How a bee colony makes decisions

The best-studied case: choosing a home

When a colony swarms, several thousand bees hang in a cluster while a few hundred scouts search for a new nest site. The decision that follows is genuinely collective, and it has been studied in enough detail to be described step by step.

1. 1Independent searchScouts fly out separately and assess cavities on volume, entrance size, height, draughtiness and exposure.
2. 2AdvertisingA scout returns and dances for the site she found. The vigour and duration of her dance is proportional to how good she judged it to be.
3. 3RecruitmentOther scouts follow the dance, inspect the site themselves, and dance for it independently if they agree. Nobody takes another bee's word for it.
4. 4DecayEach scout dances progressively less for her site on successive returns. A mediocre site loses support because its advocates give up.
5. 5QuorumWhen about fifteen to twenty scouts are found at one site simultaneously, that site has won. The scouts begin piping and the swarm prepares to fly.

Why this produces good decisions

Three features do the work. Scouts assess sites themselves rather than accepting a recommendation, which prevents an early enthusiast from dominating. Dance vigour is proportional to quality, so better sites recruit faster. And the decay of enthusiasm means support must be continually renewed, so a poor site cannot accumulate votes simply by being found first.

There is also an active stopping mechanism. Scouts advertising one site deliver short head-butt signals to bees dancing for a rival, which inhibits the rival dance. This cross-inhibition is what allows the colony to break a deadlock between two comparably good sites rather than splitting.

Other decisions a colony makes

• Where to forage. Foragers dance for patches, and the colony's effort shifts towards the best available without any bee comparing patches directly.
• When to swarm. Driven by congestion, queen pheromone dilution and forage availability rather than by any decision point.
• When to supersede a queen. Workers begin raising replacement queens when her pheromone output falls.
• How much to invest in drones. Colonies rear drones when resources allow and evict them when they do not.

What this is and is not