They spend more time choosing between a strong and a weak smell if the difference is small. The research links this deliberation to a particular gene, FoxP, and the activity of fewer than 200 neurons.
Mutations in FoxP, also associated with cognition and language in humans, made flies' decisions even slower without affecting which choice they made.
Gathering information before committing to a decision is a hallmark of intelligence. If the information is unclear, the choice is trickier and the decision takes more time. We do it, other primates do it, even rats and mice do it - but now it seems that flies do too.
"This is the clearest evidence yet of a cognitive process running in a very simple brain," said Prof Gero Miesenböck, whose team did the work at the University of Oxford's Centre for Circuits and Behaviour.
"People tended to think of insects as tiny robots that just respond reflexively to signals from the environment. Now we know that's not true."
After training fruit flies to avoid a new smell at a specific intensity, the researchers offered them a choice between that dangerous odour level and a weaker one. The flies did well when the safe option was four or five times weaker, but chose randomly if the difference was only 10%.
Crucially, as the differences became smaller and trickier to distinguish, the flies took more and more time to make a decision, waiting much longer in an intermediate zone between the two odour levels.
This is a pattern that psychologists have studied for many decades. "The same mathematical models that describe human decision-making also capture the flies' behaviour perfectly," said Prof. Miesenböck.