6.10: Order Primates

Some of the characteristics of primates (monkeys and apes) are thumbs and big toes that are opposable to some degree, flattened fingernails instead of claws, sensitive finger pads, prehensile tails (but not in all species), dentition suited to an omnivorous diet, and stereoscopic vision. These features are partly due to the fact that primates are arboreal, or live in trees. Of course, primates are not the only animals to live in trees. Individuals that were able to judge distances between themselves and branches more accurately had a competitive edge over other individuals, which led to the development in primates of stereoscopic vision. Stereoscopic vision evolved convergently in carnivores that judge distances to capture fast-moving prey. Many early primates ate insects and the visual predation hypothesis says that primate features evolved to help catch these insects in trees.

However, most primates today are not full insectivores. The reason primates have unique features is because they live (and have lived for millions of years) in trees and that they co-evolved with flowering plants. This is called the angiosperm radiation hypothesis (angiosperm means flowering plant). The plants needed primates to eat them to spread their seeds and primates needed the calories and sugar for their energy and larger brains.

Characteristics of Primates: Forward-facing eyes; eyes protected by bone; nails instead of claws (claws are part of the skeleton while nails grow on top of a nail bed over the skin); larger brain; longer periods of maternal investment of offspring and giving birth to fewer offspring at a time.

250 or so species of primate exist today; most taxonomists group them into 13 families. All share a lengthy list of defining features, mostly related to the following broad categories:

• Limbs and locomotion. The hands (and often the feet) are grasping, with mobile fingers and toes, generally with touch-sensitive pads at the tips. The first digit (the thumb or the hallux, i.e. big toe) is normally divergent (i.e. points outwards) and in many species can be swiveled to bring its tip into contact with other digits; in other words, it is opposable, to a greater or lesser degree. (Try this with your own hand to verify it.) Rather than a curved and rigid claw, at least some of the digits of primates have flat nails, making manipulation (e.g. of food) practicable. They have very flexible shoulder joints; hind-limbs are normally dominant in locomotion.
• The senses. The eyes point forwards and are set close together. The fields of view of the eyes overlap which, together with a distinctive 'wiring' arrangement linking the eyes and the brain, imparts stereoscopic, '3-D' vision. In general, vision (like hearing) is more significant than smell, and color vision is widespread. The face is flattened, with the muzzle (i.e. the area of skin around the nostrils) foreshortened; this anatomical change may, in part, be a reflection of the diminished importance of smell.
• The brain. This organ is relatively large. The cerebral cortex - in primates, more often called the neocortex - is elaborately folded and complex in structure. This part of the brain is involved with the highly complex processes that include learning, reasoning, and memory.

Figure \(\PageIndex{1}\): Brain size. This chart illustrates the absolute brain size of a selection of animals--a rat, a cat, a ring tailed lemur,a gorilla, a baboon, a chimpanzee, a talapoin monkey, a capuchin monkey, and a human--and the chart on the left shows the size of the brain relative to the repective body size. it's noteworthy that the human brain has the largest absolute size and size relative to the body size.

In all, up to 30 or so diagnostic features of the type described above have been identified for primates, though not all primates display each trait. The others include: their distinctive dentition, linked with a generally omnivorous diet; the structure of the ear; and reproductive features, such as small litter sizes and long gestation periods, relative to body size. Primates have an extended period of juvenile growth and their overall rates of growth and reproduction are generally low, which some biologists interpret as the price paid for a large brain. Brain tissue is metabolically a very expensive tissue to develop and maintain, so for large-brained mammals, less energy is available for growth and reproduction. The benefits of a large brain are very significant. For example, the elaborate social behaviour of primates is seen by many biologists to be as much a defining feature of primates as the types of anatomical feature just listed.

Prosimian/Strepsirrhines

Primitive primates are called Strepsirrhines. Primitive means ancestral, or not much changed since the original ancestor. Traditionally these primitive primates were called Prosimians and the monkeys and apes were called Anthropoid. Classification was mainly based on physical features. However, genetic analysis of a small primate in Indonesia, the Tarsier, showed that it had features of both groups (along with unique features not shared with any other primate), but that its DNA was more like the Anthropoids. The Tarsier was moved into the Anthropoid group and scientists renamed the groups Strepsirrhine (primitive) and Haplorhine (monkeys and apes).

Examples of Prosimian/Strepsirrhines: lemurs in Madagascar, lorises and galagos in Asia, and bush babies in Africa.

Unique features of Prosimian/Strepsirrhines: grooming claw (retention of one claw alongside nine nails); tooth or dental comb (lower set of incisors stick forward and are used for grooming); post-orbital bar (semi-protection of the eye with bone). Some Prosimian/Strepsirrhines still use smell more than monkeys and apes and some are nocturnal.

Figure \(\PageIndex{2}\): The hands and feet of the potto

Anthropoid/Haplorhines

In three particular respects, Anthropoid/Haplorhines have evolved features that have given them a competitive edge over other animals, especially prosimian primates. They have a diurnal instead of a nocturnal pattern of activity, they form all year-round male-female relationships and live in social groups (which evolved differently and independently in lemurs), and they communicate extensively through gestures and vocalization.

A diurnal pattern of activity means that, like us, nearly all anthropoid primates are awake during the day and asleep at night. Prosimian/Strepsirrhines, the great majority of which are nocturnal, have adaptations for nocturnal living, such as large eyes, sensitive night vision with a resultant loss of color vision, and a well-developed sense of smell. Many also have large ears that can move independently of each other. In contrast to prosimians, the diurnally living anthropoids have evolved superior stereoscopic and color vision with an associated reduction in the structures and brain processes related to smell. Being diurnal also correlates to having rods and cones in the eyes to see the color red. This helps primates find fruit that is ripe and ready to eat. Also, as stereoscopic color vision developed, the sense of smell became less important.

The suborder Anthropoidea includes monkeys, apes and humans. Taxonomists group monkeys according to the shape of their nose: Old World primates (found in Africa and Asia) are Catarrhines and New World monkeys (found in the Americas) are Platyrrhines. In fact, apes and humans originated in the Old World, so they too belong to the Catarrhrines, whereas the New World monkeys are sufficiently distinct to be contained within a grouping of their own. Other differences between these groups are in dental formula, tails and eye protection.

Dental formula refers to how many teeth are found in one quadrant (1/4) of your mouth. You can also think of this as one half of the upper jaw or one half of the lower jaw. Primates are heterodont with different types of teeth: front incisors and canines, and back premolars and molars. Starting between the two front teeth and counting to the back of the mouth, the human dental formula is 2 incisors, 1 canine, 2 premolars and 3 molars (the third would be the wisdom tooth). Therefore our dental formula is 2:1:2:3. All Catarrhines have this formula. Plattyrrhines, however, have an extra premolar and their dental formula is 2:1:3:3.

Another difference is the tails. Platyrrhines have prehensile tails that can grasp like a hand; Catarrhine monkeys have a regular tail that does not grasp and Catarrine apes (including humans) do not have tails at all. Finally, while Prosimian/Strepsirrhines have semi-protection of the eye with a post-orbital bar, Anthropoid/Haplorrhines have full protection of the eye with post-orbital closure.

The marmosets, tamarins and capuchin-like monkeys of the New World are found in Central and South America; they comprise two related families in Platyrrhine. Old World monkeys, such as baboons, mandrills, mangabeys, guenons, macaques, colobus and langurs, are found in Africa and Asia; they belong to a single family in Catarrhine called Cercopithecidae.

Unique features of Cercopithecoid monkeys are: bilophodont molars that have 4 cusps. Apes (gibbons, orangutans, gorillas, chimpanzees and bonobos) and humans comprise the Catarrhine category of Hominoidea. Unique features of the hominoid apes are: no tail and Y-5 molars with five cusps.

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Primate Classification Chart

Figure \(\PageIndex{3}\): Primate classification chart created by Sarah Etheridge

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