Primates: Characteristics, Evolutionary Trends, Taxonomy, Adaptations and Behaviour

1.5 Characteristics of Primates; Evolutionary trend and primate Taxonomy; Primate Adaptations (Arboreal and Terrestrial) Primate Taxonomy; Primate Behaviour; Tertiary and Quaternary fossil primates; Living Major Primates; Comparative Anatomy of Man and Apes; Skeletal changes due to erect posture and its implications

Official Syllabus: Paper I, Unit 1.5

This article covers the characteristics and evolutionary trends of primates, primate taxonomy, arboreal and terrestrial adaptations and primate behaviour. The fossil primates, the living apes, the comparative anatomy of man and apes and the skeletal changes of erect posture are in Unit 1.5b.

1. Man’s Place in Nature: Primates as Mammals

In 1758 the Swedish naturalist Carl Linnaeus, founder of modern classification, placed Homo sapiens in the order Primates beside monkeys and apes (Systema Naturae, 10th edition), a century before Darwin and purely on anatomy. In 1863 the British biologist Thomas Henry Huxley (Evidence as to Man’s Place in Nature) compared ape and human skeletons and concluded that man differs from the great apes less than they differ from monkeys. Genetics later confirmed both.

1.1 The classification ladder

Each step down the ladder adds traits that humans share with a smaller group of animals.

Level	Name	Defining trait shared with humans
Kingdom	Animalia	Many cells; ingests food; moves
Phylum	Chordata	A notochord (a stiff rod along the back) at some stage
Subphylum	Vertebrata	Backbone protecting a spinal cord with a brain at its head
Class	Mammalia	Hair, milk glands, constant body temperature, four kinds of teeth
Order	Primates	Grasping hands, forward eyes, large brain, slow life history
Suborder	Haplorhini	Dry nose, no reflecting layer in the eye, closed eye socket
Superfamily	Hominoidea	No tail, broad chest, swinging arms
Tribe	Hominini	Habitual bipedalism (humans and their fossil relatives)
Genus, species	Homo sapiens	Large brain, chin, symbolic culture

Most of what makes us “human” was inherited in layers: warm blood and milk are mammalian; hands, eyes and long childhood are primate; the tailless, broad-chested body is hominoid. Only the bottom rows are uniquely human.

1.2 Primates as mammals

• Mammals are endothermic (they keep a constant body temperature from inside), have hair, suckle their young and have heterodont teeth (teeth of different shapes: incisors cut, canines grip, molars grind).
• Of the three subclasses, egg-laying monotremes (platypus) and pouched marsupials (kangaroo), primates belong to the placental mammals (Eutheria), whose foetus grows long inside the womb.
• Why this matters: suckling and long dependence create a lasting mother–infant bond, the setting in which young primates learn most of their behaviour, the root of culture.

1.3 Man is a primate

The claim “man is a primate” rests on three kinds of evidence.

• Anatomy: nails, an opposable thumb, a clavicle, forward eyes with colour vision, a large brain, single births and a long childhood (§2).
• Genetics: humans and chimpanzees differ at only about 1.2 per cent of DNA letters, and humans have 46 chromosomes because two ape chromosomes fused into our chromosome 2 (Unit 1.5b §3).
• Behaviour: group living, grooming, dominance, mother–infant bonding, tool use and learned traditions all have primate roots (§5).

Humans differ mostly in degree, with a few true novelties: habitual bipedalism, a brain about three times a chimpanzee’s, articulate language and cumulative culture. Man is a “unique primate” because the combination is unique, not each part.

2. Characteristics and Evolutionary Trends of Primates

Look at your own hand. It has five fingers, a thumb that can touch each fingertip, flat nails instead of claws, and ridged skin (fingerprints) on the fingertips. A squirrel climbs a tree by digging its claws into the bark; a monkey climbs by wrapping its hand round a branch. That single contrast (gripping versus clawing) explains much of primate anatomy.

2.1 Le Gros Clark’s evolutionary trends

The British anatomist Wilfrid Le Gros Clark (The Antecedents of Man, 1959) noticed that primates are anatomically generalised, lacking the specialisations of other orders (hooves, flippers, horns). So he defined the order by evolutionary trends: tendencies developed to different degrees in different members, rather than fixed traits.

Group	Trend	What it means
Limbs	1. Generalised limbs, pentadactyly (five digits), clavicle kept	Limbs stay flexible for climbing; the collarbone holds the arm out to the side
	2. Mobile digits, opposable thumb and big toe	Digits move separately; thumb tip can meet the other fingertips
	3. Nails replace claws; tactile pads	Sensitive, ridged fingertips grip and feel
Head	4. Shorter snout	Face shrinks as smell matters less
	5. Better vision, binocular and stereoscopic	Eyes face forward; fields overlap; depth is judged
	6. Reduced smell apparatus	Smaller smell areas of the brain and nose
	7. Fewer teeth, simple molar cusps	Primitive 44 teeth reduced to 36 or 32
	8. Larger, more complex brain, especially the cerebral cortex	More learning, memory and coordination
Life cycle	9. More efficient nourishment of the foetus	Better placenta, longer pregnancy
	10. Longer life stages	Long infancy, late maturity, long life

The trends concern three areas: the limbs (1–3), the head (4–8) and the life cycle (9–10). They are trends, not characteristics, weak in lemurs and lorises and strongest in monkeys, apes and humans; and they are interrelated, all linked to life in the trees.

2.2 Limbs and hands: grasping, not clawing

• Generalised limbs (trend 1): primates keep the ancient mammal plan of one bone in the upper limb and two in the lower (radius and ulna, tibia and fibula). Two forearm bones let the hand rotate palm up and down; the clavicle braces the shoulder so the arm can reach in every direction. Hoofed mammals, which only run, have lost it.
• Prehension (trend 2): prehension means grasping. The opposable thumb can rotate so its tip faces the other fingertips. It is absent in tree shrews, incomplete in lemurs and lorises and full in Old World monkeys, apes and humans. Humans lost it in the big toe, which became part of a weight-bearing platform (Unit 1.5b §4).
• Nails and pads (trend 3): flat nails back tactile pads carrying dermatoglyphics (friction ridges, as in fingerprints) and many nerve endings, so the hand grips without slipping and feels. Marmosets keep claws for clinging to trunks while eating gum, and lemurs and lorises a grooming claw on the second toe.
• Why it matters: a hand that grips and feels could later make tools; culture borrowed a climbing organ.

2.3 Eyes, nose and brain: seeing replaces smelling

• Stereoscopic vision (trend 5): the eyes moved to the front of the face, so their fields overlap and the brain compares two slightly different images to judge depth. For a leaping animal, a misjudged distance can be fatal.
• Bony eye protection: lemurs and lorises have a postorbital bar (a ring of bone at the side of the eye socket); monkeys, apes and humans have postorbital closure (a full bony cup behind the eye).
• Colour vision: Old World monkeys, apes and humans all have three-colour (trichromatic) vision; in most New World monkeys only some females do. Colour picks out ripe fruit and young red leaves against green.
• Reduced smell (trends 4 and 6): the snout shortened and the smell areas of the brain shrank; the moist nose tip (rhinarium) of a lemur is lost in monkeys and apes. Smell is reduced, not lost (§5.4).
• Brain (trend 8): the cerebral cortex expanded and folded, especially its visual and association areas (which link senses, memory and movement). The result is behavioural flexibility: primates solve problems by learning rather than by bodily specialisation, a trend carried furthest in man.

2.4 Teeth: fewer, simpler, more general

Primates eat a mixed diet, so their teeth stay generalised. A dental formula counts the teeth in one quarter of the mouth (incisors–canines–premolars–molars); the total is four times that.

Group	Dental formula	Total teeth
Primitive placental mammal	3.1.4.3	44
Lemurs and lorises	2.1.3.3	36
Tarsiers	2.1.3.3 upper / 1.1.3.3 lower	34
New World monkeys (most)	2.1.3.3	36
Marmosets and tamarins	2.1.3.2	32
Old World monkeys, apes, humans	2.1.2.3	32

Every primate has lost at least one incisor and one premolar from the primitive 44, and the catarrhines share 2.1.2.3. Since teeth are the commonest fossils, the formula is a first clue for placing a fossil (Unit 1.5b §1).

• Molar patterns: Old World monkeys have bilophodont molars (four cusps in two cross-ridges, which keep working when worn by leaves); apes and humans have the Y-5 pattern (five cusps separated by Y-shaped grooves), a sign of close relationship.
• Canines: monkeys and apes have large projecting canines; a gap (diastema) in the opposite jaw receives them, and a blade-like (sectorial) lower premolar hones the upper canine. Canines serve display and defence as much as feeding.
• Dental comb: lemurs and lorises groom with a tooth comb of forward-slanting lower incisors and canines.

2.5 Life cycle: few young, long childhood

• Single births: most primates bear one infant at a time, which an arboreal mother can carry (marmosets bear twins).
• Placenta: monkeys, apes and humans have a haemochorial placenta (the mother’s blood bathes the foetal tissues directly); lemurs and lorises a simpler epitheliochorial one, with more layers between the bloodstreams.
• Slow life history: primates grow more slowly than other mammals of their size.

Species	Pregnancy	Sexual maturity	Growth complete
Rhesus macaque	about 5½ months	about 3–4 years	about 7 years
Chimpanzee	about 8 months	about 10–13 years	about 15 years
Human	9 months	about 12–15 years	about 20 years

Each step from monkey to ape to human lengthens every stage: the K-strategy of few, heavily invested offspring. The long childhood is when a young primate learns, linking reproduction to intelligence and culture; the cost is a low birth rate that leaves apes vulnerable today (Unit 1.5b §2).

2.6 How well do the trends hold?

• The trends are not universal. The aye-aye has claws and rodent-like incisors; humans have lost the grasping foot. Trends describe the overall direction, not every species.
• Old and new together: primates combine old features (five digits, clavicle) with new ones (big brain, forward eyes), an example of mosaic evolution (Unit 1.4b).
• Ladder bias: older textbooks lined up tree shrew, lemur, monkey, ape and man as steps towards man. Each living primate is fully adapted to its own niche, as the British primatologists John and Prue Napier (A Handbook of Living Primates, 1967) stressed.

3. Primate Taxonomy

The tarsier of the Philippines, Borneo and Sulawesi is a palm-sized, night-hunting primate with huge eyes and long leaping legs. It looks like a lemur and was long classed with lemurs and lorises as a “prosimian” (meaning “before the monkeys”). Yet it has a dry nose, a free upper lip, no reflecting layer in its eyes and a placenta like a monkey’s, and DNA places it closer to monkeys, apes and humans. The tarsier is the case on which the two systems of primate classification divide.

3.1 Two ways of classifying

• Phenetics groups animals by overall similarity.
• Cladistics (the German entomologist Willi Hennig, 1950) groups them only by shared derived traits (new features inherited from a common ancestor), so each group holds an ancestor and all its descendants.
• Evolutionary taxonomy (the American palaeontologist George Gaylord Simpson, 1945) adds the degree of adaptive change, so it accepts grades (levels of organisation) as groups.

Two schemes follow. In the table, notice that only the tarsier and the naming of the great apes differ.

Basis	Older (gradistic) scheme	Current (cladistic) scheme
Suborders	Prosimii (lemurs, lorises, tarsiers) and Anthropoidea (monkeys, apes, humans)	Strepsirhini (lemurs, lorises) and Haplorhini (tarsiers plus anthropoids)
Basis	Grade: “primitive” versus “advanced”	Clade: common descent
Tarsier	A prosimian	A haplorhine, sister to anthropoids
Great apes	Pongidae (orangutan, gorilla, chimpanzee) apart from Hominidae (humans)	All in Hominidae; humans in tribe Hominini

The cladistic scheme is standard because it reflects descent, which DNA can test; the gradistic terms survive in older books and past questions. Strepsirhine (“twisted nose”, moist and comma-shaped) and haplorhine (“simple nose”) were revived by the British zoologist Reginald Pocock (1918). As Unit 1.4a explains, “hominid” once meant only humans and their upright ancestors; the human line is now “hominin“.

3.2 Strepsirhini and Haplorhini

The strepsirhines are the lemurs, lorises and galagos. The table lists the traits that separate them from the haplorhines (tarsiers, monkeys, apes and humans).

Trait	Strepsirhini	Haplorhini
Nose	Moist rhinarium; upper lip tied to the gum	Dry nose; free, mobile upper lip for facial expression
Eye socket	Postorbital bar only	Postorbital closure (partial in tarsiers)
Night vision	Tapetum lucidum (reflecting layer that makes eyes shine at night)	No tapetum; a fovea (spot of sharpest vision) instead
Teeth	Tooth comb for grooming	No tooth comb
Grooming claw	On the second toe	Absent (tarsiers have claws on the second and third toes)
Lower jaw	Two halves unfused at the chin	Fused in anthropoids
Placenta	Epitheliochorial	Haemochorial

The pattern is consistent: strepsirhines keep the ancestral toolkit for smell and night activity; haplorhines invest in daylight vision and facial expression, which the free upper lip makes possible.

Geographical distribution of the strepsirhines:

• Lemurs: only Madagascar (and the nearby Comoros), where over a hundred species evolved without monkey competitors, from mouse lemurs (the smallest primates) to the indri.
• Galagos (bushbabies) and pottos: sub-Saharan Africa.
• Lorises: South and South-East Asia; India has the slender loris of the Western and Eastern Ghats and the Bengal slow loris of the north-east.

3.3 Platyrrhini and Catarrhini

Within the anthropoids, the monkeys of the Americas and those of Africa and Asia have been separated for over 30 million years. The table compares them.

Trait	Platyrrhini (New World monkeys)	Catarrhini (Old World monkeys, apes, humans)
Region	Central and South America	Africa and Asia
Nose	Flat, nostrils wide apart, facing sideways	Narrow, nostrils close, facing downward
Teeth	2.1.3.3 (36); marmosets 2.1.3.2	2.1.2.3 (32)
Tail	Often prehensile (grasping) in the atelids	Never prehensile; absent in apes and humans
Ear	Bony ring, no bony ear tube	Bony ear tube (tubular ectotympanic)
Rump pads	None	Ischial callosities in Old World monkeys and gibbons
Colour vision	Usually only some females trichromatic (howlers excepted)	All trichromatic

New World monkeys form the families Callitrichidae (marmosets, tamarins), Cebidae (capuchins, squirrel monkeys), Atelidae (spider, howler, woolly monkeys) and the titis, sakis and owl monkeys. The spider monkey’s prehensile tail is a fifth hand with a ridged pad at its tip; no Old World primate evolved one. Ischial callosities (hard rump pads) let Old World monkeys sleep sitting on branches. How monkeys reached South America, probably by rafting from Africa, is in Unit 1.5b §1.4.

3.4 Old World monkeys and apes

The catarrhines divide into the Cercopithecoidea (Old World monkeys) and the Hominoidea (apes and humans).

Trait	Cercopithecoidea	Hominoidea
Tail	Usually present	Absent
Chest	Narrow and deep, like a dog’s	Broad and shallow
Shoulder blade	On the sides of the chest	On the back
Arm movement	Mainly forward and back	Full rotation above the head
Molars	Bilophodont	Y-5
Lumbar spine	Long and flexible	Short and stiff
Brain	Smaller for body size	Larger and more folded

The hominoid body is built for hanging and reaching overhead: the broad chest and back-placed shoulder blades let the arms swing above the head, and the short lower back keeps the trunk upright. This orthograde (upright-trunk) plan is the starting point for human erect posture (Unit 1.5b §4). Old World monkeys form two subfamilies:

• Cercopithecinae: cheek pouches; omnivorous; many terrestrial: macaques, baboons, guenons.
• Colobinae: multi-chambered stomachs in which bacteria ferment leaves, as in cattle: langurs (Asia), colobus (Africa).

3.5 Primates of India

India has about 25 species of non-human primates: two lorises, about ten macaques, about ten langurs and two gibbons.

• Common: the rhesus and bonnet macaques and the Hanuman langurs, which live alongside people in towns and temples.
• Threatened forest species: the lion-tailed macaque (Western Ghats), the golden langur (Assam–Bhutan), the Nilgiri and capped langurs, and the hoolock gibbons of the north-east, India’s only apes.
• New species: the Arunachal macaque (2004) and the Sela macaque (2022) show that primate taxonomy is still being written.

4. Primate Adaptations: Arboreal and Terrestrial

Watch two primates in an Assamese forest. A hoolock gibbon crosses the canopy by swinging under the branches on very long arms, covering several metres a swing. Below, a troop of rhesus macaques walks on all fours along the forest floor and into a nearby village. Their bodies show the difference: the gibbon’s arms are far longer than its legs; the macaque’s arms and legs are about equal.

4.1 Why primates became what they are: three hypotheses

Three explanations compete for why the primate package (grasping hands, forward eyes, reduced smell) evolved.

Hypothesis	Proposed by	Core idea	Weakness
Arboreal hypothesis	Grafton Elliot Smith (British anatomist, 1912); Frederic Wood Jones (Arboreal Man, 1916)	Life in the trees selected grasping hands, depth vision and reduced smell	Squirrels live in trees with claws and side-facing eyes
Visual predation hypothesis	Matt Cartmill (American anthropologist, 1974)	Forward eyes and grasping hands evolved to stalk and seize insects on thin branches, as in owls and cats	Many early primates ate fruit, and some hunters (squirrel-like insect eaters) lack these traits
Angiosperm (fruit) hypothesis	Robert Sussman (American anthropologist, 1991)	Primates evolved alongside flowering, fruiting trees; grasping hands reach fruit at branch tips; vision picks it out	Explains grasping better than forward eyes

The fossil Carpolestes (about 56 million years ago, Wyoming; Jonathan Bloch and Doug Boyer, 2002) had a grasping big toe with a nail but sideways eyes: grasping came before stereoscopic vision, favouring the fruit view for the first step. Most primatologists combine the three: early primates exploited the fine terminal branches of fruiting trees for fruit, flowers and insects.

4.2 Modes of locomotion

The British primatologists John Napier and Alan Walker (1967) defined vertical clinging and leaping and helped build the standard classification of primate movement; each mode marks the skeleton.

Mode	How the animal moves	Body adaptations	Examples
Vertical clinging and leaping	Clings upright to trunks; leaps with hind legs	Long hind limbs, upright trunk	Tarsiers, galagos, indri, sifakas
Slow climbing	Moves slowly with a vice-like grip	Reduced index finger, wide-angled thumb and big toe	Lorises, pottos
Arboreal quadrupedalism	Walks and runs on top of branches	Limbs about equal; tail for balance	Most monkeys, macaques in trees
Semibrachiation	Arm-swinging plus leaping and climbing	Long arms; prehensile tail (New World) or reduced thumb (colobines)	Spider and howler monkeys; langurs, colobus
Terrestrial quadrupedalism	Walks on the ground on palms and soles, or on fingers	Equal limbs, short fingers, longer thumb	Baboons, patas monkeys, rhesus macaques
Knuckle-walking	Walks on the backs of bent fingers	Long arms, long fingers, locking wrist	Chimpanzees, bonobos, gorillas
Brachiation	Swings hand over hand under branches	Very long arms, hook-like hands, short thumb	Gibbons, siamangs (true); orangutans (modified, slow, four-handed climbing)
Bipedalism	Walks upright on two legs	Long legs, bowl-shaped pelvis, arched foot	Humans only (habitually)

Read the “Body adaptations” column: structure follows function. Brachiators need hooks, so their thumbs are reduced (spider monkey, colobus) or tucked away; leapers need springs, so their legs are long. A gibbon hangs from one hand at a branch tip and pulls fruit in with the other, so brachiation is above all a feeding adaptation.

Two cautions. Structure and behaviour do not always match: chimpanzees and gorillas have the long arms and short thumbs of brachiators but mainly knuckle-walk, probably because they became too heavy to swing. And every primate uses several modes; a classification names the most typical.

4.3 Reading locomotion from bones: the intermembral index

The intermembral index (IMI) measures arm length against leg length:

IMI = (humerus + radius) ÷ (femur + tibia) × 100

An index below 100 means longer legs; above 100 means longer arms.

Locomotion	Typical IMI	Examples
Vertical clinging and leaping	about 50–70	Tarsiers, galagos
Bipedalism	about 70	Humans
Quadrupedalism	about 80–100	Macaques, baboons, langurs
Knuckle-walking	about 100–120	Chimpanzee about 105, gorilla about 115
Brachiation	about 125–150	Gibbons; orangutan about 140

Worked example: humerus 30 cm, radius 25 cm, femur 32 cm, tibia 28 cm gives (30 + 25) ÷ (32 + 28) × 100 = 55 ÷ 60 × 100 ≈ 92, a quadruped. So a few long bones show how an extinct primate moved. Humans and leapers both score low because both push with the legs.

4.4 The primate hand

Napier also ranked hands by how the thumb works: non-opposable (tree shrews; digits move together), pseudo-opposable (lemurs, lorises, New World monkeys; the thumb converges but does not rotate) and truly opposable (Old World monkeys, apes, humans; the thumb rotates to meet the fingertips, giving separate power and precision grips). Ground-living baboons have long thumbs and short fingers for picking up seeds. Humans refine the precision grip furthest (Unit 1.4a §2.2).

4.5 Terrestrial adaptations

The ground brought predators, open country and scattered food. Savanna baboons and macaques show a consistent set of answers.

• Body: arms and legs about equal, short fingers, a digitigrade stance (walking on the fingers and toes, heel raised) in baboons, and tough ischial callosities.
• Defence: marked sexual dimorphism (size difference between the sexes): male baboons weigh about twice as much as females, with long canines and a mane for threat displays; tree monkeys, which simply flee, show little.
• Society: troops of 10 to 100 with clear dominance hierarchies that reduce fighting and organise defence.
• Diet: wider: grass, roots, seeds, insects, some meat; the Ethiopian gelada lives mainly on grass.
• Humans alone are fully terrestrial; gorillas and chimpanzees still sleep in tree nests.

4.6 Adaptive radiation and the adaptive tradition of primates

Adaptive radiation (one ancestral group branching rapidly into many forms suited to different ways of life; Unit 1.4b §6.5) describes primate history well. From small insect- and fruit-eaters of about 56 million years ago, primates radiated into:

• night-active insect-hunters (tarsiers, lorises, galagos);
• gum-eaters with claws (marmosets);
• leaf-eaters with fermenting stomachs (colobines) or long guts (howlers, gorillas);
• fruit-eaters with colour vision (most monkeys and apes);
• ground-living omnivores (baboons, macaques, humans).

The vivid case is Madagascar, where lemurs, isolated from monkeys, radiated into over a hundred species. An adaptive tradition is the lasting adaptive pattern a lineage keeps while it diversifies: for primates, the arboreal tradition of grasping, seeing and learning, inherited even by ground-living species.

5. Primate Behaviour

In 1960, at Gombe Stream, Tanzania, the young English researcher Jane Goodall watched a chimpanzee she called David Greybeard strip a twig, poke it into a termite mound and pull it out covered in termites. Tool-making had been the mark of humanity. Her mentor, the Kenyan-British palaeoanthropologist Louis Leakey, replied: “Now we must redefine tool, redefine Man, or accept chimpanzees as humans.” One observation changed how anthropology defines culture.

5.1 Primatology and anthropology

Primatology (the study of non-human primates) belongs to biological anthropology (Unit 1.3) for four reasons:

• Models of the past: fossils preserve bones, not behaviour; living apes and ground monkeys suggest how early hominins foraged and grouped.
• Roots of culture: ape tools, learning and bonds show which human behaviours are old and which are new.
• Biocultural approach: primate studies show how ecology shapes society, anthropology’s question for human groups too.
• Applied value: conservation, zoonotic disease (HIV came from chimpanzee and sooty mangabey viruses) and human–monkey conflict in Indian towns.

Pioneers of field primatology. The table lists the studies that built the field, including several conducted in India.

Primatologist	Species and site	Key finding
C.R. Carpenter (American, 1934)	Howler monkeys, Barro Colorado Island, Panama	First systematic field study of a wild primate group
Kinji Imanishi (Japanese, from 1948)	Japanese macaques, Koshima	Founded Japanese primatology; individual identification; learned traditions
Sherwood Washburn and Irven DeVore (Americans, about 1960)	Savanna baboons, Kenya	Baboons as a model for early hominins in open country
Phyllis Jay (Dolhinow) (American, late 1950s)	Hanuman langurs, north India	Relaxed hierarchy, shared infant care
Yukimaru Sugiyama (Japanese, 1960s)	Hanuman langurs, Dharwar, Karnataka	First report of infanticide after male takeover
Jane Goodall (British, from 1960)	Chimpanzees, Gombe, Tanzania	Tool-making, hunting, warfare
Dian Fossey (American, from 1967)	Mountain gorillas, Karisoke, Rwanda	Gentle family groups; poaching threat (Gorillas in the Mist, 1983)
Sarah Blaffer Hrdy (American, 1970s)	Hanuman langurs, Mount Abu, Rajasthan	Infanticide as a male reproductive strategy (The Langurs of Abu, 1977)

Goodall, Fossey and Biruté Galdikas (orangutans, Borneo, from 1971) were all sent by Louis Leakey, who believed long-term ape studies would illuminate human origins. India was an early centre of the field; Indian primatologists such as Mewa Singh (lion-tailed and bonnet macaques) now lead much of the work.

5.2 Forms of social organisation

The social organisation of a species is the size, sex make-up and stability of its groups. Primates show every major form found in mammals.

Form	Group make-up	Examples	Ecological logic
Solitary (dispersed)	Adults forage alone; ranges overlap	Lorises, galagos, aye-aye; orangutans	Scattered food; darkness hides them
Monogamous pair	One male, one female and young	Gibbons, siamangs, titi and owl monkeys, indri	Defended territory; little dimorphism
Polyandrous group	One breeding female, two or more males	Marmosets, tamarins	Twins need several carriers
One-male group (harem)	One male, several females and young	Gorillas (silverback), Hanuman langurs, geladas	Males compete for females; strong dimorphism
Multi-male, multi-female	Several adults of both sexes	Savanna baboons, macaques, ring-tailed lemurs	Large group defends against ground predators
Fission–fusion community	Large community splits into changing parties	Chimpanzees, bonobos, spider monkeys	Party size tracks fruit supply
Multi-level society	One-male units nested in bands and herds	Hamadryas baboons, geladas	Units forage; herds sleep safely

Two factors explain most of this variety. Predation pushes animals into larger groups, especially on the ground; food distribution limits group size, so scattered fruit favours small or fluid parties. John Crook and Stephen Gartlan (1966) first linked social systems to ecology; Richard Wrangham (1980) added that where females must jointly defend food, groups form round related females (female-bonded groups, as in macaques).

• Who leaves home: in most monkeys males leave at maturity and females stay, forming matrilines; in chimpanzees males stay (male philopatry) and, being related, defend territory together. Dispersal by one sex prevents inbreeding, a parallel to exogamy (Unit 2.3).

5.3 Dominance and grooming

• Dominance hierarchy: a ranking that sets priority of access to food and mates, based on size, age, confidence and allies. It reduces fighting, because each animal knows whom to avoid. It is rigid in savanna baboons, relaxed in Hanuman langurs, and political in chimpanzees, where high rank depends on alliances, as the Dutch primatologist Frans de Waal showed (Chimpanzee Politics, 1982).
• Social grooming (allogrooming): cleaning another’s fur is hygiene but mainly social cement: it calms, repairs relationships and builds alliances.
• Grooming and language: the British anthropologist Robin Dunbar (Grooming, Gossip and the Evolution of Language, 1996) argued that when human groups grew too large to groom everyone, talk took over as the way to bond.
• Reconciliation: chimpanzees kiss and embrace opponents soon after a fight (de Waal): peace-making is primate, not only human.

5.4 Communication and the role of smell

Primates send signals in four modes, which differ in strength and reach.

Mode	Examples	Strength and range
Olfactory (smell)	Scent-marking, urine-washing, gland rubbing	Long-lasting; works in the dark and in absence
Visual	Stares, bared canines, eyebrow flashes, sexual swellings	Immediate, close range
Auditory	Alarm calls, gibbon songs, chimpanzee pant-hoots	Long range; many receivers at once
Tactile	Grooming, embracing, hitting, biting	Strongest and most intimate

Smell as a signal. Reduced smell in monkeys and apes is a trend, not an absence; in strepsirhines smell remains a main channel.

• What smell does: a scent mark stays after the sender has gone, carrying identity, sex, rank and reproductive state, which suits nocturnal and solitary animals that rarely meet.
• Ring-tailed lemurs rub secretions from wrist and shoulder glands on their tails and wave them at rivals in “stink fights“; in the breeding season males release a fruity, floral scent that attracts females (Japanese researchers, 2020).
• Others: lorises and galagos urine-wash their hands and feet, leaving a trail; many New World monkeys scent-mark; the vomeronasal organ (a nose organ for chemical signals) is well developed in strepsirhines, reduced in catarrhines.
• Old World monkeys and apes shifted to vision: a chimpanzee’s sexual swelling advertises fertility where a lemur would use scent. Smell still matters, as when a mother recognises her infant.

Do monkeys have words? Vervet monkeys give different alarm calls for leopards, eagles and snakes. Played from hidden speakers (Robert Seyfarth, Dorothy Cheney and Peter Marler, 1980), the calls made vervets run up trees, look up or search the ground respectively: they carry specific meaning.

Ape language studies. Because apes cannot shape human speech sounds, researchers tried other channels.

Study	Ape and method	Result
Keith and Cathy Hayes (1940s–50s)	Chimpanzee Viki, spoken words	Only about four words; the ape vocal tract cannot form speech
Allen and Beatrix Gardner (from 1966); Francine Patterson (from 1972)	Chimpanzee Washoe; gorilla Koko; American Sign Language	About 130 signs (Washoe); several hundred (Koko); some combined
Sue Savage-Rumbaugh (1980s)	Bonobo Kanzi, lexigrams (keyboard symbols)	Learned symbols by watching his mother’s lessons; understood spoken English sentences
Herbert Terrace (1979)	Chimpanzee Nim Chimpsky, sign language	Critic: Nim mostly imitated trainers’ cues (the “Clever Hans” effect), with no grammar

Terrace’s videos showed that apparent sentences were often unconscious prompting by trainers (Clever Hans was a horse that seemed to count but read its owner’s cues). Apes can learn symbols and simple combinations, best when young and by observation (Kanzi), but not grammar.

5.5 Sexual behaviour, infant care and infanticide

• Mating systems vary: gibbons pair; baboons form short consortships; chimpanzees mate promiscuously; bonobos use sex to reduce tension.
• Oestrus and concealed ovulation: most female primates are receptive only around ovulation (oestrus), often advertised by sexual swellings; human females have concealed ovulation. The American anthropologist C. Owen Lovejoy (1981) linked this to pair-bonding and male food provisioning. Critics led by Adrienne Zihlman replied that human monogamy is not universal, that women in foraging societies supply much of the food, and that bonobos mate outside oestrus without pair-bonds: the model projected modern family ideals onto the past.
• Mother–infant bond: the American psychologist Harry Harlow (1958) gave infant rhesus monkeys a wire “mother” with milk and a soft cloth one without; they clung to the cloth mother. Contact comfort, not only food, builds attachment, and isolated monkeys grew up unable to mate or mother normally.
• Infanticide: a male taking over a Hanuman langur troop may kill unweaned infants. Sugiyama first reported it at Dharwar; Hrdy, at Mount Abu, explained it as a male strategy: a female who loses her infant returns to oestrus sooner and bears the new male’s young. Behaviour is shaped by individual reproductive interests, not the good of the group, which is why langur females sometimes jointly drive off invading males.

5.6 Tool use and culture

• Chimpanzee tools: termite fishing with prepared twigs (Gombe), leaf sponges, stone hammers and anvils for cracking nuts (Christophe and Hedwige Boesch, Taï Forest, Côte d’Ivoire) and sharpened sticks to stab bushbabies (Fongoli, Senegal, 2007). Young chimpanzees take years to master nut-cracking by watching their mothers.
• Traditions: on Koshima in 1953 a young macaque, Imo, began washing sweet potatoes; the habit spread to her playmates and mother, then down the generations. Andrew Whiten and colleagues (Nature, 1999) found 39 chimpanzee behaviours that vary between communities for no ecological reason, that is, culture as learned group tradition.
• Hunting and sharing: Gombe chimpanzees hunt colobus monkeys in groups and share the meat, mostly with allies.

How far is this “culture”? Ape culture is real but limited. Human culture is cumulative: each generation builds on the last, so techniques ratchet up in complexity. Ape traditions rarely accumulate, are seldom deliberately taught and are not symbolic. Primates show the foundations of culture; humans added symbol, language and accumulation (Unit 2.1).

5.7 Aggression and conflict

• Display before violence: most aggression is ritualised: baboons stare, flash pale eyelids, slap the ground and yawn to show their canines, and the hierarchy settles most disputes.
• Lethal intergroup conflict: in the Gombe “Four-Year War” (1974–78), males of one chimpanzee community killed every male of a group that had split away and took its territory. Such raids, now recorded at several sites, raise the question of whether warfare has primate roots.
• The bonobo contrast: bonobos have female-led societies, rarely kill, and ease tension through sex and play. Two sister species thus warn against treating any one ape as “the” model of early humans.

Key Terms

Term	Meaning in simple words
Primates	The mammal order of lemurs, lorises, tarsiers, monkeys, apes and humans
Evolutionary trend	A tendency that runs through a group to different degrees, not a fixed trait
Opposable thumb	A thumb that can rotate to meet the other fingertips
Stereoscopic vision	Depth perception from two overlapping fields of view
Dental formula	Count of incisors, canines, premolars and molars in one quarter of the jaw
Y-5 molar	A five-cusped molar with Y-shaped grooves, found in apes and humans
Strepsirhini	Wet-nosed primates: lemurs, lorises and galagos
Haplorhini	Dry-nosed primates: tarsiers, monkeys, apes and humans
Platyrrhine / catarrhine	Flat-nosed New World monkeys / narrow-nosed Old World monkeys, apes and humans
Brachiation	Swinging hand over hand beneath branches
Intermembral index	Arm length divided by leg length, times 100
Dominance hierarchy	A ranking that sets priority of access to food and mates
Philopatry	Staying in the group of one’s birth

Contemporary Developments

• Death of Jane Goodall (1 October 2025): the primatologist who showed chimpanzee tool-making, hunting and warfare at Gombe from 1960 died aged 91.
• Grammar-like calls in bonobos (April 2025): bonobo call combinations were found to combine meanings like compositional human phrases (Mélissa Berthet and colleagues, Science), narrowing, not closing, the gap with language.
• An orangutan treats its own wound (May 2024): the wild Sumatran orangutan Rakus applied chewed leaves of the medicinal liana Fibraurea tinctoria to a facial wound, the first recorded wound treatment with a known medicinal plant by a wild animal (Isabelle Laumer and colleagues, Scientific Reports).
• A new Indian primate (May 2022): the Zoological Survey of India described the Sela macaque (Macaca selai) of Arunachal Pradesh, cut off from the Arunachal macaque by the Sela Pass about two million years ago.
• Primates in crisis (2017): about 60 per cent of the world’s primate species are threatened with extinction and about 75 per cent have declining populations, mainly through farming, logging and hunting (Alejandro Estrada and colleagues, Science Advances).

Diagrams to Master

1. Topic map: characteristics, taxonomy, adaptation and behaviour.
2. Evolutionary trends: three boxes, limbs (1–3), head (4–8), life cycle (9–10), all linked to arboreal life.
3. Classification tree: Strepsirhini and Haplorhini → tarsiers and anthropoids → platyrrhines and catarrhines → Old World monkeys and hominoids; mark the prosimian grade and the old Pongidae.
4. Locomotion spectrum: eight modes with an example and the intermembral index for each.
5. Social organisation: small group diagrams (△ male, ○ female) from solitary to multi-level, one example each.

Revision Grid

Scholar	Work (year)	Key idea
Linnaeus	Systema Naturae, 10th edition (1758)	Placed humans in the order Primates
Huxley	Evidence as to Man’s Place in Nature (1863)	Man closer to great apes than these are to monkeys
Elliot Smith; Wood Jones	1912; Arboreal Man (1916)	Arboreal hypothesis
Simpson	1945	Gradistic primate classification
Le Gros Clark	The Antecedents of Man (1959)	Ten evolutionary trends
Goodall	Gombe, from 1960	Chimpanzee tool-making
Crook and Gartlan	1966	Social organisation follows ecology
Napier and Walker	1967	Vertical clinging and leaping; locomotor categories
Cartmill	1974	Visual predation hypothesis
Hrdy	The Langurs of Abu (1977)	Infanticide as male strategy
Lovejoy; Zihlman	1981; 1970s–80s	Pair-bond model; its critique
de Waal	Chimpanzee Politics (1982)	Alliances, reconciliation
Sussman	1991	Angiosperm hypothesis
Dunbar	Grooming, Gossip and the Evolution of Language (1996)	Language replaced grooming