Disturbingly Slender Waists
- The Ants by Bert Hölldobler and E.O. Wilson
Springer, 732 pp, DM 198.00, March 1990, ISBN 3 540 52092 9
This is a stunning book. Overwhelming. It achieves the impossible – combining excellent systematics (without dulling the senses) with natural history, biology, biochemistry, and a wealth of extraordinarily interesting detail. I have no doubt that E.O. Wilson is the most distinguished biologist of our times, but it is surprising, even so, that he not only combines profound knowledge of these ‘little creatures who run the world’ with considerable insight into the future trends of biological thought and progress, but manages to involve us personally in the ant world. The book is divided into 20 sections and it is worth listing them: the importance of ants; the colony life cycle; altruism and the origin of the worker caste; colony odour and kin recognition; queen numbers and domination; communication; caste and division of labour; social homeostasis and flexibility; foraging strategies, territory and population regulation; the organisation of species; symbiosis among ant species; symbiosis with other anthropods; symbiosis among ants and plants; the specialised predators; the army ants; the fungus-growers; the harvesting ants; weaver ants; collecting, culturing, observing. The illustrations, furthermore, are first-rate, giving adequate scientific data but also agreeable portraits and a feeling of swarming activity, in some cases tinged with horror. No doubt a feeling of anxiety engendered by vast numbers – whether of men or mites – is a genetic trait common to most of us. The joint author of this magnum opus, Bert Hölldobler, is himself a skilled and original myremecologist, an infectious, ebullient enthusiast and great photographer: he is able to bring considerable grist to Wilson’s mill.
The book begins by emphasising the importance of ants in our world. We are apt to think of Japan as an unusually densely populated country, with its 120 million human inhabitants occupying 377,708 square kilometres, but when we read that along the Ishigari coast of the Northern Island of Hokkaido a super-colony of the ant Formica yessensis contains 306 million workers and 1,080,000 queens, occupying a mere 2.7 square kilometres, we suddenly become aware that Japan belongs to the ants, not to the Japanese. When one considers that these insects are apparently impervious to hard radiation, that colonies exposed to cesium-based irradiation seem unaffected, and that some species survive when exposed to industrial pollution, then one may feel that the future as well as the past may well belong to the ants.
This group of insects can be described as a hundred-million-year success story. The ants and termites compose one-third of the biomass of the tropical rainforests of the Brazilian Amazon. Wilson ascribes their worldwide ecological dominance (they are only absent from the tops of the highest mountains, Antarctica, Iceland, Greenland and Polynesia east of Tonga) to their close connection with the ground: ‘the first group of eusocial predatory insects that lived and foraged primarily in the soil.’ Blessed are the meek, for they shall inherit the earth.
Another key to the ants’ success is their social organisation, a rare evolutionary achievement which gives them an advantage over solitary insects. For one thing, it allows two or more generations to overlap in the society and permits care of the young by adults and a division of labour: kings and queens, on the one hand, and non-reproductive workers, on the other. Wilson also emphasises the importance of the so-called metapleural gland, which secretes phenylacetic acid and is a substance active against bacteria and fungi. The ants appear to disseminate this throughout their nests. Without a good defence against such organisms, life in the soil would be difficult if not impossible. Although these insects have diverged and evolved into an estimated eight thousand different species, all but a few phyletic lines of arboreal ants have hung onto the highly characteristic and important metapleural gland, which distinguishes them from the related bees and wasps and other Hymenoptera.
Although the various lifestyles of ants are extremely seductive – the army ants, fungus-growers, harvesting ants, weaver ants and so on – I fancy the most important and interesting part of the book deals with the ant colony. Chapter Three is divided into sections describing the stages of colony growth, nuptual flight and mating, colony founding, brood care and larval reciprocation, demography of colony members, colony movements and alternative strategies in colony life-cycles. Wilson points out that the ant colony is an almost exclusively female society whose activity pivots on the welfare of the queen. The number of workers varies from species to species. The average size of a colony of fungus-growing ants is about thirty-five workers, while in the case of army ants workers may exceed hundreds of thousands – even a million. Each species has adapted to the special requirements of the environment in which it can survive and its numbers are tailored accordingly.
The founding of an ant colony begins with a virgin queen abandoning her mother and sisters – mostly sterile workers or virgin reproductives like herself. She is inseminated by one or two males whom she encounters on her nuptual flight: they die after copulation, without returning to their colony. She then finds a suitable site, sheds her wings and builds her first nest cell, in which she rears a brood of workers. The second stage begins when the workers start to enlarge the colony by foraging and brood care. Meanwhile the queen continues to lay eggs. After a period varying in different species from one summer to five years, the reproductive stage is reached, and new queens and males are produced – the sexual forms which eventually sally forth to engender new colonies.
The hazards encountered by virgin queens leaving their homes are brought to our attention in a forceful manner. Wilson points out that it is not uncommon for one colony of, say, fire ants, to release thousands of young winged queens in less than an hour, the great majority of which die – from overheating, dessication, drowning, or else they are destroyed by predators. A successful fire ant queen, however, lays up to three hundred thousand eggs. In some species of ants the females congregate on the surface of their nests, release sex pheromones from the terminal segments of their abdomen, and await the arrival of winged males which respond to their call, and with whom they eventually pair.
A fascinating aspect of the ant colony is the division of labour highly characteristic of social insects. Wilson notes that an ant has about a million cells in its brain, compared with 12 billion in man. But owing to the caste system and division of labour (about fifty categories of behaviour in individual physical castes and about fifty categories of chemical signals used in communication), the colony can work to perfection. The sterile worker caste is ‘by far the most significant feature with reference to the further evolutionary potential of social life’. The book provides excellent tables illustrating the roles of the physical castes (which involve increased size variation) and temporal castes (with changing roles according to age).
Ants, the authors conclude, because of their abundance, and the ease with which their lifestyle can be studied, are ideal subjects for the elucidation of community ecology. The authors provide examples – such as hierarchy in control processes, the shaping of the organisation, the development of societies by natural selection, and the effects of competition on community structure. They have no doubt that, at both the individual and the colonial level, biological organisation can be studied most effectively by means of detailed investigation of the ants. The book gives a brilliant justification of their belief that the future of biology lies in the study of particular groups of organisms across all levels of organisation – and they prophesy that more emphasis will be placed on taxonomic groups of organisms. The eight thousand described species of ants – there are probably just as many as yet undiscovered – and the various systems around which their societies are organised bring home to us most vividly that the laws of biology are written in the language of diversity. Biology is destined to turn back deliberately to study such groups of organisms for their own sake – since this has a strong intellectual as well as practical justification.
A truly excellent feature of this magnificent monograph is the illustrations to the genera, in the taxonomic section. Such drawings are usually rather meaningless, for everyone other than the specialised systematist (I sighed enviously, reflecting on my own attempts at generic keys), but these are generously spaced out on the page, and since they depict the whole animal (enlarged), they awaken a keen interest in their differences, instead of discouraging the beginner with incomprehensible linear convolution illustrating the hidden genitalia.
There is so much of interest in this book that it is difficult to select any single section for special attention. Personally, I am attracted to the chapter on communication, for it represents a different world from our own. Our lives are governed principally by sight and sound, whereas the ants are dominated by a world of chemical communication – primarily of taste and smell. Pheromones – that is to say, glandular secretions released as signals by one ant and received by another of the same species – play the central role in the organisation of the ant society. One of the great attractions of this chapter is the clear diagrams (chiefly the work of Hölldobler) illustrating the position within the ant’s body of the glands which secrete the relevant chemicals. Wilson mentions six key exocrine glands, and the great diversity and complication of the pheromonal system can be appreciated when it is realised that one of them, the versatile Dafours gland, is known to produce alkases, alcohols, ketones, esters, acids and lactones – to mention only a few. Pheromonal communication includes trail-laying, alarm signals, attraction, recruitment, recognition of nest mates and particular castes, caste determination, nest markers, sexual communication, and synchronisation of sexual activity.
Another aspect of ant life which particularly excites my own interest is the question of so-called ‘altruism’, without which the colonial system would fail to integrate. Chapter Four is a masterly analysis of this phenomenon and of the linked question of the origins of the worker caste. Wilson, in his inimitable fashion, pinpoints the problem: how can natural selection produce selfish genes that prescribe unselfishness? The examples of the workers’ self-denying behaviour, performed for the benefit of the colony, are mind-boggling: desert ants, which store the honeydew they collect in their crops, and which are specialised to serve as food storage receptacles (the repletes); foraging workers, in whom the level of sacrifice approaches suicide – 15 per cent of those engaged in dangerous searches for food outside the colony perish, usually falling prey to spiders and robber flies; the ageing workers which engage in defence and suffer a weight loss of 40 per cent and increased mandible wear. One tropical Asian ant contracts so violently in the face of an intruder that it bursts and discharges a sticky fluid which incapacitates the enemy. Wilson observes within brackets that whereas humans send their young men to war, the more successful ants send their old ladies.
In a massive compilation such as this it is usual for authors to incorporate observations and ideas culled from earlier work without referring to their source. Wilson rigorously avoids this temptation, yet never spoils the flow of his agreeable prose. His paragraphs on kin selection – considered fundamental to general sociobiology – are a model in this respect, for he allocates the origins of the hypothesis to Darwin’s insight, but emphasises that the modern theory of kin selection and sterile castes is due to W.D. Hamilton, with an important extension by Trivers and Hare. This approach adds immensely to the value of such a monograph.
Kin selection involves collateral gene descent which boosts the welfare of relatives, not necessarily direct offspring, who possess the same characters also by reason of descent. Wilson gives us one of the most reasoned and satisfactory discussions of a very complicated but key facet of eusocial evolution.
‘Classical’ insects are those which, as a result of certain outstanding features of their behaviour, have initiated unusual attention. Wilson gives as examples bumblebees, honeybees, driver and leaf-cutting ants and fungus-growing termites. I entirely agree with him that the weaver ants can be included in this select company. They are one of the most cooperative of all insects. These ants construct communal silk nests, which enable colonies to attain huge populations and dominate several trees simultaneously, thus becoming one of the most successful social insects in the Old World Tropics. Wilson points out that they have been identified in Baltic amber – which means that recognisable species were present about thirty million years ago.
The silk with which the ants construct their nests is produced by their own larvae, from enlarged silk glands. They never construct cocoons for themselves, but disgorge the silk at the beginning of the final in star. The workers draw out the silk from the cocoons by spinning movements which turn them, according to Wilson and Hölldobler, ‘into passive dispensers of silk’.
During nest construction, the ants form living chains of individuals by seizing one another’s petioles (waists), and are thus able to exert a strong pull on the leaves they wish to manipulate. Workers then form rows which hold the leaves in position in a tent-like form. Larvae of the appropriate age are carried into this edifice and are employed as living reels of silk with which the leaves are bound together. The authors are surprised that communal nest-weaving has arisen only four times during the hundred million years of ant evolution. Reading this text, with its descriptions of the incredible plethora of modifications and specialisations involved, one is inclined to marvel that it has evolved at all.
There are many delightful asides to be found in this volume – casual pieces of information that make it almost a coffee-table book as well as a scientific tour de force. The glossary, for example, if we begin with the letter A, tells us that antbirds (tropical) follow raiding swarms of ants and feed on the prey they disturb; that ant butterflies, also following the raiding swarms, feed on the droppings of the antbirds; that ant plants grow specialised structures for housing ant colonies; that appeasement substance is a secretion presented by a social parasite to reduce aggression in its host.
In the opening paragraphs of Chapter One it is suggested that ants have been somewhat neglected by scientists and virtually overlooked by others, despite the fact they are everywhere and run most of the terrestrial world as premier soil-turners and channellers of energy. Wilson’s brilliant survey gives an indication of why this should be so. It is not only their pullulating, seething numbers, their remorseless activity, their superior organisations, or their improbable physique and disturbingly slender waists. It is simply this: ants frighten us. The bibliography is extensive and excellent – approximately 64 pages, over 2800 titles, of which 94 are Wilson’s own papers and 88 Hölldobler’s. It would be sheer ingratitude to criticise anything, but I suspect that the publisher made the index: I found it frustrating. But who is ever satisfied with an index? An enormous compliment was paid to the authors by Sir Vincent Wigglesworth who, in the words of the late Howard Hinton, made entomology into a subject. The Prof, now aged 92, an honoured guest of the Entomological Club, sat for an hour and a half on a sofa, ignoring the VIP guests, oblivious of the champagne, engrossed in a review copy of The Ants, with an expression of quiet, smiling satisfaction on his face.