Final answer:
New World species, particularly tamarins and marmosets, are known for their monogamous family units, forming cooperative pair bonds to raise their young. These primates stand in contrast to many other primates, which have complex social structures with varying roles for males and females.
Step-by-step explanation:
The primate species that generally has a monogamous family unit as their basic type of social group is found among New World species, such as tamarins and marmosets. These primates form pair bonds, which are partnerships between a mating pair that last at least one season, and they cooperatively raise their young. Unlike Old World monkeys, which do not typically form monogamous relationships, tamarins and marmosets show very little size difference between the sexes and share in the care of their offspring, indicative of their monogamous social structures.
In contrast, many primate social groups are focused around related females with males often being temporary members. These groups are characterized by their complex social strategies, where both males and females compete for resources, protective relationships, and mating opportunities. This extends to our closest primate relatives, chimpanzees and bonobos, which exhibit differing social structures and behaviors.
Ultimately, primate social organization is incredibly diverse, with various species adopting different social strategies ranging from solitary lifestyles to large, multi-male/multi-female groups. Yet, when considering monogamous family units specifically, certain New World monkeys like tamarins and marmosets are exemplars of this social structure.