Stanford Today
Edition: May/June, 1996
Section: Science & Medicine News
WWW: Stanford Today - Down on the (Ant) Farm

Thousands of years ago, one of the authors of the Old Testament observed nature carefully enough to notice that ants keep busy without anyone telling them what to do. Modern biologists have long known the same thing ­ that ants, honeybees, termites and other social insects can marshal just the right number of forces to forage for food, repair the nest, care for eggs and battle off intruders, without any central or hierarchical control.

There’s no ant boss, termite overseer or bee middle manager shouting, “Hey you! The nest needs urgent repair, so stop collecting food!” says Deborah M. Gordon, assistant professor of biological sciences.

Yet somehow social insects divide up tasks and switch from one task to another when the need arises. Gordon and other biologists would like to know how they do that.

In a progress article in the March 14 issue of the journal Nature, Gordon writes that research on social insects has shown that the task a worker insect performs partly depends on internal factors, such as the individual’s size or age. But in the past decade, studies have shown that the insects also respond to external factors. They choose to rest or rush to work and they switch tasks rapidly and often, in response to cues from the environment and from the actions of other individuals.

Gordon says that the actions of a colony of ants or bees are like the many specialized cells produced as an embryo develops, or like the firing patterns of neurons in the brain. In each case there is no central headquarters giving orders, and the individual cells do not start out with a predetermined task.

“A single neuron does not think ‘10’ or ‘coffee cup,’ ” she says. “Its function depends on what other neurons are doing at the same time. No single neuron can think, but the brain can think.”

In the 1970s and early ’80s, most researchers thought that social insects were like super-specialized assembly line workers, with each individual suited to only one task. In some ways this is true, Gordon says: For example, some species of ants come in two sizes, small foragers and giant-sized soldiers. However, recent research has shown that even worker insects predisposed to do one task sometimes will switch to another if the colony’s need is urgent enough.

Gordon says that an insect colony is like a computerized neural network, or like a mammalian brain, in the sense that individuals making simple decisions together do complicated things.

But how does an individual ant know what to do and when to do it?

“Workers might use some simple rule based
on the rate of encounter with others,” Gordon says. “Say a forager expects to meet another every 2 seconds, and if she does, she goes on foraging. But if she starts to meet other foragers every 0.5 seconds, she stops foraging.

“If the number of foragers goes up, she will experience a higher interaction rate. Using this simple rule, the worker can respond to a change in worker number without having to count anything global, only having to assess the interval between contacts that she experiences.” ST