Science: To see like the animals

Being human means we miss out on a dazzling world of ultraviolet colour that is vital to the survival of many species. By Jerome Burne

Imagine being able to see the world in black and white only. The attraction of sunsets, flower gardens, coral reefs and most painting would be lost on you. Then imagine that for some reason people couldn't explain about colour to you, but you had to find theories to explain why they spent so much time daubing at canvas or snorkelling. Most of your explanations would probably be pretty daft.

This is not an idle exercise. It pretty well describes the problems that arose in study- ing animal behaviour until very recently. Humans can't see the short- wave ultraviolet (UV) end of the spectrum, but most birds and insects and some reptiles can. "Without an adequate understanding of the perceptual cues to which an animal is responding it is impossible even to start to explain its behaviour," says Dr Martin Tovee of Newcastle University. "Many behaviour studies, especially on birds, will have to be junked."

Not just birds: a recent report in "Trends in Ecology and Evolution" pointed out that spiders' webs, far from being passive devices that insects fly into, actually lure them in by reflecting UV. In fact, some spiders whose silk is not UV-reflective put UV reflecting strips on their webs, and increase their capture rate as a result. The UV factor is only just beginning to be taken into account.

To get an idea of how much humans are missing by not being able to see in the UV range, just visualise the rich gradation of colours we can see between red and green. Animals with UV capability have a similar range available to them between blue and ultraviolet that we lack completely. Seeing UV probably involves the same brilliant contrasts that we experience under long-wave ultraviolet light in a nightclub, where every speck of dandruff shows up on the shoulders of a dark suit - but with the benefit of full colour as well.

Human colour vision is composed from three pigments, and we can just about take a stab at imagining what it must be like to be a bird with four, or even five, pigments - a range which makes our much-vaunted 16 million colours on a TV monitor look pretty lame. But we can only contemplate in awe the capabilities of the mantis shrimp, the colour vision champion of the animal world. This amazing creature has no fewer than 16 pigments, six of them in the ultraviolet spectrum. As yet, we have little idea of how this incredible array is integrated or how it is used. A major problem for researchers is that, after one meal, mantis shrimps are often inactive for days.

Quite a number of aquatic animals have UV vision which they use for hunting and to avoid being eaten themselves. The "nightclub" effect of ultraviolet - putting objects into sharp contrast - is very useful because, without it, the silvery bodies of fish become almost invisible in the sea. Among salmon, only the juveniles can see UV. This is because they stay near the surface and feed on plankton, which reflects those wavelengths. However, as they get bigger and swim deeper in the ocean they lose the ability.

Because UV light has a short wavelength - compared with, say, red light - it is more easily broken up and scattered by tiny particles of dust or oxygen molecules in the air. This means that light sources such as the sun usually have a fuzzy ring of polarised UV light round them, even when covered by cloud. It is this feature of UV that was probably first exploited by animals for navigation.

Ants, for instance, have an array of ultraviolet receptors around the side of their retina that they can use to orient themselves to the polarised light still visible from the sun when it is obscured. Birds use a slightly different system to locate the sun, which exploits the difference between the shorter UV waves, which get broken up, and the longer waves which are more evenly distributed across the sky.

But UV is now widely used for all sorts of functions. The activity in your garden on a summer's evening, for instance, might look rather different if you could see in ultraviolet. Insects landing on apparently white flowers could be seen to be homing in on clear markings, pointing up the best landing pad. (One of the effects of humans' insensitivity to UV is that, in breeding plants for smell or colour we may unknowingly lose these UV markings, cutting down the plant's chances of pollinated.) Female butterflies would be looking out for particular markings on the male's wings, only visible in the UV range, which indicate that they are younger, and so more likely to donate a hefty packet of sperm. If you were lucky enough to have a hawk circling above, it could be looking out for the UV-reflective urine trails of voles and other rodents.

"Sensitivity to UV light seems to present in all major animal groups," concludes Dr Tovee. "But we are only just beginning to understand how it works and how they use it. We are almost certainly in for some surprises."