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. ("Leaf Cutter Ant", 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.
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. ("Ant", 2003; Pi, et al., 2000)
Although no one has yet observed the complete Atta sexdens mating process, it is theorized that A. sexdens queens are polyandrous. When the queen goes on her mating flight, called the revoada, she needs to obtain enough sperm to last her entire lifespan. Mated queens may contain more that 300 million sperm when all the mating is done. No one male is likely able to produce enough sperm, so it is likely that the female mates with three to eight individuals during her revoada.
Only queens and males are sexually active and able to mate. All other castes are sterile. ("Leaf Cutter Ant", 2002)
Queens of Atta sexdens are the only females that mate. Mating usually occurs from late October through the middle of december. After gaining enough sperm to found a new colony, the queen will fly away from her old colony, up to approximately 11 km, shed her wings, burrow into the ground, and begin egg-laying. ("Leaf Cutter Ant", 2002; Wirth, et al., 2003; Wilson and Holldobler, 1994)
The queen, when founding a new colony, cultivates a fungus garden from a scrap of the garden in her previous colony. Rather than eat the fungus, she lives on her own fat reserves, eggs, and her wing muscles (which she won't need anymore). The first batch of eggs, cared for by the queen, hatch worker ants of the gardener caste. After there are enough gardener-nurses to take care of both brood and fungus, the queen becomes strictly an egg-laying machine for the rest of her life. Queens may produce up to 150 million daughters during their lifetime. The next batches of eggs are the specialists and forager-excavators, to help expand the colony and find more material for the fungus to grow on. From this point, all four castes of ants may appear in the queen's eggs. (Wirth, et al., 2003)
Atta sexdens queens determine what amount of what worker castes to produce depending on the size of the colony and the age of the queen. Wilson and Holdobler (1994) experimented with a mature colony, reducing its size in proportion to give it the appearance of a young colony. Instead of going by age and producing large workers and soldiers, the queen reverted to producing small and mid-size workers as if she were in command of a younger colony. (Wilson and Holldobler, 1994; Wilson and Holldobler, 1990)
The eggs, larvae, and pupae of Atta sexdens are generally cared for by gardener-nurse caste workers, with some help from the within-nest specialists if the larvae or pupae are extremely large. The one exception to this rule is the first few clutches of eggs laid by a founding queen, as she has no workers to take care of the larvae and must take care of them herself.
These nurses feed the larvae with trophic eggs and fungus, sometimes by regurgitation. They also protect, manipulate and groom the eggs, larvae, and pupae, assisting them through their metamorphosis by licking off the last vestiges of a previous stage. (Wirth, et al., 2003; Wilson and Holldobler, 1994)
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, et al., 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. ("Leaf Cutter Ant", 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 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)
Humans in some areas of Mexico eat these ants as a source of protein. ("Leaf Cutter Ant", 2002)
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)
Atta sexdens is not listed in the IUCN Red List, CITIES appendices, or the United States Endangered Species Act. ("2002 IUCN Red List of Threatened Species", 2002; "CITIES Appendices", ; "Endangered Species List", 2001)
Ashley Dowling (editor), University of Michigan-Ann Arbor.
Matthew Wund (editor), University of Michigan-Ann Arbor.
Alex Byrne (author), University of Michigan-Ann Arbor, Phil Myers (editor), Museum of Zoology, University of Michigan-Ann Arbor.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
living in the southern part of the New World. In other words, Central and South America.
uses sound to communicate
living in landscapes dominated by human agriculture.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
uses smells or other chemicals to communicate
used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.
a substantial delay (longer than the minimum time required for sperm to travel to the egg) takes place between copulation and fertilization, used to describe female sperm storage.
ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
the condition in which individuals in a group display each of the following three traits: cooperative care of young; some individuals in the group give up reproduction and specialize in care of young; overlap of at least two generations of life stages capable of contributing to colony labor
parental care is carried out by females
union of egg and spermatozoan
A substance that provides both nutrients and energy to a living thing.
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
Referring to a burrowing life-style or behavior, specialized for digging or burrowing.
An animal that eats mainly plants or parts of plants.
having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.
fertilization takes place within the female's body
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.
having the capacity to move from one place to another.
an animal that mainly eats fungus
the area in which the animal is naturally found, the region in which it is endemic.
active during the night
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
chemicals released into air or water that are detected by and responded to by other animals of the same species
Referring to a mating system in which a female mates with several males during one breeding season (compare polygynous).
"many forms." A species is polymorphic if its individuals can be divided into two or more easily recognized groups, based on structure, color, or other similar characteristics. The term only applies when the distinct groups can be found in the same area; graded or clinal variation throughout the range of a species (e.g. a north-to-south decrease in size) is not polymorphism. Polymorphic characteristics may be inherited because the differences have a genetic basis, or they may be the result of environmental influences. We do not consider sexual differences (i.e. sexual dimorphism), seasonal changes (e.g. change in fur color), or age-related changes to be polymorphic. Polymorphism in a local population can be an adaptation to prevent density-dependent predation, where predators preferentially prey on the most common morph.
rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.
communicates by producing scents from special gland(s) and placing them on a surface whether others can smell or taste them
scrub forests develop in areas that experience dry seasons.
breeding is confined to a particular season
remains in the same area
reproduction that includes combining the genetic contribution of two individuals, a male and a female
digs and breaks up soil so air and water can get in
mature spermatozoa are stored by females following copulation. Male sperm storage also occurs, as sperm are retained in the male epididymes (in mammals) for a period that can, in some cases, extend over several weeks or more, but here we use the term to refer only to sperm storage by females.
uses touch to communicate
Living on the ground.
defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
an animal which has an organ capable of injecting a poisonous substance into a wound (for example, scorpions, jellyfish, and rattlesnakes).
movements of a hard surface that are produced by animals as signals to others
uses sight to communicate
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