resident females showed increases in suprapubic scent- marking in the presence of intruders. Both males and females discriminated between the species of intruders, exhibiting more threats, scent-marking, piloerection, and approaches in the presence of conspecific intruders. There are pronounced sex differences in the signals and behaviour patterns that are elicited in an aggressive or territorial encounter with unfamiliar conspecifics in this monogamous primate. There are a variety of mechanisms by which groups of primates can promote and maintain intergroup spacing or distance. First, groups can maintain social spacing through the use of long- distance acoustic or olfactory signals that medi- ate avoidance between groups. Waser (1975) studied groups of grey-cheeked mangabeys (Cereoeebus albigena) and found that they show a net movement away from the site of a playback of a recorded 'whoopgobble' from an adult male of another social group. Panamanian tamarins (Saguinus oedipus geoffroyi) scent-mark more frequently in areas of home-range overlap than in the centre of their home range, suggesting that scent-marks may demarcate home-range boundaries and contribute to intergroup spacing (Dawson 1979). Second, intergroup spacing can be mediated through visual, acoustic, or ol- factory threats and agonistic displays when groups meet. Titi monkeys (Callicebus moloch) engage in elaborate displays, in which vocaliza- tions, piloerection, back-arching, and tail- lashing are prominent, when groups meet at the boundaries of their home ranges (Mason 1968). Third, intergroup distance can be established and maintained through physical confronta- tions and aggression between neighbouring groups. In free-ranging rhesus monkeys (Macaca mulatta), groups usually avoid contact with each other; however, when contact between groups is made, severe fights occur. Wounds are often in- flicted on members of the subordinate group, and this group usually retreats after several minutes (Southwick et al. 1965). In her recent review of monogamy in mam- mals, Kleiman (1977) indicated that in mono- gamous species there is a reduction of sexual dimorphism, both behavioural and morpho- logical. Male and female mates may contribute equally to the maintenance of group cohesion, the exclusion of non-family animals, and the promotion of intergroup distance. This appears to be the case for marmosets and tamarins (Family Callitrichidae). Unfamiliar conspecifics presented to captive groups of both common marmosets (Callithrix jaechus) and saddle-back tamarins (Saguinus fuscicollis) receive intense threats and aggression from group members. In addition, group members show significant in- creases in scent-marking with specialized cir- cumgenital and suprapubic glands during these encounters (Epple 1970, 1978). Both adult males and adult females show these responses toward intruders. It has also been noted that in aggres- sive encounters, both within family groups and with unfamiliar intruders, male and female marmosets and tamarins direct aggression almost exclusively toward individuals of their own sex (e.g. Epple 1975; Kleiman 1979). These findings have led to the suggestion that threats, aggression, and scent-marking elicited by un- familiar conspecifics in tamarins may serve to drive off potential sexual competitors and to maintain reproductive exclusivity between the pair-bonded adults as well as being effective intergroup spacing signals (Epple 1978). Recent field reports on the cotton-top and Panamanian tamarins (S. oedipus ssp.) suggest that these species may show sexually dimorphic spacing behaviour (Dawson 1978, 1979; Neyman 1978). In these studies it appeared that females initiated scent-marking bouts in areas of home- 822