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Bees Like It Hot: Pollinators Prefer Warm Flowers, Study Reveals

Scott Norris
for National Geographic News
August 2, 2006
 
Bumblebees prefer their food warm and learn to locate hotter flowers using color as a cue, scientists say.

The findings may have broad implications for the evolution of flowering plants.

To attract insect pollinators, flowers offer a nutritious reward of nectar and pollen. Now biologists say many flowers may encourage visits by offering a "heat reward" as well.

By consuming warmer nectar, bees may save energy they would otherwise have to spend maintaining their own body temperature.

"Bees can raise their body temperature to above 37 degrees Celsius [98.6 degrees Fahrenheit], even if it is just a few degrees above zero outside," said Lars Chittka of Queen Mary College, University of London.

"But this is costly, so collecting warm nectar is a clever idea."

Many flowering plants have features that allow them to increase the temperature of their flowers.

The scientists suggest that providing hot meals might be a shrewd evolutionary adaptation for plants, whose own reproduction depends on attracting pollinators.

The research will appear in tomorrow's issue of the journal Nature.

(Related story: "Buzz Kill: Wild Bees and Flowers Disappearing, Study Says" [July 21, 2006].)

Heat Seekers

Previous work has shown that some insects are attracted to warmer plants. In tropical rain forests, for example, scarab beetles spend much of their time deep inside flowers capable of generating heat through chemical reactions.

But scientists didn't know if flower temperature is important to more active and widespread pollinators such as bees, which visit each blossom for only a short time.

Chittka and colleagues designed a set of experiments to test the effects of flower temperature on the behavior of pollinating bumblebees.

They laid out a range of flower-shaped feeders holding equal concentrations of sugar solution that varied in temperature. The bees gravitated toward the feeders offering the warmest nectar.

The researchers then placed food solutions in pink or purple feeders.

The bees quickly learned to distinguish between colors and concentrated their foraging on the warmer nectar source.

The results suggest that the bees learned to use color as a signal of temperature and utilized this information in choosing which flowers to visit.

University of Arizona biologist Daniel Papaj says the study is intriguing. But it remains to be seen whether bees in nature commonly use sensory cues to identify warmer flowers, he notes.

If bees do this, Papaj said, "one might expect such behavior to have demographic consequences.

"For instance, flowers in sunnier microhabitats would be more likely to be pollinated."

Bag of Tricks

The fact that pollinating insects may be choosy about temperature suggests a new explanation for a number of features that help plants keep their flowers warmer than the surrounding environment.

A small number of plant species generate heat through metabolic processes, similarly to animals.

Many others use more passive means to gather and retain warmth. The shape of many flowers—and the ability to track the sun's movements—helps make them efficient collectors of solar energy.

Some species even have cone-shaped cells in their petals that focus sunlight, increasing the temperature of the flower.

"Many of these features had been previously thought to only enhance the color of the flower, or else warm the flower itself up to help its seed develop faster," said study co-author Heather Whitney of the University of Cambridge in England.

"Now we know that warming structures could be part of the bag of tricks that flowers have evolved to attract pollinators."

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