Reproductive biology and social strategies of cooperatively breeding adult male cottontop tamarins (Saguinus oedipus oedipus) in two life-history phases: Fathers and adult sons

Cottontop tamarins are cooperatively breeding primates in which adult males provide considerable infant-care. The presence of multiple adult males, who also provide most vigilance, has been linked to increased infant survival. Male tamarins mate monogamously or polyandrously with a single breeding female. The opportunity for shared paternity may explain the affiliative and tolerant relationships and cooperative infant-care demonstrated by polyandrous males. However, philopatric adult sons serve as principle alloparents in groups where their mother is the breeding female. In this thesis, I investigated mechanisms that breeding cottontop tamarins may use to facilitate cooperative interactions and group membership by their adult sons. I studied several captive families at the University of Wisconsin -- Madison. Sons achieved the weight, hormone levels, and gonad size of breeding males, indicating no physical suppression. Adult sons engaged in the most frequent aggression with brothers, whereas fathers remained equally tolerant of all sons, potentially due to the alloparental benefits sons provide to fathers. High rates of contact affiliation and sociosexual mounting exhibited between brothers could maintain cooperative potential or mediate age-graded relationships. Parents engaged in more mutual affiliation than either did with their adult sons. However, expectant parents groomed each adult son at rates predicted by his previously-demonstrated infant-carrying, suggesting a strategy to retain alloparenting services. In contrast, aggression between parents and adult sons was low and not related to alloparenting. Sexually competent adult sons mounted their expectant mothers at a rate similar to that of the fathers, but did not copulate with their mothers. Fathers' mounts contained more copulatory components, solicitous behavior, and mutual post-mount affiliation with the dams. Fathers did not interfere with sons' mounts, and instead, dams regulated mounting. Tolerance of adult sons' sexual behavior may be necessary to retain alloparents. Nonconceptive mounting by adult sons may parallel mounting between pair-mates during late pregnancy, thought to strengthen social bonds. Sexual activity of sons may also reflect readiness for extragroup breeding opportunities. In summary, affiliation and tolerance characterized relationships between expectant parents and their sexually-active adult sons, in contrast to other cooperatively breeding species, where aggression by breeders may coerce alloparenting and restrict sexual behavior.