Fungus-growing ants
Fungus-growing ants comprise all the known fungus-growing ant species participating in ant–fungus mutualism. They are known for cutting grasses and leaves, carrying them to their colonies' nests, and using them to grow fungus, on which they later feed.
Their farming habits typically have large effects on their surrounding ecosystem. Many species farm large areas surrounding their colonies and leave walking trails that compress the soil, which handicaps growth of plants. Attine colonies commonly have millions of individuals, though some species only house a few hundred.
They are the sister group to the subtribe Dacetina. Leafcutter ants, including Atta and Acromyrmex, make up two of the genera. Their cultivars mostly come from the fungal tribe Leucocoprineae of family Agaricaceae.
Attine gut microbiota are often not diverse due to their primarily monotonous diets, leaving them at a higher risk than other beings for certain illnesses. They are especially at risk of death if their colony's fungal garden is affected by disease, as it is most often the only food source used for developing larvae. Many species of ants, including several Megalomyrmex, invade fungus-growing ant colonies and either steal from and destroy these fungal gardens, or they live in the nest and take food from the species.
Fungus-growing ants are only found in the Western Hemisphere. Some species stretch as far north as the pine barrens in New Jersey, USA and as far south as the cold deserts in Argentina. This New World ant clade is thought to have originated about 60 million years ago in the South American rainforest. This is disputed, though, as they could have possibly evolved in a drier habitat while still evolving to domesticate their crops.
Evolution
Early ancestors of attine ants were probably insect predators. They likely began foraging for leaf sections, but then converted their primary food source to the fungus these leaf cuts grew. Higher attines, such as Acromyrmex and Atta, are believed to have evolved in Central and North America about 20 million years ago, starting with Trachymyrmex cornetzi. While the fungal cultivars of the 'lower' attine ants can survive outside an ant colony, those of 'higher' attine ants are obligate mutualists.Generalized fungus farming in ants appears to have evolved about 55–60 Mya, but early 25 Mya ants seemed to have domesticated a single fungal lineage with gongylidia to feed colonies. This evolution of using gongylidia appears to have developed in the dry habitats of South America, away from the rainforests where fungus-farming evolved. About 10 million years later, leaf-cutting ants likely arose as active herbivores and began industrial-scaled farming. The fungus the ants grew eventually became reproductively isolated and co-evolved with the ants. These fungi gradually began decomposing more nutritious material like fresh plant-material.
Shortly after attine ants began keeping their fungus gardens in dense aggregations, their farms likely began suffering from a specialized genus of Escovopsis, which are mycopathogens. The ants evolved cuticular cultures of Actinomycetota that suppress Escovopsis and possibly other bacteria. These cuticular cultures are both antibiotics and antifungals. The mature worker ants wear these cultures on their chest plates and sometimes on their surrounding thoraces and legs as a biofilm.
Behavior
Mating
Typically, one queen ant lives per colony. Every year after the colony is about three years old, the queen lays eggs of female and male alates, the reproductive ants that will pass on the genes of the queens. Before leaving the nest, queens stuff some of the fungus' mycelia in her cibarium. These winged males and queens then take their nuptial flights to mate high in the air. In some areas, species flights are synchronized with all local colonies' virgin royalty flying at the same time on the same day, such as Atta sexdens and Atta texana.Some species' queens mate with only one male, as in Seriomyrmex and Trachymyrmex, while some are known to mate with as many as eight or 10, such as Atta sexdens and many Acromyrmex species. After mating, all males die, but their sperm stays alive and usable for a long time in the spermatheca, or sperm bank, of their mate, meaning that many male ant's offspring hatch years after their death.
Colony foundation
After their mating flights, queens cast off their wings and begin their descent into the ground. After creating a narrow entrance and digging straight down, she creates a small chamber. In here, she spits on a small wad of fungus and starts her colony's fungal garden. After about three days, fresh mycelia are growing out of the fungus wad and the queen has lain three to six eggs. In a month, the colony has eggs, larvae, and often pupae surrounding the ever-growing garden.Until the first workers have matured, the ant queen is the sole worker. She grows the garden, fertilizing it with her fecal liquid, but does not eat from it. Instead, she gains energy from eating 90% of the eggs she lays, in addition to catabolizing her wing muscles and fat reserves.
Though the first larvae feed on the eggs of the queen, the first workers begin growing and eating from the garden. Workers feed malformed eggs to the hungry larvae while the garden is still fragile. After about a week of this underground growth, workers open the closed entrance and begin foraging, staying close to the nest. The fungus begins growing at a much faster rate an hour. From this point on, the only work the queen does is egg-laying.
Colonies grow slowly for the first two years of existence, but then accelerate for the next three years. After around five years, growth levels out and the colony begins to produce winged males and queens.
The founding of a nest by these queens is highly difficult, and successful cases are not likely. After three months, newly founded colonies of Atta capiguara and Atta sexdens are 0.09% and 2.53% likely to still exist, respectively. Some species have better odds, such as Atta cephalotes, which are 10% likely to survive a few months.
Caste system
Attines have seven castes performing roughly 20–30 tasks, meaning the potential exists for development of more specialized castes performing individual tasks for AttaWorkers
Description
Lower attines have very minor polymorphism within the minor workers, though higher attines commonly have very different sizes of worker ants. In the higher attines, though, head width varies eight-fold and dry weight 200-fold between different castes of workers. The size differences in workers is nearly nonexistent in newly founded colonies.Due to the variety of tasks needed to be performed by a colony, the widths of workers heads are important and good measures of what jobs workers are likely to perform. Those with the heads about wide tend to work as gardeners, although many with heads wide participate in brood care.
Workers need heads only about 0.8 mm wide to do the work of caring for the very delicate hyphae of the fungus, which they care for by stroking with their antennae and moving with their mouths. These tiny workers are the smallest and most abundant and are called minim. Ants of appear to be the smallest workers that cut vegetation, but they cannot cut very hard or thick leaves. Most foragers have heads around wide.
Attines, particularly the workers that cut leaves and grass, have large mandibles powered by strong muscles. On average, 50% of worker ants' head mass and 25% of their full body mass is the mandibular muscles alone.
Behavior
Though all castes defend their nests in the event of invasion, a true soldier caste, with individuals called majors, exists. They are larger than other workers, and use their large, sharp mandibles, powered by huge adductor muscles, to defend their colonies from large enemies, such as vertebrates. When a foraging area is threatened by conspecific or interspecific ant competitor, the majority of respondents are smaller workers from other castes, since they are more numerous, and therefore better suited for territorial combat.Tasks are divided not only by size, but by the age of individuals workers, as well. Young workers of most subcastes tend to work inside the nest, but many older workers take on tasks outside. Minims, which are too small to cut or carry leaf fragments, are commonly found at foraging sites. They often ride from the foraging site to the nest by climbing onto the fragments carried by other workers. Most likely, they are older workers that defend carriers from parasitic phorid flies that attempt to lay eggs on the backs of the foragers.
All size groups defend their colonies from invaders, but older workers have been found to attack and defend territories most often. At least three of four physical castes of A. sexdens change their behavior based on their age.