By Mark Kinver
Science and nature reporter, BBC News
Bumblebees prefer to visit warm flowers and can use colour to predict the bloom's temperature, research suggests.
Bees like it hot (J Tautz, M Kleinhenz and B Bujok/BEEgroup Wurzburg)
The findings challenge the long-held belief that the insects seek out flowers that contain the most nectar or pollen.
UK researchers say the bees might use warmer blooms to help maintain their body temperatures and save energy.
The study by scientists from Cambridge University and Queen Mary College is published in the journal Nature.
To test whether bumblebees (Bombus terrestris) could use colour to identify warmer flowers, the team used a variety of differently coloured artificial plants.
In a "foraging bout" the creatures were given a choice between four purple flowers or four slightly cooler pink ones that were placed in a random order.
In one test, 58% of the bees chose the warmer purple flowers. When the colours were switched and warm nectar was placed in the pink petals, 61.6% headed for the pink blooms.
Lars Chittka, from Queen Mary College, University of London, and a co-author of the paper, said the tests showed that bees preferred warmer plants and could learn to identify the hotter species by the colour of flowers.
"If we do not give the bees any cues by which they can identify the warmer flowers then they fail the task. If the flowers are visually identical then they will visit them all.
"What the bees need to do is collect individual experience," Professor Chittka said. "They have to probe the flowers, learn which ones have higher temperatures and then to identify them they use colours or spatial positions."
The team suggests that the temperature can serve as an additional reward from pollinating insects in a context where there are also nutritional rewards available.
"Bees need to warm up to fly, they need to have body temperatures of at least 30C (86F)," Professor Chittka said.
If the air temperature was relatively cold, it took a considerable amount of the insects' energy reserves to reach this temperature, he added.
"In that sense, seeking out flowers with warmer nectar is a direct metabolic reward; it supplies them with energy that they would otherwise have to invest."
The researchers believe the findings could also affect the current understanding surrounding the evolutionary link between plants and pollinators.
"About 80% of flower species have a peculiar structure in their flowers; the skin is made up of little cells that are cone-shaped. It has never been fully understood what function they served," Professor Chittka said.
"But one effect it does have is that the cones act as little lenses to focus light directly into the parts of the cells that contain the floral pigment; because more light is absorbed it warms the flowers - that's a clever trick.
"We think the fact that 80% of floral species have this, it could be a broad evolutionary innovation in order to generate warmth and thus lure pollinators to collaborate with them," he suggested.
Professor Chittka's co-authors on the paper were Adrian Dyer, Heather Whitney, Sarah Arnold and Beverley Glover from Cambridge University's Department of Plant Science.