Whether by compelling us to follow the latest fashion or to practise good table manners, the company we keep is known to have a strong influence on our behaviour. But the effect of society, it seems, can reach even deeper. Joel Levine and colleagues have found that internal biological clocks — those that regulate our sleeping rhythm, for example — can be reset by social interactions. Although this work was done in fruitflies, and the external cues were found to be olfactory ones, it is possible that the clocks of other species respond to the social environment in a similar way.

In all experiments, flies were exposed to a light–dark regime for five days, then placed in constant darkness for two weeks, after which time their clock rhythm — based on locomotor activity — was scored. The clocks of grouped flies were more synchronous after this treatment than were those of flies treated in isolation. If, as this result implies, company keeps flies in time, then genetically asynchronous flies would be expected to disrupt the harmonized clock when added to a wild-type group. This was tested by mixing flies mutant for the period gene (per), which have no sense of time, with wild-type ones, which indeed lost their synchronicity. Curiously, per mutants with early activity peaks ('early birds') were able to influence the activity of 'later-rising' per mutants, but not the other way around.

So, what is it about company that synchronizes the clocks of these flies? Simply exposing individual animals to the air from a chamber in which a group of flies was kept was enough to synchronize their clocks. As the effect was abolished when the receiving flies had no sense of smell (because of a specific class of mutation in the paralytic gene), the authors settled on olfactory cues as being a good explanation.

Clocks and social interactions have been linked in many species, from humans to bees, but this is the first study to sniff out — genetically — the underlying sensory cause.