4.5 What Is a Primate?

What Is a Primate?

Figure 4.23 Orangutans, the only great ape from Asia, are one of many living primate species. Others include lemurs, monkeys, gibbons, and human beings. (credit: Dawn Armfield/Wikimedia Commons, Public Domain)

Primates—including human beings—are characterized by a number of distinct physical features that distinguish them from other mammals. These include

• opposable thumbs and (in nonhuman primates) opposable big toes;
• the presence of five digits (fingers or toes) on the appendages;
• flat nails instead of curved claws;
• pads at the tips of the fingers made up of deposits of fat and nerves;
• reduced reliance on sense of smell and a relatively small snout;
• depth perception;
• binocular vision (being able to see one image with both eyes);
• a relatively slow reproductive rate;
• relatively large brain size; and
• postorbital bars (bony rings that completely surround the eyes).

Figure 4.24 The hands of this bonobo, including its opposable thumbs, look very similar to human hands. Opposable thumbs or toes are a primate trait shared by no other group of mammals. (credit: “Bonobo Plankendaal” by Marie van Dieren/flickr, CC BY 2.0)

The first four traits enhance dexterity and enable primates to use their hands and feet differently from other mammals. Other traits on this list represent a shift in emphasis among the sense organs between primates and other mammals. Primates are characterized by a greater emphasis on vision and a reduced reliance on smell relative to other mammals.

Primate Behavioral Variation

Anthropologists regularly ask, “What makes us human?” Comparative studies of humans with nonhuman primates help answer this question. Comparing the behavior of nonhuman primates and the behavior of human beings helps anthropologists identify what culture is and develop operational definitions for it. Without the comparative perspective provided by primatology, anthropologists would be missing an important piece of the puzzle of what makes humans human. Without primatology, anthropologists would not be able to fully understand humankind.

Studying nonhuman primates in their environment is key to understanding variations in behavior and can shed light on humanity’s ancient past. Primatologists are studying the chimpanzees at Gombe National Park in Tanzania, where they live in the rainforest. The behavior of chimpanzees that live in the tropical regions of Africa is quite different from the behavior of chimpanzees that live in the savanna at Fongoli in Senegal, in West Africa. Gombe chimps hunt red colobus monkeys without the use of tools, just catching them with their hands, while the Fongoli chimpanzees hunt galagos (also known as bush babies) using sticks that they adapt and used as spears (Pruetz, J.D, et al, 2015). The two environments also show differences in gender roles with both males and females in the Fongoli savannah group involved in hunting while only male chimpanzees hunt in the rainforests. Studying how these nonhuman primates both make and use tools is critical for understanding how humans’ fossil ancestors may have used and constructed tools.

An important question that primatologists and biological anthropologists seek to answer is the question, do nonhuman primates have culture? Whenever we see an exchange of ideas where one individual is involved in teaching another and when that knowledge is passed on to others in a group is according to anthropologists, a form of culture. We see this happen in chimpanzee groups where older chimpanzees teach the young how to use sticks to termite-fish, the process of extracting termites from a termite mound using a stick.

Figure 4.25 This chimpanzee lives in the Gombe National Park in Tanzania. Chimps living in Gombe’s rainforest environment have developed a very different set of hunting techniques and tool use from their relatives living in the grassy savannah. (credit: “Chimp Eden Sanctuary – Mimi” by Afrika Force/flickr, CC BY 2.0)

Explaining Primate Success

Why primates evolved as they did and how they filled and exploited the range of ecological niches they now fill are questions that have not yet been adequately addressed. Over the last century, various hypotheses have been raised to account for the evolution of primates and their unusual anatomical characteristics. These theories include the arboreal theory, the visual predation hypothesis, and the angiosperm theory.

The arboreal theory proposes that primates evolved the traits they did as an adaptation to life in the trees. Specifically, primates evolved thumbs and big toes that are perpendicular to the other digits to help them grasp onto branches.

Matt Cartmill, a professor of anthropology at Boston University who spent his career trying to understand why primates evolved the way they did, has complicated this theory. Cartmill recognized that forward-facing eyes are characteristic not only of primates but also of predators such as cats and owls that prey on small animals. Thus, forward-facing eyes, grasping hands and feet, and the presence of nails instead of claws may not have arisen as adaptations to an arboreal environment. Rather, they may be adaptations that helped early primates succeed as predators. According to the visual predation hypothesis, primate features are adaptations for hunting insects and other small prey in the shrubby forest undergrowth and the lowest tiers of the forest canopy.

The angiosperm theory states that the basic primate traits developed in coevolution with the rise of flowering plants, also known as angiosperms. Flowering plants provide numerous resources, including nectar, seeds, and fruits, and their appearance and diversification were accompanied by the appearance of ancestral forms of major groups of modern birds and mammals. Some argue that visual predation is not common among modern primates and that forward-facing eyes and grasping extremities may have arisen in response to the need for fine visual and tactile discrimination in order to feed on small food items, such as fruits, berries, and seeds, found among the branches and stems of flowering plants.

Primate Classification and Taxonomy

Scientists generally classify the order Primates into two suborders: Strepsirrhini (prosimians) and Haplorrhini (tarsiers and anthropoids).

The Strepsirrhini or Prosimians

The Strepsirrhini are considered to be primitive primates that evolved much earlier than other primates. This suborder includes lemurs and lorises. All the Strepsirrhini primates, or strepsirrhines, possess numerous anatomical traits that distinguish them from the Haplorrhini primates, or haplorrhines. These include a clawlike nail on the second toe, referred to as a grooming claw, and incisors in the lower jaw that are tightly packed together and protrude from the mouth, forming what is called a toothcomb. There are seven families of living strepsirrhines, and all of them are found in what anthropologists refer to as the Old World, which consists of the continents of Africa, Asia, and Europe. Five groups of living strepsirrhines are found only on the island of Madagascar off the coast of Africa. Two additional families are found in Africa and Asia.

Figure 4.26 The pygmy slow loris (Nycticebus pygmaeus) is an example of a Strepsirrhini primate. Pygmy slow lorises be found in Vietnam, Laos, and a province of China. (credit: Lionel Mauritson/Wikimedia Commons, Public Domain)

The Tarsier Puzzle

The tarsier, which belongs to the family Tarsiidae, has both prosimian and anthropoid characteristics, which has made it difficult for scientists to classify. Tarsiers are currently classified within their own classification under the haplorrhines. One of the characteristics that tarsiers share with other haplorrhines, (including humans) is the inability to manufacture their own Vitamin C. They are the smallest known primate and are nocturnal, with extremely large eyes that take up much of the space in their skull. Due to their size of the eyes, the tarsier cannot rotate them; instead, it can rotate its head 360 degrees like an owl. Tarsiers are also the only primate carnivore, eating largely flying insects and sometimes small animals like bats and lizards. Tarsiers do not do well in captivity. They are extremely sensitive to noise and can become easily stressed. In fact, they can become so stressed that they die by suicide by banging their heads against tree trunks.

Figure 4.27 The Philippine tarsier (Carlito syrichta) is found only in the southern portion of the Philippine islands. The tarsier has been challenging for scientists to classify, exhibiting both prosimian and anthropoid characteristics. (credit: “8thApril2007 – ‘Tarsier’ Monkey” by Jacky W./flickr, CC BY 2.0)