The mean on the midterm was about 82%.
He'll guarantee that if you get a 90% it's an A, 80% a B, etc, no matter what the curve is turns out to be. So the curve can only make the grading system easier, not make it harder.

Solitary vs. Gregarious Life in Primates

There are two basic questions regarding group living:
Why live in groups at all?
Why live in the size and composition that they do?

Costs and Benefits of Group Living

Summary of the costs:
1 feeding competition
2 increased costs of movement
3 increased transmission of diseases and parasites.

So what are the benefits which make it more worthwhile to live in groups?

Predation as an evolutionary force

Predation rates and problems of studying predation

Also, when there's researchers around, predators are less likely to come around, so just by observing you're usually changing the likelihood.There was the 1993 study by Lynn Isbel in Kenya, studying vervets, during which they noticed the 'Nairobi effect.' They noticed that a lot of vervets would disappear when they went to Nairobi to get supplies. Not only would more vervets disappear but there'd be more signs of leopards. The rate of disappearances while the observers were present was .04/day, but the rate of disappearances when they'd gone to town was .13/day. This is another reason which makes it not easy to study predation.

Predation trends

Larger species are less vulnerable than smaller animals. Also, big animals seem to have more different type of predators, but they get eaten less often.

Mortality due to intra-specific killings is generally higher than rates of inter-specific predation. So more primates are killed by members of their own species than from members of other species. A particularly common cause of mortality is infanticide.

Do primates respond behaviorally to predation risk? Three behaviors:

Alarm call

A lot of animals use alarm calls, primates included. How important they are depends on the size of the species. For instance, there are studies on vervet monkeys' alarm calls by Cheney and Seyfarth. Vervets have several different patterns of alarm calls. One call is for airborne predators. When this call is heard, they'll run down from the trees. There is a different alarm call for ground predators, which when heard they run into the trees. The last is for snakes, and they don't run anywhere but instead stand up on their hind legs and look around carefully. Other primates, such as Gorillas for example, have alarm calls too but they are used less often and aren't as specific.

Mobbing

A lot of animals do this too. You can see songbirds mobbing owls or crows right around campus. They move toward the predator harassing and screaming until they drive it off. Examples in primates: baboons will mob any small carnivore, even cheetahs and jackals. When an animal lives in large groups, it can mob large predators which it couldn't do if it was alone. Chimps have been seen to mob leopards and lions. Rhesus monkeys have been seen to mob tigers.

Avoidance

Longtailed macaque groups will go up into trees at night to sleep, and they repeatedly use the same trees for sleeping. However, if they encounter a python in a tree they're unlikely to use it ever again. The interesting thing is, they don't show the same response in feeding trees; they'll go back and eat at a tree even if they saw a python there before. They do alter their behavior while eating, however; they stay closer to other individuals after having seen a python.The difference probably is that they feed in daytime so they can see better, so they're a little more secure.

Primate examples- the two articles for today's lecture

It also came out in this study that individual habits vary greatly. Some chimps liked hunting better than others. One chimp in particular was really good at it and he got about 20% of the colobus killed.

Crowned eagle predation on monkeys
Monkeys are about 84% of their prey. Struhsaker studied eagles and checked out what they brought back and checked out the bones which fell out of the nest. When the data was analyzed, they found that two species of monkey were taken less often then their percentage of population in the forest would predict. These two monkeys go around in poly-specific groups, they run around together) and it has ben hypothesized that this helps them avoid predation.

Possible predation-related benefits of living in groups

Dilution effects

Also known as the "selfish herd" effect. You can protect yourself by jumping into a group because your chance of being taken is less when you're in a large group.

This theory is correct under the following conditions:

1 If predators only take a few individuals per attack.
2 If attack rate is independent of group size.
In other words, if you join a group with 10 individuals but the group gets attacked 10 times more often, then it wouldn't pay off to be in the group.

There's not a lot of data to back up these theories, but there is suggestive evidence; When chimps are around, red colobus stay close to their neighbors (less than 2 m to neighbor), but when chimps are not around they're more like 2.5 m from their neighbors.

Increased group vigilance

If you live in a larger group, there are more of you looking around and it's more likely that when a predator arrives someone will happen to be looking up.

Predator deterrence through mobbing

Again, data from red colobus. Sometimes they mob chimps and sometimes they don't. This data was collected at Gombe in 1974.

	defense	no defense
successful	2	29
unsuccessful	12	21

Of course there's also contradictory data:

Site	Successful	Unsuccessful	Total
Gombe	16	3	19
Tai	2	8	10

Effects of predation on grouping patterns

Another study was done on longtailed macaques: They live in Borneo where there are a lot of predators, but there's also a group who lives on an offshore island where there aren't so many predators. The researcher compared the group sizes in these two places. The large groups that he saw were only where there was high predation risk, while on the smaller island there were only small groups.

In a final study on chimps in Tai national park, they didn't follow the prediction. When predation pressures were high, most group sizes were 2-5. Under lower predation pressure, group sizes were 11-20, which was the most. The thing is, most predators which prey on chimps and sit-and-wait type predators, so it may be more helpful for chimps to be in smaller groups.

So, predation, while not the only factor, it is an important factor for determining why primates live in groups and what types of groups they are.

If we have a disagreement about the exam, we should write an explanation about what we think is wrong and give it to him within one week of getting our exam back.

Solitary vs gregarious life in primates: a review

Hypothesized benefits
1 benefits associated with the acquisition of food
2 benefits associated with reducing predation risks

Three responses to predation:
1 alarm calls
2 mobbing
3 avoidance

Potential predator-related benefits of being in group
1 increased vigilance
2 dilution (remember the specific conditions which must be true)
3 enough numbers to mob them

Possible food-related benefits of group living

Improving the ability to find food

"The distribution of food may be a primary factor influencing grouping patterns of weavers.
1 In the savannah, seeds are patchy in distribution and locally superabundant.
2 It is more efficient to find patches of seeds by being in a group because together animals can search a wider area.
3 Patches of food, once found, contain so much food that there is little competition, and birds tend to flock together.
4 in the forest insect food is scattered. Thus birds feed alone and defend it against others."

So what about primates?

An anecdotal example from study by Peter Wasser on grey-cheeked mangabeys: He was following them when they were under severe food stress. The group started to follow a very old female who didn't normally lead the group and she went to a patch of fruit trees that the others apparently didn't know about. The basic idea is that if you're in a group, you can parasitize the knowledge of other individuals. It's kind of the same idea as increased vigilance against predators, but you have increased vigilance for looking for food and remembering good sites.

A cautionary note: Just because this female led them to the trees doesn't mean she was worried about their hunger and was trying to find them all food. Maybe she was just hungry and she went there an the others just followed her.

Another example about howlers sampling food from the CP article. In it, it talks about how one individual went into the tree and tried it first while the others waited. You might think this is an individual doing something nice for group, using himself as a tester so that the whole group wouldn't get poisoned. However, maybe he was just hungrier than the rest and the others just took advantage of it.

Why do we pick the selfish theory over the altruistic? Because it's much easier to explain selfish behavior and how it would spread through natural selection. Altruism is not impossible but it's much harder to explain and it should not the first explanation that you choose.

Intergroup feeding competition

Take vervets. There's a lot of competition between groups over food resources, so for them, better feeding competition may be an important benefit of living in groups. Dorothy Cheney and Richard Seyfarth did this study on the effects of groups size and resources. Groups often clash over a rich fruiting source. There was a lot of range overlap and the larger groups generally won. They found that the number of surviving infants a female yielded depended on the success of the group.

When a territory boundary shifts, it's usually a larger group pushing into a smaller group's territory. Sometimes two small groups blended into one large group and they were thus able to defend a larger area and so their females had more offspring live to maturity. Also, they noted that the smallest group in their area had the smallest predation rate- so in this population, decreased predation is not a benefit of living in groups.

Catching prey

The only primate who routinely hunts in groups is the chimpanzee. Do they need to be in groups to hunt successfully? When you look at data from Tai forest in Uganda comparing the number of hunters to the hunting success rate, there is a positive correlation between them. It is a very clear trend. They don't seem to organize their hunts very complexly- they just go after a group at the same time, but it still means that there are more chimps cutting off escape routes so more monkeys get caught.

How likely a group is to catch a monkey is one question, but another question is how well are the individuals doing? To see this, we break it down to per capita hunting success. If you're alone and you catch one monkey then it's 100% success. If you're in a group of ten and you catch a monkey then that's only 10% success. Looking at the data this way shows that up to a group size of about 4, cooperative hunting benefits the individuals, but over that it drops off sharply.

It turns out that most hunting goes on when there's lot of estrous females in the group. Females in estrous are a common choice for meat-sharing. Males seem to get really interested in hunting when there's a lot of estrous females about.

People have paid a lot of interest to this because it might tell us something about early human development and maybe they began living in groups because they were hunting cooperatively.

Why do primates form multi-male groups?

Wrangham said that it was up to the females, being a matter of how many males they'd allow to join the group. He first looked at the ecological distribution of food and how easily they could defend the territory. This theory hasn't really matched up to the data and people have moved from ecological explanations to sexual selection explanations. Now they don't look so much at what's good for the group; they focus more on what's good for the individuals. Remember that females are interested in taking care of offspring but males are seeking more matings. So the simplest scenario is that the females are pursuing food and males are pursuing females.

People have come up with two hypotheses; (from the male's perspective)

Breeding seasonality

Determines whether or not a single male can monopolize all the females or if they all come into estrous at the same time. If there's not a distinct breeding season then one male could theoretically control access to them all. However, if it is synchronous then other males might join the group since they can benefit by maybe getting sex when the original male is otherwise occupied. One flaw, you might say, is that it doesn't do males much good to join groups where the females are all already monopolized by someone else. Think about it, though; what are you going to do if all the females are in these groups? You have to at least try to pass along your genes, and so you join the group as a subordinate and do the best you can.

Female group size

This is just a simple correlation theory- the more females there are in a group, the more males you'd expect to see in the group.

So these two hypotheses give pretty clear predictions. Enough data has accumulated to use the comparative approach and see which theory works better. John Matani did all these comparisons on 49 different species. He factored out the effects of phylogeny and checked if the number of males in the group was higher when the breeding system was shorter. But actually it turned out to be the opposite- short breeding systems have fewer males in the group than those with longer seasons.
When he compared group sizes, though, it worked. The more females there were in the group, then the more males there were in the group.

Next time: Yet another aspect of variation in groups- some group members are related in different ways than others.

Kin Groups in Monkeys and Apes

It was only in the 50's that people got the clue that kinship groups existed in primate societies. A group of Japanese researchers were studying Japanese macaques and once they started tracking who was related to who, they began to notice that there were groups of female relatives who had a lot of interactions with each other within their kin groups and not so much interactions outside of the groups. This is a female-bonded society. You get several matrilines who hang out together. A single group might include several matrilines, and interactions go on within matrilines and not between matrilines. When the males reach sexual maturity, they transfer to another group. So in a group, the males are not socially interactive because they aren't related and they all came from different groups.

Kin-Correlated Behaviors

Spatial proximity

Among old world monkeys, related individuals living in multi-male multi-female groups are often found near and in contact with each other. Data about spatial proximity can be seen in macaques, yellow baboons, and vervets. (All cercopithecines.)

Grooming

Grooming is distributed preferentially among related individuals. It probably plays a hygienic role in terms of removing parasites and keeping wounds clean, but it also has a social role; creating or strengthening social relationships. This type of kin-correlated behavior is seen in many types of macaques, patas monkeys, vervets, and savannah baboons.

Rank

In cercopithecines, rank is inherited from a female's mom. Males' rank is affected a little too, but what rank they have as kids they lose when they transfer to another group. Daughters however stay in the group and they keep whatever rank they inherit from mom. Note that highest rank is the old lady. Next highest is her youngest daughter. So every time a daughter is born, she assumes a rank higher than any of her sisters. When the daughters have kids, they rank just below their moms but above any of their mom's older sisters.

Rank acquisition and alliance formation

Why does it work this way? Why do younger kids have more rank than older? Rank acquisition depends on agonistic support from kin; because of the agonistic support, the more siblings you have the more backup you have in any fights. You can get a tiny little monkey dominating adults simply because they know that this kid's mom will back her up. Note that there is some alliance development outside of kinship but most alliances are formed within kin lines. Maybe about 20% are formed outside of family.

Mate choice and inbreeding avoidance

Three sets of proof that female mate choice is involve with inbreeding avoidance; in Japanese macaques, females do not mate with their nephews and cousins (Ones related through females, that is; no one knows who is related through the males.) Male gorillas do not mate with their daughters, and female chimps avoid mating with their brothers. (Usually actually they're half-brothers)

Kinship and Male Behavior

Other kin behavior shown by males is in species where the females disperse and the males form the social cohesion. These groups have patrilines instad of matrilines. A group is made up of a bunch of males related to each other with some disconnected female peripherals. Cercopithecines are called female-bonded societies while species like chimps, where the females disperse, are called male-bonded societies. For instance, in Goodall's work there's the case of Figan reaching alpha male because he had the support of his brother Faben. When Faben disappeared, Figan lost power.

A bunch of scientists did some DNA checking on male-bonded societies and they found that the males in a group are more genetically related than the females.

How General are the Patterns?

Most of the old world monkeys are male-dispersal monkeys. There are of course some exceptions in the old world monkeys; chimps, gorillas, red colobus, hamadryas baboons.

In new world monkeys, the females emigrate and the males have closer social relationships. These are primates like tamarins, howlers, spider monkeys, and marikis. New world exceptions are the cebus monkeys.

Explanations of Dispersal

First, consider the costs of dispersal

You can also see this from data on times when monkeys are moving into new areas; they tend to disappear more often than when they're circulating into the same old areas.

Our conclusion? Leaving your home turf is dangerous! Be careful! Watch out for lions!

So, Why do animals disperse?

Incest avoidance

When you begin mating close relatives, you invariably have lower fertility, reduced numbers of offspring produced, and the offspring tend to be less fertile. This really happened a lot in the early days in large mammal zoo populations because they didn't have large enough populations.

Mate choice

To avoid inbreeding, females, being the choosy sex, will often refuse to mate with related males. The males might be ok with mating with any female but if all the females are refusing them sex, then the males will run out of mating opportunities and will be rewarded for looking farther afield.

Why male dispersal?

A benefits to males is greater than that to females (More mating opportunities)
B costs to females is greater than that to males (Less efficient resource acquisition)

Males disperse because it's a good way to get more mates which is what they are driven by. Females, on the other hand, are more limited by access to resources. When you move, you don't know the food sources as well and you don't have allies to help in competition over food access, so females don't gain anything by leaving their natal group.

So then, why female dispersal?

Males usually stay in a group as long as the costs of leaving outweigh the benefits. So if the male tends to leave the group after a period which is shorter than it takes a female to grow up, then a female can stay because whoever fathered her has most likely moved on. If males tend to stay in a group longer than it takes a female to mature, then the female must leave because her dad is probably still in the group.

Next time:
We'll be looking at an evolutionary explanation for kin-directed behaviors. For instance, avoidance of inbreeding. We will also be looking at explanations for things like agonistic support and other apparently altruistic behaviors.