Not dead

Hiking through a Costa Rican rain forest some years ago, I had to keep stopping for animals. Sadly, the first was dead: a small monkey who must have lost his hold while trapezing through the branches and plummeted to his end. Later, several frogs, no larger than my thumbnail, hopped daintily across the trail. Later still, another frog, size of a shoe, also hopped across, not very daintily. There were others: an elegant ferret-like creature, yellow-billed toucans, macaws like jet fighters.

But the ones we saw most often were the tiniest. Every now and then, we had to step over a line of moving green stretching across the trail and into the undergrowth.

Ants. Thousands of ants, each carrying a piece of leaf like a small sail, scurrying along in determined single file. Each piece was several times larger than the ant carrying it. So the overall effect from some distance away was of a green thread that moved steadily, but jiggled and shook like a sensuous conga line.

These were Central America's notorious leaf-cutter ants, heading home with their loot. Sweating my way through this forest, grumbling about exertion I had never known before, I had stumbled on the animal world's hardest workers. These creatures are so efficient that a colony of them can strip a citrus tree of its leaves in a single night.

But this day, kneeling to look at them, it was hard to remember that. Instead, I was captivated by the energy, even a mysterious elegance in these leaf-toting scurriers.

I didn't look close enough to see it then, but later, when I read more about them, I learned something still more beguiling. Some still tinier ants ride on the leaf fragments that their cousins carry aloft. Not because the little ones are lazy, oh no. They have a very specific, crucial task to perform. These little fellows protect the porters from a particular fly that lays its eggs on living ants. Rushing around the leaf, they swipe tiny jaws at the menacing flies. That's their job, and lives depend on it. These minors, as they are called, have one of the most specialized roles to be found anywhere in the forest.

Life among ants, in fact, is all about such roles. They live in huge colonies in which strict discipline is enforced; each resident is a specific kind of ant, with a specific, rigid role to play.

Among leaf-cutter ants, for example, some are gardeners. They tend the growth of a peculiar fungus that cannot live outside the nest. Others feed the fungus to larvae. Still others chew leaves into a mush that is used to fertilize the fungus. Then there are soldiers, who guard the colony and fight off intruders. Workers slice leaves into little bits and carry them home; they are called medias, and do just that job. To find their way to and from their colonies, medias follow a trail blazed by scouts. And there are the minors, the mandible-wavers who ward off flies.

And if each of these are critical functions, the most important ant of all is the queen, the regal and fat colony head. She lies deep inside the colony, several times larger than the workers who attend to her. She has just one job too: to reproduce. Not that she is busy mating all the time. She carries a bank of sperm from a single mating that happened before she started the nest. That store fertilizes all the eggs Her Antship will produce through her life.

So they all have distinct roles. But ants must also communicate with each other constantly. They don't have ears and cannot make sounds; while they can see, their world-view is necessarily limited by how small they are. Yet the leaf-cutter ant travels a hundred metres or more to chop up leaves to bring home. Weaver ants patrol territories as large as 1600 square metres. Ants can identify intruders, alert other warriors to them and lay a trail for other ants to follow.

How do they accomplish all this? How do they communicate?

In a word, pheromones. Specialized chemicals secreted by glands in their bodies. In his autobiography Naturalist, Edward O Wilson, the world's foremost authority on ants, tells a delightful story that illustrates nicely what pheromones mean to ants.

When an ant dies, it crumples to the floor of the colony and lies there. Its mates take no notice at all: they carry on with their work, stepping around and over the corpse as they scurry about. After a couple of days, a few worker ants will suddenly pick up their dead comrade and carry him out to a kind of ant garbage dump.

Why wait two days for this funeral? Because the dead ant releases a chemical after that time. This is his announcement that he has really died. This was Wilson's hypothesis, but he took a while to isolate the chemical itself. When he did, he found dramatic confirmation of its efficacy.

What Wilson did was to smear some of the chemical on a very alive worker in a colony. Immediately, several of its companions picked up the struggling, squirming little animal and carted it off to the dump. The ant wandered back in, only for the performance to be repeated. Driven by the inexorable logic of the chemical, the ants on the graveyard shift paid no attention to their comrade's frantic antics to show life. It smelled dead: it had to be dead. It was only when the odour wore off that they left it alone once more.

Wilson had found just how powerful those pheromones are. He had also left one thoroughly bewildered ant. All in the cause of science.