To study and encourage popular interest in all branches of Science.

Newsletter August 2006

Dear Member,

I enclose a copy of the Society's lecture programme for the 2006/2007 session.

The first lecture on TORNADOES by Paul Knightley takes place at 8.15 pm on Thursday 21st September in the Crypt Room of St John's Church, Church Row, Hampstead.

Council for 2006/7

Membership of Council for this year is:

Festival of Geology

Julia Daniels reports that this will be held at University College London on Sat 4th November.

This is an annual event organized by the Geologists' Association but this year is open to the public from 10 till 4.30. Associated with it, UCL will be opening the Regional Planetary

Image Facility (the UK archive of NASA images and data), the Petrie Museum of Egyptian Archaeology and the Grant Museum of Zoology; the Discovery Room exhibits will be greatly expanded, with a stunning range of activities for all ages. There will be local society displays and 3 talks, including one on the Geology of the Moon. There will be geological walks on the Sunday morning.

Better Colour Vision?

Peter R Wallis

With a few exceptions all human beings have colour vision based on the cone cells in the retina of our eyes [1]. Each cone cell contains a pigment which is a variant of the protein opsin, linked to a small molecule called retinal. When the pigment absorbs light, the added energy causes the retinal to alter its shape and trigger the neurons which send information to the brain via the optic nerve. We are able to see trichromatically because we have three types of cone cell that are sensitive to different frequencies of light; each covers a wide band, but they have maximum sensitivities at light wavelengths of 424, 530 and 560 nanometers (nm) respectively. What we describe as "colour" is not an absolute property of light or of the object, but the brain's interpretation from the levels of activation of the three different cones.

All this is well known to us. However, I was greatly intrigued by an article by Timothy H. Goldsmith [2]describing the superior colour vision of birds, based upon four rather than three types of cone, one being sensitive to the near ultraviolet of 300 to 400nm which we cannot detect [3]. With some space free in the August newsletter, I decided to pass on his information to you.

Ultraviolet vision in insects was discovered in the 19th C., at first by Sir John Lubbock (Lord Avebury), a friend of Charles Darwin, from his observation of ants. Then Karl von Frisch and his students demonstrated in the early 20th C. that bees and ants not only see UV but use it to navigate. Work of the past 35 years has shown that birds, lizards, turtles and many fish have UV receptors in their retina. But we don't; why not?

Goldsmith says that the different opsins in different species offer a way to study the evolution of colour vision. Examination of the DNA sequencies that code for the opsins reveal the evolutionary trees for different species. He claims that opsins are ancient proteins that existed before the emergence of the dominant groups that exist today. He traces four lineages of vertebrate cone pigments named after the spectral regions where they are most sensitive: long-wavelength, mid-wavelength, short-wavelength and ultraviolet.

Modern birds have four spectrally distinct cone pigments, typically peaking at 370, 445, 508 and 565 nm. Mammals, however, generally have only two pigments, one peaking in the violet and the other at longer wavelengths. He offers a 'likely' explanation for their loss of two. He suggests that it occurred in the Mesozoic Period (245 Myrs to 65 Myrs ago) when Saurians were the dominant species and mammals were nocturnal; in low light conditions they would need the more sensitive rod cells to see. With the extinction of the dinosaurs 65 Myrs ago, the mammals were able to diversify, taking up diurnal life.

He believes that one group of mammals - among which were the ancestors of today's humans and other primates - spread out into trees and made fruit an important diet. But, with only one long-wavelength cone, they would have been unable to discriminate between green, yellow and red. He suggests that mutations occurred to provide a second long-wave cone; other research supports this hypothesis [4]. We primates thus have today three types of cone and trichromatic vision, unlike most other mammals. But our colour vision is far from ideal. Not only do we have no UV cone, but our long-wave cones are 'something of an evolutionary reclamation job'. The genes for both lie on the X-chromosome; males only have one copy of this so mutations of either gene can damage their ability to discriminate between red and green.

The presence of four types of cones in birds, sensitive to different wavelengths, certainly offers much better colour vision and Goldsmith and his students have done colour-matching tests on budgerigars to prove this explicitly, including ultraviolet. How might the birds make use of the wealth of colour information? One possibility is mate selection; researchers have shown from measurements of the light reflected from their plumage that the eye of a bird sees differences between males and females not known to ornithologists. UV receptors may give the animal an advantage in foraging for food. Jussi Viitala has also reported that kestrels can detect the voles whose scent trails of urine and faeces reflect UV light.

Some conclusions can be determined by experiment, as above, but others may remain as speculation in view of our much more limited colour capability. Evolution is however an immensely powerful process. It gave us our brains and power of speech and we now think of ourselves as masters of the world. But it has given others their own special capabilities that we lack - the colour vision of birds, the infrared sensors of the pit vipers, the echo-sounding of bats - and we do not know what the future has to offer.


[1] The rod cells do not contribute to colour vision but provide more sensitive vision in low light conditions. Back

[2] "What Birds See" by Goldsmith T. H., Scientific American, July 2006, p51. He is a professor emeritus at Yale and has studied the vision of crustaceans, insects and birds over 5 decades. Back

[3] Ultraviolet light of wavelength less than 300nm is absorbed by the ozone in the upper atmosphere and plays no part in the birds' vision. Back

[4] Jeremy Nathans & David Hogness at Stanford University suggest that an extra copy of the long-wave opsin gene that can arise naturally in a reproductive cell subsequently suffered a mutation varying its peak wavelength, conferring a selective advantage. Back


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