Download Chapter 6 - Cynthia Clarke

Chapter 6
Primate Behavior and Ecology
Overview
• Humans have a fascination with the behaviors of non-human primates.
o
It is not uncommon for the primate exhibits at zoos to be among the most popular.
o
For researchers, many much of the earliest research was conducted in zoo, but this shifted as the realization of
natural habitat research is critical to understanding primates.
• Next, then primate studies focused on habitat.
o
The focus is on habitat, specifically contrasting terrestrial and arboreal species.

Arboreal-terrestrial contrast suggests some basic relationships between habitat and social organization,
territoriality, group size, and home range (size of the geographical area normally occupied by a group).

Simple contrast in habitats, however does not fully explain variation in primate behavior.
o
Additional studies showed that the arboreal-terrestrial focus did not explain all the variation in primates and new
studies cropped up.

Crook and Gartlan (1965) and Jolly (1972) suggest breaking down primate species into groups defined by
habitat and diet.

Other research focuses on food density, mating strategies, and predation to help explain differences in primate
behavior and social organization.
Primate Behavioral Ecology 1

Primate studies by the mid-1980’s had shifted toward behavioral ecology (the study of behavior from an ecological
and evolutionary perspective; strategies).
o
“Strategy” in behavioral ecology studies is understood to mean behavior that has been selected for because it
increases an individual’s overall fitness.
o
It does not imply a conscious decision.
•
Today, primate studies most often use an ecological approach to explain variation in primate behavior between
different species and within a single species.
o
Primate behavior is best studied from the perspective of behavioral ecology, which looks at the ecology and
evolution of traits by focusing on their adaptive value and by placing behaviors in the context of trade-offs
between costs and benefits.
o
Primates are highly social and have strong mother–infant bonds. All primates strengthen social bonds through
grooming.
o
Primate societies have dominance hierarchies, although their influence on social interactions is variable.
o
Dispersal affects social relationships that develop between kin. Studies of the behavior and ecology of individual
primate species reveals a great deal of social behavior.
Primate Behavioral Ecology 2
•
Advantages of being social
o
One illustration of the value of behavioral ecology is that which addresses the question: Why social groups?

Advantages to being social.

Facilitation of reproduction.

Predator avoidance and defense.

Foraging.
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The learning process.

Division of labor and females do slightly different things.

Disadvantages of being social. There are some obvious disadvantages to being social.

The most important is the obvious increased competition for resources by members of a group. This
applies to food and other resources
o
The set of trade-offs represents a primate strategy

A strategy is a behavior that has been favored by natural selection and that increases an individual’s fitness.

The following slides represent a number of common primate strategies.
Primate Behavioral Ecology 3
•
Reproductive strategies
o
Reproduction is central to evolution. And it is useful , in terms of behavior, to examine the costs and benefits
associated with reproduction
o
Life history theory deals with characteristics of an organism’s life cycle and how they affect quantity and quality
of reproduction.

Age at reproduction, gestation length, interval between births, and overall life span are some considered
characteristics.

Energy allocation is a central concept of life history.

Focus is on the tradeoff between energy needs of the number of offspring and their fitness.

Some animal species have ‘fast’ and ‘slow’ life histories.

Fast life histories are those where individuals reach maturity early and have large numbers of offspring.
This is often called a r-selected species

Slow life histories are those where maturity is delayed until later in the lifespan and the number of
offspring is lower. This is often called a K-selected species
Primate Behavioral Ecology 4
• Reproductive strategies (continued)
o
Primates generally have ‘slow’ life histories; are K-selected

Basically this strategy requires: More efficient fetal nourishment; longer periods of gestation; reduced number
of offspring; delayed maturation; extension of life span and less widely distributed to reduce loss of offspring
in novel environments.

The mother-infant bond

Evolutionary explanations often focus on the maximizing of fitness.

A related concept is that of parental investment, the set of behaviors that increases the probability that
offspring will survive

The mother-infant bond is the single most important primate behavior and represents a huge energy
investment.

This bond often lasts beyond infancy

This bond is more than the need for nourishment and has been shown by Harlow’s work

Psychologist Harry Harlow’s maternal depravation studies with rhesus monkeys and surrogate
mothers confirmed the importance of motherhood beyond nutrition. Warmth and comfort were
necessary as well.

Harlow’s experiments clearly demonstrated the importance of social contact with peers, showing
abnormal behaviors in monkeys kept from other infants.

The isolated primates lacked both the opportunity to learn and the important experiences associated
with physical contact.

Harlow’s findings suggest that behaviors associated with successful motherhood are somehow innate.
Primate Behavioral Ecology 5

Reproductive strategies (continued)
•
Adult males and infant care

Adult males and infant care is an important focus for research, even as we see that the mother-infant bond is
the strongest among primates.

Paternal care is variable between primate groups both in its presence and its intensity

Some monogamous primate species (those with a bond between a single male and female) show the most
intensive levels of infant care by males than those which are polygamous species (those with a bond between
a single male and 2+ females)
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If a male is the father, caring for an offspring makes sense to increase his reproductive fitness.

There is a risk for the male who nurtures another male’s offspring, though
•
It is not that simple, though.

Some males in monogamous societies engage in little parental care

Females have choices

In monogamous species, females have been seen engaged in sex with more than one partner

It seems likely that in all cases, infant care by adult males is sometimes a function of females
choosing mates in that females in polygamous species see females mating more with nurturing males.
Relethford Chapter 6 Page 2
Primate Behavioral Ecology 6

Reproductive strategies (continued)

Alloparenting

Parents are not the only individuals who care for an infant. When one watches primate social groups you can
see the evidence of this

When an individual other than a parent cares for an infant this is called alloparenting. Alloparents are those
who care for an infant but are not their biological parents.

Primate females are often alloparents,

More often those who have not given birth than those who have not

This is often adaptive if the natural mother dies.

Also gives a break to the mother with alloparental care

Alloparenting increases one’s genes being passed to next generation

I often joke that I am engaged in alloparenting when I give presents to my nieces and nephews (am I really
joking?).
Box 6.1: Infanticide in Nonhuman Primates

Infanticide is the killing of an infant.

A male can increase his own reproductive success if he kills all the offspring of the male he vanquished.

Adult males are often observed doing this in some nonhuman primate species. While advantageous for the
individual male, is counterproductive for the species.

Sarah Hrdy conducted work with langurs (monkeys) where there is social group composed of only 1 male.

When a new male takes over the group, he often kills all the infants

It is thought that the single adult male kills all infants fathered by a previous male to increase his own
fitness.

Also females go back into estrus more quickly if they are not parents

Other theories have been proposed to explain this behavior as well.

Overcrowding has been suggested as a trigger for deviant behavior

Insanity has also been suggested

What does the data show?

In one study of 16 langur deaths all were among infants not related to the dominant male

This supports the adaptive strategy explanation.

Growing Up

The extended period of infant and juvenile growth in primates is of extreme importance for learning motor skills and
social behaviors.
o
The mother is not the only means by which a child learns
o
The process of socialization is largely dependent on close contact with peers
•
Social Groups
o
Primates are essentially social creatures, but there is variation in the size and structure of their primary social
groups.
o
A social group is a group within which there is frequent communication or interaction among members.
o
Most primate species live in large, complex social groups.
•
Types of social groups:
o
One-male/one-female groups have a single adult male and a single adult female in a monogamous relationship
and their offspring.
o
There are two types of social groups which consist of a single male and multiple females:

One-male/multifemale groups have a single adult male and several adult females in a monogamous
relationship and their offspring.
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One-female/one-male groups have a single adult female and several adult males in a monogamous
relationship and their offspring.

Multimale/multifemale group is the most common type of primate social group. These groups are quite
complex, and mating may be promiscuous.

Remember, though, these categories can vary within a species
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Social Behaviors 1
•
Affiliative and agonistic behaviors
•
Affiliative relationships are those indicating strong and friendly bonds and are important to maintaining large
social groups.

Grooming is a type of behavior among all nonhuman primates involving the practice of handling and cleaning
another individuals fur. It is a form of social activity.

Think of when you rub your child’s back to put them to sleep.

Only occasionally is grooming allogrooming (self-grooming); it is usually a way to cement social bonds.

Altruism is a form of risking oneself in social groups and is often analyzed in relation to evolutionary fitness.

Altruism is sometimes analyzed relative to kin selection and its genetic consequences

Idea that one will sacrifice self for sake of kin

Sacrificing oneself will contribute their alleles and save the offspring and part of their DNA.

There is a from of altruism that is NOT related to kin: reciprocal altruism

Reciprocal altruism is behavior that is directed at non-kin if it increases the probability that the
recipient will reciprocate

One hand washes the other.

Social Behaviors 2
•
Affiliative and agonistic behaviors (continued)
•
Agonistic relationships are those indicating unfriendly and often, aggressive relationships.

Conflict within a group develops out of competition for mating partners and food items.

This can be as serious as death or more display-based

Most intra-group aggression occurs in the form of signals and displays within the context of a dominance
hierarchy.

Primates resolve the majority of tense situations using submissive and appeasement behaviors.

Between groups, aggression is used to protect territories

Primate groups are associated with a home range (area where they remain permanently)

Within the home range is a core area

The core area is portion of a home range containing the highest concentration and most reliable
supplies of food and water.

The core area is defended.

Generally, territoriality is associated with species whose ranges are small enough to be patrolled and protected

Some species are not territorial

Some are not aggressive to other primate species
Social Behaviors 3
•
Affiliative and agonistic behaviors (continued)
•
Dispersal and social behavior

Dispersal occurs when an individual leaves their birth group and moves to another social group.

It is important because it reduces inbreeding, and introduces new genetic material.

It also changes social relationships and may reduce competition of males for mates.

In many primate groups males move, but females move in other groups, and even both males and females of a
species can move.
•
Social organization and dominance

Primates are essentially social creatures with variation in size and structure of their primary social groups.

Nonhuman primate societies rank individuals in terms of their relative dominance in the group.

A dominance hierarchy is the ranking system within the society and reflects which individuals are most and
least dominant.

They are found in most nonhuman societies but vary widely in importance

Usually ruled by those with the greatest access to food or sex or those who control the behavior of others
to the greatest extent.
Social Behaviors 4
•
Affiliative and agonistic behaviors (continued)
•
Social organization and dominance
Relethford Chapter 6 Page 4
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When there is strong male dominance, there is also often the expression of sexual dimorphism in body size.
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There is a strong competition for females

Males are larger and those pass this on to offspring
Whose genes?

In a study of baboons, 81% were the offspring of the top-ranking male
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This is a short-term finding, as males gain/lose position over time
What affects dominance rank?

Individual’s size, age, and ability to form alliances

In some species, the social position of the mother is important

In some species, female hierarchies are more stable and can be strong, even with strong male hierarchies
in place
Case Studies of Primate Behavior 1
•
Strepsirhines and monkeys
•
This book provides 3 examples of the diversity in this diversity.
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Lemurs of Madagascar

Different from other living prosimians (lorises and tarsiers), primarily do their isolation
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A large proportion (more than 25%) live in monogamous groups with one adult male and one adult
female

Which may be an adaptation to food resources
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These resources are patchy, but regularly distributed

Female dominance is prevalent
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Among other prosimians males tend to be dominant
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Female lemurs aggressively defend territory (even with offspring clinging to backs)
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Males eat after females
Case Studies of Primate Behavior 2
•
Strepsirhines and monkeys (continued)

Howler monkeys
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Found in Mexico and in South America and consist of 14 different species, 13-18 pounds
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Social structure

Adults are much larger than females and males make deeper, louder noise

Small groups of 10-15 with 1-3 adult males

Make sounds that can be heard at a considerable distance.
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This is made possible by an enlarged hyoid bone in the throat

Serve many purposes, including warning and defense.

Might use vocalizations in replacement of active fighting
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Tend to defend where they are as compared to a specific delineated territory (defined as a home range
that is actively defended).
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Here is a YouTube of howler monkeys vocalizing
Case Studies of Primate Behavior 3
•
Strepsirhines and monkeys (continued)

Baboons
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One of the most widely studied and interesting.
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Found on the African savanna.
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Social structure

Live in large multi-male/multi-female social groups 20-200)

Adult males are dominant and aggressive actions play a role in this continual struggle.

Males often form coalitions.
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Males are significantly larger (50 pounds) than females (25 pounds)
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Home range

They have large home ranges

They forage as a group and eat a diverse diet of both plants and small mammals & birds
Relethford Chapter 6 Page 5
The environment affects social hierarchies; among forest-living baboons, the dominance hierarchies are
less rigid
Adults females tend to remain, offering stability, and males often move.


Case Studies of Primate Behavior 4

The Asian apes
•
Gibbons
•
Gibbons are found in the tropical rain forests of Southeast Asia (Thailand, Vietnam, Burma, and the Malay
Peninsula).
•
Some researchers split the gibbons into 2 groups: the gibbons and the siamangs.
•
The gibbon is the smallest of the living apes (11-33 pounds) and represent 12 species
•
The physical traits of gibbons reflect adaptation to live in the trees.

Their usual form of movement is brachiation (hand-over-hand swinging) from branch to branch.
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Their arms are extremely long relative to their trunks and legs.

Their fingers are elongated and their thumbs are relatively short.
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Many gibbons have a monogamous family structure (adult male, adult female and offspring).
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But may have evolved out of a multi-male/multi-female structure

And there is greater variation is social structure than originally thought

Territory

They actively defend territories.

Food resources are not clumped together and so groups come into frequent contact

There is little sexual dimorphism
Case Studies of Primate Behavior 5
•
The Asian Apes (continued)
•
Orangutans
•
The orangutan is a large ape found only in certain areas of Southeast Asia (Sumatra and Borneo).
•
The word orangutan means “man of the forest” in Malay.
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There they are endangered
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Physicality
•
Its most obvious physical characteristic is its reddish brown hair.
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Males (190 pounds) are about twice the size of females (86 pounds)
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Another signal of sexual dimorphism is seen in the cheek pouches of males
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Locomotion is by ‘cautious climbing’, but young do brachiate
•
Orangutans are agile climbers and hangers and are largely arboreal.
•
They slowly rock the tree they are on in the direction of the next and move over when they are close
together.
•
They spend more time on ground than was previously thought
•
Social structure
•
Orangutans tend to be solitary.
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Their primary social group consists of mother and infant.
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Males are not needed for protection as there is little danger of predation.
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They are polygamous.
•
Small group size may reflect an environment with widely scattered food resources.
Case Studies of Primate Behavior 6
•
The African Apes
•
Gorillas

They are found only in equatorial Africa and only in the forested areas in Africa.

Physicality

The gorilla is the largest living primate, with males at (350 pounds) and females (158 pounds)

Males also have large canines and at maturity their black hair turns silver (silverbacks)

Social structure

Gorillas live in small social groups of an adult (silverback) male, several adult females, and their
immature offspring.
Relethford Chapter 6 Page 6
The adult silverback is dominant.
Locomotion

They are predominantly terrestrial and are knuckle-walkers (walk on all fours with the weight of the arms
resting on the knuckles of the hands).

Stay up and beat chests as a warning to intruders.
Diet

Despite many myths concerning their desire for human flesh, they eat a diet of primarily leaves and fruit.

Have been seen using a digging stick
Threats

Poaching and encroachment by humans has caused their range to rapidly disappear.

Because of their size, gorillas have few problems with predators (except for armed humans).




Case Studies of Primate Behavior 7
•
The African Apes
o
Chimpanzees
•
Chimpanzees are found in Africa in rainforests or in mixed forest-savanna environments.
•
Chimpanzees are perhaps the best known of all nonhuman primates.
•
Most of us experience captive or trained chimpanzee behavior and consider it as being “just like human.”

Social structure
•
Chimpanzees live in large communities of 50 or more individuals.
•
Their social structure constantly changes.
•
They recognize and interact with others in thee group.
•
The most important social behaviors revolve around mother and infant. Adult males are generally dominant.

Locomotion
•
Like gorillas, they are knuckle-walkers with longer arms than legs, but they are more agile than gorillas.
•
They are both terrestrial and arboreal.

Diet
•
Their diet consists of fruit, leaves, seeds, nuts, insects, and meat (small animals like monkeys).
•
Famous for termite hunting
•
Now seen making spears and hunting bush babies (another primate)
Case Studies of Primate Behavior 8
•
The African Apes
o
Bonobos
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They are unique to a restricted rainforest region of Zaire, central Africa.
•
Bonobos are the least well known of the African apes.
•
They are closely related to the chimpanzee, and commonly considered a separate species of chimpanzee
known as “pygmy chimpanzee.”
•
Diet: They eat a leaf and plant diet.
•
Physicality
•
Bonobos are distinguished from chimpanzees by their relatively longer legs, narrower chest, and higher
center of gravity.
•
Their forehead and face are also shaped differently.
•
They can walk upright more easily than other apes.
•
Males weigh 95 pounds and females 73 pounds
•
It has been suggested that the first hominoids may have been quite similar in many ways.
•
Social structure
•
Bonobos live in large multimale/multifemale groups like chimpanzees.
•
Strongest social bonds exist between adult females and they are dominant (even though smaller than
males)
•
Sexual activity is often for the purpose of decreasing tension or cementing bonds
Box 6.2: Why Do Male Chimpanzees ..?
•
Social structure and testes size in primates
•
Size ranges from 0.04 ounces to about ¼ pound in chimps (who are known for their large sexual prowess)
Relethford Chapter 6 Page 7
•
•
•
Body size alone does not explain testes size

Humans are 2/3 the size one would predict by body size

Chimpanzees are 2.5 times larger than predicted
Findings

Those species with multimale social groups tend to have larger testes size

Single adult male groups tend to have sizes below body weight prediction
Why?

Competition for mates in that larger testes means more sperm?

Seasonality of mating?
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