Monday, Sept. 9 -- Variability in Social Systems & Behavior:
Problems and Methods

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Tinbergen's 4 Questions-- ways to answer the question"why"

The immediate factors that influence behavior
Development of individual- maturation or learning
What the behavior contributes to survival or reproduction
Evolutionary history

Answers are also described as 'proximate' or 'ultimate':
Proximate- In terms of causation
Ultimate- In terms of function

Example: "Why do some birds sing?"
Causation; Change in daylight hours triggers hormonal changes
Development; A "sensitive period" when young enables them to learn songs from their elders
Function; To defend territory and attract mates
Evolution; Songbirds have the same evolutionary ancestor- others don't.

We'll focus on functional explanations in here.

The Scientific Method

  1. Observe
  2. Formulate hypothesis
  3. Generate predictions based on hypothesis
  4. Test predictions through empirical observations
  5. Either reject or tentatively accept the hypothesis
  6. Repeat steps 1-5 indefinitely

Three ways to do step 4

1. Controlled experiments; good, but sometimes not possible.

Best to only vary ONE variable at a time. Example; singing and territoriality. Theory was that singing defends territory. They went out where birds' territories were established and put up some speakers with territorial song, some speakers with equally loud but meaningless noise. Birds moved right away into the area with just noise, but not as quickly into the areas with canned song until later.

2. Comparative Method; use correlations between two variables to infer causality.

Mostly developed by Darwin. No one got serious about using it for social behavior til the 60's when Crook looked at weaver birds. Some are solitary, some flock. Some nest apart, some together. Some monogamous, some polygamous. He noticed that they're mostly split into two categories;
Category HabitatDietSocial LifeTerritorial?NestsSex Life
1 forest insects solitary territorial cryptic monogamous
2 savannah seeds colonial non- conspicuous polygamous
He decided density of food and predation caused the birds to split up into these two groups. See ch. 2 of book for details.
Then they tried to apply the same analysis to primates. See chart in book. It's a little messy. Not all primates fit neatly into the same categories. Best to just try to control for compounding variables.

Another example: Range size of Macaques (fruit-eaters) and Leaf Monkeys (leaf-eaters). Leaf-eaters have smaller ranges. You might think fruiteaters have larger ranges since it's harder to find fruit, but this ignores the fact that bigger animals need more food so more range. When compared to body-weight, actually leaf-eaters have larger ranges than fruit-eaters. Doing this type of comparison will factor out body weight and keep just comparisons between diet.

remember, correlation doesn't mean causation

Another problem you might encounter: Comparing among species might be misleading since some families have more branches therefore more data points. So do comparisons among best group; family, order, genus, etc. Also, figure out what evolutionary ancestry the animals you are comparing have, and then do the comparisons on the entire family, ancestors and all. See pg. 36 on how to do this properly.

3. Cost-Benefit Analysis; to study behavioral 'decisions' that animals make.

Every behavior has costs and benefits. Uses time, energy, at least. Since animals were designed by natural selection, you'd expect them to do the thing that has the best ratio. Gibbons defend territory, orangutans do not. Why does it cost more for orangs? Maybe 'cause they're slower, have bigger territories, are solitary animals.
To do a proper comparison, you must be able to measure the costs and benefits. They must be in the same currency, i.e. hours, deaths, calories etc.

Wednesday, September 11 -- Natural Selection

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The problem: How does one explain the great functionality of so many different species?
The answer: Natural Selection

Definition of natural selection:
Differences in reproductive success that result from heritable differences between individuals.

Particulate heredity/Mendelian genetics:
Genes do not all blend: some behave as we think today, i.e. dominant/recessive. When people put together the gene discoveries along with Darwin's ideas they got it.

3 Types of Natural Selection:

Stabilizing Selection-- The extremes are selected against.
Example: height; mostly beings tend to the average height- not too many really short ones or really tall ones.

Directional selection-- One extreme value is selected for.
Example: speed; faster is always better so a population will tend to get faster over time.

Disruptive selection-- The extremes are both selected for.
This type of selection is not as common as the first two. Example: Prey-type animal with distinctive markings which the predators know will over time move away from the norm in both directions.

Micro- vs. Macro- Evolution
micro= Short-term, small changes that we can observe
macro= Big changes over long-term that we can't really observe

Taxonomy is a difficult problem. There are millions of different ways to classify all the living things on earth. But if we classify according to evolutionary relations, then there is really only one right way. We may argue about what is more closely related to what since we don't know what the right answers are, but there is only one ultimate truth.

A little on sexual selection: (more detail will be forthcoming in a later lecture)
The catch-phrase shouldn't be survival of the fittest, but reproduction of the fittest. You only need to survive long enough to breed. Prolificness is better than longevity: Animals who live a long time but don't rear many young will be overrun by animals who live short lives but multiply better. This'll show up in female choice and male-male competition.
How did such a catch-phrase come about? (No, Darwin didn't invent it- one of his later proponents did.) "Fitness" in the olden days used to mean how the organism fit into its environment. Now it just means how well it can reproduce.

The gene as a unit of selection: Dawkins' 'The Selfish Gene'
Think not of individuals as being selected for, but genes as being selected for. Individuals don't make exact copies of themselves, but genes do. So, you may ask, where does altruism come from? How come any mutation which causes altruism doesn't just get selected right the heck out of there? Think about it this way: If you sacrifice your life for 1 sibling, then only half your genes get passed on. This doesn't bode well for your genes. However, if you sacrifice your life for three siblings, then 1.5 copies of your stuff get passed on so this altruistic gene would survive better than a selfish gene. This leads to:

Hamilton's equation: Which determines whether an individual will be altruistic.
C < r B
In English, Cost should be less than the Benefit times the Relatedness.

So if you have an opportunity to sacrifice yourself for a sibling, who is 1/2 related to you, then you'll sacrifice only if his benefit is more than twice your cost. (Because altruism genes who didn't follow this equation wouldn't have gotten passed along as much as genes who did.)
(Hamilton's is not to be tested)

Common misunderstandings of evolutionary theory:

Group selection: Entire groups do *not* limit their reproduction for the good of the group. A single mutation would quickly take over the population. A group can't be selected really, unless it's completely isolated which is really rare.

Evolution should lead to perfection: You might think to yourself, "Why after all these years isn't everything perfect? Take primates with a prehensile tail; wouldn't it be better to just have some more arms? How come evolution hasn't made arms?" The problem is, that particular variation wasn't present in the population to work with- only genes which are present can be selected and refined.
Another example: smarter is better, right? So how come all animals aren't so smart? Well actually brains use a lot of fuel and lots of animals don't even need 'em.

Evolution is striving for a specific goal: Good evolutionists don't use the terms 'higher' and 'lower.' Certain organisms are not inherently better than others. Other organisms are not sitting around in primordial soups trying to evolve into humans.

And here, by the special request of my grandfather, is the paper I wrote on natural selection for this class.

Friday, Sept. 13 -- Primate Adaptations

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New Office Hours: Monday 1-3, not 2-4

Today we're going to be looking specifically at primates, not at behavioral theories or terminologies.

Describe physical characteristics of primates
Describe general features of behavior and ecology

Question we're going to answer today:

What makes a primate?

How do we come up with a definition of the order primates? It is difficult because there's quite a range of differences in characteristics. For instance, there is a two-to-three-thousand-fold size difference between largest and smallest primates.

Primate Anatomy

A definition from 1873, written by St. George Mivart:
"Unguiculate, claviculate, placental mammals, with orbits encircled by bone; three kinds of teeth, at least at one time of life; brains always with a posterior lobe and calcarine fissure; the innermost digits of at least one pair of extremities opposable, hallux with a flat nail or none; a well-developed caecum; penis pendulous; testes scrotal; always two pectoral mammae."

A link to the Oxford English Dictionary (Available only to UM-affiliated users, sorry)

Seriously though, they're not very physically specialized. Not when compared to say a giraffe or a platypus! There are some defining characteristics, though.

Eyes: Primates are very visual. Compared to other mammals, the eyes have moved more to the forepart of the head. This gives good binocular vision.

Teeth: Incisors, canines, and molars. (The three kinds referred to in the definition.) Diversity of tooth types is because of diversity in diet. The canine teeth rip food and get it into mouth, while molars grind the food to prepare it for digestion. Felines, who have a less-varied diet, have less varied teeth.

Brain: A lot of development in the neo-cortex which implies heavier reliance on learning and memory. Also, olfactory bulb is rather reduced, especially in the ones with bigger neo-cortex. Older primates, like the prosimians, have a larger olfactory bulb but a smaller neo-cortex. More recent developments have a smaller olfactory bulb and larger neo-cortex.

Skeletal layout: Fairly generalized so it can be used for many different modes of locomotion. Skeletal proportions have great variation among species, mostly related to locomotion type.

Ecology and Behavior


Worldwide geographic distribution: Primates are fairly widely distributed; on 5 out of the 7 continents (Not Australia or Antarctica.) They're more restricted now than before, mostly due to climate changes- they're basically a tropical order. Mostly they're found in the rain forests of SE Asia, west-central Africa, and South America. Exceptions: Barberry Macaque in N Africa, Japanese Macaque in Japan, and some mountainous types.

Habitat types: These differences in habitat are caused by differences in rainfall, going from highest to lowest.
Primary forest: Tall trees with a dense, multi-level canopy.
Secondary forest: Spots where primary forest has been disturbed so it doesn't have tallest trees- more dense understory due to more light getting in.
Gallery forest: Develops along waterways in drier areas.
Woodland: Trees are sparser and shorter, with more shrubbery.
Savannah: Made up of grass and scattered trees (pretty dry here).
Within any given habitat, there are microhabitats:

Microhabitats: These are smaller divisions within the same habitat: You might have altitudinal gradience where two species live in the same habitat but at different altitudes. Also, within a forest, they might use different levels of the canopy: Highest is the emergent trees (up to 300ft tall) that poke out above canopy. Next comes the main canopy, which has a lot more horizontal branches and vines and things. The main story comes next, with more trunks and vines, and finally we find the ground. Certain species may specialize in one particular height level, rarely venturing out of it. (This type of microhabitat is also known as vertical distribution)

Activity Patterns

Most mammals are nocturnal: they sleep during the day. Primates on the other hand are mostly diurnal. The holdouts are the prosimians (the less-developed primates who also have smaller neo-cortexes.) All apes are diurnal. All monkeys except the 'night monkey' are diurnal. Some monkeys could be called crepuscular maybe, but not nocturnal.


Primates, being 'unguiculate,' not only have nails or claws, but usually have flat nails instead of claws. This is because hands and feet have become modified for grasping.The exception is the prosimians; they still have nails.

The Classifications a.k.a Different ways to move:
Quadrapedalism; May be arboreal(african monkeys) or terrestrial (macaques and baboons). A specialized form called knucklewalking is seen in african apes, gorillas, chimps, and bonobos.

Leaping; Seen in squirrel monkeys. Vertical leaping from a clinging position on a trunk is seen in the prosimians such as the tarsiers and indries (who still have claws).

Suspensory climbing; Hanging down from hands from branches. Orangutans do this. Gibbons do a special type of suspensory travel; they brachiate.

Bipedalism: Examples are us; also chimps and spider monkeys do it. Mostly they bipdedal (if that's a word) for shorter distances or when carrying something, but don't use it as their primary mode of movement.


'...well-developed caecum...' This is a sac in the digestive system which comes off at the top of the large intestine but leads nowhere. Its harbors bacteria used in the digestion of some nutrients that we can't break down, such as cellulose or complex carbohydrates. Bacteria in the caecum digests this stuff for us.

Gross dietary categories
frugivore fruit-eater: orangutans
folivore leaf eater: leaf-eating monkeys
insectivore insect eater: tarsiers
gummivore gum eater (saps and gums from injuries to trees): tamarins and marmosets
omnivore animal & plant eater
faunivore animal eater (includes insectivores) but this category also includes invertebrate-eaters
herbivore plant eater (includes leaf and fruit eaters)

*Note if we say an animal is one of these types it doesn't mean that's all they eat! Some animals have stricter diets than others.

Spacing systems-- Nomadic-migratory vs. Philopatry

Many animals move around quite a bit. Primates don't move so much. They are called philopatric which means they stay in the same place. This is ‘cause they live in such a complex environment; they need to know where to find food, sleeping areas, and predators. Therefore they stick around within an area that they know. Maybe an animal will move to another territory once when it reaches adulthood, but that's usually it.

Range terminology:
How far they go in one day; day range (often measured in length)
How far they go in a longer period of time (like a year); home range (area)
The area in home range which is used most; core area (area)
If they actively defend their home range, then it's called a territory and if they don't, then it's just called a home range.

Examples: Gibbons have territory and they defend it. They move pretty quickly, so their day rage is pretty long, but their territory is relatively small since they do defend it. The orangutan moves more slowly so its day range is shorter. Its home range, not being a territory, is pretty big, however. When undefended, home ranges often overlap quite a bit.

Social Groups: Solitary vs. Gregarious

Mostly, primates are social animals. Most mammals aren't. Holdouts, as usual, are the prosimians many of whom are solitary. Monkeys and apes are almost all in groups, the prototypical example being baboons who live in huge groups. An exception is the orangutan who is a solitary beast.

A Classification of Social Group Types
Note that these often correspond to mating system classifications.

Noyau: Animals have overlapping home ranges, and the sexes don't live together. There's no territoriality. Each female has a home range while males have larger home ranges that cover several female ranges. This is the system seen in orangutans. Usually goes with a promiscuous mating system.

Monogamy: One territory for each pair and their offspring. An example is the gibbons. Goes hand-in-hand with the monogamous mating system.

Polyandry: Each territory has one female and many males. Guess which mating system they use.

Multi-male group: (Should really be called multi-male, multi-female group, since each territory has many of both sexes.) Macaques are an example. Usually a promiscuous mating system.

One-male group: Such as the leaf-monkey. Usually a polygynous mating system. The leftover males form bachelor herds who raid periodically to take power and women from aging reigning males.

Fission-fusion society: This type is less common, but can be seen in chimps, bonobos, and spider monkeys. A group has shared territory. All the members are friendly to each other and work to keep non-group members out, but they don't travel toge ther as a group. They have smaller subgroups that join and split almost constantly.

Hamadryas baboons: This category is used almost just to describe their systems, so they got the naming of it. It is a complex hierarchical system, with several levels. The basic unit is a one-male unit, with accompanying females, but the units co mbine into larger groups called clans. These are made up of related males' groups who merge for a while to forage and socialize, but don't share women. Clans will sometimes merge to form troops who share common sleeping sites, often on cliffsides. Sometimes a troop will move as unit to a new location, but usually during the day they split into clans. A similar system is seens in the golden monkey and proboscis monkey, but it hasn't been so well described.

Monday; film will begin right at 11 since it's exactly 60 mins.

Discussion --

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First meeting: We received the section syllabus and introduced ourselves. Then we discussed four different primates: The orangutan, leaf-eaters, macaques, and gibbons. We compared their social structure, diet, and territoriality, discussing which factors influenced the differences.

Let me know your thoughts:
Last modified: Saturday, September 14, 1996