Anthro 101: Human Biological Evolution

Lecture 6: Primate Anatomy and Social Behavior

Basic primate phylogeny

Lemurs, lorisesTarsiers

ProsimiiAnthropoideaNew World Monkeys

Old World Monkeys

Apes & humans

Haplorhines

Strepsirrhines

Primates

(Hominoidea)

Forms of Locomotion

• Slow quadrupedal climbing• Vertical Clinging & Leaping• Arboreal & Terrestrial

Quadrupedalism• Semi-brachiation• Brachiation• Knuckle walking• Bipedalism

Primate Skeleton - generalized

http://www.bristol.ac.uk/centenary/look/cabinet/monkey.html

Skeletal features to noteas you do the in-classactivity:• Body position• Spine shape• Length & shape of trunk (torso)• Position of shoulder blade (scapula)• Limb length• Finger length (thumbs)• Length of tail

Intermembral Index

• humerus + radius x 100femur + tibia

• hindlimbs vs. forelimb longer in the part that does the most work

• Bipeds & vertical clinging and leaping hindlimb longer than forelimb

• Quadrupeds Approx equal lengths

• Brachiators forelimbs longer than hindlimbs

Slow quadrupedal climbing (prosimians)• Slow lorises of South Asia• Robust muscles & limb bones• Mobile ankle, wrist, hip• Short tails

Vertical clinging and leaping - prosimians

• Vertical body Orthograde

• Push off branch with hindlimbs• Turn midair and land vertically

again

• Evolved two or three different times in prosimians

• Bush babies• Sifakas• Lemurs

Sifaka

Arboreal quadrupeds – monkeys• Arboreal - run & leap• Walk on palms on top of branches• Push off with hindlimbs• Stabilize with tail (long)• Pronograde (horizontal) posture• Longer fingers & toes to grab branch• Prehensile tails in NWM • No terrestrial species in NWM Owl Monkey

Squirrel MonkeyDiana Monkey

Semibrachiation

• New World Hands + prehensile tail

• Old World Hands, no tail use

Terrestrial quadrupeds - Old world monkeys

• Lots of time on ground, but also in trees• Pronograde (horizontal) posture• Walks on palm/toes (shorter digits)• Move limbs in limited plane

(baboons, dogs)• Rarely hang full weight from arms• Shorter tail – not balancing• Barrel shaped rib cage

baboon

Mandrill © Robert Young

Brachiation - gibbons

• Shoulder blade on the back rather than top of torso

Full range of arm motion• Cone-shaped rib cage• Long, curved fingers• Small thumbs• Long arms• Ball and socket wrist• No tail

Siamang

Slow quadrumanous climbing - orangutans

• Grasp branches with hands and feet as slowly move through trees

• Females almost totally arboreal• males distance travel on the ground

Fist walking rather than knuckles

• Brachiating ancestor Longer arms than legs Cone shaped torso No tail Long fingers

Knuckle-walking - chimps & gorillas

• Great apes too large to brachiate frequently Still use upper body & arms, suspend

body when they do• Stronger wrists bones than brachiators in

order to support weight on arms

• Brachiating ancestors Longer arms Longer fingers No tail Scapula on back Cone shaped torso

Chimpanzee

Hominoids: Homininae: Hominini Bipedalism

• Unique to humans (hominins)

• Modifications to: Pelvis Knees & legs Feet

Changes in the pelvis narrowly center our weight

Human pelvis is a bowl Supports internal organs Short and broad ilium Shorter ischium abductor muscles attach to

wide surface of ilium Maintains the center of gravity

over one foot while walkingChimp pelvis is longer and narrower

Organs “hang” below Strong muscles on ischium

• “S-shaped” curves in spinal column keep trunk centered over pelvis

• Lumbar curve

• C-shaped curve in chimps more

Changes in the spine center our weight over the pelvis

To walk efficiently, knees must be close to center line of body

• Pelvis wider and shorter• Femur slants inward from

wide pelvis• Close-knee stance• Centers weight for balance

and efficiency (less muscle work to hold upright)

• Long femur neck = more surface for abductor muscle attachment

• Longer leg bones Increased stride chimp human

• Arched foot “spring”-like shock

absorbers Return energy to the next

push Heel-toe stride

• Loss of opposable toe necessary to act as a platform for body - big toe in-line

• Stiff foot makes better platform

Bipedality also causes changes in the feet

Comparative anatomy of manipulation

• Apes long curved fingers & short thumb

Hook over branches

• Humans shorter fingers & longer thumb

Allows precision grip and greater opposability of hand

Bones in palm rotate more to allow finger tips to meet the thumb

Human hand

Changes in the skull: Foramen magnum• Hole in skull where spinal cord and brain connect• Position indicates body posture• Nuchal ridge on back of skull (occipital bone) show

where muscles attachHuman skull is balanced on top!

Brain Size

Lemur

Howler monkey (NWM)

OWM

Features of the skull

Prognathism

Enclosed eye

Orbits

Dental arcadeDiastemaCanine size

Why do primates live in groups?

• Benefits of group life

• Costs of group life

• Why do primates live in so many kinds of groups?

Balancing costs & benefits

• How do:

Resources (diet), predators, climate, and “conspecifics” affect this balance?

1. Why do primates live in groups?

• Most mammals are solitary

• Many prosimians solitary

• Why are diurnal primates

social?

Major benefits of group life• Protection versus predators • Better access to resources• Access to potential mates

Who likes primate snacks?

leopard

These guys do!

Some larger primates also like primate snacks

Chimps hunt red colobus monkeys Baboons prey on vervets

Humans kill primates, and sometimes eat them

We don’t see predation very often, but there are signs indicating why someone’s missing

leopard paw print

drag marks

baboon jaw & hair

Severe injuries are also likely caused by predators

• Wound observed • Healthy animals disappear

overnight

Oryx, back wound

Juvenile, scalp wound

For diurnal primates, living in groups is an effective anti-predator strategy

= the Predation Model

1. Detection

2. Dilution

3. Defense

Detection: In larger groups, there are more eyes to watch out for predators

Dilution: In groups, any particular individual less likely to be caught by predator

Imagine chance of being caught = 1/n, where n = group size

risk = 1/2

risk = 1/12

Defense: Many strategies for diurnal primates

• Sleep in trees, cliffs• Defensive weaponry • Large body size • Vigilance• Alarm calls• Mobbing• Interspecific associations

Nocturnal primates use different strategies

• Hide during day• Park infants while feeding• Solitary• Quiet• Cryptic

Living in groups also has costs 1. Competition

2. Contagion

3. Cuckoldry

4. Inbreeding

5. Cannibalism

6. Infanticide

2. Natural selection shapes social organization to balance the costs & benefits of group living

• Socioecology = study of how ecological forces shape the size and structure of social groups

Solitary (but differentiated social relationships)

OrangutanLoris

• each individual lives alone, occasionally meet up for mating

• may chose to neighbour with kin, meet more often

Monogamous (territorial pairs + offspring)

Titi monkeys

Gibbons

Mountain gorillas

Black and white colobus

One-male, Multi-female groups (polygyny)

Pygmy marmoset

marmoset

Callitrichids

One-female, two-male groups (Polyandry)

Ring-tailed lemurs

Savanna baboons

Multi-male, multi-female groups

Spider monkeys

Chimpanzees

Communities (fission-fusion social organization)

Some argue that primates live in groups to better compete for resources

= Resource Defense Model

Between group competition

To understand competition you need to understand:• Diet – How big is the species? How much space is needed

to find enough food?• Food distribution –Where & how does the food grow?

• Female reproduction – Timing & Frequency?• Protection – Are males a threat to females & infants? Are

males needed for protection?• Affect what kinds of groups primates will form and when

Body size affects required quality & amount of food in diet

• Larger bodies need more food, but less energy per pound

can get enough by eating lots of lower quality leaves

• Smaller bodies need less food, but more energy per pound

To get enough need smaller amount of high quality insects, fruit

The distribution of food affects the type of competition

• Dispersed --> scramble competition Food is distributed evenly Food items not worth fighting over Scramble to get enough food no direct competition

• Clumped --> contest competition Resources are scarce & valuable Resources are worth fighting over Contest access to particular resources

Groups will form based on the species’ diet

• Is a food defensible? If yes, then species will form groups that cooperate to defend food resources.

• Larger groups will defend more successfully than small groups

• Larger groups need more resources than small groups

• Defend territories• Defend resources within homerange