Chapter 1 | Ant Butterflies

The floor of the rainforest is a silent space, despite the diversity of life it sustains. The creatures living there are well concealed indeed. They have been getting better at deception over quite a long time. And their predators—the smaller dinosaurs, which have evolved into the modern birds—have been keeping pace. They have been getting better at seeing through the deception. They have ways of finding the hidden inhabitants that live there on the ground in front of them.

The low-living, ground-dwelling invertebrates are cryptic, clothed in the colors of mold and fallen leaves. They don’t draw attention to themselves by moving. But on rare occasions a susurration arises from the undergrowth. It transforms the flat spaces into a sea of commotion. An advancing tide hisses through the deadfall on hundreds of thousands of tiny feet.

The opportunistic birds in the branches notice every sound that disturbs the early morning stillness. They are attuned to all the signs of life in their realm, and they were listening for this particular disquiet. They glide down for a closer inspection, converging from every direction. Their feathered friends from farther afield follow the calls of the earlier birds.

The rustling in the leaves is a raid of army ants, attacking everything in its path. The horde behaves as a single being—each individual connected with all the others. They organize their formations with no central commander. They pass pheromone signals back and forth through the air or through deposits they leave on the ground. Their advance covers every surface, flowing through the leaves, down into burrows, and up into trees—a boiling flood that stings anything it touches.

These ants have given up the subterranean ant-farm lifestyle. They are nomadic, moving from one camp to the next. They live in the open, in tents woven from their own linked bodies. The advance of their raid is quiet, a ripple in the leaf-mold accented by the occasional stroke of a leaping insect falling back to Earth. But the commotion turns raucous when the birds arrive. They are hailing their mates and calling out their pecking-order foes.

This specific guild of forest birds moves with the ants. These birds have also given up the home-based life. They no longer defend specific territories in the forest—they defend moving territories. They are wanderers, following the campaigns of the wandering ants. They are the antbirds.

Hundreds of antbird species have diversified into all the forest niches that sustain army ants. These birds make up less than ten percent of the tropical avifauna species. But their numbers can make up more than a quarter of the birdlife in areas where raiding ants march. Each of these antbirds is a specialist; many of them live solely by following the ants.

They travel one hop ahead of the advancing army. They have mastered the skill of wading through the tide without being bitten. In constant motion, they flit away just before the swarm engulfs their perch—just before they feel their feet crawling. They stand aside and peer down from vertical surfaces or from dangling vines. They see everything in fine detail, from just inches away.

The antbirds do not eat ants. They eat what the ants eat. The ants flush their prey from every hiding place. The birds pursue the spiders, scorpions, earwigs, centipedes, and roaches that crawl, jump, and run from the advancing ant army. Crickets that spring into the air may never come down—there are antbird species that launch from the sidelines to intercept them mid-arc. Beetles that fly straight up will fall prey to other kinds of antbirds, which pursue targets farther above the fray.

The bigger birds, like the ocellated antbird, command the space at the front of the raid. Smaller species, such as the bicolored antbird or the rufous-collared antbird, take the flanks. Still smaller, the scale-backed antbird hangs back and watches to hawk insects that escape into the air.

The woodcreepers perch on the wider trunks, while horizontal perches accommodate birds like ant tanagers or ant wrens. The largest species—ant pittas, or ground cuckoos—walk through the foliage ahead of the ant columns. At any time there may be scores of birds from a dozen species accompanying a large raid. They call out to each other and squabble over possession of the best overlooks.

The antbirds take advantage of the misfortune that has befallen their prey. They are raptors of disaster, like the rain birds that track the water’s rising edge to hunt creatures flooded from concealment, or the firebirds scouting the advance of the ground fire that chases their prey from dry grass.

The antbirds steal much of the pillage that would otherwise be taken by the army ants. The ants themselves are blind, unaware of the theft going on all around them. When the grub they are holding is lifted off the ground and shaken until they fall off, all they can do is run faster, until they find their scent trail again. The birds can rob the army of most of its plunder.

After the front lines of raiders have passed, the commotion recedes into the distance and the forest settles back down. The calling of the birds fades; the vacated foreground lies still. A narrow, quiet rivulet of ants runs both ways through the center of the glade, carrying nondescript fragments of raiding booty back to the ant bivouac.

The musty odor of army ants lingers in the air. It attracts those forest dwellers attuned to that particular airborne scent: the ant butterflies. Gliding in silence, these sprites materialize in the aftermath of the raid. Some of them float ghost-like on invisible, transparent wings. Their flight is lazy and slow; they frequently dip down to ground level. But they are not pursued by the antbirds that bring up the rear of the raid. The birds know that these butterflies are distasteful. Many of them share the red-and-black warning coloration worn by the long-wings—butterflies that feed on toxic passion vines.

Ant butterflies are of different sizes and color patterns. Each species has its own ways of avoiding its predators and parasites. The larvae of each feed on different host plants. But all the ones that have showed up here have one thing in common: they are all female. The males of the species are off elsewhere in the forest.

Another thing they share is a niche—one of the many hyper-specialized niches in the tropics. Like the antbirds, these insects know how to detect and follow ants. But what they are really following is the antbirds.

The swath of ground passed over by the army ant raid is speckled with white dots, dropped along the route by the birds. The butterflies touch down on these droppings, which are sources of essential, high-quality nutrients. The quiet flyers attend close to the rear of the raiders, to sup from the white spots while they are still moist.

The ant butterflies can assimilate nitrogenous compounds excreted by the birds, for use in the production of their own eggs. Other butterflies lay a relatively small number of eggs; they are limited by the amounts of nutrients they were able to accumulate in their bodies as larvae. But adult ant butterflies can add to the amount they hatched with. They are longer-lived than others of the lepidoptera. They extend their egg-laying success across their longer life—sustained by what they can find to forage.

The activity in this green world is a zero-sum game, where losses in one place are balanced by gains elsewhere. Nothing in the forest is wasted. The addition of butterfly eggs to the foliage, and the addition of the antbirds’ eggs to their own hidden nests, balances the ruination wrought by the army ants. Alternating cycles of destruction and growth sustain the diversity of life here—maintaining the vitality concealed beneath the trees.

The ant butterflies gather where the ants have gathered. They fly in from far and wide to the place where they will find antbirds. These butterflies spend their lives in pursuit of a movable feast, the most concentrated source of butterfly nutrients to be found in the forest. Then, when the feast is over, they glide higher into the forest canopy. Like the departing souls of the creatures that died below, they rise in the calm after the catastrophe has moved on, returning peace to the clearing.

Notes
Eciton burchelli, in the American tropics, is the archetypical army ant legionnaire (Franks et al., 1991). Their communication with each other and with their environment is olfactory (by pheromones and scents) and tactile (by touch). A soldier’s eye consists of only a single optical facet. It tells them the difference between night and day and not much more. The operations they launch are diurnal and predictable, well suited to kleptoparasitism by the antbirds (Willis & Oniki, 1978). There are more than a hundred species of army ants. E. burchelli is exceptional in the broad net it casts for prey. Many of the other species are specific predators on the hives of other ants or social insects.

Many different butterflies follow army-ant raids, including skippers (Lamas et al., 1993) and members of the Ithomiidae, long-winged mimics of the Heliconiid butterflies (Ray & Andrews, 1980). This guild of understory butterflies follows the birds that follow the ants. In the first published description of the ant butterflies, Drummond (1972) did not notice the connection between these butterflies and the high-nitrogen sustenance they derived from the birds. Their dependence on antbird droppings has since been documented, e.g., by Ray & Andrews (1980), who noticed the preponderance of females among them. The ithomiids carry dissuasive alkaloids in scent pockets in their clear wings. Those toxins protect them from predatory birds, and from spiders—which cut them out of their webs (Brown, 1984).

The antbirds are raptors of disaster, as are the firebirds, such as the black kites. That kite, from the Australian outback, has been reported to rekindle the fires that flush their prey from the tall grass; they have been seen carrying twigs that are burning on their far ends (Montague, 1970). This raises the possibility that once upon a time, at the dawn of human history, these raptors, by their example, taught another two-legged animal about the handling of fire.

References
Brown. K. S. 1984 Adult-obtained pyrrolizidine alkaloids defend ithomiine butterflies against a spider predator. Nature 309, 707–9.
Drummond, B. A. 1972. Butterflies associated with an army ant swarm raid in Honduras. Journal of the Lepidopterists Society 30, 237–38.
Franks, N. R., et al. 1991. The blind leading the blind in army ant raid patterns: Testing a model of self-organization. Journal of Insect Behavior 4, 583–607.
Lamas, G., et al. 1993 The Ahrenholz technique for attracting tropical skippers (Hesperidae). Journal of the Lepidopterists’ Society 47, 80–82.
Montague, A. 1970. A remarkable case of a tool-using bird. American Anthropologist 72, 610–14.
Ray, T. S., & C. C. Andrews. 1980. Ant butterflies: Butterflies that follow army ants to feed on antbird droppings. Science 210, 1147–48.
Willis, E. O., & Y. Oniki. 1978. Birds and army ants. Annual Review of Ecology and Systematics 9, 243–63.

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