I have carefully measured the proboscis of a specimen of Macrosila cluentius from South America in the collection of the British Museum, and find it to be nine inches and a quarter long! One from tropical Africa (Macrosila morganii) is seven inches and a half. A species having a proboscis two or three inches longer could reach the nectar in the largest flowers of Angræcum sesquipedale, whose nectaries vary in length from ten to fourteen inches. That such a moth exists in Madagascar may be safely predicted; and naturalists who visit that island should search for it with as much confidence as astronomers searched for the planet Neptune, – and they will be equally successful!
In 1903, after Darwin’s death but well within Wallace’s long lifetime, a hitherto unknown moth was discovered which turned out to fulfil the Darwin/Wallace prediction, and was duly honoured with the sub-specific name praedicta. But even Xanthopan morgani praedicta, ‘Darwin’s hawk moth’, is not sufficiently well endowed to pollinate A. longicalcar, and the existence of this flower encourages us to suspect the existence of an even longer-tongued moth, with the same confidence as Wallace invoked the predicted discovery of the planet Neptune. By the way, this little example gives the lie, yet again, to the allegation that evolutionary science cannot be predictive because it concerns past history. The Darwin/Wallace prediction was still a perfectly valid one, even though the praedicta moth must already have existed before they made it. They were predicting that, at some time in the future, somebody would discover a moth with a tongue long enough to reach the nectar in A. sesquipedale.
Insects have good colour vision, but their whole spectrum is shifted towards the ultraviolet and away from the red. Like us, they see yellow, green, blue and violet. Unlike us, however, they also see well into the ultraviolet range; and they don’t see red, at ‘our’ end of the spectrum. If you have a red tubular flower in your garden it is a good bet, though not a certain prediction, that in the wild it is pollinated not by insects but by birds, who see well at the red end of the spectrum – perhaps hummingbirds if it is a New World plant, or sunbirds if an Old World plant. Flowers that look plain to us may actually be lavishly decorated with spots or stripes for the benefit of insects, ornamentation that we can’t see because we are blind to ultraviolet. Many flowers guide bees in to land by little runway markings, painted on the flower in ultraviolet pigments, which the human eye can’t see.
The evening primrose (Oenothera) looks yellow to us. But a photograph taken through an ultraviolet filter shows a pattern for the benefit of bees, which we can’t see with normal vision (see colour page 5). In the photograph it appears as red, but that is a ‘false colour’: an arbitrary choice by the photographic process. It doesn’t mean that bees would see it as red. Nobody knows how ultraviolet (or yellow or any other colour) looks to a bee (I don’t even know how red looks to you – an old philosophical chestnut).
A meadow full of flowers is nature’s Times Square, nature’s Piccadilly Circus. A slow-motion neon sign, it changes from week to week as different flowers come into season, carefully prompted by cues from, for example, the changing length of days to synchronize with others of their own species. This floral extravaganza, splashed across the green canvas of a meadow, has been shaped and coloured, magnified and titivated by the past choices made by animal eyes: bee eyes, butterfly eyes, hoverfly eyes. In New World forests we’d have to add hummingbird or in African forests sunbird eyes to the list.
Hummingbirds and sunbirds are not particularly closely related, by the way. They look and behave like each other because they have converged upon the same way of life, largely revolving around flowers and nectar (although they eat insects as well as nectar). They have long beaks for probing nectaries, extended by even longer tongues. Sunbirds are less accomplished hoverers than hummingbirds, who can even go backwards like a helicopter. Also convergent, although from a far distant vantage point in the animal kingdom, are the hummingbird hawk moths, again consummate hoverers with spectacularly long tongues (all three types of nectar junkie are illustrated on colour page 5).
We shall return to convergent evolution later in the book, after properly understanding natural selection. Here, in this chapter, flowers are seducing us, drawing us in, step by step, lining our path to that understanding. Hummingbird eyes, hawk-moth eyes, butterfly eyes, hoverfly eyes, bee eyes are critically cast over wild flowers, generation after generation, shaping them, colouring them, swelling them, patterning and stippling them, in almost exactly the same way as human eyes later did with our garden varieties; and with dogs, cows, cabbages and corn.
For the flower, insect pollination represents a huge advance in economy over the wasteful scattergun of wind pollination. Even if a bee visits flowers indiscriminately, lurching promiscuously from buttercup to cornflower, from poppy to celandine, a pollen grain clinging to its hairy abdomen has a much greater chance of hitting the right target – a second flower of the same species – than it would have if scattered on the wind. Slightly better would be a bee with a preference for a particular colour, say blue. Or a bee that, while not having any long-term colour preference, tends to form colour habits, so that it chooses colours in runs. Better still would be an insect that visits flowers of only one species. And there are flowers, like the Madagascar orchid that inspired the Darwin/Wallace prediction, whose nectar is available only to certain insects that specialize in that kind of flower and benefit from their monopoly over it. Those Madagascar moths are the ultimate magic bullets.
From a moth’s point of view, flowers that reliably provide nectar are like docile, productive milch cows. From the flowers’ point of view, moths that reliably transport their pollen to other flowers of the same species are like a well-paid Federal Express service, or like well-trained homing pigeons. Each side could be said to have domesticated the other, selectively breeding them to do a better job than they previously did. Human breeders of prize roses have had almost exactly the same kinds of effects on flowers as insects have – just exaggerated them a bit. Insects bred flowers to be bright and showy. Gardeners made them brighter and showier still. Insects made roses pleasantly fragrant. We came along and made them even more so. Incidentally, it is a fortunate coincidence that the fragrances that bees and butterflies prefer happen to appeal to us too. Flowers such as ‘stinking Benjamin’ (Trillium erectum) or the ‘corpse flower’ (Amorphophallus titanum), which use flesh flies or carrion beetles as pollinators, often nauseate us, because they mimic the smell of decaying meat. Such flowers have not, I presume, had their scents enhanced by human domesticators.
Of course, the relationship between insects and flowers is a two-way street, and we mustn’t neglect to look in both directions. Insects may ‘breed’ flowers to be more beautiful, but not because they enjoy the beauty.* Rather, the flowers benefit from being perceived as attractive by insects. The insects, by choosing the most attractive flowers to visit, inadvertently ‘breed for’ floral beauty. At the same time, the flowers are breeding the insects for pollination ability. Then again, I have implied that insects breed flowers for high nectar yield, like dairymen breeding massively uddered Friesians. But it is in the flowers’ interests to ration their nectar. Satiate an insect and it has no incentive to go on and look for a second flower – bad news for the first flower, for which the second visit, the pollinating visit, is the whole point of the exercise. From the flowers’ point of view, a delicate balance must be struck between providing too much nectar (no visit to a second flower) and too little (no incentive to visit the first flower).
Insects have milked flowers for their nectar, and bred them for increased yield – probably encountering resistance from the flowers, as we have just seen. Have beekeepers (or horticulturalists with the interests of beekeepers in mind) bred flowers to be even more productive of nectar, just as dairy farmers bred Friesian and Jersey cows? I’d be intrigued to know the answer. Meanwhile, there’s no doubt of the close parallel between horticulturalists as breeders of pretty and fragrant flowers, and bees and butterflies, hummingbird
s and sunbirds doing the same thing.
YOU ARE MY NATURAL SELECTION
Are there other examples of selective breeding by non-human eyes? Oh yes. Think of the dull, camouflaged plumage of a hen pheasant, compared with the splendiferous male of the same species. There seems little doubt that, if his individual survival were the only thing that mattered, the cock golden pheasant would ‘prefer’ to look like the female, or like a grown-up version of how he was as a chick. The female and the chicks are obviously well camouflaged, and that’s the way the male would be too if individual survival were his priority. The same is true of other pheasants such as Lady Amherst’s and the familiar ring-necked pheasant. The cocks look flamboyant and dangerously attractive to predators, but each species in a very different way. The hens are camouflaged and dull-coloured, each species in pretty much the same way. What is going on here?
One way to put it is Darwin’s way: ‘sexual selection’. But another way – and the one that better suits my primrose path – is ‘selective breeding by females of males’. Bright colours may indeed attract predators, but they attract female pheasants too. Generations of hens chose to mate with bright, glowing males, rather than the dull brown creatures that the males would surely have remained but for selective breeding by females. The same thing happened with peahens selectively breeding peacocks, female birds of paradise breeding males, and numerous other examples of birds, mammals, fish, amphibians, reptiles and insects where females (it’s usually females rather than males, for reasons we needn’t go into) choose from among competing males. As with garden flowers, human pheasant-breeders have improved upon the selective handiwork of the hen pheasants that preceded them, producing spectacular variants of the golden pheasant, for example, although more by picking one or two major mutations rather than by gradually shaping the bird through generations of breeding. Humans have also selectively bred some amazing varieties of pigeons (as Darwin knew at first hand) and chickens, descended from a Far Eastern bird, the red jungle fowl Gallus gallus.
Varieties of chicken: three illustrations from Darwin’s The Variation of Animals and Plants under Domestication
This chapter is mostly about selection by eyes, but other senses can do the same thing. Fanciers have bred canaries for their songs, as well as for their appearance. The wild canary is a yellowish brown finch, not spectacular to look at. Human selective breeders have taken the palette of colours thrown up by random genetic variation and manufactured a colour distinctive enough to be named after the bird: canary yellow. By the way, the bird itself is named after the islands,* not the other way around as with the Galapagos Islands, whose name comes from a Spanish word for tortoise. But canaries are best known for their song, and this too has been tuned up and enriched by human breeders. Various songsters have been manufactured, including Rollers, which have been bred to sing with the beak closed, Waterslagers, which sound like bubbling water, and Timbrados, which produce metallic, bell-like notes, together with a castanet-like chatter that befits their Spanish origins. Domestically bred songs are longer, louder and more frequent than the wild ancestral type. But all these highly prized songs are made up of elements that occur in wild canaries, just as the habits and tricks of various breeds of dogs come from elements to be found in the behavioural repertoire of wolves.*
Once again, human breeders have only been building on the earlier selective breeding efforts of female birds. Over generations, wild female canaries inadvertently bred males for their singing prowess by choosing to mate with males whose songs were especially appealing. In the particular case of canaries it happens that we know a little more. Canaries (and Barbary doves) have been favourite subjects for research on hormones and reproductive behaviour. It is known that in both species the sound of male vocalization (even from a tape recording) causes the females’ ovaries to swell and secrete hormones that bring them into reproductive condition and make them more ready to mate. One could say that male canaries are manipulating females by singing to them. It is almost as though they were giving them hormone injections. One could also say that females are selectively breeding males to become better and better at singing. The two ways of looking at the matter are two sides of the same coin. As with other bird species, by the way, there is a complication: song is not only appealing to females, it is also a deterrent to rival males – but I’ll leave that on one side.
Now, to move the argument on, look at the pictures opposite. The first is a woodcut of a Japanese kabuki mask, representing a samurai warrior. The second is a crab of the species Heikea japonica, which is found in Japanese waters. The generic name, Heikea, comes from a Japanese clan called the Heike, who were defeated at sea in the battle of Danno-Ura (1185) by a rival clan called the Genji. Legend tells that the ghosts of drowned Heike warriors now inhabit the bottom of the sea, in the bodies of crabs – Heikea japonica. The myth is encouraged by the pattern on the back of this crab, which resembles the fiercely grimacing face of a samurai warrior. The famous zoologist Sir Julian Huxley was impressed enough by the resemblance to write, ‘The resemblance of Dorippe to an angry Japanese warrior is far too specific and far too detailed to be accidental . . . It came about because those crabs with a more perfect resemblance to a warrior’s face were less frequently eaten than the others.’ (Dorippe was what the crab was called in 1952 when Huxley wrote. It reverted to Heikea in 1990 when somebody rediscovered that it had been so named as early as 1824 – such are the strict priority rules of zoological nomenclature.)
Kabuki mask of samurai warrior
Heikea japonica crab
This theory, that generations of superstitious fishermen threw back into the sea crabs that resembled human faces, received new legs in 1980 when Carl Sagan discussed it in his wonderful Cosmos. In his words,
Suppose that, by chance, among the distant ancestors of this crab, one arose that resembled, even slightly, a human face. Even before the battle of Danno-ura, fishermen may have been reluctant to eat such a crab. In throwing it back, they set in motion an evolutionary process . . . As the generations passed, of crabs and fishermen alike, the crabs with patterns that most resembled a samurai face survived preferentially until eventually there was produced not just a human face, not just a Japanese face, but the visage of a fierce and scowling samurai.
It’s a lovely theory, too good to die easily, and the meme has indeed replicated itself through the canon. I even found a website where you can vote on whether the theory is true (31 per cent of 1,331 voters), whether the photographs are fakes (15 per cent), whether Japanese craftsmen carve the shells to look that way (6 per cent), whether the resemblance is just a coincidence (38 per cent), or even whether the crabs really are manifestations of drowned samurai warriors (an amazing 10 per cent). Scientific truths are not, of course, decided by plebiscite, and I voted only because I was otherwise not allowed to see the voting figures. I’m afraid I voted with the killjoys. I think, on balance, that the resemblance is probably a coincidence. Not because, as one authoritative sceptic has pointed out, the ridges and grooves on the crab’s back actually signify underlying muscle attachments. Even on the Huxley/Sagan theory, the superstitious fishermen would have to have begun by noticing some kind of original resemblance, however slight, and a symmetrical pattern of muscle attachments is exactly the kind of thing that would have provided that initial resemblance. I am more impressed by the same sceptic’s observation that these crabs are too small to be worth eating anyway. According to him, all crabs of that size would have been thrown back, whether or not their backs resembled human faces, although I have to say that this more telling source of scepticism had a large bite taken out of it when I was taken out to dinner in Tokyo and my host ordered, for all the company, a dish of crabs. They were much larger than Heikea, and they were thickly encrusted in stout, calcified carapaces, but that didn’t stop this superman picking up whole crabs, one by one, and biting into them like an apple, with a crunching sound that seemed to presage hideously bleeding gums. A crab as small as Heikea wou
ld be a doddle to such a gastronomic champion. He would surely swallow it whole without batting an eyelid.
My main reason for scepticism about the Huxley/Sagan theory is that the human brain is demonstrably eager to see faces in random patterns, as we know from scientific evidence, on top of the numerous legends about faces of Jesus, or the Virgin Mary, or Mother Teresa, being seen on slices of toast, or pizzas, or patches of damp on a wall. This eagerness is enhanced if the pattern departs from randomness in the specific direction of being symmetrical. All crabs (except hermit crabs) are symmetrical anyway. I reluctantly suspect that the resemblance of Heikea to a samurai warrior is no more than an accident, much as I would like to believe it has been enhanced by natural selection.
Never mind. There are plenty of other examples not involving humans, where animal ‘fishermen’, as it were, ‘throw back’ (or don’t see in the first place) would-be food because of a resemblance to something sinister, and where the resemblance is certainly not due to chance. If you were a bird, out hunting caterpillars in the forest, what would you do if you were suddenly confronted with a snake? Leap back startled, would be my guess, and then give it a wide berth. Well, there is a caterpillar – to be precise, the rear end of a caterpillar – that bears an unmistakable resemblance to a snake. It is truly alarming if you are frightened of snakes – as I shamefacedly confess I am. I even think I might be reluctant to pick this animal up, despite knowing perfectly well that it is in fact a harmless caterpillar. (A picture of this extraordinary creature appears on colour page 7.) I have the same problem with picking up wasp-mimicking or bee-mimicking hoverflies, even though I can see, from their possession of only one pair of wings, that they are stingless flies. These are among a vast list of animals that gain protection because they look like something else: something inedible like a pebble, a twig or a frond of seaweed, or something positively nasty like a snake or a wasp or the glaring eyes of a possible predator.
The Greatest Show on Earth Page 6