Primate

• 1. Introduction

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  Primates range in size from Madame Berthe's mouse lemur, which weighs 30 grams, to the eastern gorilla, which tips the scales at over 200 kilograms. Between those two extremes lives an order of mammals that includes lemurs, galagos, lorisids, tarsiers, monkeys, apes, and one species reading these words. The group arose between 74 and 63 million years ago, beginning as small terrestrial mammals that climbed into tropical forests and never fully came down. Carl Linnaeus thought them the highest order of animals, and gave them a name drawn from the Latin for first rank. How did life in the treetops shape a brain, an eye, and a hand? Why do counts of living primate species swing from 376 to over 500? And what makes about 60% of these animals threatened today? The answers begin in the canopy.

• 2. Life In The Canopy

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  Tree tops are a challenging environment, and many primate traits read as solutions to it. Large brains, binocular vision, color vision, and vocalizations all trace back to this arboreal pressure. Shoulder girdles that allow a wide range of upper-limb movement, and opposable thumbs in most but not all species, enabled better grasping and dexterity. These features grew more developed in monkeys and apes, and noticeably less so in lorises and lemurs.

  Visual acuity rose at the expense of smell, the dominant sense in most other mammals. The arboreal life pushed primates toward a tactile, visually dominant system, shrinking the olfactory region of the brain. Gorillas, humans, and baboons live primarily on the ground rather than in trees, yet every species keeps adaptations for climbing.

  Locomotion splits along this divide. In the trees, primates leap from branch to branch or swing between them, a method called brachiation. On the ground, some walk on two hindlimbs in bipedalism, while others use a modified four-limbed walk that includes knuckle-walking. Those moving bodies turned out to be unusually social ones, which is where the order grows strange.

• 3. Societies And Strategies

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  Primates are among the most social of all animals, forming pairs, family groups, uni-male harems, and multi-male, multi-female groups. Richard Wrangham argued the systems are best sorted by how much females move between groups, and proposed four categories. In female transfer systems, seen in chimpanzees, related males stay and cooperate to defend territory. In male transfer systems, common among ring-tailed lemurs and capuchins, the adolescent males leave instead. Monogamous species like indri and gibbons form a male-female bond with shared parental care. Solitary species like lorises, aye-ayes, and orangutans simply overlap home ranges.

  Moving females or males away from the natal group likely guards against inbreeding. Breeding records from captive colonies show inbred infants die at higher rates than non-inbred young, a result of deleterious recessive alleles surfacing. Transfer keeps relatives from breeding.

  Jane Goodall, studying in Gombe Stream National Park, noted fission-fusion societies in chimpanzees. The main group splits to forage by day, then fuses at night to sleep together. The gelada takes this further, with smaller groups merging into temporary herds of up to 600 monkeys. Humans in hunter-gatherer societies do the same, splitting and rejoining to gather different resources.

• 4. The Primate Body

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  The endocranial volume of a human skull is three times that of the greatest nonhuman primate. The mean reaches 1,201 cubic centimeters in humans, against 469 in gorillas, 400 in chimpanzees, and 397 in orangutans. The brain's neocortex drove this growth, handling sensory perception, motor commands, spatial reasoning, conscious thought, and in humans, language. The visual acuity of humans and other hominids is the most acute known among vertebrates, save certain predatory birds.

  Forward-facing eyes sit on the front of the skull, granting binocular vision and accurate distance perception useful to brachiating ancestors. Strepsirrhines protect their eyes with a postorbital bar, a bone ring around the socket. The haplorhines, the higher primates, evolved fully enclosed sockets instead.

  Most primates carry five digits per limb, capped with keratin fingernails and sensitive pads on the fingertips. The combination of opposing thumbs, short nails rather than claws, and long inward-closing fingers is a relic of gripping branches. Prosimians keep a clawlike nail on the second toe of each foot, a toilet-claw used for grooming. The prominent collar bone gives the shoulder broad mobility, and apes extend this with flatter ribcages, shorter spines, and in some species, the loss of the tail entirely.

• 5. Seeing In Color

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  Color vision in primates is unique among most eutherian mammals. The remote vertebrate ancestors had three-color vision, but nocturnal mammalian ancestors lost one of three retinal cones during the Mesozoic. That left most mammals as dichromats or monochromats, totally color blind, with primates and marsupials the exceptions. Nocturnal primates like night monkeys and bush babies are often monochromatic themselves.

  Catarrhines are routinely trichromatic thanks to a gene duplication of the red-green opsin, dated to 30 to 40 million years ago. Platyrrhines manage it only in a few cases. A female must be heterozygous for two alleles of the opsin gene on the same X-chromosome locus, so males can only be dichromatic while females may be either.

  Howler monkeys, a platyrrhine family, evolved routine trichromatism through a separate, recent gene duplication. They are among the most specialized leaf-eaters of the New World monkeys, and fruit is not central to their diet. The young, nutritive, digestible leaves they favor can only be detected by a red-green signal. Fieldwork on their dietary preferences suggests the environment itself selected for that vision.

• 6. Minds And Tools

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  In 1960, Jane Goodall watched a chimpanzee poke pieces of grass into a termite mound, then raise the grass to his mouth. Unsure what she had seen, she approached the mound and repeated it, finding the termites bit onto the grass with their jaws. Soon after, she observed chimpanzees stripping leaves from twigs to fish for insects. That change of a leafy twig into a tool was a major discovery. Until then, scientists thought only humans manufactured and used tools, and that this set humans apart.

  The first observation of a non-ape primate using a tool in the wild came in 1988, when Sue Boinski watched an adult male white-faced capuchin beat a fer-de-lance snake to death with a dead branch. The black-striped capuchin became the first non-ape primate with documented routine tool use, cracking nuts on a stone anvil with another stone. In Thailand and Myanmar, crab-eating macaques use stone tools to open nuts, oysters, bivalves, and sea snails.

  The aye-aye fills the woodpecker's niche through anatomy rather than learning. It taps trees with rodent-like teeth to find insect larvae, gnaws holes in the wood, and inserts a long thin middle finger to pull the larvae out. Comparative studies show intelligence rising from prosimians to New World monkeys to Old World monkeys, and higher still in the great apes.

• 7. Voices In The Forest

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  The Philippine tarsier reaches a high-frequency limit of auditory sensitivity near 91 kilohertz, with a dominant frequency of 70 kilohertz, among the highest recorded for any terrestrial mammal. Those ultrasonic vocalizations may form a private channel that hides the animal from predators, prey, and competitors. Male howler monkeys take the opposite tack, roaring loud enough to be heard up to 4.8 kilometers away to manage spacing between groups.

  Lemurs, lorises, tarsiers, and New World monkeys lean on smell, marking territories with pheromones detected by the vomeronasal organ. In Old World monkeys and apes this ability is mostly vestigial, having regressed as trichromatic eyes became the main sensory organ. Facial musculature took over instead, especially in monkeys and apes, allowing complex expression. Chest-beating in male gorillas is visual and non-vocal sound that signals fitness to rivals and females alike.

  The vervet monkey gives a distinct alarm call for each of at least four predators, and other monkeys react differently to each. Chimpanzees, Campbell's mona monkeys, and Diana monkeys combine vocalizations in sequences, hinting that syntax may not be uniquely human but evolutionarily ancient. The time range for human language stretches from the divergence of Homo, 2.3 to 2.4 million years ago, to full behavioral modernity some 50,000 to 150,000 years ago.

• 8. A Vanishing Order

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  The International Union for Conservation of Nature lists more than a third of primates as critically endangered or vulnerable, and 75% of species have declining populations. The losses concentrate sharply by region: 87% of species in Madagascar are threatened, 73% in Asia, 37% in Africa, and 36% in South and Central America. More than 90% of primate species live in tropical forests, and large-scale clearing of those forests is the process that most threatens them. In Indonesia, lowland forest cleared for palm oil cost an estimated 1,000 Sumatran orangutans per year in the Leuser Ecosystem during 1998 and 1999.

  Body size compounds the danger. Primates over 5 kilograms draw poachers through their greater profitability, mature later, and space births further apart, so populations recover slowly. In some African cities, half of all protein consumed comes from the bushmeat trade, and endangered animals like guenons and the drill are hunted past sustainable levels.

  Madagascar has seen the greatest recent extinction. Since human settlement 1,500 years ago, at least eight classes and fifteen of the larger species have vanished, among them Archaeoindris, a lemur larger than a silverback gorilla. Miss Waldron's red colobus was declared extinct after no trace was found between 1993 and 1999. A few hunters have killed individuals since, but its prospects remain bleak.