Female Squirrel Monkeys may develop a short, consortlike bond (also seen in heterosexual interactions) during which they interact sexually with one another. In addition, a number of other types of female bonding occur in this species. Females frequently have one close female “friend” with whom they travel and rest; often this relationship develops into a highly affectionate one and may even include coparenting. The two females frequently touch hands, kiss each other on the mouth, and huddle together. If one of them is a mother, the other helps her raise her infant; if both are mothers, they help each other with parenting, including nuzzling and carrying each other’s infant and protecting them from predators. Often the infant develops a strong bond with the comother, although some females act as coparents for infants belonging to several different mothers. The coparenting female is sometimes known as an “aunt,” although she need not be genetically related to the mother; her relationship with the mother often outlasts the duration of parenting.
Homosexual mounting sometimes occurs between male Squirrel Monkeys, especially younger individuals or between an older and a younger (adult) partner; adult males also perform the genital display to each other. During intense displays, one male will thrust his erect penis into the face of the other male while holding him down with his hands and may even climb onto the back of the other male (who also sometimes develops an erection). Several males can be drawn into the activity, forming a ball or “pileup” of three or four individuals all twisting and climbing on one another as they try to perform genital displays.
In Rufous-naped Tamarins, homosexual behavior takes the form of same-sex mounting, including pelvic thrusting (as in heterosexual copulation); both males and females participate in homosexual mounts.
Nonreproductive and Alternative Heterosexualities
In addition to the female coparenting arrangement described above, other alternative family configurations and nonbreeding individuals occur in Squirrel Monkeys. One female sometimes adopts another female’s infant, raising it along with her own baby, while some male Squirrel Monkeys never copulate at all during the mating season. Interestingly, these may be the highest-ranking males in the troop, who are often more aggressive and less patient than other males and therefore more likely to disturb females or fail to attract willing mates. Several other aspects of Squirrel Monkey heterosexual life reveal considerable antagonism and separation between the sexes. Females often form groups or COALITIONS during the mating season to chase off males who are pursuing unwilling females; females (and occasionally males) may also directly disrupt heterosexual copulations in progress. At other times, females persistently harass males so that they remain spatially segregated from the females, either on the periphery of the troop or closer to the ground. When a willing female is found and the sexual interaction is not disturbed, often several males will participate, all joining in by kissing the female on the mouth, displaying their genitals, and sniffing or nuzzling her genitalia. The mating system is promiscuous, as both males and females copulate with multiple partners.
A number of nonprocreative sexual activities are also found in these New World monkeys. Male Squirrel Monkeys masturbate by either sucking their own penis or rubbing it with one or both hands, while females may copulate when not in heat or during pregnancy (up to the fourth month). In addition, females sometimes produce a vulvar plug from sloughed vaginal cells when they are in heat, which may serve to limit inseminations. Rufous-naped Tamarin males on occasion mount females without thrusting or penetration. Male Squirrel Monkeys also have a pronounced sexual cycle: for three to four months out of the year, they are sexually active, more aggressive, and develop a characteriscic appearance—heavier, with more fluffed fur—during which time they are known as FATTED MALES. For the remainder of the year, however, their testes are essentially dormant; they lose their “fatted” appearance and live largely separate from the females. In Rufous-naped Tamarins, as in other tamarins and marmosets, all but the highest-ranking female in a group forgo reproduction, perhaps through a complex mechanism of “self-restraint” that is mediated by pheromones from the lead female. As a result, only about half of all mature females actually reproduce at one time; however, nonbreeding individuals often continue to copulate. In addition, most matings outside the breeding season do not result in offspring, and it is thought that many embryos may be reabsorbed, aborted, or the young die soon after birth.
Other Species
Same-sex activity occurs in several other species of Central and South American monkeys. Homosexual mounting (in both males and females), including pelvic thrusting and genital rubbing on the partner, has been observed in a variety of Tamarin species, including the Saddle-back Tamarin
