Some (Bumblebees) Like It Hot

Flower colour is traditionally viewed as a method by which a plant advertises its nectar, a sweet reward for ensuring pollination, to hungry insects. However, a recent collaborative study by the University of Cambridge and Queen Mary University London has revealed that bees will choose flowers of a certain colour if they have learned that it indicates warmth as well.

Dr Heather Whitney, Miss Sarah Arnold, Dr Adrian Dyer and Dr Beverley Glover, Department of Plant Sciences, University of Cambridge, and Professor Lars Chittka, Queen Mary University London, published their findings in this week's issue of Nature.

Dr Whitney stated, "It has been observed that flowers with warming structures attract basking insects, and previous work has shown that insects can obtain a metabolic reward from warmer flowers. However, this is the first time it has been shown that insects can use other cues, such as colour, to preferentially seek out warmer flowers."

These findings indicate that varying temperatures of plants may be an adaptation to encourage pollinators to visit flowers. As flower temperature varies widely, it is believed that the heat may influence which plant the pollinator chooses to visit (depending on the insect's temperature preference).

Like many other insect pollinators, bumblebees invest energy in keeping their body temperature above the ambient level. On cold days, they must warm themselves up before they can fly far. Researchers discovered that, in an effort to conserve energy, bees will select warmer flowers identified by their colour to stay cosy. When tested, bumblebees consistently chose warmer flowers over cooler flowers containing the same nectar reward.

Dr Glover stated, "We're very excited by this result as it suggests that a whole range of structures act as potential pollinator attractants. We can now re-evaluate the roles of lens-shaped petal cells, sun-tracking by flowers, light and heat absorbing pigments and specialised surface structures, all of which may be part of a plant's bag of tricks for attracting pollinators."

The scientists hope that this study will shed some light on why specific floral structures have evolved, as well as on the connection between floral sensory cues, floral temperature and pollinator behaviour.