Atta sexdens - Megan Mullan

~Atta sexdens~

by Megan Mullan

~TAXONOMY~

Kingdom: Animalia

Phylum: Arthropoda

Class: Insecta

Order: Hymenoptera

Family: Formicidae

Subfamily: Myrmicinae

Genus: Atta

Species: A. sexdens

~DISTRIBUTION~

Although the genus Atta is mostly Neotropical, these leaf-cutting ants are also found in the southern part of the United States. They consume more vegetation than any other group of species in the Neotropical ecosystem (Dugatkin, 2009). In Central and South America, they are found in tropical and semi-tropical environments. Though the map below indicates that it is uncertain whether the genus Atta is found in a few different South American countries, it is certain that the species Atta sexdens has not been found in Chile. It is possible that there are other countries it is present in, but the species has not been identified elsewhere. In North America, they are most commonly found in Texas, but have also been found in Arizona and Louisiana. It is assumed that because they have been seen in Texas and Arizona, that they would also be present in New Mexico. Leaf-cutter ant species such as Atta sexdens require an environment in which they can cultivate fungus. This is why they are located in tropical regions with high temperatures and high humidity.

~Description~

Leaf-cutting ants such as Atta Sexdens are dominant, eusocial insects (Fowler and Robinson, 1979). These ants live in nests containing hundreds of thousands of workers (including a single queen) and have evolved an elaborate, caste-based system for obtaining the leaves on which the subsist (Dugatkin, 2009). Just like all other arthropods, Atta Sexdens are composed of a head, thorax and abdomen with a hard exoskeleton composed of chitin. Atta sexdens are polymorphic with a size range from .5 to 4.5 mm (Bass and Cherrett, 1995). They have slight anatomical variances apart from other ants in that they are comparatively large with long legs and extraordinarily strong mandibles which are used for picking up bits of leaves. They vary in body length with male ants being up to half of an inch long and the queen potentially being over one inch long. Typically though, soldier ants are the largest caste among the colony as they are the ones that carry pieces of leaves up to ten times their weight back and forth between the nest. Leaf-cutter ants can be red, brown, black or orange but Atta Sexdens are normally found to be more of a red color.

~Relationship With Fungus~

The species Atta Sexdens maintains a highly specific and unique mutualism with fungus in order to maintain their colony. The symbiotic relationship benefits both the ant and the fungus, as they are dependent on each other for survival. The ants cultivate their own food source similar to the way humans cultivate crops as a food source. During the cultivation process, the fungus benefits from the ant colony to keep it enriched with nutrients while also gaining protection. The ants travel into the forest where they are living and cut leaves into pieces to be brought back to the colony. Once the leaf has been brought back to the colony, it is inoculated with a fungus. The harvest fungus grows and is then a viable food source for the colony of ants. The ants also carry a bacterium with them on their skin to prevent other fungi species from disrupting or invading the colony. This is one of the most unique symbiotic relationships seen in invertebrate species.

Along with producing ordinary hyphae, leaf-cutting ants also produce clusters of swollen hyphae called staphylae (Bass and Cherrett, 1995). In a study done by Bass and Cherrett, it was found that Atta sexdens workers lived longer when their diet included staphylae as opposed to only hyphae. They starved worker ants and then exposed them to fungus for three hours. The workers gained 7.3 times more weight when they were given a fungus garden containing staphylae than when the fungus garden only contained hyphae (Bass and Cherrett, 1995). Hyphae and staphylae of the fungus provide an excellent source of nutrients to Atta sexdens and their larvae. Other than that, the staphylae seem to not serve a direct purpose.

~Biochemistry of Atta sexdens and Fungus~

The relationship between Atta sexdens and the fungus has a fascinating and complex biochemical process involving the breakdown of leaves. After the cut leaf pieces have been taken back to the nest, they are chewed and licked by the garden worker ants (North et al., 1997). After they are chewed and licked by the workers, they are treated with enzymes, and then they are placed into the "fungus garden" where they are subsequently cultivated (Dugatkin, 2009). The leaves serve as a food source for the fungus while the overall material of the fungus and leaves mixed together is what composes the ant nest. The garden ants perform “weeding” behaviors to maintain this hyphae fungus garden from being infected by competitive micro-organisms (North et al., 1997). Certain contaminants can be removed from the garden ants by a licking behavior. Although the ants maintain the nest by keeping invasive organisms away, the fungus itself can also produce anti-microbial substances for protection. The ants themselves produce a more important antimicrobial substance that is spread over the surface of the workers by grooming activity and through touching the nest (North et al., 1997). Some of the compounds secreted by the ants are indoleacetic acid, phenyltacetic acid and myrmicacin.

It has been found that the fungus Leucocoprinus gongylophorus that Atta sexdens is associated with produces enzymes that break down cellulose (North et al., 1997). This fungus breaks down the cellulose into many different forms such as microcrystalline cellulose, carboxyimethyl-cellulose and cellobiose (North et al., 1997). Along with different forms of cellulose, the fungus contains bacteria that can break down casein, gelatin and starch (North et al., 1997).

~Foraging by Atta sexdens~

The life of a leaf-cutting species such as Atta Sexdens depends on the foraging behavior of the ants and their ability to collect leaves to supply the colony with. Although temperatures and humidity levels remain elevated in most parts of South America where Atta Sexdens are located, the ants’ foraging behavior does respond to slight changes in weather. This is due in part to the differentiating availability of vegetation that correlates with seasonal changes.

In a study done by Harold G. Fowler and S. W. Robinson, colonies near San Lorenzo, Paraguay were selectively observed for foraging activity and efficiency of the multi-stage foraging strategy that the Atta sexdens species uses. The ants use a strategy where they climb up the trunks of trees, cut off portions of leaves, and ants underneath the canopy forage for the dropped leaves. They found that seasonal changes occurred in the rate of foraging, the numbers and lengths of foraging trails, and the size of the foraging territory (Fowler and Robinson 1979). Cooler months showed lower numbers of foragers, a smaller foraging territory, and numerous but short foraging trails (Fowler and Robinson 1979). Also, during winter months the loads were smaller than those carried by summer foragers (Fowler and Robinson 1979).

One of the most interesting observations that Fowler and Robinson made was that there was a shift from nocturnal to diurnal foraging during the cooler months of the year. Atta sexdens thrive by being able to forage in optimal temperatures. Foraging during the day during cooler months and foraging at night during hot months allows them to forage for leaves in these optimal ranges.

~Chemical and Vibrational Communication~

With foraging ants such as Atta sexdens having to travel relatively far distances to forage for food supplies, they must have a way to communicate information back to those in their nest about where food supplies are located. They do this by using chemical markers to produce trails leading to the food supplies, and they also produce vibrations on the leaves they are cutting. All members of the genus Atta use these communication tactics, not only Atta sexdens.

There are two chemicals that the ants use for long distance foraging communication: methyl 4-methylpyrrole-2-carboxylate and 3-ethyl-2,5methylpyrazine (Dugatkin, 2009). These pheromones are left on the trails that the foraging ants take so that worker ants back at the nest know exactly where to travel to find food.

The other mode of communication leaf-cutter ants use is stridulation, which is the act of producing sound by rubbing together certain body parts. Atta sexdens do not randomly cut sections of leaves from a tree, but instead they cut and cut from the same leaf until almost all of that leaf is gone (Dugatkin, 2009). Holldobler and Roces used noninvasive laser Doppler vibrometry to test whether workers in the genus Atta stridulate while they cut leaves, and they found that they do (Dugatkin, 2009). The ants also take into consideration the quality of the leaf they are cutting into. Leaf cutting ants prefer to cut into tender leavers as opposed to tough leaves, and they also stridulate more frequently on the leaves that are tender (Dugatkin, 2009).

~Nutrient Cycling~

Nitrogen fixation is the process by which atmospheric nitrogen is assimilated into organic compounds. Besides providing a symbiotic, mutualistic relationship with fungi, Atta sexdens also have a positive impact on nitrification of the soil in the Eastern Amazonia Forest. This in turn has a significant impact on resource availability and ecosystem function (Vercho et al., 2003). They create nests with complex below-ground heterogeneity (Vercho et al., 2003). By creating the nests with fungus and leaves, naturally the ants disturb the soil around them. They can change the density and texture of the soil, which increases nitrogen fixation. Though they do not create impacts on the deep soil, the major effects have been found to be on the surface layers (Vercho et al., 2003). Although they may not be thought of as the most important species in the Amazon, Atta sexdens do have an ecological impact on the nutrient content of the soil they create their nests in.

~References~

Fowler, Harold G., and S. W. Robinson. "Foraging by Atta Sexdens (Formicidae: Attini): Seasonal Patterns, Caste and Efficiency." Ecol Entomol Ecological Entomology 4.3 (1979): 239-47. Web.

North, R. "Evolutionary Aspects of Ant-fungus Interactions in Leaf-cutting Ants." Trends in Ecology & Evolution 12.10 (1997): 386-89. Web.

Bass, M., and J. M. Cherrett. "Fungal Hyphae as a Source of Nutrients for the Leaf-cutting Ant Atta Sexdens." Physiological Entomology Physiol Entomol 20.1 (1995): 1-6. Web.

"Geographic Range of Leaf Cutter Ants (Don, Indianapolis, IN, USA)." - Ant Blog. N.p., n.d. Web. 28 Nov. 2015.

"Leaf Cutter Ants." Bugpage2. N.p., n.d. Web. 27 Nov. 2015.

Picture of leaf cutter ant from: https://s-media-cache-ak0.pinimg.com/736x/62/f4/fe/62f4fe359fdbae00119ede52417fba62.jpg

Verchot, Louis V., Paulo R. Moutinho, and Eric A. Davidson. "Leaf-cutting Ant (Atta Sexdens) and Nutrient Cycling: Deep Soil Inorganic Nitrogen Stocks, Mineralization, and Nitrification in Eastern Amazonia." Soil Biology and Biochemistry 35.9 (2003): 1219-222. Web.

Dugatkin, Lee Alan. "Communication." Principles of Animal Behavior. New York: W.W. Norton, 2009. 428-31. Print.

Picture of leaf cutter ants cutting leaf: http://www.alamy.com/stock-photo-leaf-cutter-ants-atta-cephalotes-cutting-leaf-fragments-in-costa-rica-22836210.html