Variability in Social Systems & Behavior

There are approximately 200 living primate species. There is a lot of variability in the primates.

Mating systems:

male dominance hierarchy--> dimorphism

3 Sources of Variation:

1. Ecological
Food is clumped--> animals are clumped.
Food is scattered --> animals are scattered.

2. Phylogeny (=Evolutionary descent)
Some animals are more closely related to each other and so are less varied than a non-related animal.

Different environments engender different social behaviors / genetic developments.

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.