Variability in the Social Lives of Red Titi Monkeys ( Callicebus discolor) and Equatorial Saki Monkeys (Pithecia aequatorialis), Two Pair-Living Primates in Ecuador

Despite decades of research on the evolution of monogamy, it has only been within the last 10-15 years that more attention has been given to the social lives of pair-living species, which can be highly variable with respect to cohesion between pair-mates, levels of paternal care, and flexibility in group structure. The social environments of pair-living animals can be dynamic and pair-mates are likely interacting with extra-group individuals much more regularly than is typically detected. These encounters can provide opportunities for extra-pair copulations as well as opportunities to assess individuals as potential partners, which can sometimes lead to "divorce" or the inclusion of additional adults into the group. It is not well understood how potential social pressures, coupled with the different lifestyles (e.g., presence vs. absence of paternal care), shape pair-mate relationships and either promote or constrain alternative reproductive strategies.;This dissertation explores the social lives of red titi monkeys ( Callicebus discolor) and equatorial saki monkeys (Pithecia aequatorialis), two pair-living primates that differ in their degrees of paternal care. Titi monkeys, unlike saki monkeys, exhibit strong paternal care. In Chapter 2, I show that pair-mate relationships are more mutually maintained among titis than sakis and that among sakis, pair-mate relationships are maintained more by males. In Chapter 3, I show that saki pairs can be flexible in their reproductive strategies, even to the extent that multiple same-sex adults live in the same group. I found that this can occur through immigration of adults and by retention of adult offspring. I describe the process by which a putatively unrelated adult male saki joined a group containing an established pair of seven years along with their adult daughter and juvenile daughter. I provide preliminary evidence that this may occur because secondary males assist resident animals with territorial defense (possibly in exchange for mating opportunities) and show that putatively unrelated males are able to form affiliative relationships. I also show that dynamic social environments can disrupt pair-mate relationships, which in this study resulted in the resident male saki abandoning his home territory, his pair-mate of seven years, and a neonate, to pursue additional mating opportunities along with the immigrant male. In Chapter 4, I describe the process of replacement of the resident male saki by another adult male, which was then followed by successful reproduction of both the natal adult female and her mother.;Collectively, my study adds to our knowledge of variability in the social lives of pair-living animals by documenting some of the ways in which social pressures can influence mating and parenting strategies.