Passage IV

NATURAL SCIENCE: This passage is adapted from The Earth Dwellers: Adventures in the Land of Ants by Erich Hoyt (@1996 by Erich Hoyt).

In the steamy first light of the tropical rain forest in Costa Rica, life is buzzing. The forest echoes with the whooping calls of the oropendolas [tropical birds]. At lower volume but ever more persistent is the hissing of thousands of cicadas and other insects. In the distance howler monkeys are on dawn patrol.

No matter the racket, nocturnal animals are winding up their day and preparing to sleep, while those that need light to hunt are just awake and looking for breakfast. Still others—including some of the insects—have been working all night and will continue into the day.

From overhead a shaft of light beams down on a scene of frenetic activity. A worker ant—a foraging scout, female, as are all ant workers—stands on the leaf of a low-growing bush. Her hind legs are digging in, her head is down, hard at work. Her serrated mandibles, her jaws, are moving, sawing through a leaf. The air is pungent with leaf sap. As it drips from the leaf, she stops to lick a drop or two for refreshment. Two seconds later she's back at work.

As she saws, the sharp blade of her mandibles cuts through the leaf tissues, and her round head bobs u and down with the rhythm of a woodcutter. Slowly she begins to pivot on her hind legs to inscribe a curved line. She turns steadily, finally coming all around in a nearly perfect circle. She nudges the leaf panel to break the final perforations, and it falls to the ground. She jumps down, squats over the leaf piece, and marks it with a drop of liquid from the underside of her rear abdomen, her gaster. This marking will make the piece more attractive to the others in the colony. It also contributes enzymes that will eventually help break down the leaf. Then she grabs the leaf piece with her mandibles, hoisting it high so that it almost rests on her antennae like a floppy, oversized hat. She walks a few feet into the full sunlight of the forest clearing and stops. Her leafy hat is heavy.

In the speckled light of the clearing, the scout's entire body, seen in profile, glows reddish brown—except for the protruding black eyes. She has the distinctive spines of a leafcutter, or parasol, ant. They look like thorns rising from her back. She is covered in fine, tiny hairs from her head to her rear feet. The two antennae droop under the weight of her green load—the piece of leaf is three times her body mass. She struggles to press the green prize up high. And then another, much smaller leafcutter ant appears out of nowhere, climbs aboard the leaf piece, and clings to one end of it. She is less than a quarter the size of the scout, her nest-mate. This smaller worker keeps alert for enemies. She, too, marks the leaf piece with a drop or two from her abdomen, contributing more enzymes to help break down the leaf.

Today the scout has what may well be a new leaf species—an untried plant for her colony. Her leaf-prize thrives on a few low-lying bushes at the edge of a clearing, some distance from the nest. It is an exotic plant—an introduced (that is, not native) species, perhaps a plant hat escaped from a nearby farmer's field. As an exotic, it has fewer of the built-in defenses that many indigenous plants have evolved through living among insects for millions of years. Therefore, herbivorous insects, mammals, and other animals may be able to make easier use of it.

In a survey of forty-two plant species from the forests of Costa Rica, 75 percent contained terpenoids, steroids, waxes, and other defense compounds that repel leafcutters. Many tropical forest leaves are tough and have sharp spines that discourage monkeys and other animals from eating them. But the persistent leafcutters aren't much bothered. Their razor-sharp mandibles can slice through some of the fleshiest leaves and even twigs and stems. With regard to defense compounds, the leafcutters are selective about what they cut, but in species-rich tropical forests they have a lot of choice. Surprisingly, the rarity or commonness of the plant species has no real bearing on its appeal to the prospective leafcutter scout. Neither is proximity a crucial factor; leafcutters will walk a few hundred feet to a favored plant species. Researchers estimate that the tribe of gardening ants. which leatcutters belong to, cuts 12 to 17 percent of the leaves and flowers produced in the forests they inhabit. On one island in Panama, leafcutters annually cut almost as 250 pounds of foliage per acre. To put it in terms of a colony, every day a single mature colony uses as much vegetation as a cow.