Thieves Deceivers and Killers Read online

  PRINCETON UNIVERSITY PRESS PRINCETON AND OXFORD

  Copyright © 2001 by Princeton University Press

  Published by Princeton University Press, 41 William Street, Princeton, New Jersey 08540

  In the United Kingdom: Princeton University Press, 3 Market Place, Woodstock, Oxfordshire OX20 1SY

  All Rights Reserved

  Library of Congress Cataloging-in-Publication Data

  Agosta, William C.

  Thieves, deceivers, and killers : tales of chemistry in nature / William Agosta.

  p. cm.

  Includes bibliographical references (p. ).

  ISBN 0-691-00488-9 (alk. paper)

  1.Chemical ecology. I. Title.

  QH541.15.C44 A49 2000

  577—dc21 00-032627

  This book has been composed in Palatino

  The paper used in this publication meets the minimum requirements of ANSI/NISO Z39.48-1992 (R1997) (Permanence of Paper)

  www.pup.princeton.edu

  Printed in the United States of America

  10 9 8 7 6 5 4 3 2 1

  CONTENTS

  PROLOGUE

  The Protos and Their Slaves

  1

  From Protos and Lepts to Nature’s Special Chemicals

  2

  Paying Ants for Transportation

  3

  Getting Pollinated

  4

  Flies and the Misery They Bring

  5

  Eavesdropping as a Way of Life

  6

  Success through Mimicry and Theft

  7

  Bacteria: Chemical Complexities in Simple Cells

  8

  Delving into Nature’s Chemicals

  9

  Stocking the Medicine Chest

  10

  Loose Ends and New Beginnings

  11

  Complexity in the Real World

  12

  Capitalizing on Ecology

  Glossary

  Further Reading

  Index

  PROLOGUE

  The Protos and

  Their Slaves

  The three Proto scouts moved closer. They saw what looked like a Lept camp up ahead but wanted to make sure. Soon they were close enough to spot the entrance and, sure enough, there the Lepts were, going in and out, carrying food. The Protos had been right: It was a medium-sized Lept camp and near enough to their own for a quick attack. This must be the camp of the Lepts they had been encountering in recent days. Two of the Proto scouts turned back toward home to gather reinforcements, while the third stayed behind to guide the raiding party to the camp entrance.

  The two scouts made good time returning home and announced their news at once. Excitement ran through the Proto compound as the story spread. A Lept camp only a short distance away! For as long as anyone could remember, the Protos had kept Lepts as slaves, and like slave-owners everywhere they needed to renew their supply of captives from time to time. For the Protos, nothing really worked well without Lepts. Without an ample supply of slaves, their whole society would disintegrate. It was not merely that the Protos were lazy, but they had kept slaves for so long that they were truly unable to care for themselves. The slaves could raise young, gather food, and keep the place clean, whereas the Protos themselves really excelled only at capturing Lepts. Otherwise, they spent a great deal of time lolling around their camp, acting bored, and asking the Lepts for something to eat. Lept slaves did all the work and were remarkably faithful to their Proto masters, even becoming ferocious participants in slave-raids on their own kind.

  As the scouts made their report, a well-armed raiding party of both Protos and slaves gathered around them. When the group was large enough, the scouts led it out toward the Lept camp. One scout led the way over the carefully marked trail, the eager party briskly following in an orderly column. They soon made their way to the Proto scout left behind earlier and grouped excitedly for the assault.

  Intimidating the few Lepts encountered on the way, the raiders advanced directly to the entrance. They forced their way through the entrance, attacking and pushing their way into the camp. The Lepts did not give up easily, and soon the fighting was fierce. As the battle proceeded, the distinctive fighting styles of master and slave became apparent. The Lept slaves fought hard, slashing and stabbing the free Lepts who were defending their home. As Lept struggled against Lept, combatants were equally matched, and the clashes often ended with one or both participants injured or dead. Encounters between Lepts and Protos, however, were generally quite different. Though the Lepts were brave, they rarely attacked the Protos. The Protos themselves almost never struck a blow. If a Lept did strike a Proto, the fast-moving Proto slipped neatly away. Instead of direct physical combat, the Protos preferred using an impressive chemical weapon that each one carried. The Protos needed to spray only a few drops of this chemical and the Lepts were reduced to panic and confusion. They forgot about challenging the invaders but turned instead on one another, suddenly fighting among themselves in chaos and turmoil. All Protos carried this unusual chemical spray but did not share it with their slaves. Although the slaves fought fiercely, the Protos’ weapon really won the battle. In fact, the Protos generally had no trouble defeating Lepts even when there were no slaves in the raiding party.

  Meanwhile, as the conflict continued, several raiders pushed deep into the camp, searching out the Lepts’ communal nursery. Here, where the Lepts kept their young, was the Protos’ real goal. Because adult Lepts would never adapt to enslavement, the Protos had no interest in them. A Lept captured at birth, however, and raised among the Protos would accept servitude without question. Thus the raiders began removing infants from the nursery for the trek back to the Proto compound. Lept nurses rushed about trying to save their wards, grabbing up as many as they could and running for the camp entrance and the safety beyond. Many reached the entrance, but two Protos stationed just inside barred their way to freedom. These guards did not harm the nurses but allowed them to escape only after they released their precious loads. In this way, the young Lepts were captured and the adults were either destroyed or cleared from their camp and dispersed.

  After the battle, several Protos carried the little ones back to the Protos’ camp, making repeated trips until the Lept nursery was emptied. They installed the captured infants in the Proto nursery where, under the eye of slave nurses, the little Lepts would learn their place in the world. This Proto home would be the only one they would ever know, and they would grow to adulthood unaware of what might have been. Any free Lept they met they would recognize only as an enemy.

  For the Protos, it had been another successful raid. Several slaves had died, but the Protos themselves suffered few injuries. Owing to their chemical weapon, the Protos could raid the Lepts with little loss of life to themselves or their victims. If any society based on slavery could be characterized as “advanced,” the Protos certainly seemed so. Other slave-raiders that preyed on Lept nurseries killed as many adults as possible during their assaults. Some even ate their victims. In comparison, the Protos could pass as relatively civilized. From the Proto point of view, of course, charity made good sense. By letting the adult Lepts escape, the Protos assured themselves of future Lept nurseries to plunder. The day after the raid, in fact, the scattered Lepts straggled home to pick up their lives and begin replenishing their looted nursery.

  From this portrayal, we might condemn the Protos as barbarous creatures, but as you may have known all along the Protos are not wicked slave-keeping humans. Protos and Lepts are tiny ants that live out their entire lives in a world no larger than a dinner table. A Proto camp consists of one or two dozen indolent masters cared for by perhaps tw
ice as many Lept slaves, all sheltered in an empty milkweed stem or perhaps within a hollow acorn. Free Lepts form similar colonies of two or three dozen individuals. The Protos’ formal name is Protomognathus americanus, and the Lepts they enslave are Leptothorax curvispinosus and two other closely related species. Their little domains occur throughout the broad-leaved woodlands of eastern North America, from Ontario to Virginia and as far west as Ohio.

  Slave-making ants’ remarkable habits have made them a favorite of natural scientists for almost two hundred years. Generations of entomologists (scientists who study insects) have delighted in watching them in the field and arranging their raids in the laboratory. If the ants’ behavior seems almost human, scientists have also noted a contrary aspect that we should not overlook. Whereas sight and sound are human beings’ primary modes of communicating with one another, many of the ants’ interactions depend on releasing and sensing chemicals.

  If Lepts and Protos were people, they would identify friend and foe by sight. They would mark their trails with visible signs and exchange information using written and spoken language. Instead of depending on sight and sound, however, Lepts and Protos communicate with chemicals. Other kinds of ants do the same. On meeting, two ants touch each other with their antennae, “smelling” the chemicals on each other’s body for identification. They readily distinguish other ant species from their own, and within their own species they discriminate members of their home colony from outsiders. To mark their trails, scout ants deposit minute amounts of chemicals on the ground behind them as they move along. The Proto scouts that returned home to report finding the Lept camp used chemicals in communicating their discovery. A chemical recruitment signal they released helped assemble the raiding party. The Protos probably identified and located the Lept brood by odor as well. Overall, ten to twenty different antenna-detected chemical signals are necessary to assure an ant colony’s smooth operation.

  Chemicals also have a critical place in ant warfare. Most ants possess a sting that delivers poisonous chemicals to their enemies. In the strenuous fighting of the slave raid, Lept slaves and free Lepts frequently stung one another severely. Some kinds of ants employ other chemical warfare agents as well, such as the Protos’ unusual weapon that created turmoil among the Lepts and caused them to attack one another rather than the invading Protos.

  It was ants’ extensive dependence on chemicals that brought Lepts and Protos to the opening of our story. These little insects vividly illustrate chemical interactions in creatures’ lives. Just as we considered Protos and Lepts, we want to look at other living creatures as they employ their chemicals, sending messages, defending themselves, and carrying out many other activities.

  The world of chemical relations among plants, animals, and other organisms was completely hidden from us until only a few decades ago. Many scientists thought that chemicals now recognized as central to these relations were nothing more than cellular waste products. Interactions among organisms were much less well understood in the past, and the role of chemicals in many of them was not yet acknowledged. Biologists and chemists opened up this world of interactions among living creatures only as appropriate chemical and biological investigative tools started to appear around mid-century. Only then did the story of these universal chemical activities begin to take shape.

  As the story of the Protos’ raid demonstrates, a familiarity with chemistry or biology is certainly not required to delve into these activities. An up-close look at the chemicals used by the Protos provides a marvelous entry into a wider biological world filled with events as remarkable and fascinating as the raids of slave-making ants. Other creatures have equally amazing stories and, from a broader point of view, their behaviors play a key role in the health of the earth, the preservation of environments and biodiversity, and the extinction of species.

  The chemicals featured in the behavior of many organisms also touch our own lives in important ways. They provide a sizable fraction of modern medicines, as well as perfumes, pesticides, and other products ranging from textiles to glue. Some of these chemicals have been in use for thousands of years and have intriguing histories. Others offer ways to save threatened environments; all affect our own lives, and some do so profoundly.

  Most of us now live far from the close daily contact with the natural world that was taken for granted until well into the nineteenth century. It is easy to forget that, as living creatures, we too are part of this natural world and that our high-tech existence today depends on this world and its creatures no less than life did in former times. In what follows, we shall see what the chemicals of this natural world are all about.

  CHAPTER

  From Protos and

  Lepts to Nature’s

  Special Chemicals

  1

  The Protos’ warfare on their Lept neighbors depended heavily on chemicals, but ants are by no means unique in making extensive use of chemicals for communication and warfare. From one-celled organisms to complex plants and animals, many living creatures do the same. As species develop over evolutionary time, it is relatively easy for them to adapt their cellular machinery to producing chemicals for communication, warfare, and other purposes. These chemicals facilitate the way of life of organisms spread all across the biological spectrum.

  Like the signals of the Proto scouts, one large group of chemicals carries messages that pass between members of the same species, messages that humans can express in words. Such signals are conveyed by substances called pheromones, which can transmit different kinds of information. Some pheromones are attractants, bringing male and female together for mating, or perhaps assembling a group of creatures for feeding or defense. Others carry such varied messages as “Danger! Flee!” “This is my territory,” or “I am pregnant.”

  Other types of chemical signals that occur may pass between members of different species. These interspecific signals, as they are called, may benefit one or both of the species involved. The Protos’ chemical warfare weapon, for example, can be viewed as an interspecific signal that benefits only the sender as it reduces the recipient Lepts to fighting among themselves. On the other hand, sometimes the receiver is the sole beneficiary of a message, as when a predator locates its prey by following the distinctive odor the victim haplessly communicates to its enemy. Other signals between species serve both sender and receiver, as when a flower’s delightful fragrance entices a foraging insect to linger and explore. In an exchange profitable to both organisms, the insect pollinates the flower and receives a drop of nectar in return.

  ILLUSTRATION 1 About half the 15,000

  to 20,000 hairs on this male silkworm moth’s

  feathery antennae are specialized for

  detection of the female’s sex attractant

  pheromone.

  Chemicals may also supply more general information about a creature’s environment. Salmon return from the sea to their native stream to spawn, guided by the distinctive odor of their birthplace. Thirsty animals follow their noses to locate life-saving water. Other chemicals are closely associated with organisms’ characteristic ways of life, such as the silk spun into a silkworm’s protective cocoon or the pearl an oyster fashions around a grain of sand in response to the gritty irritation.

  Organisms, wherever they are, interact with one another to live, and chemicals mediate many of these contacts. Some interactions may be optional and occasional, whereas others are absolutely necessary to sustain the organism’s way of life. Protos can survive only by keeping Lepts as slaves. Various species of slave-making ants differ in their self-sufficiency, but Protos have lost the ability to care for themselves. Deprived of its slaves, a Proto colony soon deteriorates.

  Before continuing with the role of chemicals in the living world, we should clarify certain words and concepts that we have been using. The chemicals we have been talking about are chemical compounds made of molecules that are three-dimensional arrangements of atoms joined together by chemical bonds. A molecule is the s
mallest existing unit of a chemical compound, just as an atom is the smallest existing unit of a chemical element. The specific arrangement of the atoms in a molecule—how the atoms are joined together—is called its molecular structure. It is this structure that makes a molecule what it is. Molecules come in a multitude of sizes, from as few as two atoms to many thousands. The atoms themselves are of many types, but the biologically active molecules that interest us here are usually limited to atoms of carbon, hydrogen, oxygen, perhaps nitrogen, and, less frequently, a bit of sulfur or phosphorus. Many of these biomolecules are medium sized, with twenty to eighty or ninety atoms. This size range also includes the molecules of such familiar natural compounds as cocaine, penicillin, and cholesterol.

  Chemists can make, or synthesize, many biologically active chemical compounds in the laboratory. (Chemical, chemical compound, and compound are interchangeable terms.) One starts with readily available simple molecules and proceeds to build more complex structures, one step at a time, through chemical reactions. This procedure is called stepwise synthesis, and chemists have used it to prepare thousands of compounds that are found in nature, as well as many thousands more that have never been found in living systems. The chemist’s repertory comprises many different synthetic reactions that can be carried out in different combinations, so that numerous ways are often available for preparing a specific compound. Living organisms, too, make molecules by stepwise syntheses, employing their enzymes (natural proteins that speed up, or catalyze, chemical reactions) and biochemical machinery rather than the chemist’s laboratory reactions. Typically, nothing about an ordinary biomolecule reveals its history or origin, so a particular compound synthesized in the laboratory is indistinguishable from the same one obtained from nature.