Scientists just found out that lots of ants are "unemployed," and it might be the only reason their colonies survive disasters.
We think of ants as the hardest workers in nature. "Go to the ant, thou sluggard," the Bible says. But anyone who watches a colony long enough notices something strange. A lot of the ants aren't doing anything.
The ants that never work
For years, scientists tracking individual ants kept seeing the same thing. In most social insect colonies, about 20 to 30% of workers are inactive at any time. In one study of the acorn ant Temnothorax rugatulus, Daniel Charbonneau found that about 72% of workers spent at least half their time standing still.
And it isn't just ants taking turns. The same ants tend to stay inactive day after day. Some workers basically specialize in doing nothing.
That's a puzzle. Natural selection should favour colonies that get the most work done. So why would a colony feed and house thousands of workers that don't seem to do their jobs?
Taking out the workers
To find out, Charbonneau, Takao Sasaki and Anna Dornhaus ran a simple experiment. They painted individual ants so they could track them, then removed workers from 20 colonies.
- When they removed the busiest 20% of workers, the colonies kept working at the same level. Within a week, ants that had been inactive started working and filled the missing jobs, mainly foraging and caring for the young.
- When they removed the laziest workers, nothing much changed. The busy ants didn't slow down, and the colony didn't replace the lazy ones, even two weeks later.
So the colony protects its workforce, not its idlers. When workers are lost, "lazy" ants step up. The researchers concluded that inactive workers act as a reserve labour force. They might also work as living food stores for the colony.
What the simulations show
In Japan, Eisuke Hasegawa and his colleagues tested the same idea with computer simulations. They compared two kinds of colonies:
- colonies where every ant responds to work at the same time
- colonies where ants differ in how quickly they respond to work, so some ants usually stay idle
When workers never got tired, both kinds of colonies lasted equally long. But real workers get tired. Once fatigue was added, the colonies with "lazy" ants survived longer. When the hard workers wore out, the rested ants took over jobs that someone always has to do, like caring for eggs. In colonies where everyone worked at once, everyone got tired at once, and the colony suffered.
The team also found evidence of this happening in real ant colonies, with inactive workers replacing active ones. Their conclusion was that inactive workers lower a colony's short-term productivity, but they're necessary for it to survive in the long run.
The problem with being 100% efficient
It's tempting to see a colony as a machine that should run at full speed. But a machine running at full speed has nothing left when something goes wrong. A nest gets flooded. A raid kills the foragers. A disease spreads. A colony with spare workers can absorb the hit. A colony where every ant is already maxed out can't.
Researchers describe this as a kind of insurance, a bet-hedging strategy against an unpredictable world. The colony accepts getting a bit less done every day so that it can keep going when disaster strikes.
Because the problem with being 100% efficient is that it comes with being 0% prepared.
To learn how ants divide their work, read Ant jobs and the division of labour.
Further reading
- Hasegawa, E., Ishii, Y., Tada, K., Kobayashi, K. & Yoshimura, J. (2016). Lazy workers are necessary for long-term sustainability in insect societies. Scientific Reports.
- Charbonneau, D., Sasaki, T. & Dornhaus, A. (2017). Who needs 'lazy' workers? Inactive workers act as a 'reserve' labor force replacing active workers, but inactive workers are not replaced when they are removed. PLOS ONE.
- Charbonneau, D., Hillis, N. & Dornhaus, A. (2015). 'Lazy' in nature: ant colony time budgets show high 'inactivity' in the field as well as in the lab. Insectes Sociaux.
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