Visual visionaries of the animal kingdom
Most of us humans have 3 different types of cone cells, which are responsible for the wide variety of colour we can perceive out there in the world. But we’re hardly known as a species for having the best vision on Earth, so how do our eyes compare with others' in the animal kingdom? Katie Haylor met Cambridge University neuroscientist Simon Laughlin at the university's museum of zoology, where he explained how us humans compare to other animals...
Simon - Well, the majority of mammals only have two types of cone. They have two colour channels. And this means that their colour vision is not as good as ours. In general a dichromat can only see something like one 10th the number of colours that are trichromat can see. In humans we have split the long wavelength part of the spectrum into two components, a green and a red. Whereas in most of the mammals, they have a blue or an ultraviolet and a green.
Katie - So what makes us so special? Why have we got three?
Simon - Almost certainly because a lot of primates are vegetarian. They eat fruit and leaves and trichromatic colour vision gives you much better ability to judge the colour, the quality, the ripeness of leaves and fruits. You can get around the world perfectly well with dichromatic colour vision. Indeed, as everybody knows from black and white photographs, you can see many of the objects that we're interested in without any colour vision at all. What colour vision is particularly good for is judging the quality of objects, things like ripeness, constituents, chemical composition, and also to produce signals which can be used for mate recognition and for feeding and pollination and all sorts of things like that.
Katie - So let's talk about another trichromat. We're standing in front of a pretty impressive collection of bees.
Simon - So what we have is a display of many of the different types of hymenoptera, the family of bees and ants. You can see there's an astonishing diversity among the hymenoptera in body form and size. There's a giant wasp here, which is almost as large as the palm of your hand.
Katie - Some would say it's terrifying, others might disagree!
Simon - They do sting, although many of them would prefer not to. Most of them are not as aggressive as say, wasps domestic wasps. So bees have three different colour types of photoreceptor, but compared with our visual system, they're displaced to shorter wave lengths. So they have an ultraviolet receptor, a blue receptor and a green receptor. And probably next to human beings, bees are the animal whose colour vision is best documented and best understood. It's been really intensively studied because bees use their sense of colour to identify flowers in order to get nectar and pollen.
Katie - Does that mean when we look at a pretty flower and see the different colours, bees are seeing a whole host of other things going on in that same flower?
Simon - Yes. In the 1950s the famous bee neuroethologist Karl von Frisch took pictures of flowers in the ultraviolet and showed that they had patterns on them, very distinct ultraviolet patterns, that we couldn't see, but would be very salient for a bee.
Katie - I'm imagining they're a little bit like the air traffic control guys, who sort of signal a landing strip, that kind of thing?
Simon - So the flowers are using a combination of shape and color and odor to attract bees.
Katie - That's trichromatic vision. Does anyone have tetrachromatic vision? These are four types of cone cells.
Simon - Yes. In fact, tetrachromatic vision is probably the standard pattern for vertebrates. They're only eight and a half thousand species of mammal in the world, so they're really quite a small group of vertebrates.
Katie - So we're the odd ones out.
Simon - We're the odd ones out, yes. So fish and amphibia and reptiles and birds and therefore, by implication, dinosaurs, the standard pattern there is to have four different classes of colour receptor. And these generally extend from the UV through to the red, so they have the bee-type UV receptor plus three others.
Katie - Unfortunately, Simon didn't show me any dinosaurs, but we headed downstairs to find a rather impressive display of tetrachromats, some a little closer to home.
Simon - Down here we've got one of the smaller birds, which is commonly found in people's gardens. Many of you will have seen them. It's a blue tit. And the blue tit has this blue crown on its head. About 20 years ago it was discovered by taking images in the UV that in fact the crown is much more visible in the ultraviolet, to a blue tit's ultraviolet preceptors. And it led to a classic paper, which is called Blue Tits are Ultraviolet Tits. And this is a very nice example of animals generating a signal in a part of the spectrum that many other animals are unable to resolve. Mammalian predators like cats and so on will be unable to see this really vivid signal, which stands out to blue tits.
Katie - Oh, I see. So if you are blue tit and you're looking for a mate, then you can see this beacon flashing on their head that something that was trying to eat and wouldn't be able to see?
Simon - Yes. And there's some evidence that the brightness of the ultraviolet signal in your plumage is an indicator of what a good blue tit you are. You've been well nourished and so on. And so therefore you would be a more desirable blue tit to mate with. It's by using the ultra violet cone that the blue tits can see this very vivid signal. But of course, so can the eagle up there, and the hawks, it's not fail safe.
Katie - Back upstairs in the museum gallery, Simon and I continued upping the anti when it came to the number of cone cells in animal eyes. And this next creature is really impressive.
Katie - There's a lot of legs going on here, Simon! What's in the display cabinet in front of us?
Simon - Crustacea, that's crabs and lobsters. And we're looking at a particular group called the mantis shrimp. You can just about cover it with your hand. So it's a really big shrimp. So the mantis shrimp has the most complicated eye in the animal kingdom for analysing the wavelength of light coming in. It has 14 different colour channels. I should say that many birds have up to eight colour channels. Some butterflies have as many as 13. So having more than four colour channels is quite common, particularly among brightly coloured animals.
Katie - So what's the advantage of having such a massively complex visual system?
Simon - They probably do not have eight dimensional or 14 dimensional colour vision. It would just be too difficult for the brain to handle 14 different colour channels all at the same time. They're probably using very narrowly tuned receptors in order to be able to discriminate very vivid colour signals. The mantis shrimps are lethal animals. They have a giant claw on their forelimb, which they wind up with an elastic band. They release it and as it whips through the water, the tip of the claw actually is going faster than the speed of sound in water.
Katie - Wow.
Simon - And they smash it into shellfish and other crustacea and kill them instantly. They also kill and stun small fish. Now when you have such a lethal weapon, it's very important that you only use it on the right animals. And so the different species of mantis shrimp are very, very brightly coloured and they recognise mantis shrimps of the same species using these colours. And so they have this very accurate system for determining differences in the wavelength spectrum of light coming into the eye.
Katie - I see. So you've got a really precise visual system because you don't want to mistake a potential mate for lunch.
Simon - Yes, that's right. But they go much further. If you think that you had a retina with 14 different receptors, then any point on the retina is only going to be monitoring a small portion of the visual field and they get round this problem by putting all of the receptors in a big fat band across their compound eye. All the photoreceptors in that band are all looking in the same direction, and then they scan the band across the image to determine the spectrum of light at different locations.