Ants foraging for food

Directions Click on the applet to specify a new food position. Use the scroll bar to control how many ants are used. Check/Uncheck the box to control whether the ants will follow pheromones or not.

Ants are great. Each individual ant is relatively small and simple. On there own they are capable of running around quite well, and when they find food they are able to take it back home. Different ant species use quite a few sensory techniques to find their way home. Some use the sun, some remember how much they have turned and how far they have moved. Whatever most ants can find "home", as long as it is not too far away.

Finding home is quite clever, but the really clever bit about ants is that they can cooperate. Obviously for cooperation you need some sort of communication. Well ants often use a fairly indirect method of communication, they lay pheromones. Pheromones are just chemicals that ants can "smell" using their antenae. A high concentration of pheromones can attract ants.

The strategy used by the "ants" in the applet on the right is quite simple.

• Each ant has two states, foraging and going home.
• If an ant is going home it does just that, until it gets home, where upon it turns around and starts foraging again. As an ant goes home it drops pheromone, which spreads slightly and dissipates over time.
• If an ant is foraging it tests in front for pheromone and turns towards the direction where there is a greater concentration of pheromone. The ants also randomly turn slightly, so that they will explore new area, when there is no pheromone to be found. If an ant collides with some food it picks it up and starts going home.

Using this strategy ants will quickly form a "trail" between home and the food source. Most of the ants will be on this trail somewhere. When the ants get home they turnaround, then they are facing in the direction of the food. Because there is a pheromone trail to the food they are more likely to stay in the direction of the food as they move.

The pheromone evaporates with time. So if there are not enough ants collecting food and dropping pheromone on the way home, the trail may disappear completely, and won't reappear until an ant randomly walks onto the food source again. You can experiment with how many ants are needed to keep this trail going, with the slide bar on the applet.

It should be noted that using pheromones is only worthwhile when food sources are large and small in number. If one ant can carry all (or most) of the food it finds back to the nest then there is little point in getting more ants to help out.