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Atta sexdens

By Alex Byrne

Geographic Range

Atta sexdens can be found throughout Central and South America from as far north as Texas down to Southern Argentina. ("Leaf Cutter Ant", 2002)

Habitat

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)

Physical Description

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)

  • Sexual Dimorphism
  • sexes shaped differently
  • Average mass
    .02 g
    0.00 oz
  • Range length
    3 to 24 mm
    0.12 to 0.94 in

Development

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)

Reproduction

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)

  • Breeding interval
    Queens and males breed only during one mating season of their lives. Non-queen females cannot breed.
  • Breeding season
    End of October to Middle of December

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)

Lifespan/Longevity

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)

  • Typical lifespan
    Status: wild
    10 to 15 years

Behavior

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)

  • Range territory size
    60 (high) m^2

Communication and Perception

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)

Food Habits

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)

  • Other Foods
  • fungus

Predation

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)

Ecosystem Roles

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)

Mutualist Species
  • Subphylum Basidiomycotina (species currently unknown)

Economic Importance for Humans: Positive

Humans in some areas of Mexico eat these ants as a source of protein. ("Leaf Cutter Ant", 2002)

  • Positive Impacts
  • food

Economic Importance for Humans: Negative

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)

  • Negative Impacts
  • crop pest

Conservation Status

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)

Contributors

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.

Glossary

Nearctic

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.

World Map

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

acoustic

uses sound to communicate

agricultural

living in landscapes dominated by human agriculture.

bilateral symmetry

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.

chemical

uses smells or other chemicals to communicate

colonial

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.

delayed fertilization

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.

diurnal
  1. active during the day, 2. lasting for one day.
dominance hierarchies

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

eusocial

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

female parental care

parental care is carried out by females

fertilization

union of egg and spermatozoan

food

A substance that provides both nutrients and energy to a living thing.

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

fossorial

Referring to a burrowing life-style or behavior, specialized for digging or burrowing.

herbivore

An animal that eats mainly plants or parts of plants.

heterothermic

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.

internal fertilization

fertilization takes place within the female's body

iteroparous

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).

metamorphosis

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.

motile

having the capacity to move from one place to another.

mycophage

an animal that mainly eats fungus

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

pheromones

chemicals released into air or water that are detected by and responded to by other animals of the same species

polyandrous

Referring to a mating system in which a female mates with several males during one breeding season (compare polygynous).

polymorphic

"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.

rainforest

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.

scent marks

communicates by producing scents from special gland(s) and placing them on a surface whether others can smell or taste them

scrub forest

scrub forests develop in areas that experience dry seasons.

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

soil aeration

digs and breaks up soil so air and water can get in

sperm-storing

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.

tactile

uses touch to communicate

terrestrial

Living on the ground.

territorial

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

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

venomous

an animal which has an organ capable of injecting a poisonous substance into a wound (for example, scorpions, jellyfish, and rattlesnakes).

vibrations

movements of a hard surface that are produced by animals as signals to others

visual

uses sight to communicate

References

IUCN. 2002. "2002 IUCN Red List of Threatened Species" (On-line ). Accessed 03/13/03 at http://redlist.org/.

EnchantedLearning.com. 2003. "Ant" (On-line ). Accessed 03/18/03 at http://www.enchantedlearning.com/subjects/insects/ant/Antcoloringpage.shtml.

CITIES Secretariat. "CITIES Appendices" (On-line ). Accessed 03/13/03 at http://www.cities.org.

United States Fish and Wildlife Service. 2001. "Endangered Species List" (On-line ). Accessed 03/13/2001 at http://endangered.fws.gov/50cfr_animals.pdf.

Discovery Communications Inc. 2002. "Leaf Cutter Ant" (On-line ). Animal Planet. Accessed 03/11/03 at http://animal.discovery.com/fansites/jeffcorwin/carnival/crawler/leafant.html.

Fowler, H., S. Robinson. 1979. Foraging by Atta sexdens (Formicidae: Attini): seasonal pattern, caste, and efficiency. Ecological Entomology, Vol 4, Issue 3: 239-247.

Hill, P. 2001. Vibration and Animal Communicaton: A Review. American Zoologist, Vol 41, Issue 5: 1135-1142. Accessed 03/14/03 at http://proquest.umi.com/pqdweb?Did=000000110143115&Fmt=3&Deli=1&Mtd=1&Idx=4&Sid=4&RQT=309.

Howse, P., C. Lofgren, R. Vander Meer. 1986. Chemical communication in Leaf-Cutting Ants. Pp. 192-199 in Fire Ants and Leaf Cutting Ants. Boulder: Westview Press.

Pi, P., R. Rencsok, A. Schmidt. 2000. "Frequently Asked Questions" (On-line ). Accessed 03/18/03 at http://www.antcolony.org/FAQ2.htm.

Poulsen, M. 2002. "Leaf-cutting ants and their fungus" (On-line ). Accessed 03/19/2003 at http://www.webspawner.com/users/michaelpoulsen2003/index.html.

Wade, N. 1999. For Leaf-Cutter Ants, Farm Life Isn't Simple. The New York Times, Aug 3: F1, F4.

Wilson, E. 1979. Caste and Division of Labor in Leaf Cutter Ants - I. The Overall Pattern in A. Sexdens. Behavioral Ecology and Sociobiology, Vol 7, Issue 2: 143-156.

Wilson, E., B. Holldobler. 1994. Journey to the Ants. Cambridge, Massachusetts: Kelknap Press of Harvard University Press.

Wilson, E., B. Holldobler. 1990. The Ants. Cambride, Massachusetts: Belknap Press of Harvard University Press.

Wirth, R., H. Herz, R. Ryel, W. Beyschlag, B. Holldobler. 2003. Herbivory of Leaf-Cutting Ants. New York: Springer.

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