Darwin's Island (30 page)

CHAPTER VIII

WHERE THE BEE SNIFFS

A gift of orchids is a statement of a gentleman’s intentions towards a potential partner. A man willing to spend so much on his mate must be devoted indeed - or rich enough not to care, which comes to more or less the same thing. An orchid, with its extravagant flowers and a price tag to match, is a real test of his readiness to invest in a relationship.

The plants feel the same. Their Latin name, Orchidaceae, means ‘testicle’ after the unexpected shape of their roots. Orchids advertise their prowess with expensive and often bizarre blooms. So impressive are their carnal powers that the English herbalist Nicholas Culpeper called for caution when they were used as aphrodisiacs. In
The Descent of Man and Selection in Relation to Sex
Charles Darwin had shown how, in the animal kingdom, the battle to find a mate was as formidable an agent of selection as was the struggle to stay alive. Males, in general, have the potential to have far more offspring than do females - if, that is, they can fight off their rivals and persuade enough members of the opposite sex to play along with their carnal desires. Losers in the conflict reach the end of their evolutionary road for their genes go nowhere. Evolution as played out in the universe of sex is as pitiless as is that in the battle for survival. Sexual selection, as Darwin called it, can lead to rapid change: to the evolution of gigantic antlers, a vivid posterior or - for species interested in such things - gold watches and flashy clothes.

Later in his career, Charles Darwin examined the sexual struggles within the second great realm of life, the plants. He showed how the search for a partner can be as much of a challenge for them as it is for stags or peacocks. Plant reproductive habits were obscure and their mere existence often denied until the seventeenth century, but within a hundred years or so the basic machinery had been worked out. Flowers were both the home of the reproductive organs and an eloquent statement of erotic need. Darwin found that they evolved in rather the same way as an animal’s sexual displays and were subject to the same forces of selection, which often achieved ends equally - or more - bizarre than those found in animals. In addition he discovered (although he found it hard to believe) that for orchids sex was full of dishonesty and discord, with all those involved ready to cheat whenever necessary.

Any botanical marriage is - by definition - more crowded than its animal equivalent, for a third party is needed to consummate it by moving male sex cells to the female. For some species, wind or water step in to help, but most flowers need a flying penis - a pollinator - to carry their DNA to the next individual (Ruskin, with his passion for the beauties of nature, strongly advised his young female readers not to enquire ‘how far flowers invite, or require, flies to interfere in their family affairs’). Darwin himself saw how antagonism between the plant and animal partners is as powerful an agent of selection as is the process of female choice and male competition that gives rise to the peacock’s tail. Flower and pollinator each become trapped into the embrace of the other and enter an evolutionary race that may end with the emergence of structures as unexpected, and tactics as devious, as anything in the animal world.

The interests of those who manufacture the crucial DNA and those who deliver it are quite different. From a female flower’s point of view, or that of the female part of a hermaphrodite plant, one or a few visits by a winged phallus is enough to do the job (although the more callers she gets, the more choice she has of which sex cell to use). To beat its rivals, however, a male is forced to attract the distribution service again and again - and that can be expensive.

In his 1862 volume
On the Various Contrivances by which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing
, Darwin studied the divergence of interests between the two parties. He used the showiest and most diverse of all flowers as an exemplar. He found that ‘the contrivances by which Orchids are fertilised, are as varied and almost as perfect as any of the most beautiful adaptations in the animal kingdom’. As well as an exhaustive account of the structure of the orchids themselves (‘I fear, however, that the necessary details will be too minute and complex for any one who has not a strong taste for Natural History’), his work introduced the idea - much developed nine years later, in
The Descent of Man, and Selection in Relation to Sex -
that large parts of evolution depend on an ancient and endless sexual conflict that crafts the future of all those who are drawn in.

The war between flowers and insects became an overture to a wider world of biological discord. It has led to spectacular bonds between improbable partners, As it does, it reveals many of the details of the mechanism of natural selection, including its uncanny ability to subvert the tactics of any opponent. The orchids and their pollinators were, for Charles Darwin, an introduction to the dishonesty that pervades the world of life.

Pollination has attracted attention since ancient times. Both Aristotle and Virgil were interested in bees, but only because they made honey (or collected the stuff, for the Greeks imagined that it fell from the air: ‘air-born honey, gift of heaven’) rather than because they were essential for reproduction. The Egyptians, in contrast, understood that dates would not grow on cultivated palms unless male flowers were shaken on to the females. They used their slaves as pollinators. A diversity of other creatures has been called in to act as marital aids and quite often that duty drives their own evolution. Two hundred thousand insects (the male malaria mosquito included) are known to transfer pollen and their own vast radiation into a variety of forms began soon after the origin of flowers. From the tropics to the sub-Arctic, hundreds of species of bird are busy shifting genes. Some, such as humming birds, can almost never afford to stop as they need a constant supply of nectar to keep their tiny bodies at a high level of activity. Mammals are also involved, and a certain Ecuadorian bat has a tongue half as long again as its own body - in relative terms the longest of all mammalian tongues. It is coiled up in a special cavity in its chest, except when the animal is feeding. An African tree is even adapted for pollination by the giraffes that browse upon its leaves. In Australia, too, marsupials have taken up the job for the honey possum has lost many of its teeth and gained a long tongue. The sugar glider - a marsupial that floats through the air from flower to flower - is much the same.

 

Plants want their go-betweens to be cheap, trusty and eager, while pollinators would prefer to be fat, wanton and as idle as possible. The flower shows that a reward is on offer while the other party must decide whether the hard work needed to get it is worthwhile. The struggle between the two parties leads to the evolution of displays that dwarf the efforts of any animal. A bunch of flowers is an advertisement - a silent scream from the sexually frustrated. Like all advertisements it attempts to reassure those who see it that a high-quality product is on view. In commerce, as in life, the temptation to cheat is never far away; to make false promises with no reward, or to take the prize and fail to complete the task.

Plants and animals make signals of many kinds. They advertise their qualities as a mate, their willingness to fight for territory or food, or their ability to escape from a predator who might as a result be dissuaded from bothering to attack. One surprise is that the signals are so often honest when the reward for dishonesty is so high, be it in the form of sex, food or safety. Some signs are direct and impossible to fake: large tigers make scratch marks higher up a tree trunk than can their smaller rivals and can as a result hold bigger territories. Often, though, the information is indirect. Thus, a black and yellow wasp warns predators about its dangers without the need to sting all of them.

Such secondary signals, too, are sometimes pricey and hard to simulate. Giant antlers, vivid tails or spectacular blooms can be made only by those who can afford them: the healthiest, the sexiest or the super-aggressive. Most of what we interpret as the joys of nature costs a lot, for a stag may die in battle, and a male nightingale loses a tenth of its body weight after a night spent singing in the hope of sex. Testosterone itself, that signifier of masculine identity, is costly in many ways. It suppresses the immune system, so that a red deer male in sexual frenzy is open to attack by parasites - and if he can keep roaring in spite of his tapeworms he might have particularly fine genes. Elephants go even further. Now and again, one falls into a state of ‘musth’, in which its testosterone level goes up by fifty times. The agitated beast becomes very aggressive, and a small animal will fight even to the death against a larger rival.

Flowers, too, are not cheap. Orchid fanciers pay tens of thousands of dollars for prize specimens and the trade as a whole has a worldwide turnover of several billion. The orchids themselves invest far more of their limited capital into sexual display than does the most avid gardener, for if they do not their genetical future is over. The cost of sex to each orchid and to those that market them is manifest in the fact that, in the world of the garden centre, many of the specimens are grown from cells in culture rather than by persuading the plants to go through the expensive ritual of sex.

Orchids and other flowers are, like the peacock’s tail, animated billboards that advertise sexual prowess. For all signals, two parties are involved: those who transmit a message and those who receive it. A system of checks and balances tests whether the information is accurate; that those with the biggest antlers or brightest blooms really are the fiercest or most generous. The system is always under test by potential fraudsters at both ends and sometimes cheats get in. Often, they do well. For insects, black and yellow is no more difficult to manufacture than is brown or blue - and a whole group of harmless flies does just that, with bright stripes that make a false claim of a waspish nature. That costs the wasps a lot when a hungry bird attacks under the assumption that the pattern advertises good taste rather than potential danger.

Such swindlers also flourish in the botanical world - and in orchids most of all. To his considerable surprise, Charles Darwin found that among those elegant flowers dishonesty pays. Many of his specimens had gorgeous displays, but gave no payment to their pollinators. He found it hard to believe that Nature could be so fraudulent or that insects were so foolish as to fall for ‘so gigantic an imposture’ and suggested, wrongly, that his plants had an as yet undiscovered reward. His finding throws light on a question that he posed but failed to solve: how can natural selection favour the dishonest? The orchids give part of the answer.

 

The battle for sex is a war of all against all. It may end in an arms race; a tactical struggle in which every move made by one party is countered by the other. Sometimes, as in the Cold War, each antagonist is forced into massive investment, and, as in those days, negotiation may end in stalemate. To an untutored eye that may look like peace, but it is in truth no more than battle deferred. The orchids, beautiful as they are and exquisite as their adaptations to the needs of their pollinators might be, show such a struggle hard at work and show how propaganda - false information - is useful in both love and war.

Many of Darwin’s own observations were made on the ‘Orchis Bank’, close to his home, where he found eleven species of the plants. As he noted with a certain pride, ‘no British county excels Kent in the number of its orchids’, but he also studied specimens sent from all over the world. He soon saw how the conflict between plant and pollinator had led to change. He speaks of an orchid, ‘the
Angræcum sesquipedale
, of which the large six-rayed flowers, like stars formed of snow-white wax, have excited the admiration of travellers in Madagascar’. It had ‘a whip-like green nectary . . . eleven and a half inches long, with only the lower inch and a half filled with very sweet nectar. What can be the use, it may be asked, of a nectary of such disproportional length? . . . in Madagascar there must be moths with probosces capable of extension to a length of between ten and eleven inches! . . . As certain moths of Madagascar became larger through natural selection in relation to their general conditions of life . . . those individual plants of the Angræcum which had the longest nectaries . . . and which, consequently, compelled the moths to insert their probosces up to the very base, would be fertilised. These plants would yield most seed and the seedlings would generally inherit longer nectaries; and so it would be in successive generations of the plant and moth. Thus it would appear that there has been a race in gaining length between the nectary of the Angræcum and the proboscis.’ In 1903, that long-tongued insect, product of an endless contest with its plant, was at last discovered and named as Morgan’s Sphinx Moth. A long conflict of interests had forced both parties to adapt themselves to each other’s demands.