Taxonomic groups of animals (for example, species) can share characteristics because of two different processes. First, descent from a common ancestor can cause similarity. When shared characters between two groups of animals occur because they both inherited the character from a common ancestor, the characters are said to be homologous. Second, similar selection pressures can cause similarity in unrelated groups. When shared characters between two groups of animals evolve independently in each group, the characters are said to be analogous. This process is also known as convergent evolution.

An example of homology is flight in sparrows and robins. Both inherited flight from a common ancestor. An example of analogy is flight in birds and bats. Bats are mammals that evolved flight independently of the birds. The following diagram illustrates the difference (horizontal lines indicate the evolution of flight).

Primates are defined by the following suite of shared, derived characters: (1) opposable toes, (2) nails and tactile pads on hands and feet, (3) hindlimb dominance, (4) reduced olfaction, (5) stereoscopic vision, (6) small litters and longer gestational and juvenile periods, (7) large brains, and (8) unspecialized molars.

Many of the traits that define primates are functionally integrated with each other. For example, opposable toes and thumbs, nails and tactile pads, and stereoscopic vision allow primates to skillfully maneuver through trees, grasping fruit and mobile insects. Also, primates have a slower life history relative to other mammals. Longer juvenile periods, smaller litters, longer gestational periods, and larger brains reflect an increased parental investment per offspring and an increased reliance on complex behavior, learning, and behavioral flexibility.

Primates are most often found in tropical areas. The tropics are characterized by a great deal of rainfall and relatively constant temperatures year-round.

Lemuroidea: The lemurs are found only on Madagascar and the Comoro Islands. Most of the extant lemurs are medium in body size and travel quadrupedally or by leaping in arboreal environments. Lemurs exhibit a range of activity patterns including diurnality, nocturnality, and activity during both day and night. In some lemur species, females regularly dominate males, supplanting them at food sources and defeating them in aggressive encounters.

Lorisoidea: The Lorisiformes, or galagos and lorises, are small, nocturnal, arboreal primates found in forested areas of Africa and Asia. They feed on similar types of food but have very different locomotor styles. Galagos are often referred to as ìleapers and hoppersî while lorises are ìstealth-likeî in their movements. Lorisiformes are generally solitary, and females leave their infants in nests or park them in vegetation when foraging.

Tarsioidea: The tarsiers are small, nocturnal, arboreal primates living in the rainforests of Borneo, Sulawesi, and the Philippines, where they move about by clinging and leaping. The social organization of tarsiers varies from monogamy to multimale/multifemale groups. Tarsiers are unusual because they are the only primate known to have a diet made up exclusively of animal matter.

Ceboidea: The New World monkeys are quite diverse in body size, diet, and social organization. Body size varies from 600 g to 9.5 kg. Some species are highly insectivorous, while others eat mainly fruit and still others leaves. Types of social organization include monogamy, polyandry, one-male/multifemale groups, and multimale/multifemale groups. However, all but one New World monkey are diurnal, all are forest living, and all are primarily arboreal. All are quadrupedal with the exception of the subfamily Atelinae, who are suspensory, moving and hanging underneath branches.

Cercopithecoidea: The Cercopithecoid monkeys, or the colobines and cercopithecines, are found throughout Africa and Asia. One feature that unites them is specialized molars that have prominent anterior and posterior cusps. The colobines are primarily arboreal and typically live in one-male/multifemale groups. They have a diet of mostly seeds and leaves which they digest efficiently in their multichambered stomachs. Infanticide has been documented in colobines. Compared to the colobines, the cercopithecines are more variable in bodysize and diet. Cercopithecines typically live in multimale/multifemale groups, and females usually stay in the natal groups while males disperse at adulthood.

Hominoidea: The hominoids include gibbons, siamangs, orangutans, gorillas, chimpanzees, and humans. The hominoids share broad noses and palates, large brain size, Y-shaped lower molars, and the absence of a tail. The nonhuman apes are found in restricted areas of southeast Asia and central Africa. See question 7 for a description of each genera.

The modern prosimians are nocturnal and have a number of adaptations related to nocturnal life. They rely heavily on sense of smell and have a reduced visual sense, large eyes, and independently movable ears. They consume more insects than anthropoids. They are mostly solitary, with the sexes associating mainly during mating. The lemurs of the Malagasay Republic (formerly Madagascar) have more derived characters than the prosimians living on the African continent. This may be attributed to their relative isolation, a lack of competition with other primates, and a lack of natural predators.

This diversity is the result of total isolation of lemurs on the island of Madagascar from other primates and many predators and other competitors. The availability of so many open niches resulted in a spectacular adaptive radiation for the lemurs.

Orangutans are among the most solitary species of primates and are found only on the islands of Sumatra and Borneo. Orangutans are highly frugivorous. Orangutans are very sexually dimorphic; heavier males are primarily terrestrial while females and their dependent offspring travel arboreally. Males spend most of their time alone and hold territories that overlap with the home ranges of several females.

Gorillas are the largest of the living apes and are restricted to several small areas in central Africa. Mountain gorillas live in groups containing one or two adult ìsilverbackî males and several females and their offspring; they have a diet that is mainly folivorous and are primarily terrestrial. Silverbacks coordinate their groupís activities and determine where the group travels. Conversely, lowland gorillas live in larger, less cohesive groups, consume large amounts of fruit, and are more arboreal.

Found in areas of central Africa, chimpanzees and bonobos form large multimale/multifemale groups that often break down into smaller subgroups. In chimpanzees, males form the closest bonds, in bonobos, females form close bonds with each other and with their adult sons. Chimpanzees make and use a variety of tools including sticks to extract insects from mounds, rock hammers and anvils to open nuts, and leaf sponges to soak up water.

Primates are threatened by habitat destruction, hunting, and live capture for trade and export. By far, the major hazard facing primates is habitat destruction as areas where primates live are losing about 125,000 square kilometers of forest each year. Primate populations are also impacted by hunting, especially for the growing bushmeat trade. However, capture and trade of live primates has been reduced in recent years through the enactment of the Convention of Trade in Endangered Species of Wild Flora and Fauna.

This is an open-ended question and students may say something like the following. Balancing the needs of people in developing nations with the needs primates that live around them will be complicated. One issue that needs to be addressed is habitat destruction due to subsistence farming. Agricultural techniques that use less land but provide the same yields need to be developed.

Tropical forest destruction is the result of economic and demographic pressures. Timber harvesting and agriculture in tropical forests have increased as third world governments struggle to raise funds to repay large amounts of foreign debt. At the same time, the human population in these countries has increased rapidly, resulting in the clearing of huge areas of tropical forest for subsistence farming.