In 1947, pioneer ethologist Karl von Frisch reported that foraging bees, on returning to the hive, perform a `waggle dance' on the surface of the honeycomb. The dance, a figure-of-eight movement in which the worker waggles its abdomen in the middle, is used to transmit information about the location of a food source to other workers. In subsequent decades, the waggle dance has revealed some of its secrets, but a fundamental mystery remains: how do bees know that a dancing forager is on the comb? A beehive is a dark, crowded and noisy place, full of rustling bees, and the space between honeycombs is often extremely narrow – the insects cannot use visual cues to see if one of their sisters is dancing.
A group of researchers led by David Gilley from Tucson, Arizona, decided to see whether chemical communication could help explain the ability of the bee dance to recruit workers – this is what happens in bumblebees, which do not dance. Using a technique called solid phase micro-extraction, the authors put a highly absorbent fibre into the narrow space above the `dance floor' and then placed the fibre into a gas chromatograph to identify the substances that had stuck to the fibre. They found that the air around dancing forager bees contained substantial quantities of four long-chain hydrocarbons (two alkanes and two alkenes) whereas the air around non-dancing foragers showed significantly lower levels.
To test whether these chemicals had a behavioural function, they dissolved synthetic versions of three of these compounds in hexane, allowed the mixture to evaporate and injected the gas into an experimental bee hive where workers were dancing naturally. They discovered that the number of bees leaving the hive increased when the synthetic compounds were injected, compared to a hexane control.
These data suggest that dancing bees emit a set of pheromones that encourage foragers to leave the hive and forage, following the directional and distance information encoded in the angle and duration of the dance. The authors suggest that these pheromones may act by attracting foragers to the`dance floor', where they would then come into contact with the dancing bee.
However, we are still far from understanding the full story. How specific is the bees' response? The authors used only hexane as their control –it is possible that bees require a particular set of compounds or that they show similar responses to a wide range of stimuli. How does the message get transmitted? The substances involved are relatively heavy hydrocarbons(23°C and 25°C), which ooze out from underneath the cuticle. These waxy substances are widely used by insects for chemical communication, but they are not really `odours' and they are generally considered not to be volatile.
Furthermore, cuticular hydrocarbons are very sticky and would end up smeared all over the honeycomb, taking a substantial amount of time to evaporate. How do bees recognise the difference between a new signal emitted by a dancing bee and one left a few minutes, hours or even days earlier?Perhaps the bustling activity of the hive means that every worker around the stimulus rapidly gets a tiny amount of these substances on them, and the intense signal emitted by the dancer is soon `dissolved' among hundreds or thousands of insects.
By adding chemical communication to the rich mystery of the bee waggle dance, Gilley and his colleagues have shown that the buzz of the beehive includes its smell – or rather, its taste.
