Primates are a diverse group of mammals, including lemurs, lorises, tarsiers, monkeys, apes, and humans. With over 500 species, the order Primates is divided into two suborders: strepsirrhines (such as lemurs and lorises) and haplorhines (which include monkeys, apes, and tarsiers). These species evolved 85 to 55 million years ago from small terrestrial ancestors that were adapted to life in the trees of tropical forests.
Key characteristics of primates include large brains relative to body size, a strong reliance on vision over smell, and the development of grasping hands and feet, often with opposable thumbs and flat nails, enabling greater dexterity. While most primates are arboreal, some, like humans and gorillas, have adapted to living on the ground. They are an extremely social group of animals living in groups that grow slowly and have a longer lifespan than other similar-sized mammals.
Primates use various forms of locomotion, such as brachiation (arm swinging), quadrupedalism (walking on all fours, sometimes with knuckle-walking), and bipedalism (walking on two legs). Cognitively advanced, many primates use tools and communicate through complex languages. Their social structures are equally varied, ranging from pair bonds to intricate multi-male and multi-female groups.
Currently, more than 60% of primate species are threatened by habitat destruction, hunting, and the illegal wildlife trade.
As a broad group, primates display remarkable variation in size. The smallest living primate, the pygmy mouse lemur, weighs around 1.05 oz (30 g), while the largest, the Eastern gorilla, can weigh up to 385 lb (175 kg). These mammals exhibit sexual dimorphism in body mass, tooth size, and sometimes even skin color.
The primate skull is characterized by a large, prominent cranium that encases the brain.
It has an elaborate neocortical region that controls sensory perception, motor commands, spatial reasoning, and cognition. In general, the brains of apes and monkeys are larger than those of lemurs, lorises, and tarsiers. Of all primates, humans have the highest mean endocranial volume (the volume within the skull) of about 1,200 cm3. A unique fissure, the calcarine sulcus, sets apart the first and second visual areas on each side of the brain.
The front of the skull bears two sockets for forward-facing eyes. These sockets are guarded by a bony ridge, which controls facial muscles during chewing. In strepsirrhines, a protective bone called the postorbital bar is present around the eye socket, whereas in haplorhines, the sockets are completely enclosed.
Snout
All primates exhibit a general evolutionary trend toward a reduced snout, though some, like baboons and mandrills, possess relatively longer snouts. While New World monkeys have their nostrils set wide apart, facing sideways, nostrils in Old World monkeys are relatively close, facing down.
Teeth
Primates, like all mammals, are heterodonts having incisors, canines, and cheek teeth. Their dental arrangement is represented by the formula: 0-2/1-2, 0-1/0-1, 2-4/2-4, 2-3/2-3 = 18-36. While some primates lost most of their incisors with evolution, all species have retained at least one lower incisor. Old World monkeys have only eight premolars, whereas New World monkeys possess twelve. Apes and monkeys are differentiated by the number of cusps on their molars. Apes typically have five, while monkeys have four, and humans possess four to five molars.
In strepsirrhines, the incisors are often modified, with many species having lower incisors that form a comb-like structure called the tooth comb, used for grooming or feeding. In fork-crowned lemurs and needle-clawed galagos, this comb is further adapted for scraping bark, piercing fruit, or grooming their fur.
The vertebral column in primates follows the basic mammalian pattern, including the anticlinal vertebra. This bone lies in the mid-thoracic region and marks the transition from the forelimb to the hind limb segments. However, this bone is absent in apes.
They have a general evolutionary trend of shortening of the lumbar, sacral, and caudal or tail regions. In gibbons, the great apes, and humans, the lumbar region is extremely short, and the caudal region is lost.
All primates, except spider monkeys, woolly spider monkeys, and colobus monkeys, have five digits on their hands (pentadactyly). Both their hands and feet are prehensile (gripping), though humans have lost the prehensility of their feet.
The most crucial component of these prehensile hands is an opposable thumb, capable of being moved freely and independently. The apes, particularly, have a short thumb and long fingers.
In contrast to most other mammals, primates have a pendulous or scrotal penis, which hangs freely and is not attached to the abdominal wall. The testes lie permanently in the scrotal sac, except in lemurs, where they are withdrawn into the body seasonally. The color of the scrotal sac varies considerably among certain groups, like the guenon (genus Cercopithecus), drills, and mandrills. In most primates, except tarsiers, humans, and some South American monkeys, a small bone called the baculum lies in the penis.
Female primates exhibit several variations of the uterus, from the two-horned (bicornuate) uterus to single-chambered (simplex), as found in humans and other higher primates. Most primates have a bone in the clitoris called the baubellum. This bone is homologous to the baculum of the penis.
While olfaction is a key sensory mode for most mammals, in primates, the sense of smell is less prominent. Instead, their well-developed visual and tactile systems play a dominant role in perception.
The eyes of most primates are well-adapted to color vision. As mammals, they have both rod and cone cells in their retina, with at least two types of cones: a short-wavelength (blue type) and a medium or long-wavelength (red-green type). However, some nocturnal primates, like night monkeys and bush babies, are monochromatic or color-blind.
In contrast, all catarrhines (parvorder Catarrhini) are trichromatic (three-color vision) due to a gene duplication on closely linked loci on the X chromosome. Conversely, in platyrrhines (parvorder Platyrrhini), the males are always dichromatic (two-color vision), whereas females are either dichromatic (when homozygous) or trichromatic (when heterozygous).
Like catarrhines, howler monkeys, too, exhibit routine trichromatism due to duplication of a particular gene.
External stimuli are sensed by Meissner’s corpuscles, the primary tactile receptors found in hairless parts of their skin. Though present in all primates, these corpuscles are most well-developed in apes and humans.
The receptivity of these tactile sensors is enhanced by anatomical features, like the absence of pads on the palms and soles, along with patterns on the skin (dermatoglyphics).
Carl Linnaeus coined the term ‘primates’ from the Old French word primat or the Latin primus, both meaning’ prime or first rank.’ He proposed such naming because he considered the order of primates to be the highest of all animal orders.
This order, currently considered monophyletic based on cladistics, is placed in the clade Euarchontoglires, which in turn is nested within the clade Eutheria. The three orders, namely primates, colugos, and treeshrews (once considered primates), together constitute the clade Euarchonta. This clade combines with Glires (a clade containing lagomorphs and rodents) to form Euarchontoglires.
In 2001, Primate Taxonomy listed about 350 species, followed by an increase to 376, as estimated by the author Colin Groves, in the third edition of Mammal Species of the World (MSW3). Currently, around 500 species are classified under two suborders: Strepsirrhini and Haplorhini.
New primate species are still being discovered. More than 25 species were added in the 2000s, 36 in the 2010s, and 6 in the 2020s.
Members of the genus Purgatorius are considered the earliest primates, dating back to the Early Paleocene Epoch (66 million years ago). Altiatlasius might have been the oldest known (and undisputed) primate found in Africa (Late Paleocene, 57 million years ago). Some molecular clock studies estimate the origin of primates to have occurred in the mid-Cretaceous Period (85 million years ago).
Strepsirrhines are believed to have split from the primitive primate lineage around 63 million years ago. Two strepsirrhine groups, adapiforms, and the omomyids, were abundant in most northern continents during the Eocene Epoch, but omomyids perished about 30 million years ago, followed by the extinction of adapiforms around 10 million years ago.
Genetic studies suggest that around 75 million years ago, lemurs diverged from the lorisoids in Madagascar. It is believed that a single oceanic dispersal (rafting event) caused a parent population of lemurs to reach Madagascar from Africa.
Haplorhines possibly originated in Asia, as is evident from the fossil of a 55 million-year-old tarsier-like Archicebus. Simians, like monkeys and apes, also probably originated in Asia around 40 million years ago. They soon dispersed to Africa via the Tethys Sea. Catarrhines evolved in Africa, whereas humans and other apes, as well as New World monkeys, diversified in South America.
Primates, except humans, are predominantly found in the tropical latitudes (25°N to 30°S) of Africa, Asia, and the Americas. However, some species live outside the tropics, such as the Japanese macaque in the Honshū and Hokkaido islands, the Barbary macaque in North Africa, and various langur species in China. While they mostly thrive in tropical rainforests, some are also found in temperate forests, deserts, savannas, coastal regions, and mountainous areas.
They can be found living at various elevations, from lowland regions to high mountainous areas. Some species, like the mountain gorilla, dwell at altitudes of over 13,000 ft (4,200 m) in the Virunga Mountains, while others, such as the black-snub-nosed monkey, thrive at even greater heights, surpassing 15,000 ft (4,700 m) in the Hengduan Mountains and geladas reaching 16,000 ft (5,000 m) in the Ethiopian Highlands. In contrast, many primates like gray langurs and rhesus macaques are also well-adapted to life at lower elevations, inhabiting dense forests, coastal zones, and even urban landscapes.
They feed on various food items, like leaves, fruits, seeds, nectar, tree gum, and invertebrates, like insects and insect larvae. The only primate that feeds on grass is the gelada baboon.
Since the primary habitat of primates is the forest, they have evolved four major forms of locomotion owing to their arboreality. Within each of these types, there are different subtypes, and within these subtypes, there are numerous variations between species.
As a diverse group, different primates employ various feeding and hunting strategies.
These mammals have multiple ways of communicating with their conspecifics, ranging from olfactory and auditory cues to hand and arm gestures.
Some primates, like lemurs, lorises, tarsiers, and New World monkeys, rely on olfactory signaling for socializing and reproduction. They scent-mark their territories with pheromones secreted by special glands, which are then detected by the vomeronasal organ.
They are a noticeably vocal group, producing specific sounds for communication.
Since facial features are well-developed in primates, particularly chimpanzees and humans, they can communicate through facial gestures. Additionally, hand and arm gestures help convey information among conspecifics.
There are four primary reproductive phases in primates: copulation, gestation, birth, and lactation. They have slower rates of development and undergo prolonged juvenile and adolescent stages before reaching sexual maturity.
While most lemurs and lorises have one or more breeding seasons per year and may undergo more than one estrous cycle, the ring-tailed lemur has only one breeding season in a year. In tarsiers, apes, and monkeys, the reproductive cycle is continuous throughout the year, whereas some primates, especially monkeys living outside the equatorial belt, are seasonal breeders.
Primates generally have a long gestation period, with humans having the longest of about 267 days on average. Lemurs have a gestation period of about 132–134 days, whereas chimpanzees and gorillas have 230 and 255 days. Even small primates, such as bush babies, have considerably longer gestations than other non-primate mammals of their size.
Most primate young are born fully furred and with their eyes open, except mouse lemurs, gentle lemurs, and ruffed lemurs, who give birth to relatively helpless (altricial) infants and need to carry them in their mouths.
All young are breastfed by their mothers and taken care of till they are old enough to function independently. Some primates, like Galagos and New World monkeys, use tree holes to park their infants while they go foraging. Sometimes, the adults, accompanied by the young, build special nesting sites for resting. However, when the mother needs to carry their babies along, the young cling to maternal fur using their grasping hands and feet.
Primates are most susceptible to predation and starvation during infancy. Infanticide is common among gray langurs, gorillas, and other polygynous groups. Sometimes, adult males kill unborn offspring in the womb of the female, realizing they are not their own. The females then return to estrus and are ready to bear the offspring of their new mate.
Primates, particularly females, have longer lifespans than those of other similarly-sized non-primate mammals. Although the average lifespan of humans is the longest among all primates, chimpanzees typically live for as long as 60 years. Monkeys survive for about 25 to 30 years, whereas lemurs live for around 15 years.
They are usually preyed upon by carnivorans, large reptiles, birds of prey, and other primates.
More than one-third of primates are currently listed as Critically Endangered (CR) or Vulnerable (VU) under the IUCN Red List. Over 70% of primates in Asia and about 40% in South America, mainland Africa, and the island of Madagascar are considered endangered. They are also listed under CITES Appendices I and II.
In 2000, the African red colobus monkey (Procolobus badius) was the first primate since 1800 to be declared extinct. With time, more primates, like orangutans, gorillas, and some of the Madagascan lemurs, have become vulnerable to the dangers of habitat loss and fragmentation, as well as human predation.
Currently, some concerted conservational efforts have led to an increase in the population of some Critically Endangered primates, like golden lion marmosets and western lowland gorillas.
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