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Primate Diversity

Note the eyeshine (tapetum lucidum) in this nocturnal aye-aye. The tapetum lucidum is a layer of tissue at the back of the eye that increases the light available to the eye in low light situations, so many nocturnal animals have a tapetum lucidum to enhance their night vision. Aye-ayes are one of the most interesting and bizarre primates. These animals live in Madagascar and use rodent-like teeth to gouge trees and long, thin fingers to extract grubs.
Note the eyeshine (tapetum lucidum) in this nocturnal aye-aye.
Aye-ayes are one of the most interesting and bizarre primates.
These animals live in Madagascar and use large rodent-like
teeth to gouge trees and long thin fingers to extract grubs.
Click image to enlarge

Primates are one of about 21 Orders of mammals. Primates are most closely related to tree shrews, dermopterans (also known as flying lemurs), rodents, and lagomorphs (rabbits and hares).  The Order Primates can be subdivided into two Suborders based on details of the nose.  The two groups are:

  • Strepsirhines - lemurs, lorises, and bushbabies

  • Haplorhines - tarsiers, New World monkeys, Old World monkeys, apes and humans

The word ‘strepsirhine’ means wet nose, and the word ‘haplorhine’ means dry nose. Thus all lemurs, lorises, and bushbabies have a wet nose connected to the upper lip, much like the wet noses of dogs and cats. All tarsiers, monkeys, apes and humans have a dry nose that is not connected to the upper lip.

All organisms, including primates, can be classified into taxonomic categories based upon having the same anatomical or behavioral traits. Traits are divided into one of two types: primitive traits and derived traits. Primitive traits are traits that evolved a long time ago in a distant common ancestor. Derived traits are traits that evolved relatively more recently. Taxonomists used only derived traits to assign each species to a taxonomic category. 

Strepsirhines include all the lemurs, which are restricted to Madagascar, the lorises of Africa and Asia, and the bushbabies of Africa. Strepsirhines retain more primitive mammalian traits (traits they inherited from their mammalian common ancestor) than haplorhines, including a wet nose and a reflective layer behind the eye called the tapetum lucidum. The tapetum lucidum amplifies light in the eye of nocturnal animals and is the so-called "eyeshine" seen in many mammals like dogs, cats, and deer. Like all primates, strepsirhines have the derived primate traits of larger brain size relative to body size, smaller olfactory bulbs, and orbital convergence. However, in comparison to halplorhine primates, strepsirhines generally have smaller brains relative to body size, larger olfactory bulbs indicating more reliance on smell, and relatively more laterally-facing eyes (although not as distinct as in other mammals). Strepsirhines have a postorbital bar (a derived primate trait), but do not have complete postorbital closure (a derived haplorhine trait). Most strepsirhines show no or little sexual size dimorphism (body size difference between males and females).

All strepsirhines share a number of derived traits which categorize them in the Suborder Strepsirhini. Strepsirhine primates have toothcombs (except the strange aye-aye from Madagascar) and grooming claws on the second toe of each foot. Haplorhines include all the monkeys and apes as well as one small, enigmatic primitive primate from Southeast Asia called the tarsier. The tarsier is sometimes grouped in with the strepsirhines to form a group called the Prosimians.  In this classification scheme, all the monkeys and apes are classified together as Anthropoids. 

The orangutan is a large-bodied ape from Asia. The image shows the characteristic arboreal habitat of the orangutan, as well as its typical living situation. Orangutans are semi-solitary with most “groups” made up of just a mother and offspring. This is unusual for a primate, as most primates are highly social and live in larger groups.
The orangutan is a large-bodied ape
from Asia.
Click image to enlarge.

All Haplorhines share a number of derived traits which categorized them in the Suborder Haplorhini. Haplorhines have complete postorbital closure (bony eye socket), larger brain size relative to body size (even larger brains than Strepsirhines), greater reduction in olfactory abilities, and convergent eyes. The Haplorhines can be further divided into Platyrrhines and Catarrhines.  Platyrrhines are the New World monkeys of Central and South America.  Most New World monkeys are arboreal quadrupeds and some of them have grasping, or prehensile, tails.  The Catarrhines include the Old World monkeys, apes and humans.  The Old World monkeys live in Africa and Asia, and can be divided into 2 groups: the cercopithecines (also known as cheek-pouch monkeys) and the colobines (also known as leaf-eating monkeys). The cercopithecines generally eat a lot of fruit and include monkeys such as guenons, baboons, and macaques. The colobines generally eat a lot of leaves and include monkeys such as the proboscis monkey and colobus monkey. Old World monkeys display a mix of arboreal and terrestrial quadrupedalism.

Also in southeast Asia and Indonesia are the orangutans.  Orangutans are large, suspensory and climbing apes.  In contrast to the gibbons, orangutans are slow moving, large animals that cross gaps in the forest canopy using multiple limbs for support.  Male orangutans are twice the size of females. Females usually weigh about 35 kg (about 80 pounds), while males typically weigh in around 78 kg (170 lbs).  This degree of sexual dimorphism is matched in primates only by baboons and gorillas.  Orangutans eat fruits and are generally solitary. Females share their home range with juvenile offspring, but otherwise live alone.  A single male's home range overlaps the ranges of multiple adjacent females and their juvenile offspring.  This type of social organization is known as a noyau.

The other living apes are restricted to Africa and are frequently lumped together and called African Apes.  The lowland and mountain gorillas are the largest living primates.  Females weigh on average about 70-90 kg (150-200 lbs) while males can weigh up to 200 kg (400 lbs)! Gorillas are knucklewalking quadrupeds but they also do a significant amount of climbing.  Most gorillas live in small groups with a dominant male (the silverback) and multiple adult females and their offspring.  However, mountain gorillas are more variable in their social structure, ranging from single-male/multi-female (harem groups) to multi-male/multi-female groups like in chimpanzees. Mountain gorillas eat the leaves and pith of plants growing in the extreme altitudes of their habitats because fruits are generally not available.  Lowland gorillas are more ape-like in their diets, preferring to eat fruits.

The bonobo is also called the pygmy chimpanzee. As you can see in the image, its appearance is very similar to a chimpanzee and despite the “pygmy” in its name, it is not significantly smaller in size. These apes from Africa use sex as a way to resolve conflict within the group – an interesting solution to some of the problems of group-living, such as increased conflict with increased group size.
The bonobo is also called the pygmy chimpanzee.
These apes from Africa use sex as a way to
resolve conflict within the group.
Click image to enlarge.

The most well-known of the African apes is the common chimpanzee. Chimps are knucklewalking quadrupeds who, like gorillas, also engage in climbing and suspensory behaviors in the trees. Sexual dimorphism is less significant in chimps than in other apes with males weighing approximately 40-50 kg and females 30-40 kg.  Chimps eat a wide range of foods including fruit, nuts, termites, and meat.  Chimps are accomplished tool-users with different groups having distinct cultural traditions for food extraction and tool use (e.g., termite fishing, hunting, and nut cracking).  Chimpanzee social structure is more complex than other primates. Chimpanzees live in fission-fusion communities where individuals spend time in different sub-groups within the community, and these sub-groups may join other individuals or sub-groups, or an individual chimp may go off on his/her own for several days. Chimpanzees are also distinguished from other primates in the level of intergroup aggression and violence between males in different groups.

The last of the African apes is the bonobo which is also referred to as the pygmy chimpanzee even though it is not significantly smaller than the common chimp.  Bonobos eat mostly the same foods as chimpanzees such as fruits, nuts, leaves, and meat.  They are somewhat more arboreal and suspensory, but they knucklewalk when traveling on the ground. Bonobos are distinguished most readily from common chimps and other apes in their social behaviors.  Bonobos use sexual intercourse as a means of social bonding among group members.  Sex is commonly used as a greeting between two individuals, as a conflict-resolution tool, or as a reconciliation mechanism. This behavior is not limited to males and females, but includes male-male as well as female-female interactions and multiple different forms of sexual activity.



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