Bumblebees like it hot

3 August 2006

It's not just humans who warm up with a hot drink. Bumblebees do too, new research reveals. The clever insects can even learn to forecast nectar temperature before landing, using flower colour as a guide.

Antenna finds out more...

This research was published in the journal Nature on 3 August 2006.

Busy bees are warm little creatures - as you can see from this infrared picture.

Image: B Bujok, M Kleinhenz and J Tautz, BEEgroup Wuerzburg

Bees have to warm up before they can begin buzzing around. And to stay busy they have to keep their little bodies above air temperature. Like us, they can warm up by moving - uncoupling their flight muscles from their wings and shivering. But this strategy is expensive in energy terms.

Image: Katia Grimmer-Laversanne

Now scientists from the Universities of London and Cambridge have discovered that bees have a much more intelligent plan. By drinking warm nectar they can raise their body temperature without using up their own energy reserves.
But that's not all. Using petal colour like a signpost, bumblebees can learn to predict which flowers have the most warming rewards, before they bother to make a pit stop.
Putting bumblebees to the test
To uncover the bumblebees' tactics the scientists devised a simple yet effective experiment. 'We set up an artificial habitat with a nest box for a bumblebee family and a flight arena so we could watch the bees foraging.
'Then we faced the bees with a choice of artificial flowers. These are better than natural flowers because we can control the bees' foraging options - the amount, temperature and concentration of nectar available and the flower's colour,' says bee expert Lars Chittka.

These artificial flowers may not look convincing to you, but to a bumblebee they provide all a flower's rewards - nectar and warmth.

Image: Adrian Dyer

The scientists painted the artificial flowers pink and purple to look like wild snapdragon blooms. Real pink snapdragon flowers are naturally cooler than purple ones, so to start with the fake flowers followed the same pattern.

Snapdragons come in many colours, including white, pink and purple.

Image: Adrian Dyer

Most of the bees chose to feed at the warmer purple flowers. But when the scientists switched the colour cue over, so purple flowers contained cooler nectar, the bees quickly learned the new pattern.
'Bumblebees have fantastic learning ability,' says Lars Chittka. 'They can categorise objects and can even eavesdrop on each other's choices. They're very keen learners and really cooperative.'

Lars Chittka, bee behaviour expert, University of London

Petal puzzle put together
The researchers' results didn't just reveal the intricacies of bumblebees' foraging behaviour. They also explained a riddle that has baffled plant scientists for years.
About 80% of flower species have petals with a peculiar surface structure made of 'cone cells' - but it's been a puzzle why. Cone cells act like little lenses, focusing light into the middle of a cell, where a flower's colour pigments are.

This is what a petal's cone cells look like under a high-powered microscope.

Image: Heather Whitney

'We think this lens-like action helps warm the whole flower up, and the nectar produced by the flower with it,' explains plant cell expert Heather Whitney from the University of Cambridge.

Heather Whitney, plant cell expert, University of Cambridge

'Pink snapdragons - like the ones we mimicked in this experiment - have a genetic mutation that means they don't have cone cells. So their nectar is cooler than nectar from the purple variety that does,' says Lars.
'Our study sheds new light on cone cells - they're an advantage for plants because pollinators like bees are attracted to warmer nectar. And bees can learn to use colour cues to tell which plants these are.'

This is what the surface of a petal looks like when it doesn't have cone cells.

Image: Heather Whitney

So could you plant your garden with flowers to attract a bumblebee bonanza?
'This is a difficult one', says Lars, 'because we don't know the "warmer species" yet. In general it would make sense to assume that flowers that are darker, such as violet or blue, will absorb more light than white and yellow flowers. Since absorbed light is converted to heat, darker flowers should in theory be warmer.

Image: iStockphoto.com

'A comparative study to test this with a large number of plant species is on the way, but until then this is a bit of guesswork. But gardeners could of course place themselves at the forefront of science and try this out for themselves, using the rationale above - darker colours equals more warmth - and see whether these get more visits by bees!'
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