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Flowers bloom red for honeyeaters


The visual system of honeyeaters is exerting selection pressure on the colour of flowers in Australia (Source: Mani Shrestha)

Seeing red There is bias towards a special shade of red in the colour of Australian bird-pollinated flowers thanks to the honeyeaters, new research has found.

"It's the best evidence to date to support conventional wisdom that flower colour evolves in response to the visual system preferences of pollinators," says lead author Dr Martin Burd, an evolutionary ecologist at Monash University in Melbourne.

The findings are reported today in the Proceedings of the Royal Society B.

It is generally believed that plants that share the same kind of pollinator have evolved to share similar characteristics, says Burd.

Flowers that are pollinated by birds, for example, are believed to have a bias towards a colour that suits the visual system of the specific pollinator.

To study this in Australian flowering plants, Burd and colleagues used a method called spectral reflectance to measure the wavelength profile of light given off by 80 bird-pollinated species and 154 insect-pollinated species.

They converted these profiles into a single colour value, using an algorithm based on bird pollinator colour vision systems.

"Different organisms will see the same wavelength profile differently depending on how their eyes work and how their brains cognitively interpret the signals that their eyes send them," says Burd.

"What's special about what we did is we analysed the colours of flowers not as we see them but as the birds see them."

Honeyeater 'language'

Bird pollinators have two main colour vision systems, both with blue, green and red receptors like humans. One group of birds, including honeyeaters, has an extra violet receptor, while another group of birds, which includes parrots, has an extra ultraviolet receptor.

The findings showed that the range of colours of bird-pollinated flowers tended to overlap with the colours of insect-pollinated flowers, with one exception.

About half the bird-pollinated flower species had a colour value that would have stood out to birds with the honeyeater colour vision system.

From the perspective of other birds this colour, which humans would perceive as a special shade of red, would have blended into the range of colours of other flower species.

Burd says the findings are not surprising given honeyeaters are the main bird pollinators in Australia.

"The flowers are talking honeyeater language because that's where all the action is," he says.

The researchers found that the number of species that had evolved to this red colour were much greater than could be explained by chance.

This is evidence, says Burd, that there has been significant evolutionary convergence in the colour of bird-pollinated flowers in Australia.

"Presumably the driving force is the visual system of the honeyeaters," he says.

While the main bird pollinator in North and South America has a colour vision system similar to honeyeaters, the main bird pollinator in Asia and Africa has an ultraviolet vision system.

"So we would expect the same kind of evolutionary convergence to have occurred among American flowers as occurred in Australia, but we would not expect to find that in African and Asian flowers," says Burd.

He says preliminary results by him and colleagues support this.

Tags: botany, evolution