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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