Platyrrhini (New World monkeys)

There are two parvorders of the Infraorder Simiiformes: Platyrrhini (New World monkeys), and Catarrhini (Old World monkeys, apes, and humans).

How do you tell a Platyrrhini from a Catarrhini?

Families of Platyrrhini

The Infraorder Platyrrhini has five families (although older taxonomies only recognized three): Callitrichidae (marmosets and tamarins), Cebidae (capuchin and squirrel monkeys), Aotidae (night monkeys), Pitheciidae (titis, sakis, and uakaris), and Atelidae (howler and spider monkeys).

Callitrichidae (marmosets and tamarins)

Callitrichids are small, squirrel-like primates. You should watch this video of the marmoset song, but be prepared to have it stuck in your head for at least 24 hours.

Locomotion: Callitrichids are quadrupedal. Unlike most primates, they have claws on all but their big toe, which they use for tree-climbing. Watch them move through the trees here

Social Organization: Marmosets and tamarins have an unusual social organization. Most callitrichids live in family groups, with a mother, a father, and several of their children. The children stay with their parents until they are adults, and often stay past the time they would be able to have children of their own. They help their parents to raise their brothers and sisters. This help is important because callitrichid babies are quite large compared to the size of the parents, and callitrichids frequently have twins. More than one helper is needed to care for the babies. The mother feeds them, but the father and siblings carry them around, protect them, and serve as general caretakers. This type of family structure, where the mother gets help in raising her children from a number of other individuals in the group, is known as cooperative breeding. Cooperative breeding is not very common in primates, but it is characteristic of at least one other primate group: humans!

When adult females stay with their parents to help raise their siblings, their ovulation is suppressed by hormone signals from the mother. In other words, the daughters will never become fertile and have children of their own until they leave their parents' family and form a family of their own. Why don't they leave as soon as they are sexually mature? Participating in cooperative breeding is a form of inclusive fitness. Inclusive fitness is a type of reproductive fitness that doesn't depend on having children of one's own. As we learned earlier in the semester, an individual's reproductive fitness is how many of their genes they pass down to the next generation. If one individual has a lot of children, then they've passed on a lot of their genes. But, having children of your own is not the only way to pass your genes down to the next generation. A parent shares half of their DNA with their child (each child has half of their genes from one of their parents, half from the other). But each individual also shares half of their DNA with their full siblings. So increasing the number of brothers and sisters you have is just as effective in increasing your reproductive fitness as having children of your own.

Callitrichids are unique in another way. They are the only group of primates (other than humans) who frequently form polyandrous families (one female with multiple male mates). Usually, these families include one adult female, two adult males, and their children. Frequently, the males are brothers. This makes sense from the perspective of reproductive fitness. Callitrichid males help raise the infants, and having more than one male helper increases the survival of large-bodied twins. Evolutionarily speaking, however, a male shouldn't help raise any infant who is not his own child. After all, any energy that he puts into raising that child cannot go into helping children of his own, who would carry his genes into the future generations. However, if the two adult males are brothers, then even if one of the mates is not the father of the infant, he still shares DNA with that child, who would be his niece or nephew. In fact, studies on cotton top tamarins have shown that frequently both brothers are the fathers of the infants. Twins are often from different fathers, and during gestation the twins share bone marrow stem cells, essentially sharing DNA in the womb. So, in a sense, the two males are often both the fathers of both the twins.

Active time: like almost all Haplorhini animals, callitrichids are diurnal (active during the day)

Habitat: Callitrichids live in tropical rainforests and scrub forest areas. They are entirely arboreal, and very seldom go down to the ground.

Diet: Callitrichids eat insects and fruits, as well as tree gum

Atelidae

Atelidae are large-bodied (compared to other Platyrrhini), and have a prehensile (grasping) tail. Atelidae species include howler monkeys (so-called for their "howls" - which sound more like gargles to me - that can be heard here), muriquis, and spider monkeys.

Locomotion: The Atelidae are the brachiators of the New World. Brachiation is a form of locomotion similar to the "monkey bars" movement that you may be familiar with from childhood. When moving, the prehensile tail is used as another limb, to grab tree branches or to hang. This video of a spider monkey shows how it works. Spider monkeys are virtually thumbless, a common adaptation in highly arboreal animals who move via brachiation.

Social organization: Atelidae groups are usually single-male/multi-female, with one male dominating a "harem" of females. Alternatively, the groups are larger, including several males and more females. The groups have a dominance structure, but it is not as developed as in Old World monkeys. Activities like grooming are of less importance.

Active time: like almost all Haplorhini animals, atelids are diurnal

Habitat: Atelids are adapted to tree life. They are found in heavily forested areas, and usually in the tallest of trees. They do not spend any time on the ground.

Diet: they eat fruits and leaves

Cebidae

Cebids are smaller-bodied New World monkeys (although bigger than the callitrichids!), with shorter arms and legs relative to the Atelidae. This difference in body plan reflects their different adaptations. Atelids are adapted to be high-canopy monkeys, swinging from the trees. Cebids spend most of their time in trees, too, but their forms of locomotion are much more variable. Like the Atelids, they have a prehensile tail, although theirs is less maneuverable. Cebids include squirrel monkeys and capuchins.

Locomotion: quadrupedal, as seen in this video, using their partially prehensile tail as an anchor as they run across branches.

Social organization: Cebids live in large groups of males and females. Males do not help care for the infants. Squirrel monkey groups can get up to 200 individuals, and they have complex social behaviors and dominance hierarchies, much like the Old World monkeys. Partly because of their social nature, cebids tends to be more intelligent than other New World monkeys.

Active time: like almost all Haplorhini animals, cebids are diurnal

Habitat: Cebidae spend most of their time in trees, but are more likely to be found on the ground than Atelidae. They are found in a wider range of forest habitats than the atelids, which are only found in the densest forests.

Diet: They mostly eat fruits and leaves, although some eat a number of insects. Interestingly, Capuchin monkeys will use tools to obtain food, as can be seen in this bizarrely over-dramatized video of a Capuchin cracking a nut. Capuchins use stones to dig for tubers, and sticks to probe holes in trees and between rocks. They will crack seeds, nuts, and hollow branches with convenient rocks.

Aotidae

The Family Aotidae consists of one genus (Aotus), which is the night monkey. These are the only nocturnal Haplorhini (active during the night). They are medium-bodied for New World monkeys, and have many unique characteristics, including very large eyes (if you don't look closely at their teeth, you might confuse their skull for a tarsier's), no color vision, and a focus on scent.

Locomotion: Aotids are quadrupeds. Sometimes they cling and leap.

Social organization: Aotids form monogamous pairs, with males and females cooperating to raise their offspring. Males are the primary carriers of the infants, who are quite large compared to their mothers.

Active time: This family is the only nocturnal group of monkeys

Habitat: They spend most of their time in trees. They are not restricted to dense forests, but can live in a wide range of forested habitats.

Diet: they eat insects, as well as fruit and leaves

Pitheciidae

The Family Pitheciidae includes the titis, sakis, and uakaris, the most unattractive of the New World monkeys. They are medium-bodied, long-limbed, and have faces only a mother could love. Except for the titis, which are quite cute.

Locomotion: Pitheciidae are mostly quadrupedal, although some of the smaller species are clingers and leapers.

Social organization: Their social organization is variable. Some, like the titis, form monogamous pairs where the father does most of the carrying of - and caring for - the baby. Others, like the sakis, form monogamous pairs where the mother cares for the baby. Still others, like uakaris, form large multimale/multifemale groups with dominance hierarchies.

Active time: like most Haplorhini, they are diurnal

Habitat: They spend most of their time in trees. They are not restricted to dense forests, but can live in a wide range of forested habitats.

Diet: They eat fruits and seeds. Some species eat insects, as well.