Groups in Relation to Resources

Solitary vs gregarious life in primates: a review

Potential costs
1 feeding competition
2 increased costs of movement
3 increased transmission of disease and parasites

Hypothesized benefits
1 benefits associated with the acquisition of food (read on)
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

This is more a concept of behavioral ecology than of primatology, but people have begun to apply it to primates. Remember the weaver birds in Africa which we talked about early in the course. They fall into two distinct groups- 1 is group-living and feeds in flocks while the other is solitary and feeds alone. They hypothesize that the difference in social behavior is related to the difference in diet. One lives in the forest and feeds on insects. The other lives in the savannah and eats seeds, which can be difficult to locate but once found there is enough for a huge group. Insects are more evenly dispersed and a find doesn't feed much more than a single individual.

"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?

It's hard to test this idea for primates because you'd need to find members of the same species, some of whom are living in groups and others of whom are living solitarily, and compare their feeding success. This isn't practical and no one has come up with good ideas to test this quantitatively. However, there is anecdotal evidence that information exchange is important in location of food. People who talk about these theories use the phrase "information center" to refer to the group. This implies that animals can go to the group to get information about what food food is available and where.

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

This is an idea proposed by Richard Wrangham. If animals compete over food as groups, then it'll be advantageous to be in a group as opposed to being solitary, and it'll also be more advantageous to be in a bigger group.

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 idea is borrowed from other areas of biology; group-hunting carnivores can only take down large prey by hunting in groups, so maybe this is an important factor for some primates.

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?

We've been talking about group size, but there's another parameter of group living and that's the composition of the group. How do you account for single-male groups and 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.