18.4: Chapter 3- Modern Primates and Their Evolution

Learning Objectives

1. Describe the distinguishing features of mammals and of primates
2. Discuss the range of physical and behavioral traits in several different species of primate
3. Describe general principles of brain evolution as proposed by Striedter (2006) and discuss one short coming of his principles
4. List major steps in primate evolution
5. Describe the taxonomic classification of our species, Homo sapiens

Overview

Humans are primates. We belong to the taxonomic order Primates. This order encompasses humans as well as non-human primates. Non-human primates are our closest biological and evolutionary relatives. So, we study them in order to learn more about ourselves. All primates are mammals.

Figure \(\PageIndex{1}\): Golden Lion Tamarin, a primate found in Brazil. Woodland Park Zoo, Seattle, WA 9/30/2011

General Mammalian Characteristics

The earliest evidence of mammals is from the Mesozoic era (see table of geological time below), however, there is limited fossil evidence and the fossils that have been found are mouse-like forms with quadrupedal locomotion. Primates evolved from an ancestral mammal during the Cenozoic era and share many characteristics with other mammals. The Cenozoic is the era of the adaptive radiation of mammals with thirty different mammalian orders evolving.

Some of the general characteristics of mammals includes:

• mammary glands: females produce milk to feed young during their immediate post-natal growth period
• mammals have hair (sometimes called fur) that covers all or parts of their body
• the lower jaw is a single bone
• the middle ear contains three bones: stapes (stirrup), incus (anvil), and malleus (hammer)
• four-chambered heart
• main artery leaving the heart curves to the left to form the aortic arch
• mammals have a diaphragm
• mammals regulate their body temperature to maintain homeostais (a constant body temperature)
• teeth are replaced only once during lifetime

Primate Evolution in Summary

The first fifty million years of primate evolution was a series of adaptive radiations (diversification of a group of organisms into forms filling different ecological niches) leading to the diversification of the earliest lemurs, monkeys, and apes. The primate story begins in the forest canopy (overlapping branches and leaves high above ground) and understory of conifer-dominated forests, with our small, furtive ancestors subsisting at night, beneath the notice of day-active dinosaurs. From the archaic primates to the earliest groups of true primates (euprimates), the origin of our own order (the Primates) is characterized by the struggle for new food sources and microhabitats in the arboreal setting. Climate change forced major extinctions as the northern continents became increasingly dry, cold, and seasonal and as tropical rainforests gave way to deciduous forests, woodlands, and eventually grasslands. Lemurs, lorises, and tarsiers—once diverse groups containing many species—became rare, except for lemurs in Madagascar where there were no anthropoid competitors and perhaps few predators. Meanwhile, anthropoids (monkeys and apes) emerged in the Old World (Africa, Europe, and Asia), then dispersed across parts of the northern hemisphere, Africa, and ultimately South America. Meanwhile, the movement of continents, shifting sea levels, and changing patterns of rainfall and vegetation contributed to the developing landscape of primate biogeography, morphology, and behavior. Today’s primates provide a few reminders of the past diversity and remarkable adaptations of their extinct relatives.

A Simplified Classification of Living Primates

 

 

  

Figure \(\PageIndex{2}\): This image is a hand mnemonic used to help students learn a categorization of primates. The right hand (held palm upward) correlates to the apes, including the great apes and lesser apes. Humans are distinct from that group as the thumb is distinct from the other four digits of the hand. All apes evolved to be tailless, whereas species grouped onto the left hand are characterized as having tails (with certain exceptions, such as the Barbary macaque). Old world monkeys have a family name that means "tailed ape" and are indicated on the left thumb that points toward the right hand of apes. Old world monkeys are grouped with all of the apes in the parvorder called Catarrhini. New world monkeys on the left index finger form their own parvorder called Platyrrhini (meaning "flat nosed"). These are the only monkeys with prehensile tails. The group of simians (higher primates) are all apes and monkeys, so includes all of the above (Catarrhini and Platyrrhini). The three remaining digits on the left hand form the group of prosimians (lower primates).

The hand phalanges are not to be mistaken for a phylogeny as the branching geometry is not accurate. And the ten hand digits do not correspond to any single particular level of taxonomy. The specific correspondence of digits is:

• right thumb = genus Homo => 1 species: humans,
• right index = genus Pan => 2 species: common chimpanzees (4 subspecies) and bonobos,
• right middle = subfamily Gorillinae => genus Gorilla => 2 species: western gorillas (2 subspecies) and eastern gorillas (2 subspecies),
• right ring = subfamily Ponginae => genus Pongo => 2 species: Bornean orangutans (3 subspecies) and Sumatran orangutans,
• right pinky = family Hylobatidae => 4 genera of gibbons => nearly twenty species in total, including siamangs, lar gibbons and hoolock gibbons,

• left thumb = superfamily Cercopithecoidea (old world monkeys) => well over one hundred species including baboons and macaques,
• left index = parvorder Platyrrhini => superfamily Ceboidea (new world monkeys) => well over one hundred species including marmosets, tamarins, titis, howlers and squirrel monkeys,
• left middle = infraorder of tarsiers,
• left ring = superfamily of lemurs,
• left pinky = superfamily of lorisoids.

Categorization of humans has not been without controversy. One position is that homo sapiens are above apes and that it is improper to categorize the species as one of the great apes. The opposite extreme is the view that humans are "the third chimpanzee", based upon the very high percentage of genetic commonality between the species. This image follows a convention found between these two positions.

(Image and caption for Figure 18.4.2 from Wikimedia Commons; File:Primate Hand Mnemonic.png; https://commons.wikimedia.org/wiki/F...d_Mnemonic.png; by Tdadamemd; licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license).

General Primate Characteristics

While primates share traits with other mammals, such as mammary glands and regulation of body temperature, there are a number of traits that all primates share. Because humans and non-human primates share a common evolutionary history (we have a common primate ancestor), human and non-human primates have a number of similar biological and behavioral traits.

Body

Primates have a flexible and generalized limb structure that's able to move readily in many directions. Compare the flexibility of the spider monkeys on the left with the horse on the right.

Figure \(\PageIndex{3}\): Spider monkeys.

Figure \(\PageIndex{4}\): Horse.

Hands

Primates have prehensile hands (and most of them have prehensile feet also). This means that they have the ability to grasp and manipulate objects. Primates have 5 digits on their hands and feet. Note that a few primates, like spider monkeys, have what are called vestigial thumbs. This means that they have either a very small or non-existent external thumb (but in that case, they will still have a small internal thumb bone).

Figure \(\PageIndex{5}\): Primate hands and feet.

Teeth

Primates have different teeth that perform different tasks when processing food via biting or chewing. Thus, they have the ability for a more generalized diet (compared to a specialist diet), meaning that they have more dietary flexibility. Why would this be a good thing to have?

Compare the teeth of the cayman (a relative of crocodiles and alligators) with the human teeth on the right. While the cayman's teeth do vary in size, they don't vary in structure. However, the human has incisors, canines, premolars, and molars -- all of which perform different food processing tasks.

Figure \(\PageIndex{6}\): Cayman.

Figure \(\PageIndex{7}\): Human jawbone.

Senses

Compared to most other mammals, primates have an increased reliance on vision and a decreased reliance on their senses of smell and hearing. Associated with this are their smaller, flattened noses, loss of whiskers, and relatively small, hairless ears. Also associated with this are their forward-facing eyes with accompanying binocular or stereoscopic vision. This type of vision means that both eyes have nearly the same field of vision with a lot of overlap between them. This arrangement provides good depth perception (but a loss of peripheral vision)--very useful for moving through tree branches.

Note first how the eyes of the monkey on the left are more front-facing than the eyes of the cow on the right. Also note how flat the monkey's nose is and how small its ears are, when compared to those of the cow.

Figure \(\PageIndex{8}\): Monkey.

Figure \(\PageIndex{9}\): Cow. Note placement of the eyes compared to the monkey.

Brain Evolution

In relation to other mammals, primates have a more expanded and elaborate brain, including expansion of the cerebral cortex. Compare the complexity of the human brain on the left to the cat brain on the right.

Figure \(\PageIndex{10}\): Human Brain.

Figure \(\PageIndex{11}\): Cat Brain.

Clearly, significant anatomical changes have taken place during the evolution of the brain in primates, in other mammals, and in animals in general. Are there generalizations that can be made about the evolution of animal brains?

Striedter (2006) has identified a number of general principles of brain evolution applicable across a wide range of species (i.e. not just primates or mammals).

1) Embryonic brains across species are more similar than adult brains, because brains tend to diversify more as they grow toward adult form;

2) relative brain size to body size in the vertebrates (animals with backbones) has tended to increase more often than decrease over evolutionary time;

3) it appears that increases in relative brain size were generally accompanied by increases in social or food foraging complexity;

4) most increases in relative brain size were accompanied by increases in absolute body size;

5) increases in absolute brain size require changes in the brain's internal connections which imply greater modularity (specialized processing modules increasing the "division of labor") of brain anatomy and functioning;

6) evolution generally enlarges brains by extending the period of brain development while conserving (keeping the same) the "birth order" of different brain regions, so that big-brained animals tend to have disproportionately larger late-"born" (late-developing) regions ("late equals large"), such as cerebral cortex, leading to disproportionately more cerebral cortex (increased corticalization) in big-brained mammals (non-mammals don't have cerebral cortex); however there are exceptions to this rule, for example, at any given absolute brain size, there is more cerebral cortex in simians than in prosimians, and in parrots there is an unusually large telencephalon which is not accounted for by the rule;

7) changes in size proportions of brain areas, although "automatic" within the scaling (allometric) rules above, can still be adaptive and undergo natural selection;

8) as brain regions increase in absolute or proportional size, they tend to become laminated--organized into sheets of neurons--allowing point for point corresponding connections between sensory and motor maps with minimal axonal and dendritic wiring, saving space and metabolic energy;

9) as brain size increases, more regional subdivisions occur from ancestral parts subdividing into new parts, as in the dorsal thalamus, or, as in the case of neocortex, a new part was added to an ancestral set of conserved parts;

10) Deacon's rule is that "large equals well connected," meaning that as the relative size of a brain structure increases it tends to receive more connections and to project more outputs to other structures.

Striedter (2006) adds a number of additional generalizations about the mammal and primate brains, including human:

11) six-layered mammalian neocortex probably evolved from a 3-layered reptilian precursor (something like that in turtles) called dorsal cortex by adding several layers;

12) aside from neocortex, the mammalian brain is similar to the reptilian brain (which also has hippocampus, for example) but even with a "fundamental scheme" of brain regions and circuitry, many minor changes in wiring can drastically change how information flows through a brain and thus how it functions--thus, the mammal brain is not just an upscale version of the reptilian brain;

13) increasing corticalization in mammals cannot be explained in terms of the above scaling (allometric) rules and involved highly specialized changes in brain anatomy presumably due to natural selection which expanded precursor sensory and motor cortical regions;

14) bird forebrains evolved along a very different path with expansion of their dorsal ventricular ridge (DVR), the major sensorimotor region of the avian telencephalon, highly similar in function to mammalian neocortex, making "many birds at least as intelligent as most mammals."

Striedter adds a number of points about the human brain in an attempt to identify features that make it special compared to the brains of other mammals.

15) In the six million years since bipedal apes (hominins) diverged from other apes, absolute brain size increased radically (about fourfold), not gradually, but in bursts--from when genus Homo first evolved, absolute brain size doubled from 400 to 800 cubic centimeters, then remained relatively steady in Homo erectus during the next 1.5 million years, but then exploded again in the transition to Homo sapien until about 100,000 years ago, at which time absolute brain size reached its current value of about 1,200 to 1,800 cubic centimeters. The first jump in Homo brain size was likely related to change in diet involving transition to meat and later the cooking of meat. The second leap was perhaps stimulated by competition among humans for mates and other resources;

16) the principle of "late equals large" predicts large neocortex in humans (the human neocortex to medulla ratio is twice that of chimpanzees);

17) the principle of "large equals well connected" is consistent with known expanded numbers of projections from human neocortex to motor neurons in medulla and spinal cord permitting greater precision of control over muscles serving hands, lips, tongue, face, jaw, respiratory muscles, and vocal folds, required for the development of human language about 50,000 to 100,000 years ago;

18) once human language appeared, dramatic changes in human behavior became possible without further increases in brain size;

19) increase in brain size has some disadvantages: including increased metabolic costs because the brain utilizes so much metabolic energy (20% of human metabolic energy even though it is only 2% of human body weight; being so metabolically expensive increases in brain size must be paid for by improved diet or reduction of other metabolic energy demands); decreased connectivity perhaps making the two hemispheres more independent of one another, and perhaps explaining why the two cerebral hemispheres became functionally specialized; and size limits of neonatal brain due to constraints imposed by size of the human mother's pelvis and birth canal. According to Striedter, these costs may explain why human brain size plateaued about 100,000 years ago;

20) within neocortex, the lateral prefrontal cortex has become relatively enlarged in the human brain, likely increasing its role in behavior (see Chapter 14 for discussion of the the lateral prefrontal cortex and higher cognitive functions);

21) some key evolutionary changes in brain structure were not caused by increases in absolute or relative brain size, such as evolution of the neocortex of mammals, but require additional explanation; comparing distantly related species on absolute brain size alone misses important factors, for example, brains of some large whales weight 5 times as much as the human brain but whale brains have poorly laminated and thin neocortex; in cases of distantly related species, comparisons of relative brain size are more useful, for example, human and some toothed whales have relative brain sizes significantly larger than average mammals of similar body size;

22) two general hypotheses about brain evolution are that individual brain systems evolve independently by natural selection (the mosaic hypothesis) or alternatively that components of such systems evolve together because of functional constraints (the concerted or constraint hypothesis), with a third view being that all brain evolution is simultaneously both mosaic and concerted.

One problem with Striedter's approach is that it doesn't explicate the forces of natural selection that may account for more specific features of brain evolution in specific species, including our own. He admits that evolution of the neocortex cannot be explained by evolution of bigger brains and that neocortex evolved independently of absolute brain size. However, aside from restating the theory that complex social life in ancestral humans, along with competition among humans for resources, stimulated the evolution of increases in the size and complexity of the neocortex, he offers little about what role natural selection played in the evolution of neocortex, or in brain evolution in general. As Adkins-Regan (2006, p.12-13) states in her critique of Striedter's work, "There is relatively little discussion of tests of hypotheses about the selective pressures responsible for the origin and maintenance of traits . . . The author would seem to be experiencing symptoms of discomfort with the concept of adaptation. . ..Given that brain mechanisms are products of natural selection, a central strategy in neuroscience should be to use the methods of evolutionary biology, which have been so successful in helping us to understand the mechanisms and design of organisms generally." In Chapter 14 of this text, on Intelligence, Cognition, and Language, consistent with this advice, you will find an extensive discussion of the role of natural selection in evolution of brain systems involved in intelligence and thinking.

Primate Life History

Life history refers to the pattern that an organism takes from conception to death. When compared to other mammals, primates have:

• longer gestation (pregnancy) periods
• reduced number of offspring (usually one, but some species commonly have twins)
• delayed maturation, with a long infancy and juvenile learning period, reflecting substantial brain development after birth
• longer lifespan

Figure \(\PageIndex{12}\): Chimpanzee mother and baby.

 

Primate Ecology

Primate ecology refers to the relationship between primates and their environment. Their environment includes not only the physical environment (e.g., trees, water, weather) but also the other animals in the environment, including other non-human primates and even humans. Why should we care about primates and their ecology? Remember that evolution is always environmentally dependent. Also, keep in mind that primates are not only affected by their environments, they affect their environments as well (by eating plants and insects, dispersing seeds, etc.).

Figure \(\PageIndex{13}\): Monkey feeding.

There are two primary environmental factors to consider: Food and Predators.

1) Food

• Primate researchers examine the quality, quantity, and distribution of food in a primate's environment.
• Why? Because food = babies. This means that those individuals who acquire more high-quality food more efficiently are likely to have more offspring and therefore will be more evolutionarily successful.
• There is greater pressure for females than for males, because of the higher biological costs of reproduction for primate females (reproductive asymmetry)--remember that primates spend a very long time as dependent nursing infants, so primate mothers have a heavy burden when caring for them (the only exceptions are some New World monkeys: the father cares for the infants, but the mother still nurses them).

2) Predators

• the types and distribution of potential predators in a primate's ecosystem is important.
• Why? Because no one wants to get eaten, right? Those who can most efficiently and effectively avoid predation are likely to have more offspring and therefore will be more evolutionarily successful.

Figure \(\PageIndex{14}\): Jaguar, a primate predator.

Other important environmental factors are:

• weather
• the distribution of water
• the distribution of sleeping sites, and
• the primate's relationships with other individuals within their social group, other groups of their own species, and other species (including humans).

Food

Figure \(\PageIndex{15}\): Baboon examining a leaf.

Most primates are herbivores (they eat plant foods) and are dietary generalists. Some primates are omnivores and eat lots of things (plant and animal). However, some primates are more specialized.

• Folivores: eat mainly leaves.
• Frugivores: eat mainly fruit.
• Insectivores: eat mainly insects.
• Gummivores: eat mainly tree sap.

Figure \(\PageIndex{16}\): Lemur eating fruit.

One of the challenges that primates face in their day-to-day life is a type of evolutionary arms race they have with their food. Some food items, like fruit, "want" to be eaten. In other words, it's evolutionarily advantageous for a fruit to be eaten by a primate, who then carries the fruit's seeds far away from the parent plant in its stomach, to deposit them elsewhere. The fruit benefits from this seed dispersal, thereby limiting competition between the parent plant and its offspring. This is one of the hypothesized reasons that fruits have high sugar or fat contents, to make them attractive to seed dispersers, such as primates and birds.

However, some food items, like leaves, don't "want" to be eaten. It's not evolutionarily advantageous for a plant to be stripped of its leaves. A leafless tree would not be able to perform photosynthesis and would promptly die. This is why plants have developed certain chemicals in their leaves that make the leaves unpalatable or indigestible to primates. In these cases, most leaf-eating monkeys prefer to eat young leaves, which are visually identifiable (either lighter in color or a different color) and higher in nutrients than mature leaves.

Some primates have evolved color vision in order to more successfully forage for food and other primates have developed specialized digestive systems to deal with low-quality (high-fiber) foods.

Figure \(\PageIndex{17}\): Black and white colobus monkey resting.

Activity budgets

How a primate spends its time is called its activity budget. These are comprised of the major categories of:

• foraging/feeding
• mating
• social behavior
• locomotion/traveling, and
• resting/sleeping

Why should we care about activity budgets?

Examining a primate's activity budget gives a good idea of how that particular primate (or primate group) "makes a living". For example, time spent foraging, gives clues about how much food in its area and how much time it has to spend searching for food. Eating meat provides a concentrated source of calories; meat eating after a successful hunt in meat-eating primates such as chimpanzees and humans leaves more time and calories for activities other than food gathering.

Remember also that if a primate has to spend more time eating or moving around in order to find something to eat, then it has less time for either resting or socializing. Primates only have a finite amount of time in their day, so they have to maximize the use of it the best they can within their environmental parameters.

Activity budgets denote when a primate is "making a living":

• Diurnal: active during the day, generally inactive at night. Most primates are diurnal.
• Nocturnal: active during the night, generally inactive during the day.
• Crepuscular: active during dawn and dusk. Ring-tailed lemurs (pictured below left) have this activity pattern.
• Cathemeral: active during irregular periods during day and night. Black lemurs (pictured below right) are one of the few primate species who have this activity pattern.

Figure \(\PageIndex{18}\): Ringtailed lemur.

Figure \(\PageIndex{19}\): Black lemurs.

Note that these activity patterns are not hard and fast rules. As with diet, there is a lot of variation in how flexible a particular species (or group within a species) is when it comes to their activity patterns. For example, although traditionally thought of as strictly nocturnal, the activity patterns of owl monkeys have been found to be highly sensitive to environmental factors such as temperature and amount of moonlight.

Food and Feeding Competition

One of the main factors that affects a primate "making a living" is feeding competition (competing with others for food).

There are two groups that a primate competes with;

• other primates in its social group (within-group competition)
• primates (of the same species) in other groups (between-group competition)

Figure \(\PageIndex{20}\): Macaques fighting.

There are also two types of competition. The type of competition prevalent in a situation depends on the quality and quantity of food available in the environment.

• Scramble competition: happens when there is a lot of low quality food. This is more common for leaf-eating primates, because trees tend to have large quantities of edible leaves (although some types, such as young leaves, may be preferable to others). It's an indirect competition in which whoever finds food faster or eats faster gets more food than do other individuals.
• Contest competition: happens when there is a small amount of high quality food. This is more common for fruit-eating primates, because a fruiting tree has a limited amount of high-quality fruit. It's a direct competition, where certain individuals (stronger, higher social status) get more food than do other individuals via squabbling or fighting.

In total, one can have:

• within-group scramble competition
• within-group contest competition
• between-group scramble competition, and
• between-group contest competition.

Feeding competition can be severe and have serious effects on the health, well-being, and reproductive capacities of primates. In other words, it has a direct effect on their evolutionary fitness. As expected, primates demonstrate a lot of behavioral strategies to deal with the feeding competition they face. For example, a group with high scramble competition may spread out more when feeding so as to access more resources OR in a group with high contest competition, some individuals may form coalitions in order to "gang up on" other primates to take their food.

Spatial use: home ranges and territories

The area that a primate uses is its home range. The part of the home range that's used most often is the core area. Some species, such as snub-nosed monkeys, have very large home ranges (32 sq. km) and other species, such as the pygmy marmoset, have very small ones (0.003 sq. km). A primate's home range has to contain all of the resources it needs in order to survive (food, water, sleeping sites, etc.). The home ranges of different primate groups often overlap, sometimes only a little, but sometimes a lot. If the home range is physically defended, it's a territory. Chimpanzees are famous for their territorial conflicts, which are so organized and violent that scientists have begun calling it "warfare".

Primate Behavior

The behavioral traits we share with other primates include:

• a greater dependence on flexible, learned behavior, reflecting neuroplasticity of the brain and corticalization (increased development of cerebral cortex)
• a tendency to live in social groups

Figure \(\PageIndex{21}\): Baboons crossing the road. Baboons live in complex social groups.

Primate Social Behavior

One of the fundamental traits that primates share is that we are social creatures. All primates share some form of social structure with others of their own species, whether that be in a large permanent social group or as an individual who has repeated short-term interactions with others. The social relationships that primates have with other members of their own species have a huge impact on the individuals involved and are incredibly important to their health, well-being, and reproductive success. This makes the study of primate social behavior very important.

Figure \(\PageIndex{22}\): Chimpanzees, our closest living relative, in captivity.

Primate behavior studies come in three formats:

• Captive: The animals are in captivity. Variables are easy to control in these situations (for example, the number of individuals in a group or food availability) and the primates are easier to observe at close-range. However, because of the artificiality of the situation, the primates may not be exhibiting normal behavior.
• Semi-captive (semi-free-ranging): The animals are captive, but in a very large area like an island or a fenced-in compound. Variables are still relatively easy to control and the primates are easier to observe (than in the wild). The primates tend to exhibit more natural behavior patterns in this type of setting than when compared to a completely captive situation.
• Free-ranging: The animals are living in the wild, in their natural environment. This is the most logistically difficult type of primate study. The animals are more difficult to observe (often requiring a habituation period to get them accustomed to the presence of observers). However, the primates are most likely to perform their normal array of behaviors in this type of study.

Social structure: the "whys" and the "hows"

The basic premise of why primates have certain social behaviors is due to reproductive asymmetry. Females are under a lot more pressure than males to forage effectively because of the biological pressures they're under (due to the burden of reproduction). It is precisely this inequality that causes the sexes to have different evolutionary priorities:

• For females, their priority is food. Females can only reproduce at a certain rate, due to the length of time it takes to be pregnant, raise an infant to independence, and have their bodies recover to do it all over again. Access to lots of good quality food through little effort is the key to accomplishing this task.
• For males, their priority is finding mates. Males can reproduce at a much faster rate than can females. The only real constraint on their reproductive rate is how frequently they can get fertile females to mate with them.

So, primate social structure works like this:

• male distribution follows how the females distribute themselves, and
• the females distribute themselves depending on how the food is laid out in their environment.

Social groups have two immigration/emigration patterns:

Figure \(\PageIndex{23}\): Baboon social group.

Female philopatry:

• Females do not emigrate (depart) from their birth group at sexual maturity.
• Males usually do emigrate from their birth group at sexual maturity.
• Females form the core of the group, are biologically related, and have tight social bonds (often exhibited through social cooperation and grooming: seen in these baboons, pictured right). Males have few positive social relationships.
• This is the most common form of social system in primates.

Male philopatry:

• Males do not emigrate from their birth group at sexual maturity.
• Females usually do emigrate from their birth group at sexual maturity.
• Males form the core of the group, are related and have tight social bonds. Females have few positive social relationships.
• This type of social system tends to occur when resources are widely dispersed, so females are widespread and difficult for males to consistently access.

Social strategies

 

In order to achieve their priorities, male and females must utilize certain social strategies.

Dominance

• As with chickens, primates have a "pecking order" or dominance hierarchy in their social groups.
• One's position in the dominance hierarchy often gives them access to preferred resources, including not only food and mates, but other resources such as sleeping sites and water.
• Sometimes dominance hierarchies are determined through fighting, but more often they are sorted out through a series of aggressive/submissive non-contact interactions, such as approach/avoidance, facial expressions, and body postures. Primates try to avoid direct aggressive contact in an effort to avoid risk of bodily harm.

Figure \(\PageIndex{24}\): Macaques grooming.

Cooperation

• Dominance hierarchies aren't completely linear. One's rank in the hierarchy often depends on who they can get to cooperate with them during conflicts. For example, Monkey 2 may be submissive to Monkey 1 when alone, but when her buddy Monkey 3 is around, the two of them cooperate and chase Monkey 1 away from food together. Therefore, Monkey 2's position in the dominance hierarchy is situationally dependent.
• These cooperative relationships are usually between relatives. In male philopatric groups, they're usually brothers. In female philopatric groups, it's often mother-offspring or siblings.
• The relationships are cultivated through affiliative behaviors, such as play, grooming, and other forms of body contact (e.g., hugging). So, primates practice You scratch my back, I'll scratch yours both literally and figuratively.

Why be in a social group?

Figure \(\PageIndex{25}\): Harpy Eagle. Predator of smaller primates. (Copyright; author via source).

So, why be in a social group if it is just one big mess of food competition, mate competition, and political posturing? Well, because there are benefits, of course!

Living in a social group:

• provides access to mates. While there is, of course, competition for mates within the group, a social group ensures that there are at least mates available to fight over.
• provides more eyes looking for food and more brains remembering where food is in the ecosystem
• provides anti-predator defense
• There are more eyes on the lookout for predators.
• An additional anti-predator benefit is known as "The Selfish Herd" (safety in numbers). It basically comes down to "you don't have to run faster than the predator, just faster than the other guy".
• Some social groups will "mob" predators and drive them away. This only works on smaller predators, especially those who rely on surprise attacks (such as the Harpy Eagle, pictured above).

Types of primate social groups

Solitary:

Figure \(\PageIndex{26}\): Galago. These small nocturnal primates are also called "bush babies" and are native to sub-Saharan Africa. Social groups consist of closely related females and their young. Males leave the group at puberty.

• Males are alone most of the time, except when seeking out mates. Females live with their dependent offspring.
• One male will have territorial overlap with several females.
• Some prosimians have this social system, like the galago, pictured above.

Monogamy:

Figure \(\PageIndex{27}\): Titi monkey.

• A social group is comprised of a male, a female, and their dependent offspring.
• This is also called "pair bonded".
• This is common in New World monkeys (for example, titi monkeys, pictured above) and the "lesser apes" (siamangs and gibbons).

Single-male multi-female:

Figure \(\PageIndex{28}\): Diana monkey.

• A social group is comprised of one adult male, several (to many) adult females, and their dependent offspring.
• Males that don't belong to a social group may either live alone or in multimale (bachelor) groups that have no females.
• It's a difficult life for the resident male in these social groups, because not only did he have to fight his way into the group (either driving the previous resident male out or stealing females from another male), he has to continually fight to keep other males out of his group, while keeping the females in the group.
• When males take over groups, some may commit infanticide (killing the previous resident male's infants).
• This is a common social group in Old World monkeys (like the Diana monkey, pictured above)

Multi-male multi-female

Figure \(\PageIndex{29}\): Hanuman langur.

• A social group is comprised of more than one adult male, more than one adult female, and their dependent offspring
• In these groups, in order to have preferential access to females, males will form dominance hierarchies or develop biological characteristics that the females find attractive. For example, during the mating season, squirrel monkey males begin storing large amounts of water and fat on their bodies.
• Some species (such as Hanuman langurs - pictured above) have both single-male and multimale-multifemale social groups in the same population.

Fission-fusion

Figure \(\PageIndex{30}\): Bonobo. Chimps and bonobos are our closest living relatives. Humans, chimps and bonobos descended from a single ancestor species that lived six or seven million years ago.

• In this situation, individuals belong to a large group called a community. Each community has a home range and consistent community membership. Within the community, individuals form temporary foraging groups called parties. Parties have an unpredictable membership. So, on any given day (or even part of a day), one cannot predict who will be foraging with whom.
• Females always travel with their dependent offspring.
• Sometimes males form partnerships and forage together.
• This type of social group is thought to have been an adaptation to environments with patchy, unpredictable fruit availability.
• It's found in chimpanzees, bonobos (pictured above), and spider monkeys.

Single-female multi-male

Figure \(\PageIndex{31}\): Tamarins.

• A social group is comprised of one adult female, more than one adult male, and their dependent offspring.
• This is rare in primates.
• Found in marmosets and tamarins (pictured right), this type of social system is thought to be an adaptation to the twinning (giving birth to twins) that it common to these small primates. The males carry the dependent infants.

Primate Evolution

Now that you have an understanding of living primates' morphology and behavior, it is time to learn about the origins of primates. the study of primate evolution is multidisciplinary in nature and incorporates data and methods from paleontology, geology, anthropology, and archaeology to study the fossil record of primates.

Fossils

Figure \(\PageIndex{32}\): Permineralized wood.

Fossils are at the center of the study of ancestral primates. Animal fossils provide insight into morphology and behavior of ancient organisms while plant fossils help to determine what the landscape may have looked like during the time that a particular organism, including primates, lived there (remember, evolution is environmentally dependent!).

When a fossil is found, it can be dated using a variety of methods. Some methods date the fossil itself; other methods use the surrounding indicators There are relative dating techniques that provide an order of occurrence, but no absolute dates, and there are absolute, or chronometric, dating techniques that provide dates (usually a range of dates) for an object.

Geologic time

From geology we have an understanding of the time involved in the development of earth. In this section we are specifically focused on the late Mesozoic/early Cenozoic eras in relation to primate evolution (see chart below).

Figure \(\PageIndex{33}\): Geologic time scale (Ma = millions of years ago).

Tracking Primate Evolution

The Mesozoic: the origin of mammals

The Mesozoic era is known as "the age of the dinosaurs" due to their ecological dominance at the time; however, it is during this era that the first mammals evolved, including a primate-like mammalian ancestor. Research using DNA analysis and fossils suggests that by 75 mya (million years ago) all of the mammalian orders had diverged.

The Cenozoic era

The Paleocene epoch

Proto-primates, primate-like mammals, evolved in the Paleocene epoch, about 65 mya. The proto-primates from this epoch are controversial; some argue that they are related to primates but are not actually primates (hence, "proto-primates").

The Eocene epoch

The Eocene epoch (56-33 mya), with its warmer and wetter climate and diversification of rainforests and flowering plants, sees the emergence of the first true primates.

Eocene primates were (Jurmain et al. 2013: 189):

• about the same size as modern squirrels, with prehensile hands and feet and more forward-facing eyes that gave them stereoscopic vision.
• widely distributed
• mostly extinct by the end of the Eocene
• most are not ancestral to later primates

The Oligocene epoch: the monkey/ape divergence (34 to 23 mya)

The global climate shifted again, cooling and drying, during the Oligocene epoch (33-23 mya). There is a reduction in the amount of rainforests, which retreated toward the equator, but an expansion of grasslands. So, there was an increase in terrestrial (ground) niches and a decrease in arboreal (tree) niches. While most of the fossils from this time are Old World monkeys, some are ancestors of New World monkeys; at this point the American continents had separated from Africa and Eurasia.

The Miocene epoch: the Old World monkey/ape divergence (23 to 5 mya)

There were two climate shifts during this period, first there was a warm period with heavy forestation, followed by drier and cooler climates with decreasing forest and increased grasslands.

At this point, the Old World monkeys and apes split. The fossils are from east Africa.

The ape ancestors had the following traits:

• no tail
• arboreal quadrupeds (limb structure was still monkey-like)

The Old World monkey ancestors at this point in time had the following traits:

• A tail
• Had macaque-like faces
• Were medium sized 7-11 lbs

Where did primates come from?

There are three primary hypotheses about the origins of primates. The earliest hypothesis, the arboreal hypothesis, claims that the first primates evolved a suite of traits for living in trees, e.g., grasping hands and feet and stereoscopic vision. This hypothesis held sway from the early 1900s until the 1970s when the visual predation hypothesis was proposed. This hypothesis suggests that primates evolved because they provided an advantage for hunting small insects, e.g., orbital convergence that allowed for 3d-vision. Shortly thereafter, the angiosperm hypothesis was proposed, which claims that primate evolution is related to the adaptive radiation (diversification into different ecological niches) of angiosperms or flowering plants. Each of these hypotheses have been challenged over time, and it may be that each may play a part in explaining primate evolution.

The Taxonomy of Humans

At this point, it is useful to review taxonomy, the biological classification of organisms, using the human species to illustrate the taxonomic classifications.

Figure \(\PageIndex{34}\): Taxonomic classification of humans illustrating taxonomic categories used for all living things. (Image from Wikimedia Commons; File:Biological classification of Humans Britfix.png; https://commons.wikimedia.org/wiki/F...ns_Britfix.png; by L Pengo PD-User, Modified by Britfix; made available under the Creative Commons CC0 1.0 Universal Public Domain Dedication).

Summary

Humans are primates and primates are mammals. Both mammals and primates have distinguishing features, anatomical, physiological, and behavioral. One distinguishing feature of mammals with direct relevance for behavior is that only mammals have six-layered neocortex (cerebral cortex), while non-mammals lack six-layered cortex. Larger brains tend to be found in larger animals. Cerebral cortex developed as a result of natural selection, perhaps related to the increased information processing demands of living in complex social groups, although this is only one theory of the evolutionary origins of cerebral cortex. Brains are metabolically expensive organs--they require a lot of metabolic energy (in humans, the brain is only 2% of body weight, but it consumes 20% of the body's metabolic energy). Larger brains in human ancestors requiring more metabolic energy may have come about after ancestral humans began to eat meat, a concentrated source of calories and nutrients, and later to cook the meat making it easier to digest, leaving more metabolic energy from the meat to support energy requirements of bigger brains. In subsequent modules, we consider more details of hominin and human evolution.

References

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Jurmain R, Kilgore L, Trevathan W. (2013). Essentials of physical anthropology, 4th edition. Belmont (CA): Wadsworth, Cengage Learning.

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Peppe DJ, Deino AL. (2013). Dating rocks and fossils using geologic methods [Internet]. The Nature Education Knowledge Project [cited 2105 Jul 06]. Available from: http://www.nature.com/scitable/knowledge/library/dating-rocks-and-fossils-using-geologic-methods-107924044

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Attributions

"Primate Evolution in Summary" adapted and modified by Kenneth A. Koenigshofer, Ph.D., Chaffey College, from Explorations: An Open Invitation to Biological Anthropology; https://explorations.americananthro.....php/chapters/; Chapter 8: Primate Evolution by Jonathan M. G. Perry, Ph.D. and Stephanie L. Canington, B.A.; Editors: Beth Shook, Katie Nelson, Kelsie Aguilera and Lara Braff, American Anthropological Association Arlington, VA 2019; CC BY-NC 4.0 International, except where otherwise noted.

Adapted and modified by Kenneth A. Koenigshofer, Ph.D., Chaffey College, from Biological Anthropology (Saneda and Field), chapters:

Modern Primates by Tori Saneda & Michelle Field via LibreTexts

Primate Ecology by Tori Saneda & Michelle Field via LibreTexts

Primate Evolution by Tori Saneda & Michelle Field via LibreTexts

"Brain Evolution" adapted by Kenneth A. Koenigshofer, Ph.D., Chaffey College, from Striedter (2006).