Leafcutter ants

Near the upper edge of our research area, Alex pointed out an opening in the forest canopy above and the sunlight actually made it to the forest floor here where it shone on a massive dirt hill. The hill and the opening in the canopy were the work of leaf cutter ants; they had stripped the trees of their leaves above their huge colony. These ants forage into the vegetation, cutting off bits of leaves and bringing them back to their nests. It is said that the ants can defoliate a small tree in a single day. A web of ant trails spread out into the forest, each trail swarming with a line of ants coming or going. Some trials went up the trunks of the massive trees near the colony.

Large soldiers ran along the trials guarding the more numerous workers. Worker ants were carrying pieces of leaves as large as their own bodies. From a distance these trails looked like small streams of green leaves moving along toward the colony. The worker ants ran along the trails back inside the nest, left the leaves, and then scurried back the other way for more. The soldiers were always patrolling for predators that might eat the workers, including ants from other colonies.
Some smaller ants seemed to be hitching a ride on the backs of the ants that carry the leaves. My first thought is that these were the lazy ants. It turns out these are small soldiers that are the first line of defense against attack. They run along the trial unassisted but they also apparently ride on the back of the workers to fight off parasitic flies that try to lay their eggs on the ants’ heads.

Leafcutter ants cannot get nutrition from eating the leaves themselves, and tend gardens of fungi that grow upon the leaves taken into the chambers within the ant hill. Deep in the colony there is an even smaller bodied ant that tends rooms where the leaves are inoculated with the desired fungal food. The smallest bodied ants tend the fungi on the leaves, tend the larvae and attend to the queen.

The rooms below the jungle floor are the optimal humidity and temperature for the fungus. The ants weed other kinds of fungus and bacteria out and eat their preferred crop of fungus from their dark gardens. The ants also have bacteria in special glands that excrete antibiotics that keep the food fungi from being overgrown. Species of leaves that do not allow the proper fungi to grow are removed, and somehow the workers are signaled not to collect that species any more. If leaves that are growing the desired fungi are removed from the colony, the species of fungus that the ants like to eat is quickly overgrown by others. When a winged queen leaves to mate and start a new colony, she takes a bit if the fungus with her to start the new colony. The fungus would not compete without the ants, and the ants could not survive without the fungus, yet another example of coevolution.

The ants continually replenish their gardens with fresh leaf clippings for the fungi and remove the waste to the outside of the colony. The large dirt hill is made up of soil particles from the excavation of the colony and waste from the colony. Humans are not the only species that has had an “agricultural revolution”. When jungles are cut down the hills remain and can be a hazard for livestock. When cattle walk over the once active ant hill, it can collapse into the large area of caverns excavated below and a cow can become trapped. The ants damage crops and their colonies can damage roads.

These ants are common in the tropics and there are 41 species from two genera of ants, and they tend any of several species of fungi. Termites and ambrosia beetles are the only other insects known to tend fungi. This species is a fascinating result of evolution of social insects, with the adaptation to use fungi, the bacteria that helps maintain the fungi, and the six morphologies of ants all in the same species inhabiting the same colony (the smallest workers inside the nest, the slightly larger soldiers outside the nest, the still larger workers that collect the leaves, the largest soldiers, the queen, and during certain times males).

One enemy of the leaf cutters is the army ants that patrol the forest. These ants are predators that move in a continuous stream across the jungle floor. If you follow the trail of the army ants you will eventually come upon their nest. It is a bivouac that is a ball of ants as big as a watermelon suspended a few feet above the jungle floor. The queen is inside this seething ball of ants. The ants control the interior temperature of the ball by the rate they cycle to the inside. Follow the trail that leads from this ball of ants a mile or two through the jungle, and you will find the next bivouac with the next queen. These ants move through the forest, occasionally in swarms. The ants kill every insect that cannot escape and will even take smaller animals. Birds follow these army ants to snatch the insects the ants scare up.

It is fascinating to follow a trail of these ants. Where they come to a place that is difficult to cross, the ants grasp the one in front of them and make a chain of ants. Several of these chains side by side make a bridge for the remainder of the ants to cross over. If leaf cutter ants are the farmers, these are the raiders. Army ants are aptly named. They will overcome any other insect colony they encounter, or any animal that cannot get out of the way, regardless of size.