Mellow fruitfulness: UV colour clue

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  • Published: Oct 1, 2018
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Mellow fruitfulness: UV colour clue

Reflecting on UV

Fieldfare and bright red pyracantha berry, photo by David Bradley Reflections in the ultraviolet could be key to how different fruits have evolved their colours to attract those animals that eat them and help spread their side. The colours of red plums and cherries, yellow, green, and orange melons, purple figs and aubergines, and every other hue exist in nature.

Reflections in the ultraviolet could be key to how different fruits have evolved their colours to attract those animals that eat them and help spread their side. The colours of red plums and cherries, yellow, green, and orange melons, purple figs and aubergines, and every other hue exist in nature. Usually to entice a fructivore to ingest the seed-bearing fruit and release the seeds elsewhere in the neighbourhood when it discards pieces or releases ingested seeds in its faeces.

For those of us in the Northern hemisphere we know that the season of mists and mellow fruitfulness is upon us. With a few exceptions a long, hot and dry summer seems to have led to an abundance of berries and fruit in the English countryside for instance. It's as if the heat and lack of rain stimulated the plants to produce more offspring. So, we now have many bright red berries that will signal to hungry birds and especially the winter visitors to the UK such as the Fieldfares and Redwings, that those berries are good to eat. Of course, we have had a hunch since the late 1800s that the colours of fleshy fruits evolved to grab the attention of animals with colour vision.

Visionary explanation

There was, however, only mixed evidence for this evolutionary hypothesis. One of the problems that stymied a working model was that we assume, wrongly, that other animals experience the colours of those fruits in the same way as we do. But this is now known not to be the case at all. We might thing of reds, yellows, and oranges as being enticing in the fruit we select ourselves but many animals have vision that stretches beyond what we refer to as the visible spectrum, into the near ultraviolet.

There are good reasons to doubt whether red to us looks the same way it does to, say, a blackbird. Humans have three types of colour-sensing cone cells in the retina of their eyes, each one sensitive to a different band of wavelength of light. But most other mammals have only two types of cone cells. Birds are tetrachromats, they have four, which helps them see a range of hues most humans cannot see. There is evidence that some women have four types of colour cones in their retina and may have a great colour palette than the rest of us in their visionary picture of the world around them. Generally, though a fruit that looks darkly deep purple to us, for example, may actually reflect ultraviolet rays that birds perceive as bright in that part of the spectrum.

Aping the birds

"With the exception of a handful of other primates, no other animal on Earth sees colour the way that we do," explains team member Kim Valenta, an evolutionary anthropologist at Duke University, USA. Moreover, much research has neglected to take into account other reasons that might explain why fruits have evolved specific colours. For example, some plant species may simply take after their closest genetic relatives, bearing fruits that are pink or brownish because their common ancestors did too. Or fruit colour could be a product of environmental factors such as latitude, temperature, or properties of the soil.

We can see that fruits eaten mainly eaten by mammals, such as monkeys and apes, often have higher reflectance in the green part of the spectrum whereas fruits eaten by birds reflect more in the red. The team also found that those plants whose fruits reflected ultraviolet light also tended to have ultraviolet-reflecting leaves. This suggests that there may well be an environmental factor at play in those species.

Related Links

Sci Rep 2018, online: "The evolution of fruit colour: phylogeny, abiotic factors and the role of mutualists"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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