All for one and one for all: The intricate social structure of leaf-cutting ants

by: Raluca Ionescu and Laura Zeppetelli

Screen Shot 2016-04-10 at 8.56.15 PM.pngFigure 1

They are small, they seem fragile, and nevertheless, they are amongst the most remarkable and strong species on our planet! At any time of the day while walking through the Costa Rican forests, we could observe ants cutting leaves and transporting the pieces back to their nest, where they exercise their cunning gardening skills.

This article studies the ants that belong to genera of Atta from the attain tribe, regrouping about 210 species within. These particular ants, known as leaf cutters or fungi growing ants, are the perfect example of a mutualistic relationship between the ants and their fungi, since this species grow their own fungus and depend on it. To solve their problems, these ants are the champions of the collective and social work! These insects’ survival depends on the community and on the well-defined role occupied by every individual. They are among the species that reach the highest degree of social community in the animal Kingdom. It is indeed one for all, and all for one!

Castes

In order to manage these huge colonies that can lead up to more than 5 million individuals, Atta ants have learned to divide the work load into several social castes. They are a textbook example of polymorphic species: within them are many different body shapes and sizes specifically useful to a certain task for their group. Scientifics do not yet agree on an exact number of Atta cast, but they range around 5 to 12 different social groups within their species. (1) Usually, larger ants form the soldier group, whilst medium sized ones serve for foraging and other technicalities as the smaller group takes care of the nest and its fungi.

Screen Shot 2016-04-10 at 12.24.02 PM.pngFigure 2:  From the smallest to the biggest castes: the minims, mediae, majors and drones

Minims

The minims, often referred to as “garden ants” do most of their work underground. As their name suggests it, they are the smallest of all the castes and have a head width of less than 1 mm. They primarily take the role of fungi gardeners, groom other ants and act as nurses for the queen and larvae. In fact, the minims are responsible to a big degree of caste determination due the fact that they manipulate the pheromone concentrations and temperature that contribute to the “caste programming” of the future ants. (2) Minims are also charged with the task of cleaning cut sections of a leaf while they are being carried back to the nest by the mediae workers. Doing so will protect the mediae from a species of parasitic phorid fly which lays its eggs inside them. (3)

Minors

The minors are a bit larger than the minims and their head width varies from 1.8 to 2.2 mm. They are the first line of defense and constantly move around the territory surrounding the nest to attack any enemy which seems to be getting too close to the foraging lines. (1)

Mediae

The mediae, often referred to as “workers” are primarily foragers, but they also function as excavators. They are bigger than the minims and minors, but smaller than the majors. Their key tools are their mandibles which allow them not only to cut sections of leaves but also to bite any intruders. While they feed on the fungus like other castes, a part of their diet also consists of plant sap which they ingest while physically cutting out sections of various plants. (4)

Majors

The majors, very often called “soldiers”, are only produced when the colony includes at least around 10 000 workers (5) and their most distinguishable characteristic is their size. Their total body length can go up to 16 mm with head widths of 7 mm. The soldiers primarily defend the colony from other insects and vertebrate predators using their massive jaws. Also with the help of those mandibles, they move the objects that are too heavy for the smaller castes.

Males

The males of these colonies do comparatively less work than the females, yet they are crucial to the survival of the species. They are considered drone ants since only them and the queens have wings. Indeed, their sole purpose is the transfer of genetic information from one nest to another during the reproduction period. That period is the only time when they are produced since they do no other work in the colony. Surprisingly, they are mostly haploid individuals: they all carry the exact genetic information as they have a unique set of chromosomes. They die shortly after reproducing during their nuptial flight. (6)

Queen

A new queen must always have wings because without them she would not be able to move around so easily and accomplish all the tasks required of her to establish a new colony. After leaving the original nest during nuptial flight, she mates with about 3 to 8 male drones and carries a small piece of the old fungus. (6) She will then take this piece underground and spit it up, remove her wings and begin her gardening. She will tend on the crucial fungi by feeding it with her fecal matter until the first wave of worker ants is born and can carry on the leaf foraging task. As the fungus gets bigger, the colony does the same: it produces millions of ants that get bigger and more various in body types in order to increase and diversify the work done. Even though the queen does not do much of that work after starting her colony, she lives long enough and is crucial since she remains the only fertile ant and the pillar of her social network.

Screen Shot 2016-04-10 at 8.53.29 PM.pngFigure 3: The nursing minim ants on top of the much larger queen ant

References

  1. Wilson, Edward O. “Caste and Division of Labor in Leaf-Cutter Ants (Hymenoptera: Formicidae: Atta).” Behavioral Ecology and Sociobiology. 7, 1980: 157-165.
  2. Suen G, Teiling C, Li L, Holt C, Abouheif E, Bornberg-Bauer E, et al. (2011) The Genome Sequence of the Leaf-Cutter Ant Atta cephalotesReveals Insights into Its Obligate Symbiotic Lifestyle. PLoS Genet 7(2): e1002007. doi:10.1371/journal.pgen.1002007
  3. Vieira-Neto, E. H. M.; F. M. Mundim; H. L. Vasconcelos (2006). “Hitchhiking behaviour in leafcutter ants: An experimental evaluation of three hypotheses”. Insectes Sociaux 53: 326–332. doi:1007/s00040-006-0876-7.
  4. Littledyke, M.; J. M. Cherrett (1976). “Direct ingestion of plant sap from cut leaves by leafcutting ants Atta cephalotes (L.) and Acromyrmex octospinosus“. Bulletin of Entomological Research 66: 205–217. doi:1017/s0007485300006647.
  5. Hölldobler, Bert, and Edward O. Wilson. Journey to the Ants: A Story of Scientific Exploration. Cambridge, MA: Belknap of Harvard UP, 1994. Print.
  6. Wirth, H. Herz, R.J. Ryel, W. Beyschlag and B. Holldobler.  In Herbivory of Leaf-Cutting Ants. New York: Springer-Verlag Berlin Heidelberg. 2003.
  7. Fendt, Lindsay. “The Secret Lives of Leaf-cutting Ants.” The Tico Times. N.p., 03 Apr. 2015. Web. 07 Apr. 2016.

Figure 1: Courtesy of Wikipedia Commons

Figure 2: http://dx.doi.org/10.1371/journal.pgen.1002007.g001

Figure 3: http://ecolibrary.org/page/DP177

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