Questions on A105 Primate Lectures

Here are (partial) answers to some of the questions you had about lectures this week. Let me know by email what other questions you might have that would be of general interest, and I can add them to the list.

Questions about sexual dimorphism:

Q: How can monkeys be big if they have to be? Is diet the only influence?

A: You inherit the potential to grow a certain body stature from your parents. You can achieve that potential only if you have good nutritional health when you are growing/maturing. When I talked about male baboons having larger stature than females, this is a trait that is inherited. That is, it is an example of an ADAPTATION... a physical trait, in this case, that had a selective advantage for individuals, allowing them to achieve greater reproductive success and pass that on to their offspring. What is different in this case is that male and female stature can DIFFER in some species if the selection pressures on males and females differ... some alternative explanations for sexual dimorphism are described in Lewin Chapter 13.

Q: Why is it harder for males to mate with females if the species is dimorphic?

A: You've got it backwards. In polygamous species where males face alot of competition for access to mates, larger body size is one (of several) traits that can offer a selective advantage... and thus can get passed on to future offspring of successful males. The dimorphism is a result of mating pressure, not the other way around.

Think about the two examples of baboons and orangutans.

    Baboons are small monkeys that live in groups for protection in open habitats. Females, do not have to compete for access to mates... the males come to them. So access to food is a primary selection pressure for females. If they have to share food with someone, would rather share it with kin, so females stay together with mothers and sisters in the same group, and the troop cooperates to defend their food/territory against other troops. Male transfer is advantageous because it avoids incest and inbreeding. Males can compete with each other to be accepted into a new troop and gain access to new females in several ways: through aggression, through intimidation, through cooperation and altruistic social behavior. Large body size and large canines can help a male to put on a good show, and dominate fights, so these traits can be favored by natural selection.

    Orangutans are large apes that are forced to spread out through the forest to avoid competing for food with each other. So they lead solitary lives because of ecological constraints. Males must also compete with each other for access to females... to increase your chances of reproductive success, the goal is to have access to as many females as possible, and to try to maintain excluxive access to those females (by excluding other males)... sort of a "harem" structure. But it is difficult for orang males to monitor females because they are so spread out, and they come into estrus so infrequently. (Males also compete for food, and will cover 3-4x the area of a female orangutan.) What seems to happen is one dominant male will "claim" several females, and try to keep the other males at a distance, through confrontations. Large body size and large canines can help a male to put on a good show, and dominate fights, so these traits can be favored by natural selection. And occasionally, one of the "rogue" males will successfully sneak in to copulate with a female.


Q: Do the females ever get into fights?

A: Yes, sometimes physically, sometimes in more devious ways!


Questions about Mating Behavior:

Q: Are gibbons the only primate that forms monogamous relationships?

A: Gibbons are the only ape that is monogamous, but several monkey species form pair bonds too. Marmosets, a small, South American monkey, are another example. They also show little sexual dimorphism.


Q: If their mate dies, do they find a new one or stop reproducing?

A: There always seem to be plenty of young guys around to step into the breach!


Q: Where do the male monkeys and apes go after birth? Do they help raise the young?

A: In some monogamous species, the males will help provision (feed) and carry the young. Biologists explain this as an investment in the male's own offspring... in monogamous relationships there is "paternity certainty." In most polygamous primate species the fathers of individual offspring are unknown, and males play little role in raising young, other than general group defense, etc. That's why it is interesting to see males in some species, such as the savanna baboons, spending alot of time with infants. Read about this in the Barbara Smuts article on baboons in your reader.


Q: After female orangutans mate with one male, do they stay with that male or mate with different males?

A: If the dominant male leaves a female orangutan while she is still in estrus, sometimes other males will try to mate with her as well. Sometimes she has no choice, because they can dominate her physically.


Q: Why don't female apes stay with one male?

A: In some species (like gibbons and gorillas) they do, but only in circumstances where they don't have to compete for food. Gibbons are small and swift, so they can get plenty of food in a small territory, and the male gibbons can defend their resources (both food and mate) against other males. Gorillas are largely vegetarian and live "in a salad bowl"... with food so abundant it places no constraints on their social behavior. So dominant males (silverbacks) are able to protect a "harem" of females and his offspring from other males, and females will stay with him.


Terminology Confusion:

Q: Are we more closely related to a chimp or an ape?

A: Chimps ARE one type of "ape." Other "Great Apes" (family Pongidae) include gorillas and orangutans. Genetically, our closest relatives are chimpanzees, then gorillas, then orangutans.


Q: Are baboons monkeys?

A: Yes, they are monkeys that live in Africa, and are part of the Old World Monkey group.


Just curious Questions:

Q: Why is it important to study this stuff?

A: Ah, always a skeptic! Two reasons: (1) I think you can get interesting insights into human nature by comparing yourself to the animals that are most closely related to us. We have alot in common with other primates, but we are also unique in many ways. Learning about "them" helps you learn about "us." (2) The principles we use to understand how primate groups are structured are the same principles we will apply to interpret the fossil record for the lifestyles and behavior of ancient primates and our own ancestors. Example: sexual dimorphism is a clue to social structure in living primates, and extinct ones. Humans are not sexually dimorphic in body size today, but fossils suggest some of our ancient ancestors were. This gives us clues about the behavior patterns of extinct creatures, and what might have changed. Then our challenge is to figure out why.


Q: Why are we focusing so much on baboons, rather than other monkey species?

A: Well, primates are so diverse and fascinating, it is hard to know where to end. But I selected baboons because they have been studied for a long time, they illustrate some socio-biological principles well, and they live in open habitats that are often compared to habitats our own ancestors lived in, and are suggested as possible social models for proto-humans .


Q: How do primates react to close contact with humans in the wild?

A: Since many human cultures hunt monkeys and apes for food, or to capture young animals to sell to labs, or zoos, or tourists, most wild primates are very wary of people, and will run away. (In fact, many primate species are in danger of extinction, because of such hunting or because of habitat loss and fragmentation.) To study primates in their natural habitat requires a huge amount of patience to "habituate" the animals to human observers. ... Habituating a population of chimpanzees, for example, takes many years of dedication before the animals trust a researcher enough to allow them to observe them for any length of time. Habituation is very important, because it is only through long-term studies in the wild that we are beginning to learn about the complex nature of natural primate societies.


Logistical questions:

Q: Are the readings important in addition to lectures?

A: Yes! Especially for learning about examples to explain for the written questions on exams.


Q: Why does it take so long to download notes from the web?

A: I don't know... perhaps you have a slow modem connection? Try using a direct connection in one of the student labs or dorm rooms? Or, turn off the "images" in your browser. Use Netscape... its faster than Internet Explorer.


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