Final answer:
The evolutionary mechanism suggested for the behavior of subordinate females in tamarin and marmoset groups, who assist the dominant female by carrying her offspring, is kin selection, reinforcing the importance of inclusive fitness and shared genetic success within their social structure.
Step-by-step explanation:
The evolutionary mechanism that has been suggested and supported to explain why subordinate females often forgo their reproductive opportunities in tamarin and marmoset social groups is kin selection. Kin selection is a type of natural selection where an individual's inclusive fitness is enhanced by the survival of its close relatives. By assisting the dominant female in caring for her offspring, the subordinate females are ensuring the continuation of their shared genetic material.
Primate Social Life and Evolutionary Mechanisms
Research indicates that in primate social life, both males and females are competitive and cooperative, and are central actors in social interactions. The social organization is complex and includes strategizing for resources, roles, and relationships. For example, in New World monkeys like tamarins and marmosets, there is no significant size difference between the sexes, and they form pair bonds, reflecting a cooperative approach to raising young.
In New World species, the central feature of primate social life is often female kinship and solidarity, as opposed to male competition. Subordinate females carrying the offspring of the dominant female is a manifestation of such cooperation and kin-related altruism. This is a stark contrast to some Old World primates, which exhibit sexual dimorphism and have groups with a single dominant male defending multiple females.