Sunday, July 02, 2006

The Ant Farmers

Andy Suarez specializes in biological invasions and ants and this was great news for those of us interested in invasive species as well as members of the Social Insect Mafia: the 4 boys in our group who study ants and bees. Since the course began, the Mafia has been educating us all with the fascinating world of social insects. One of the flagship groups in this world are leaf cutter ants, genus atta and acromyrmex. The more I learn, the better I understand the Mafia’s fascination.

Small leaf fragments seem to creep across the forest floor of their own volition, shimmering in sun flecks. Look closer and you will see that they are being carried by ants vastly smaller than the leaves themselves. In the photo below, you can see them walking near the edge of the sidewalk:

These are the leaf cutter ants and the leaves they carry are not for the colony members but for the fungus they grow deep within the colony nest. In this mutualism, the fungus is fed fresh leaves and other plant material and kept free of pests and molds. In turn the fungus is consumed by the colony. To additionally defend the fungus garden, the ants carry on their bodies a specialized bacteria that produce antibiotics. Leaf cutter ant fungi-culture, including this three-way mutualism, likely evolved over 50 million years ago making it the most ancient form of agriculture known to exist in nature.

If the practice of agriculture doesn’t grab you, try this: a mature leaf cutter ant colony may contain more than eight million members specialized into five primary morphotypes all with specific roles and whose behavior is communally directed. The heavy lifters that cut and carry leaves back to the colony are members of the mediae caste. A closer look at individual leaf fragments will reveal minors riding on the fragments, inspecting them for potential dangers to the colony. Of course this adds to the already considerable load born by the mediae. The trail between food source and colony home can be several kilometers long and is patrolled and maintained by majors, the significantly larger soldier ants. Living out their lives entirely within the colony are the minims that tend the growing brood and are the gardeners of the fungus crop. Finally, there is the single queen who produces the various morphotypes in response to the colony’s requirements as determined by, get this, her diet. It has been shown that the colony will change the content of her diet in response to a deficiency in a particular morphotype. In turn she will produce more of the required type.