Monkeys, Introduction

by Katharine Milton, Ph.D.

In contrast to Africa and Asia, there are no apes in the forests of Central and South America. There are, however, an amazing number of monkeys, perhaps as many as 59 different species, several discovered only within the past few years. The monkeys in the New World are believed to have originated from an ancestral stock which arrived in South America from Africa during the Oligocene epoch, some 37 million years ago. At that time, the continents of South America and Africa were closer together than they are today and the ancestral primates giving rise to New World monkeys are believed to have island-hopped and drifted in clumps of vegetation across the Atlantic Ocean from Africa to South America. Here they were then able to diversify such that they came to fill a number of different ecological niches. All New World monkeys are arboreal and spend their lives in the forest canopy though some species will come to the ground to forage for fallen fruits.

New World monkeys are known as platyrrhines or flat-nosed monkeys because of their relatively broad nasal septum in comparison to Old World monkeys and the vertical orientation of their nostrils. New World monkeys fall naturally into two taxonomic divisions--the Callitrichidae and the Cebidae. Callitrichids are the small monkeys known commonly as marmosets and tamarins. In fact, the smallest monkey in the world is a New World monkey, the tiny pygmy marmoset, Cebuella pygmaea, which weighs only 107 to 140 grams, about the same weight as a North American chipmunk, and can fit comfortably into a teacup. Marmosets and tamarins are noted for their colorful pelage and elaborate hair tufts including ear tufts, thick fluffy side whiskers and elaborate curled moustaches. Callitrichids typically give birth to twins whereas all other anthropoids (monkeys, apes and humans) tend to give birth to only a single infant at a time. In callitrichids, the cost of carrying and feeding two rapidly growing infants apparently is too high a burden for the mother alone. Thus in tamarin and marmoset groups, we find that other members of the social unit, frequently the males, help by carrying the infants most of the time, passing them to the mother for nursing only when the young are hungry. Many callitrichids, particularly the marmosets, feed on tree sap or gums that they gouge out of the trunk or branches of trees in droplets, using special tusk- like teeth to pierce the bark. Marmosets and tamarins also eat plant nectars and fruits as well as some insects and, at times, small vertebrates such as lizards and frogs.

The other family of New World monkeys, the Cebidae, is composed of a very diverse group of monkeys including capuchin (or organ grinder) monkeys, howler, spider, woolly and woolly spider monkeys, bald-headed uakari monkeys, squirrel monkeys, night monkeys and callicebus monkeys. Many, but not all members of this family have a prehensile (grasping) tail in which the last few inches of the undersurface of the tail is completely devoid of hair and covered with a touch-sensitive pad just like your fingertips. The prehensile tail serves as a type of fifth hand, aiding its owners in crossing between the slender branches of trees as well as greatly enlarging their feeding sphere. There is no set rule in this family in terms of social structure and we find everything from huge mixed groups of males, females and their offspring (squirrel monkeys), to smaller one-male groups (red howlers) to monogamous couples (callicebus monkey). Cebids tend to take much of their diet from plants, eating quantities of fruit, flowers and in some species, new leaves; many of the smaller species also include animal matter in the diet, particularly insects.
 Katharine Milton, Ph.D. 1998