Why do animals of the same species form social groups? The disadvantages and advantages, or costs and benefits, of living in groups often revolve around avoiding predators and foraging for food. Animals need to protect themselves from predators and maximize their potential to obtain food.

Let’s list the benefits of social behavior to a group of animals that wants to avoid being eaten by a predator.

First, any individual animal is diluted by the other members of a group. A lion may eat a whole antelope, but the chances of the lion's finding one antelope in a herd of 100 are slim.

Second, a herd offers increased vigilance. A lion might sneak up on a single antelope, but a herd offers more opportunity to sense the lion’s presence.

Third, a predator may be confused by a large group. It may not be able to select a target as easily.

Finally, the group may be able to defend itself. A group of 100 antelope stampeding may be enough to scare a lion away.

But there are also two costs associated with social group behavior as a way of avoiding predation.

First, the group suffers from increased conspicuousness. A lion might miss a single antelope, but it wouldn’t easily miss a hundred!

Also, a large group may be a preferred target. The lion may sense that it can’t miss one out of many, although it may not give chase to a single individual.

The cost/benefit analysis for avoiding predation looks weighted toward the benefits, but the real analysis would be different for each species.

Is communal foraging for food beneficial to an animal? Let’s perform a cost/benefit analysis to see.

A group benefits from an increased search area. A number of individuals looking for a scarce food resource are more likely to find it than a single individual on its own.

Similarly, the group benefits from shared information about food location. The bee dances that we saw earlier are a good example of how cooperation between individuals can lead a group to food.

The group also benefits by being able to protect food against competitors. Wild dogs in Africa wouldn’t be able to fight off hyenas, and they’d probably starve unless they hunted in packs.

Finally, a group provides increased efficiency in foraging for large or evasive prey. Groups of killer whales cooperate to round up creatures as large as a blue whale.

What are the costs associated with social foraging?

There is competition for the food source. If a large group of animals finds only a limited food supply, not all the members of the group may receive enough food. Some individuals may steal food from others in the group.

And the competition may lead to reduced feeding rates. Members of the group may spend so much time scrambling for the food resources that they might have been better off foraging on their own.

As with predator avoidance, the cost/benefit analysis of foraging in groups depends on the individual species, as well as the size of the group.

Now that we’ve seen costs and benefits associated with several social behaviors, let’s look at how the presence of parasites might affect a group.

Decide whether the observations shown here are costs or benefits, and drag them into the appropriate column. Click Submit when you're done.

Correct answer: That’s right

Wrong answer: That’s not quite right

All: The cost of being in a group is that parasites may be spread from individual to individual. The benefits are that members of the group can groom each other to remove external parasites, and the group may actually dilute the effect of the parasites: Instead of the parasite’s eggs reinfecting the same individual, they may infect others in the group. A third benefit is that any genes that offer full or partial immunity toward a parasite may be selected for in a group more quickly than in a more dispersed population.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education