Naked Science Forum
Life Sciences => Plant Sciences, Zoology & Evolution => Topic started by: thedoc on 07/08/2012 23:30:02
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Charles Turner asked the Naked Scientists:
Hi Chris,
Can you explain how and why birds have the ability to see ultraviolet light... What is the reason for this.
Charles Calgary Canada
What do you think?
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Actually, the situation is more the other way round. Why are mammals not able to see UV light?
Current understanding based on genomic studies (published e.g. in scientific american) is the following: Initially the vertebrates had 4 different colour receptors including one into the UV wave length. Those receptor cells also had (and in birds still have) a colour filter which makes the absorption lines very sharp so that slight differences in colour can very well be distinguished. At dinosaur age the mammals were living in dark dens and were mainly night active. So they lost the filters and two of the colour receptors in favour for better sensitivity to light in the dark. Later some of the mammals (like humans) evolved a copy of one of the receptors to have 3 different colour receptors again (but not in the UV area). However these two copies are still very similar so that wrong genetic crossing over sometimes leads to loss of one of them, which is the reason for the common red-green blindness.
So compared to us birds have 4 receptors instead of 3, those receptors are more evenly distributed with regards of the wavelength they absorb and have much sharper absorbtion lines. So you can roughly assume that a bird can distinguish 4 times more colours than we do (or comapred to birds we are colour blind). The main usage of this colour sensitivity is to provide contrast. For example in UV light they can see a mouse trace in the grass (caused by the urine).
Reference: Goldsmith TH (2006) What Birds See. Scientific American July 2006.
(I have read the German version in Spektrum der Wissenschaft, but I assume it is the same content)
I hope that answers a bit your question, Jens
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Next time you are in a 1980s disco under UV light have a look at someone's eyes.
You will see that they fluoresce a bit.
The cornea and lens of the eye absorb UV (and re emit some of it as visible light).
Since this UV is absorbed, it can't reach the retina and form an image.
There are people who, having had the lens replaced by a plastic one (as a treatment for cataracts) can see into the UV.
The number of different receptors influences how many colours you can see, but it doesn't stop you seeing UV.
Most birds have eyes rather smaller than ours. The light has less material to travel through and so more of the UV gets through to the back of the eye and forms an image.
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(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2FBild2.png&hash=b4b25780e039244507e2da538328b17d)
You are right.
However, what I meant with beeing able to see UV light was not just see it as lighter and darker (via the rod cells) but actually see it with the sense to distinguish it from other light (via the cone cells). A bird can distinguish a mixture of light which contains much UV and less from another wave length from the opposite. We cannot.
In the picture below you see the same flower. The right photo is shot with an UV camera. What humans see is just the left picture. Birds are able to see a mixture of both (as different colors).
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"There are no colours!"There are only different wavelengths of the electromagnetic spectrum that our brain perceive as colour. Some birds and other creatures have detectors (Cones) in their retinas that can detect wavelengths that are beyond the ones our eyes can detect. Humans have just three cone types that detect wavelengths that we perceive as blue, green, and red which are the same wavelengths used in colour tv, using mixtures of these colours it is possible to recreate all the hues that we see as colour. pigeons have at least four types of cone and possibly more. one of these cone types is believed to be receptive to UV, but what colour a pigeon perceives it as is unknown.