Unit 4
Sex, Evolution, and Social Organization

In this unit, we examine the different types of grouping patterns found among primates, and we discuss how these patterns reflect compromises between female and male strategies to gain access to mates, food, and other resources that they and their offspring need to survive. The unit begins with an exploration of the relationship between sexual dimorphism and mating behaviour, and discusses how being a limiting resource puts female primates in an influential position over males. From how they distribute themselves in relation to their food resources, to how frequently they are capable of reproducing, to how they assert their mate choices, female primates determine the behavioural options and reproductive opportunities available to males. What emerges from these compromises between male and female strategies are the grouping patterns we can observe in primates today. These compromises are constrained by life history and physiology, and vary with ecological and demographic conditions. Given these compromises, females have very specific constraints on their reproductive fitness, including the risk of infanticide, male aggression, and competition for resources. What strategies should females adopt to counter these constraints? Should a female mate promiscuously in order to confuse paternity of her infant, or should she maintain a long-term relationship with a particular male? How should she choose a potential mate? (I.e., what should a female look for in a male?) Finally, we will look at the relationship between male rank and reproductive success.

Objectives

When you have completed Unit 4, you should be able to

1. define sexual selection and explain its relationship to sexual dimorphism.
2. describe the different mating patterns found among primates.
3. explain the major hypotheses put forward to explain why primates live in groups and the types of social groups (size and composition) they should live in.
4. explain the various ecological and phylogenetic factors that might determine the mating pattern a female primate should adopt.
5. explain the relevance of seasonal versus aseasonal breeders to mating pattern and social organization.
6. discuss female mating strategies and female mate choice in light of the constraints on female fitness.
7. discuss why some females (species) should mate promiscuously and others not; why some females (species) should advertise ovulation and other not.
8. discuss the relationship between male rank and reproductive success, and define the concept of honest advertisement.

Viewing Assignments

In preparation for the first examination, you must watch at least three videos listed in Units 1 through 5. Each unit gives you two video options, so that by the end of Unit 5 you should have watched three videos from these four options:

Five Species or Primate Patterns II
Lemurs of Madagascar or Spirits of the Forest
Darwin’s Revolution in Thought or Great Transformations
Images from the Field or Baboon Tales

It does not matter the order in which you watch your video options, but the viewing assignments for each unit offer you a guide. Remember, completing a viewing assignment differs from watching a video for pleasure. You should consider watching each video twice: once to get an overview, and once to make notes. Study questions or instructions for viewing are supplied for each video; be certain that you read them over before you begin watching, and keep them in mind as you complete the assignment and make your notes.

Unit 4 Viewing Assignment

Choose one of the two videos below.

Zeller, Anne
1997  Images from the Field: Baboons. Watertown, MA:
Documentary Educational Resources.

This video provides a closer look at the olive baboons, describing, among other things, typical behaviours of a group, sociosexual behaviour, dominance interactions, and interactions with human populations.

If you choose this video, then in preparation for the examination, be certain that you can answer the questions given below.

1. What roles do males play in the group?
2. What is a typical day for a baboon troop?
3. Why do human populations hunt baboons?
4. What functions does presenting have among baboons?

or

Strum, Shirley Carol
1998  Baboon Tales. Tamarin Productions Inc. Montreal.

This video is an in-depth look at a troop of olive baboons in their natural habitat, with a particular focus on infant development. The video also presents information about the general behaviours of baboons.

If you choose this video, then in preparation for the examination, be certain that you can answer the questions given below.

1. Describe the general development of infants, and the factors affecting it.
2. What role does alloparenting play in olive baboons?
3. Why do males form friendships with infants?
4. What is infant buffering?
5. Provide examples of body language among olive baboons.
6. What happens during group encounters?
7. What role does friendship play in baboon society?

Alternative Reading Assignment

If you cannot complete the viewing assignment before the midterm examination, you must do the alternative reading assignment instead. You may combine the reading with viewing assignments so that you have a total of three video and reading combinations. For example, you can watch two videos and do one reading assignment for the midterm. Or, if you cannot watch any videos, then you must complete three reading assignments from the four options presented in Units 1-5. The reading materials can be ordered from the library. Be sure you can answer the questions below each assignment.

Unit 4 Alternative Reading Assignment

If you cannot complete the viewing assignment, you must do the alternative assignment below.

Veit, Peter
1998  Chapter Two, Gorilla Society. In The Primate Anthology:
Essays on Primate Behavior, Ecology and Conservation
from Natural History. Ciochon, R. L., and R. A. Nisbett, eds.
Pp. 12-18. Upper Saddle River, NJ: Prentice-Hall.

In preparation for the examination, be sure that you can answer the questions below.

1. What sub-species of gorilla lives on the Virunga volcanoes?
2. What is the typical mating pattern found in gorillas?
3. How was Group 5 somewhat unusual in its social structure?
4. Why is there often a delay between the time at which males physically become silverbacks and the time they actually start gaining access to females?
5. When are dominant silverbacks particularly vigilant over estrous females?
6. What is the evolutionary function of dispersing from the natal (birth) group and starting or joining a new group?
7. Given the group structure of gorillas, how certain can males be of paternity?
8. What is the extent of parental investment typically exhibited by male gorillas?