A new study by Dr Heather Whitney from Bristol’s School of Biological Sciences, Professor Beverley Glover from Cambridge’s Department of Plant Sciences and colleagues, shows that the petals of iridescent flowers are tailored to be the perfect signal for bees.

When looking for nectar, bees need to be able to spot a flower from a distance and recognise which coloured flowers are full of food for them.  The researchers found that iridescence – the shiny, colour-shifting effect that you see on soap bubbles – makes flower petals more obvious to bees, but too much iridescence confuses the bees’ ability to distinguish the colours that help bees focus on the most food-filled flowers.

While nature can produce amazingly brilliant and vivid iridescence (such as that found on peacock’s tails) this study found that flowers use a more subtle, or imperfect, iridescence on their petals.  This ‘imperfect iridescence’ actually creates an ideal signal for bees such that the flowers are more obvious but it doesn’t interfere with the bees’ ability to distinguish subtly different colours, such as different shades of purple.  More vivid, perfect iridescence (like that found on the back of a CD, for example) would make it more difficult for bees to distinguish between subtle colour variations and cause them to make mistakes in their flower choices.

Dr Whitney said: “In 2009 we reported that some flowers can be iridescent and that bees can see that iridescence – but since then we have wondered why floral iridescence is so much less striking than other examples of iridescence in nature.  We have now discovered that floral iridescence is a trade-off that helps flowers to be detected by bumblebees, but won’t interfere with their ability to recognise different colours.”

Bees use search images to remember which coloured flowers are a good source of nectar.

Professor Glover said: “On each foraging trip a bee will usually retain a single search image so if they find a blue flower that is rich in nectar, they will then visit more blue flowers on that trip rather than hopping between different colours.  If you watch a bee on a lavender plant, for example, you’ll see it visit lots of lavender flowers and then fly away – it won’t usually move from a lavender flower to a yellow or red flower.”

This colour recognition is vital for both the bees and the plants, which rely on the bees to pollinate them.  If petals were perfectly iridescent, then bees would be unable to identify and recognise which colours are worth visiting for nectar.  Instead, flowers have developed the perfect balance of iridescence that allows them to talk to bees in their own visual language.

The researchers created replica flowers that were either perfectly iridescent (using a cast of the back of a CD), imperfectly iridescent (using casts of natural flowers), or non-iridescent.  They then tested how long it took for a colony of bees to find the flowers.

They discovered that the bees were much quicker to locate the iridescent flowers than the non-iridescent flowers, but it didn’t make a difference whether the flowers were perfectly or imperfectly iridescent.  The bees were just as quick to find the replicas modelled on natural petals as they were to find the perfectly iridescent replicas.

When they tested how fast the bees were to find nectar-rich flowers amongst other, similarly-coloured flowers, they found that perfect iridescence impeded the bees’ ability to distinguish between the flowers – the bees were often confused and visited the similarly-coloured flowers that contained no nectar.  However, imperfect iridescence, found on natural petals, didn’t interfere with this ability, and the bees were able to successfully locate the correct flowers that were full of nectar.

“Bees are careful shoppers in the floral supermarket, and floral advertising has to tread a fine line between dazzling its customers and being recognisable,” said Professor Lars Chittka from Queen Mary University of London, another co-author of the study.

“To our eyes most iridescent flowers don’t look particularly striking, we had wondered whether this is simply because flowers aren’t very good at producing iridescence,” said Dr Whitney. “But we are not the intended target – bees are, and bees see the world differently from humans.”

“There are lots of optical effects in nature that we don’t yet understand. We tend to assume that colour is used for either camouflage or sexual signalling, but we are finding out that animals and plants have a lot more to say to the world and to each other.”

Dr Whitney and colleagues in the School of Biological Sciences are now working towards investigating how the disruptive effect of vivid ‘perfect’ iridescence might be being used by animals and plants as a form of camouflage.

Paper

‘Flower Iridescence Increases Object Detection in the Insect Visual System without Compromising Object Identity’ by Heather M. Whitney, Alison Reed, Sean A. Rands, Lars Chittka and Beverley J. Glover in Current Biology