Meet Your Relatives

Why study primates? By studying primates, we're essentially studying ourselves. The primate order includes humans and our closest living biological relatives,as well as all extinct primate and human 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, which allows them to exploit many habitats within the tropics, ranging from savanna-woodland to rain forest. 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 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. 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 groups have lost some of these traits or developed others over the course of their evolutionary history.

1. opposable thumb and big toe
   • assists in grasping and manipulation behaviors
   • adaptation to arboreal lifestyle
2. flat nails instead of claws, with dermatoglyphs (fingerprints) on fingers and toes
3. hindlimb-dominated locomotion
4. relative reduction in the olfactory sensory system (smaller snouts) as compared to other mammalian orders
5. increased reliance on visual sensation
   • eyes are large and exhibit a high degree of frontation, or placement toward the front of the face
   • frontation increases overlap of visual fields, increasing binocular vision
   • each sends visual information to both hemispheres of the brain, enhancing depth perception and producing stereoscopic vision
6. tendency toward smaller litter size, longer gestation times, and extended period of juvenile growth
   • increased period of maternal investment and care
7. relatively large brains
8. 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, different divisions of the Primate order are produced by different systematic approaches--namely, the differential weighting of overall similarity versus phylogeny. Two taxonomic arrangements are generally used today--one based on the traditional division between Prosimii and Anthropoidea, and a more recent one that divides groups into the Strepsirhini and Haplorhini. The difference between these two taxonomies is that the Tarsiiformes have been moved from the traditional suborder of Prosimii into the Anthropoidea. For the purposes of this site, the traditional taxonomy consisting of a Prosimii-Anthropoidea division will be used.

Suborder	Infraorder	Superfamily	Family	Subfamily
Prosimii (prosimians)	Lemuriformes	Lemuroidea (lemurs)	Cheirogaleidae (dwarf and mouse lemurs)
			Lemuridae	Lemurinae (true lemurs)
			Lepilemurinae (sportive lemurs)
			Indriidae (indris)
			Daubentoniidae (aye-aye)
	Lorisiformes	Lorisoidea	Lorisidae	Lorisinae
	Tarsiiformes	Tarsioidea	Tarsiidae (tarsiers)
Anthropoidea (anthropoids)
	Platyrrhini	Ceboidea (New World monkeys)	Cebidae	Cebinae (e.g., capuchins, squirrel monkeys)
				Aotinae (e.g., owl monkeys)
				Atelinae (e.g., spider monkeys)
				Alouattinae (e.g., howler monkeys)
				Pithecinae (e.g., saki, uakari)
				Callimiconinae (e.g., callimico)
			Callitrichidae (e.g., tamarins, marmosets)
	Catarrhini	Cercopithecoidea (Old World monkeys)	Cercopithecidae	Cercopithecinae (e.g., macaques, guenons, vervets)
				Colobinae (e.g., colobus, langurs)
		Hominoidea (apes and humans)	Hylobatidae	Hylobatinae (e.g., gibbons and siamangs)
			Pongidae	Ponginae (great apes; e.g., gorilla, chimpanzee, and orangutan)
			Hominidae	Homininae (humans)

Source: R. Martin, 1992: Classification of Primates, in S. Jones, R. Martin, and D. Pilbeam, 1994, The Cambridge Encyclopedia of Human Evolution, Cambridge University Press, Cambridge.

---

The Prosimians

The Prosimians are divided into three groups: Lemuriformes, Lorisiformes, and Tarsiiformes. Lemurs (Lemuriformes) can only be found on Madagascar and the Comoro Islands, and 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.

The continental prosimians consist largely of Lorisiformes, which can be divided into two groups:galagos and lorises (pictured above, middle). The lorises are all nocturnal, and exist in African and Asian forest regions. 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 composition; additionally, 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 groups along geographic lines: Platyrrhini (New World primates) and Catarrhini (Old World primates).

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 Central and South American rain forests. Several unique characteristics include their dentition (they have one fewer molar in each quadrant than other Anthropoids), the fact that they have clawlike nails instead of flat nails, and habitual "twinning," i.e., females frequently give birth to twins. In addition to these physiological characteristics, callitrichids also live in polyandrous groupings (i.e., 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 also includes the only noctural Anthropoid, the owl monkey or night monkey; additionally, 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 platyrrhines; additionally, aspects of the physiology (e.g., catarrhines have only two premolars in each quadrant vs. three for platyrrhines) and ecology (e.g., catarrhines are found in many terrestrial habitats) differ between these groups. Catarrhines can be divided into two superfamilies, the Cercopithecoidea (Old World monkeys) and the Hominoidea (apes).

The cercopithecoids have 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. Additionally, Colobines tend to live in single-male, multifemale social groupings. Cercopithecines, the other goup within the Cercopithecoidea, are composed of a cohort of monkeys that exhibit a wide range of dietary, ecological, and social preferences. There are many terrestrial species, and their geographic distribution includes Africa as well as Asia (and in one case, Gibraltar as well).

The apes represent a tailless group of large-bodied primates that proliferated during the warm, moist Miocene epoch. The classification Hominoidea includes both lesser (e.g., gibbon, siamang) and great apes (orangutan, gorilla, chimp, human). The apes are generally allocated into one of three families: Hylobatidae includes the lesser apes; Pongidae includes the great apes, minus humans; Hominidae includes humans and their fossil relatives.

home|next chapter|take a quiz on this chapter