About Atta sexdens (Linnaeus, 1758)
Atta sexdens (Linnaeus, 1758) is a species of leafcutter ant in the tribe Attini, native to the Americas, with a range that extends from southern United States (Texas) to northern Argentina, and it is absent from Chile. These ants cut leaves to create a growing substrate for the fungus farms that are their main source of food. Their social colonies rank among the most complex societies observed in social insects. A. sexdens is ecologically important, but it is also considered an agricultural pest. Other species in the genus Atta, such as Atta texana and Atta cephalotes, share similar behavior and ecological patterns. Between late October and mid-December, A. sexdens colonies produce winged virgin queens and males in preparation for reproduction. Before the nuptial flight, young queens visit their natal colony's fungal gardens and store a small piece of fungal mycelium in their infrabuccal cavities. During their nuptial flights, each queen mates with multiple males, which die shortly after mating. The queen stores the received sperm in a specialized organ for the rest of her life. It is estimated that each queen is inseminated by 3 to 8 males, and the number of different fathers for workers in a single colony ranges from 1 to 5. After mating, the mated queen lands on the ground, removes her now unnecessary wings, then digs a vertical tunnel to a depth of approximately 30 centimeters. She excavates a small chamber at the end of the tunnel, then starts a new fungal garden using the mycelium piece she brought from her birth colony. Usually, the queen fertilizes this garden only with her feces, but she may sometimes need to forage for a small amount of plant material. She also lays a small number of eggs. The queen then tends to both the new fungal garden and her developing larvae, feeding the larvae on fungus and trophic eggs. The queen does not eat anything during this founding period, sustaining herself on stored body fat and her shrinking flight muscles. This process takes 40 to 60 days, after which the first brood of worker ants is mature enough to take over all colony tasks, and the queen becomes exclusively dedicated to laying eggs. New colonies grow slowly at first, but growth speeds up after two to three years. One suggested explanation for this slow early growth is that colonies benefit from staying unnoticed until they are large enough to support a full caste of large soldier workers. Soldiers typically emerge once the colony reaches a population of around 100,000 workers. Once the total worker population reaches 5 to 8 million, the colony stops increasing in size and shifts resources to producing new virgin queens and males. Very few virgin queens successfully found new colonies. If the total number of colonies in a given area stays stable over time, on average only one out of the thousands of new virgins produced by a colony will successfully establish a new colony. Mature colonies with several million workers face very few threats. Aside from the mostly subterranean army ant Nomamyrmex esenbeckii, no known predator actively attacks A. sexdens nests, even other highly aggressive army ants avoid confrontation with mature A. sexdens colonies. If a colony is not destroyed by floods or human activity, it typically only ends when the founding queen dies of old age, giving successful colonies a lifespan of 10 to 20 years. In this time, a mature colony produces many generations of males and virgin queens to found new colonies. Like all leafcutter ants, A. sexdens eats fungus, and lives in a symbiotic relationship with a fungus species from the subphylum Basidiomycota. Foraging workers bring cut leaves and other soft plant material back to the nest, where it is chewed into a pulp and fertilized with feces to act as a substrate for the fungus. A small piece of the symbiotic fungus is placed on this prepared substrate. The gardener-nurse worker caste manages fungus cultivation, transplanting fungus to fresh substrate and removing unwanted contaminating fungi, such as the parasitic genus Escovopsis that can invade ant nests. The ants also use salivary gland secretions and partner with antibiotic-producing Streptomyces bacteria to maintain their fungal gardens as a strict monoculture of their symbiotic partner. Gardener-nurse ants also cut small pieces of fungal mycelium to feed other ant castes. In addition to the cultivated fungus, adult A. sexdens also feed on plant sap. These two food sources are the only nutrition for the ants, aside from the trophic eggs produced by the queen in young founding colonies. The exact species identity of the symbiotic fungus cultivated by A. sexdens is not confirmed. It is known to belong to the basidiomycete family Lepiotaceae. Some researchers propose that all fungus-growing ants cultivate just a single species, Leucocoprinus gongylophorus. This fungus produces specialized structures called gongylidia, which have evolved specifically to be eaten by the ants that cultivate it.
