How colour can help in the mating game?
Chris - Now for the final part in our science of colour series, Naked Scientist Anna Lacey has been looking at how animal coloration can be helpful in finding a mate, and how some colours that animals use are completely invisible to the human eye. But to kick us off, Anna's going to try a little experiment, guess where? In her favourite place, the pub.
Anna - I've come down to my local pub to see if I can woo some unsuspecting males with the colour of my clothes. So excuse me there gentlemen, there's a nice table of three gentlemen here, what do you think of my maroon stripy t-shirt and my blue jeans?
Man 1 - Very attractive actually. I'm really liking the stripes on your top.
Anna - Well thanks very much, that's very kind. And what about you?
- I would say something very similar. I think the maroon suggests a very bright colour and a very vibrant colour. Suggests a lot of fun.
Anna - So now to the third gentleman here. What would you think about me based on the colour of my clothing for potential mating opportunities and perhaps being a partner, whatever?
Man 3 - Well my girlfriend would kill me for saying this but I very much like the maroon, but that the denim's a little bit standard.
Anna - Ok, well that actually turned out quite well for me. Thanks for that guys. But it turns out that a whole range of animals, and birds in particular, actually use their own version of clothes or brightly-coloured feathers to try and bag a mate. So to find out why these colours are so important, I went to the botanic gardens to speak to Cambridge University zoologist Nicola Nadeau.
Nicola - Why females would be particularly attracted to these colours is an area of debate at the moment. But it's thought they could indicate the fitness of the bird. So if these colours are particularly costly to produce, then that could indicate to the female that I'm a particularly strong and healthy male.
Anna - Isn't it also the case that the colours that we see in birds aren't necessarily what the birds are seeing?
Nicola - Yes, so we've only got three types of photoreceptor in our eyes and these can detect green and red and blue. Whereas birds have a fourth, and this can detect ultraviolet light, which we can't see. So for example blue tits, they look the same to us but the males have these ultraviolet ornaments that we can't see.
Anna - So are they using that again to try and attract the females?
Nicola - Yes, so it's thought that healthier males have brighter UV ornaments and the females can detect these and use them in mate choice.
Anna - Lots of animals use UV ornaments and patches to attract mates, but how on earth do you go about making the colour UV in the first place? Well it turns out that you can actually do it in two different ways. Now one of these is with pigments, so something equivalent to a paint, and the other is by something called structural colour, and for that to work you need to use a process called interference. Now this works by having tiny barbs or scales that act a little bit like reflectors, and these are spaced at specific distances so they reflect certain wavelengths or colours of light, and cancel out all the others. Mike Majerus, a Professor of Evolution, explains a little bit more about this and how it works in butterflies.
Mike - Butterflies are in the insect order Lepidoptera, which simply means 'the scale wings'. The wings, if you've ever touched a butterfly with your fingers, you get all these scales just rubbing off and they're literally covered with thousands of these tiny scales. That gives them this enormous capacity for fabulous colour patterns.
Anna - So how does the colour actually form through the structure of those scales?
Mike - The scales, they're like tiles on a roof. They're placed very close together at an angle, and at the top end, you'll get a tiny gap. Now the distance between it causes an interference effect as white light hits it, and just by putting the scales at greater or lesser distances, you can get different physical reflectances.
Anna - So what kind of colours can you get with this structure with the scales? What's the potential for that?
Mike - Oh for the reflectance patterns, they can be virtually any colour, but the most common are blues and greens. Once you go into the chemical pigments, you can get any colour you like; all the colours of the rainbow.
Anna - But for birds like blue tits, some pigments are actually quite rare. Here's Nicola again.
Nicola - Well in general, there aren't any blue pigments that are used in birds so all blues will be structural.
Anna - So the garden blue tit, it's not actually got any blue dyes if you like in its feathers. It's all to do with the structure of its feathers reflecting light in a certain kind of way.
Nicola - Yes exactly. So if you took the feathers and mushed them up, they wouldn't appear blue. It's just purely the structure of the feathers that's producing that colour.
Anna - So it turns out that colour's not just about slapping on a bit of paint. Because of all the different colours out there, some are made by pigments, some are made by tiny structures in the wings and feathers of the animals, and some colours just need a bit of both. And what's even more amazing is that there's a whole load of these colours that we can't even see. But what we have seen over the past few weeks is that colour spans a whole range of disciplines from biological greens to chemical purples, and also how it's completely revolutionised the world of fashion, medicine and got far enough under our skin to change our behaviour. So by just looking at the word 'colour', what we've actually done is taken a journey through physics, biology, chemistry and psychology. Meaning that we've not only learnt about the science of colour, but a little bit about the diversity and colour of science.