TLS Andrew Parker: Seven Deadly Colours

Darwin's blind spot
by John Tyler Bonner 12 October 2005

review of SEVEN DEADLY COLOURS
The genius of nature's palette and how it eluded Darwin Andrew Parker 336pp. 
Free Press. £16.99. | 0 7432 5939 4

Seven Deadly Colours is a wonderful voyage into a world that even biologists have not given the attention it deserves. It is a natural history of animal colours, which by itself is a fascinating subject, but to this Andrew Parker has added the chemical and physical basis of the colours as well as an understanding of how the eye records them and passes them on to the brain. Parker begins by pointing out that Charles Darwin, in his On the Origin of Species (1859), was seriously puzzled by the eye of an animal; it seemed so remarkably complex and complete – how could it have evolved by natural selection? This famous worry has been pursued by many biologists in the past century and has largely been dispelled by the finding of all sorts of rudimentary eyes among lowly invertebrates, and more recently by discovering that eyes, primitive and advanced, have particular genes in common; the elaborate eye of octopuses and mammals did not have totally independent origins but have a common foundation. This, however, is not Parker’s message. He shows us most convincingly that there is not one kind of eye that does one thing. No doubt we are greatly influenced by the marvels of our own eyes (as was Darwin), but the author throughout his book gives convincing evidence that among different animals there are eyes of varying capabilities that are built to see the different kinds of colours. In a series of fascinating examples he shows that the eyes of animals differ in fundamental ways, and that there is every evidence that these differences arose by natural selection. As Parker repeatedly points out, had Darwin known all that is in this book, he would have been reassured that his grand scheme of natural selection was quite safe.

The book is organized around specific colours and the different ways in which they are produced. Here, one immediately thinks of pigments, as in a painting, and indeed many animal colours are produced by pigments, but there are other kinds as well, such as the iridescent physical colours that can be seen on the surfaces on some minerals and bird feathers. In the chapter on the colour ultraviolet, Parker asks why kestrels hover over motorway verges, where the prey is well camouflaged. The answer is totally unexpected and very interesting. It revolves around the fact that animals differ in what colours they can see. Some are quite colour-blind and see things only in black and white, such as dolphins and seals, while others, such as birds and insects, can manage better than us. They not only see the range of colours we see, but also into the ultraviolet to which we are blind. The world before them looks quite different from ours, something we can only get an inkling of by special photography that records in ultraviolet. Kestrels have this gift. They prey on voles that are themselves well camouflaged, but voles establish trails and secrete various marking chemicals (mainly in their urine) along their runways, and one of these substances absorbs ultraviolet light. So the kestrel can follow the network of trails, which are abundant along roadways, and pounce on the moving furry balls on the track that they – but not we – can see.

The chapter on the colour blue is especially interesting because it involves bioluminescence. Some organisms possess a way of chemically manufacturing light in the dark: the light of fireflies and of many denizens of the oceans: fishes, various invertebrates and unicellular organisms that can be seen in the wake of a boat or the dip of an oar in warm seas. Even some fungi send off a magical glow at night, as do glow worms (insect larvae) of various sorts. This nocturnal activity serves many functions, from attracting mates, or prey, to actually seeing in the dark, a skill acquired by some of those strange abyssal fishes. They carry their headlights below their eyes, but the luminescence comes from symbiotic bacteria that glow in a special chamber that holds them. The bacterial light is on permanently, yet they flash their headlights; they do so by having a membranous shutter over the luminous pocket that the fish can open and close. There has been some very interesting recent work on these luminescent bacteria that has a direct relevance to human pathology. It has been shown that the bacteria can glow only if they are in a group of sufficient numbers, something that is automatically sensed by them. It turns out that this is true for bacterial pathogens as well: they can have their damaging toxic effect only if they are sufficiently numerous to form a quorum.

The section on the colour orange includes the particularly interesting example of milk snakes. There are some species found all the way from North to South America that are beautifully banded, with broad bands of white, black and orange. They are extremely conspicuous, which at first seems surprising since they are harmless and have numerous predators. It is assumed that they are to some degree protected because their colouration closely resembles that of coral snakes that are highly venomous. Parker himself has done some ingenious experiments to show that the milk snake has an additional way of eluding predators: it can move very rapidly and reach a speed where the colours become fused into a uniform colour such as green or pink. This happens because the eyes of their predators – birds and mammals – can capture in their eyes and brain only a finite number of images per second, and beyond that the striking bands turn into a blur.

Parker elaborates on this in an excellent discussion of mimicry, where an edible species will acquire the marking of a distasteful or dangerous one, as in the case of milk and coral snakes. This was first discovered in butterflies by Henry Bates in the middle of the nineteenth century, deep in the Amazon jungle. A few years later, Fritz Müller found that in some cases the similarly coloured species were all noxious; by flying under the same banner and widely advertising it, they spread the word: “Don’t even think of eating any of us”. In other words, there are two kinds of mimicry: in one, distasteful or poisonous species are imitated by edible and harmless ones to avoid being eaten (Batsesian mimicry); in the other all the forms are unpalatable, and to make this known they all have the same flashy warning colouration (Müllerian mimicry).

Animal colour also plays an important role in the selection of mates; the so-called sexual selection was another of Darwin’s major insights. Usually males are the painted ones, often going to extremes, such as the birds of paradise and many others. Sexual selection may be a system to find the most desirable mate, but it has the severe disadvantage of showing the predator where to attack. I am particularly fond of a case (not discussed in the book) where male guppies living in streams where there are no predators will have large and showy spots, while those in predator-infested streams have greatly subdued markings. Many of the colour patterns, as Parker shows, serve as camouflage, making the animal blend in with its natural surroundings. I particularly like the marine forms that become transparent – see-through fishes and shrimp. How can the normal pigments of an animal’s body transform so that they no longer absorb light? It seems to me that it should be a problem of interest to biochemists, but I have been unable to persuade anyone to look into it. Seven Deadly Colours is a book that has a great deal to offer, not only to the interested reader but to the biologist as well. The author makes a valiant attempt to make the physics that surrounds all colour easy and accessible, and when he only partially succeeds, this is no fault of his own. The subject is simply not easy, and he provides enough of it to give a sense of what is relevant.

At the end of Seven Deadly Colours: The genius of nature’s palette and how it eluded Darwin, Andrew Parker returns to the idea that complex eyes arose rather suddenly as part of the Cambrian explosion. It is true that in those famous deposits one finds the first fossil eyes, and that they do indeed seem remarkably elaborate. I am more persuaded, in keeping with the recent genetic findings as well as our knowledge of primitive invertebrate eyes, that the process was gradual long before the Cambrian explosion. I have a special attraction to lower beasts and there is a unicellular marine organism (a dinoflagellate) that not only has a light sensitive eyespot (equivalent to our retina with a light absorbing pigment that guides the motile cell towards light) but also a lens capable of focusing the light and producing an image. It is certainly hard to imagine why this elaborate structure arose, and what it does for the animal in its environment, especially as there is nothing equivalent to a brain to process what it sees. It is indeed very puzzling, but then from my own research on social amoebae, I know that we often underestimate the cleverness of simple and primitive organisms.