Atta sexdens
leaf cutter ant

Atta sexdens can be found throughout Central and South America from as far north as Texas down to Southern Argentina. (Discovery Communications Inc., 2002)

Colonies of Atta sexdens are found scattered throughout forest floors and fields - anywhere with an appropriate climate and abundant foliage to harvest. Rather than building upward, A. sexdens colonies will dig a nest 2-6 meters (6-18 feet) deep into the ground. (Discovery Communications Inc., 2002; Wade, 1999; Wirth et al., 2003)

Atta sexdens divides its colony members into four major castes, each differentiated by a range of head widths.

Gardener-Nurses: The smallest of the four castes, gardener-nurses have an average head width of 1.0 mm, and range from 0.6 mm to 1.2 mm.

Within-nest Generalists: The next size up, they have an average head width of 1.4 mm, and range from 1.2 mm to 1.6 mm.

Foragers-Excavators: The second-largest caste, their heads average 2.2 mm, and range from 1.6 mm to 2.4 mm, with a few individuals above 2.4 mm.

Defenders: The largest of the ant castes, defenders usually have a head width above 3.0 mm.

The queen is the largest ant in the colony, often measuring an inch or longer. (Fowler and Robinson, 1979; Wilson and Holldobler, 1990; Wilson, 1979)

Ants in general progress through four stages of development: egg, larva, pupa, and adult. Atta sexdens, like all hymenopterans are haplo-diploid, meaning males are haploid (one of each chromosome) and females are diploid (a pair of each chromosome). Sex is determined by the type of egg is laid. Unfertilized eggs will turn out to be males, which are strictly used for mating and are short-lived. Fertilized eggs produce females.

Three to four weeks after being layed, the egg hatches and a larva emerges. In Atta ants, the larvae are fed by secretions from gardener-nurse ants and trophic eggs (unfertilized eggs that are used as food). The larvae, after an additional 3-4 weeks, spin cocoons around themselves and pupate. After 3-4 more weeks, the pupae hatch into adult workers, ready to serve the queen and colony. (EnchantedLearning.com, 2003; Pi, Rencsok, and Schmidt, 2000)

Colonies of Atta sexdens have a success rate of approximately 2.5%, that is, only 2.5% of queens sent out to build new colonies actually succeed in doing so.

Those colonies that do survive, as a whole, live about as long as their queens do, which is 10-15 years. By this time, the original colony has likely sent out several mating flights' worth of queens to populate other areas and propagate their genes. (Wirth et al., 2003)

Atta sexdens is an aggressively territorial species. Members of different colonies are not spared if they happen to wander into a colony that isn't their own. Atta sexdens naturally expand their colonies to large amounts of land and resources in all directions and dimensions if given enough room to grow. Workers run on foraging trails for up to 60 m from the nest, which means that if expanded to human proportions, the ants would have about a 15 km radius of territory to forage in.

Foragers find leaves by using chemical trails left by scouts to go to a particular tree, strip it of its leaves, and then follow the same trail back to the nest to deposit the leaves. The speed at which they do this is equivalent, in human terms, of running a four-minute mile with a 300-kilogram weight strapped to their back. Not once, but multiple times do the runners go from harvest site to nest in this fashion. (Wilson and Holldobler, 1994)

The four castes of A. sexdens each have different tasks within the colony. The first caste, the Gardener-Nurses, take care of the fungus and the larva/pupae. Second, Within-nest specialists do a wide range of tasks: disposing of refuse, processing vegetation a second time to make it small enough for the gardeners to use, reconstructing fungus gardens, transporting other workers, helping the smaller Gardner-Nurses with larger pupae and larvae, and taking care of the queen. The third caste, Forager-Excavators, find food sources, lay trails to them, cut and retrieve vegetation, and also excavate the nest, making new chambers or recovering old ones that may have collapsed. The fourth caste, the Defenders, protect the colony from attacks by predators and other ants. In addition, tasks may be delegated depending on the individual's age within its caste, resulting in multiple sub-castes. (Wilson and Holldobler, 1994; Wilson, 1979)

After finding a suitable spot to build her nest and colony, the queen burrows into the ground and creates her first chamber, where she lays her first set of eggs and places the fragment of the fungus garden that she took from her old colony.

The first eggs to hatch are gardener-nurses, who take care of the fungus gardens. After suitable amounts of these workers have been born, the queen produces forager-excavators and within-nest specialists. After finding enough material to build the fungus garden up, the excavators create new chambers and pathways for cultivating more fungus and holding more eggs. Finally, the colony produces defenders and soldiers to protect the colony from predation. After the first 2 years, the colony grows in leaps and bounds, and sexually active queens and males appear. A six-year old colony has expanded to more than 1920 chambers, 248 of which contain the fungus crop. A fully formed colony can contain anywhere from 5 to 8 million individual ants. (Wirth et al., 2003)

Communication between members of Atta sexdens involves a combination of chemicals called pheremones secreted from various glands, and touch. The gaster, the organ used for applying these chemicals, can be used to mark cut leaves, cutting sites, territory boundaries, and paths to take to get to cutting sites or rubbish heaps. If the ground is already marked, the ant will not mark it again unless the scent has faded.

Atta ants also can communicate by using a file-and-scraper mechanism that produces sound both in the air and through the ground. This mechanism is usually used to recruit workers to a particular patch of good vegetation. (Hill, 2001; Howse, Lofgren, and Vander Meer, 1986; Wilson and Holldobler, 1994)

Colonies of A. sexdens, like all leaf cutter ants, are mycophagic (fungus-eaters). The ants cut fresh leaves up into small pieces to use as a substrate for their fungus crop (flower petels are often used for the same purpose). Once the leaf fragments are inside the nest each is prepared with a drop of anal liquid which acts as a fertilizer. Once the leaf is placed into position a small amount of fungus is placed onto the anal drop. In addition to leaves, flower petels are often used for the same purpose. In addition to cultivating the fungus, Atta ants feed on plant sap. (Discovery Communications Inc., 2002; Wirth et al., 2003)

Like many other kinds of ants, Atta sexdens has a specialized caste devoted to fending off predators that disturb the nest. Usually, strength (and victory) depends on the number of defender-caste (called soldiers) ants that can arrive on the scene and attack the predator. Soldier ants are the largest in the colony and have evolved very sharp mandibles for use in defense of the nest or its workers. Depending on the size of the predator, smaller or larger ants will be dispatched to fend off the attack. (Wilson, 1979; Wirth et al., 2003)

The movement of leaves and waste and the construction of new canals by colonies of A. sexdens contribute a great deal to their ecosystem. By constructing and expanding the colony, A. sexdens colonies aerate and turn over almost 40,000 kg of soil before they finish.

Atta ants also redistribute vital elements into the soil through processing and removal of waste products from the colony. (Wilson and Holldobler, 1990; Wilson and Holldobler, 1994)

Atta ants also have a mutualistic relationship with their fungal crops. The fungus provides different types of nutrition to the ants, and the ants feed the fungus and protect it from parasitic fungi (genus Escovopsis). The ants use a bacteria (genus Streptomyces) that produces antibodies to fight Escovopsis. The parasite, if left unchecked, will easily destroy the entire fungus crop, severely hampering the colony's ability to survive. Many ants, at the first sign of the infection, will abandon the crop and try to get the uninfected crop elsewhere in the colony. (Poulsen, 2002; Wirth et al., 2003)

Usually when Atta ants cross human paths, the ants are not looked upon favorably. Atta sexdens and Atta cephalotes are the principal insect pests where they are found, destroying billions of dollars worth of crops with their ability to quickly defoliate and strip crops of anything useful to the ants. In fact, Atta ants are considered the number one herbivorous pest in many areas where they are found. (Wilson and Holldobler, 1994)

Humans in some areas of Mexico eat these ants as a source of protein. (Discovery Communications Inc., 2002)

Atta sexdens is not listed in the IUCN Red List, CITIES appendices, or the United States Endangered Species Act. (IUCN, 2002; United States Fish and Wildlife Service, 2001)