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Conradt and Roper. 2005. Consensus decision making in animals. TRENDS in Ecology and Evolution 20.

In brief

Animals that live in groups regularly need to make decisions--which direction to go (e.g. migrating birds), when to feed/forage, where to nest (e.g. bees), and so on. The authors present a typology of these decisions and provide a few illustrative examples, urging further research in the field.

Typology of decision

See Figure 1 for the authors' complete typology. First, they ask whether the decision involves a conflict of interest among members of the group. Then they ask whether communication is global (i.e. all group members can communicate with any other group member) or local (i.e. group members communicate with only those group members near them). These two questions yield four possible types of consensus decision.

No conflict of interest, local communication

Example: Bees (or ants) looking for a nest choice. Since there is no conflict of interest, the bees are willing to delegate to a small subgroup. In this case, it's a small group of scouts, usually numbering no more than 5 percent of the colony. These scouts (independently) explore one or two sites each, then communicate with each other. By dancing, they try to persuade other scouts to visit their site. Once a good number of the scouts agrees, the colony moves to the new location.

Little conflict of interest, global or local communication

Example: A migrating flock of birds. Whether communication is global or local depends on the size of the flock. It's not known exactly how the flock decides which direction to fly. One model suggests something like this: Each bird has its own preferred direction to fly, but it also has a preference for staying near the other birds. Presumably, there is some weight (or relatively value) attached to each of these preferences. By observing the direction that the birds near it are flying, each bird averages between its preferred direction and the flock's current direction. As a result, the flock's aggregate path reflects a weighted average of each bird's preferred direction.

Here's an analogy (my own contribution): Picture five teenagers walking at the mall. If one wants to change direction, he starts pulling a little to the left. If the group follows, great. If not, he comes back in and keeps going in the group's direction.

Larger conflicts of interest, global or local communication

Example: A group of deer deciding whether to forage in a new location, to stay where it is, or what. Various deer will have their own preferences. One might vocalize to indicate its preference, or use ritualized signals, or do something more subtle, like face in its preferred direction or take a few steps. The rest of the herd observes these actions, tallying the "votes" of the other deer.

My analogy: Picture 40 teenagers at a beach barbecue. Suppose one wants to drift closer to the shore and look at something. He might face that direction, or try to "pull" the group by taking a couple steps in his chosen direction. But other group members might want to stay closer to the grill, or out of the water. The group either splits, or comes to an implicit agreement, as various kids constant look around and observe what others are doing.

Larger lessons

From the examples, note a few things that occurred:

• Information pooling. In all three examples, there was a mechanism by which the group decision integrated information from individual members. It wasn't necessary that any one group member learned all the information, yet all the information was ultimately incorporated in the decision.
• Decision makers. Each example involved a specific subset of the group in the decision. For the bees, the subset was quite small. For the birds and the deer, these subsets were relatively larger--perhaps involving the entire group.
• Consensus costs and conflicts of interest. Some of the examples involved potentially conflicting preferences; for a member to abandon her preference in favor of the group consensus, she will incur "consensus costs." But if she values remaining with the group, she will incur these costs willingly.
• Local vs global communication.