chapter five: Introduction to the Primates
Chapter Review
Meet Your Relatives
The term species is defined by some as a group of organisms that interbreed under natural circumstances, producing viable, fertile offspring, and that are reproductively isolated from other groups. The last chapter covered the sorts of evolutionary changes that happen in populations, and species can often be understood to be collections of populations. While microevolution produces changes in populations, macroevolutionary processes affect entire species, or even larger taxa, like genera and families. The above definition of a species is known as the biological species concept. One way of evaluating this concept is to consider that a species experiences gene flow between the populations that make it up and this tends to maintain genetic compatibility between members. Conversely, populations that do not exchange genetic information—either through geographic or behavioral isolation—experience increased isolation or genetic drift and tend to become increasingly different from other groups over time, perhaps even evolving into new species.
Why study primates? By studying primates, we are studying ourselves. The Order Primates includes humans and our closest living biological relatives, as well as all extinct primate ancestors. In addition to providing evolutionary insights into the physiological and behavioral evolution of the human lineage, primates exhibit an extraordinarily diverse array of behaviors and social systems, allowing them to exploit many habitats within the tropics, ranging from savanna-woodland to rain forest. Not only that, but some species, such as the Japanese macaque (Macaca fuscata), have adapted to the snowy winters of the island of Hokkaido. The rather cosmopolitan status that the primate lineage boasts is one of a number of reasons to study their adaptive strategies in the context of evolution. Finally, the cognitive capacity of nonhuman primates provides a window into the evolution of intelligence, providing deeper insights into the machinery that drives our own behavior and thought processes.
Common Characteristics of Primates
The following characters distinguish the primate order from other mammalian groups (Figs. 5-1, 5-2, 5-3, and 5-4). While all primates do not necessarily have every one of these traits, all primates have trait complexes that include a preponderance of the traits listed below. This is because some primate species have lost several of these traits or developed other, more specialized traits over the course of their evolutionary history.
Figure 5.1 Spider monkey.
Image credit: Robert Boyd and Joan Silk.
Figure 5.2 Vervet monkey.
Image credit: Robert Boyd and Joan Silk.
Figure 5.3 Chimpanzee.
Image credit: Joan Silk.
Figure 5.4 Two lorises at night.
Image credit: W.W. Norton.
- Opposable thumb and big toe.
- Assist in grasping and manipulation behaviors.
- Adaptation to arboreal lifestyle.
- Flat nails instead of claws, with dermatoglyphs (fingerprints) and sensitive pads on fingers and toes.
- Hindlimb-dominated locomotion and increasingly upright posture.
- Relative reduction in the olfactory sensory system (smaller snouts) compared to other mammalian orders.
- Increased reliance on the visual sense, versus the olfactory (smell).
- Eyes are large and exhibit a high degree of frontation, or placement toward the front of the face.
- Frontation increases the overlap of the visual fields, increasing binocular vision.
- Each eye sends visual information to both hemispheres of the brain, enhancing depth perception and producing stereoscopic vision.
- Tendency toward smaller litter size, longer gestation times, and extended period of juvenile growth.
- Increased period of maternal investment and care.
- Relatively large brains.
- Reduced number of teeth, with a maximum of two incisors, one canine, three premolars, and three molars in each jaw quadrant.
Taxonomy of Living Primates
The primate order is generally subdivided into four groups, which are colloquially the prosimians, New World monkeys, Old World monkeys, and apes. However, the specific subdivisions of the primate order depend on the systematic approach used—namely, the differential weighting of overall similarity in morphology and behavior versus phylogeny, or evolutionary relationship. Two taxonomic arrangements are generally used today; one is based on the traditional division between Prosimii (lemurs, lorises, galagos, tarsiers, etc.) and Anthropoidea (monkeys, apes and humans), and a more recent one divides groups into the Strepsirhini (lemurs, lorises, galagos, etc.) and Haplorhini (tarsiers, monkeys, apes and humans). The difference between these two taxonomies is that the Tarsiiformes (tarsiers—a small, nocturnal primate with large stationary eyes and long ankle bones) have been moved from the traditional suborder of Prosimii into the Anthropoidea. Phylogenetically (principally in their cranial morphology), tarsiers are more similar to monkeys and apes than they are to prosimians (e.g., lemurs and lorises). This study guide and your text, however, uses the traditional taxonomy, consisting of the Prosimii-Anthropoidea division (Table 5-1).
Table 5-1. Taxonomy of the Primate Order
The Prosimians
The prosimians are traditionally divided into three groups: Lemuriformes, Lorisiformes, and Tarsiiformes. Lemurs (Lemuriformes) are found only in Madagascar and the Comoro Islands; they represent a lineage of primates that have evolved in isolation from other such groups over the past 120 million years. Lemurs are primarily nocturnal, although some species (most notably Lemur catta) are active during the day, or during portions of both day and night (Figure 5-4).
The continental prosimians consist largely of Lorisiformes, which can be divided into two groups: galagos and lorises. All the lorises are nocturnal and live in forested regions of Africa and Asia. Their diets consist primarily of fruits, gums/exudates, and insects.
Tarsiers are the third group of prosimians. They are unique in their entirely faunivorous dietary adaptation; in addition, they are particularly specialized for a vertical clinging and leaping style of locomotion. Tarsiers can be found in Southeast Asia and Indonesia.
The Anthropoids
The anthropoids are divided into two large taxonomic groups along geographic lines: Platyrrhini (New World primates) and Catarrhini (Old World primates) (Figs. 5-1, 5-5, and 5-6).
Figure 5.1 Spider monkey.
Image credit: Robert Boyd and Joan Silk.
Figure 5.5 Rhesus monkey.
Image credit: Robert Boyd.
Figure 5.6 Gorilla.
Image credit: Robert Boyd and Joan Silk.
Platyrrhines
There are two families within the New World monkeys. The callitrichids (Callitrichidae) consist of marmosets and tamarins, which are small-bodied primates that live in the Central and South American rain forests. Several of their unique characteristics include their dentition (they have one fewer molar in each quadrant than other anthropoids), the fact that they have claw-like nails instead of flat nails, and they habitually give birth to twins. In addition to these physiological characteristics, callitrichids also live in polyandrous groupings (that is, a single female with more than one male).
The second family, Cebidae, consists of six subfamilies that encompass a wide range of ecological and social diversity. This group includes the only nocturnal anthropoid, the owl monkey (also called the night monkey). Furthermore, several species of cebids also possess prehensile tails. The cebids are all primarily arboreal and exhibit a number of different social systems.
Catarrhines
The catarrhine primates are generally larger than the platyrrhines. In addition, aspects of their physiology (for example, catarrhines have only two premolars in each quadrant instead of the three that platyrrhines have) and ecology (catarrhines are found in many more terrestrial habitats) differ between these groups. Catarrhines can be divided into two superfamilies, the Cercopithecoidea (Old World monkeys) and the Hominoidea (apes and humans).
The cercopithecoids are made up of two general groups, which can be divided along ecological and dietary lines. The colobine monkeys are distinguished by their specialized stomachs, which have been modified for a highly folivorous, or leaf-eating, diet. In addition, colobines tend to live in single-male, multifemale social groupings. Cercopithecines, the other subfamily within the Cercopithecoidea, live in larger groups of monkeys and exhibit a wide range of dietary, ecological, and social preferences. There are many terrestrial species, and their geographic distribution includes Africa and Asia.
The apes are a tail-less group of large-bodied primates that proliferated, or enjoyed an adaptive radiation, during the warm, moist Miocene epoch. The classification Hominoidea includes both lesser (gibbon, siamang) and great apes (orangutan, gorilla, chimpanzee, human). The apes are generally allocated into one of three families: Hylobatidae (lesser apes), Pongidae (great apes, minus humans), and Hominidae (humans).
Conservation
Because the majority of primates are arboreal and are most numerous in the tropical areas of the world, they are deeply affected by the continued disappearance of the earth's forests. These uniquely human-engendered environmental changes are the result of demographic pressures, political events, agricultural demands, and the increase in wildfires (also attributable largely to human actions). Many nonhuman primates are also hunted for food as well as captured and kept illegally. It is easy for those of us reading and studying this text to condemn other people for their treatment of endangered species, like the killing and exporting of chimpanzees for the "bush meat" trade; however, to continue the same example, we should also remember that people in the African countries where chimpanzees live are not strangers to political strife, social injustice, and famine conditions (to learn more about chimpanzee conservation read about the Jane Goodall Institute’s work at http://www.janegoodall.org/chimp_central/conservation/issues/default.asp). Nevertheless, while people's awareness of these dangers to our vulnerable primate relatives has increased, and numerous international organizations now work to protect them and provide alternatives for people who may try to poach them, many primate species remain endangered, some critically so.
This site and the materials contained herin ©2006 W. W. Norton and Company, Inc. unless otherwise stated. All rights reserved.