What are leafcutter ants?

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  1. Introduction

  2. Fungus culturing by Leafcutter ants

  3. The Leafcutter ant caste system

  4. The founding of the colony
  1. The Leafcutter nest

  2. Leafcutter ant waste management systems

  3. Leafcutter ants and human beings
A parade of leafcutter ants heads home.
A parade of leafcutters heads home (c) J. Wong

Leafcutter ants cut leaves from plants and trees and grow fungus on these cut fragments. The ants use this fungus to feed their larvae (the ants themselves mostly imbibe plant sap from the cut leaf fragments). The true leafcutters are restricted to two genera of ants (Atta and Acromyrmex) comprising a total of about 38 species.

Leaf cutter ants are limited to the arid, semi-tropical and tropical regions of South, Central, and North America, but they are one of the ecologically-dominant ants everywhere they are found. They are arguably the most well-known of the ants to the local people and foreign tourists in these regions, mainly because of their spectacular habit of carrying colored petals or green leaves in foraging lines that may stretch more than 250 meters from their nest!

Leaf cutter ants have one of the most sophisticated animal societies in the world. This is because of their unusual method of farming (they are the only animal besides humans who grow their own food from living matter), their extremely large colony sizes (up to 8 million individuals per colony in one species, Atta sexdens), and their fantastic caste system (with ants of different sizes and forms specialized for various tasks in the colony).
Tiny hitchhikers crowd around a leaf fragment.
Tiny hitchhikers crowd around a leaf fragment (c) J. Longino

Leafcutter ants grow fungus cultures for food

Leaf cutter ants grow their fungus cultures on a substrate made of ground up plant matter, which they obtain by harvesting prodigious amounts of leaves, petals, and various other plant parts from the vegetation surrounding the nest. When an ant scout finds a suitable bush or tree, it lays a scent trail back to the nest and summons the foragers. These medium sized ants (head widths of around 2 mm) cut out pieces of leaves and head clumsily back to the nest. All around them smaller ants weave to and fro, constantly scanning the surrounding terrain for danger. Some of these smaller ants may also hitchhike on the leaf fragments carried by their bigger sisters, although the function of this behavior is still unclear (Linksvayer et al 2002). One hypothesis is that the small ants are protecting their burdened comrades from tiny phorid flies that might lay eggs on the heads of the ants (the eggs hatch later and the growing larvae basically eat the heads of the unfortunate ants).

Once in awhile an observer can spot a large soldier as she gingerly steps over her rushing nestmates. These gigantic ants in some Atta species can have headwidths greater than 7 mm and total body lengths of more than 1.7 cm! Soldiers were generally thought to be highly specialized for defence, but new studies and observations have shown that soldiers in some species serve a variety of other functions (San Juan and Li, 2004a).
Fungus culture.
A fungus culture inside an Atta cephalotes nest (c) ASJ

When the foragers get to the nest, they hand the leaves to smaller ants, who rush it to one of the many culture gardens. The leaves are then processed into smaller and smaller fragments by smaller and smaller ants, until the thoroughly masticated result is placed into the growing culture. These fluffy-looking fungus cultures are tended by the tiniest ants, who roam inside the numerous galleries that ramify throughout the culture and harvest special nutritional bodies produced by the fungi called "gongylidia". These tiny ants then distribute their bounty to the rest of the colony (although most of the harvested nutritional bodies are fed to the ant larvae).

In order to protect their fungus cultures and combat invading fungi pests, these ants employ antibiotics produced by a Streptomyces bacteria that lives on their skin, in addition to physically removing the invading fungi. This interaction between ant, bacteria, and fingus crop is one of the most intricate examples of mutualism in nature!

The leafcutter caste system

The caste system in the Atta ants is one reason why these ants are so successful. Individual ants are specialized for various colony tasks based on their size and form. The queen is the reproductive female of the colony and is responsible for laying more eggs and increasing the colony population, while the workers (all non-reproductive females) can be divided into various subcastes, each with her own function. Younger workers, even the largest soldiers, remain inside the nest and do in-house tasks until they mature, but a worker's specialized function reveals itself sooner or later.
Leaf cutter soldier.
A spectacular Atta laevigata soldier (c) A. Wild

The smallest workers tend to the growing brood, while other small workers (head width less than 1 mm) care for the fungus gardens. They are optimized for these tasks because of their smaller sizes - it wouldn't do to have a gigantic soldier bumbling around inside the tiny tunnels of the fungus cultures!

Slightly larger minima workers (or minims) are present in large numbers in and around foraging columns. These ants are the first line of defense because they continuously patrol the surrounding terrain and vigorously attack any enemies that threaten the foraging lines.

Workers with headwidths of around 1.8-2.2 mm are the generalized foragers, who cut leaves and bring the leaf fragments back to the nest. In fact, researchers have found that this exact head size is optimum for cutting the average leaf!

The largest workers (called soldiers or majors) are mostly used to fight off large predators, although they are also an intimidating force against marauding Nomamyrmex army ants. In addition, as noted above, soldiers may perform other minor activities, such as removing obstructions from the ant foraging trails and handling very large items.

Finally, winged reproductives are produced by mature colonies. The young queens and male drones are fated to fly off and mate with young queens and drones from other colonies to produce new leafcutter ant colonies. The males die soon after mating with several females, but the mortality rate among the potential queens is frighteningly high as well. Only a very small percentage of incipient leafcutter colonies reach their 3 year anniversary. Most young colonies probably die off when their fungus fails for one reason or another, while a few are overwhelmed by vertebrate and other predators.

The founding of the colony

Before flying off to start their new colonies, the virgin queens take some fungi culture with them. They will use these to start the new fungus garden. Like most other ant species, leafcutter ants engage in a "nuptial flight", whereby virgin queens and males from many colonies take flight at around the same time in order to mate. The flight capabilities of leafcutter queens are not well known, but Atta texana queens have been recorded to travel up to 10.5 km. After insemination by several males during the nuptial flight, the queens land and search for a likely spot to start their nest. Most leafcutter colonies are founded via haplometrosis (a single founding queen), however, pleometrosis has been seen in A. texana and Acromyrmex versicolor. For awhile, the new queen has to depend on trophic eggs and muscle catalysis in order to survive, but once she has reared the first brood, the small workers can start foraging and enlarge the fungus garden.

Atta mother queens during colony founding demonstrate the most complex behavior among dealate female ants, mostly because the female has to not only rear her first brood of workers, but she also has to rear the fungus on which the future of the colony depends. The weight of dealate Atta queens varies from species to species, but all Atta queens are huge (Acromyrmex queens are much smaller), exceeded in size only by the grotesque queens of doryline driver ants. The table below from Mintzer (1990) shows some weights for dealate females:

Species Initial weights (mg)
Atta texana 300-400 mg
Atta cephalotes 500-600 mg
Atta sexdens rubropilosa 400-500 mg
Atta capiguara up to 800 mg or more

All females lose weight during colony foundation before the first workers appear, typically 30-35% for Atta mexicana and Atta texana and 17-39% for the larger Atta cephalotes. This is because the female has to use up her energy reserves to rear the new workers and fungus without outside foraging.

The future mother queen is tireless in taking care of her larval brood and fungus. She cultures the growing mat of fungus (which develops as a disk or mat of mycelium) on her own fecal fluid, and feeds the first larval brood her own eggs. It takes 35-38 days for the first workers to emerge from their pupae in Atta texana and Atta cephalotes, and these new ants are relatively small (2-4 mm total body lengths for Atta texana and Atta mexicana, although some Atta cephalotes first brood workers were larger - up to 5.5 mm) (Mintzer, 1990). Once the first workers develop and start to forage on the outside, they take over all the duties of the queen, which thereafter retires into a life of egg-laying.

Cofoundresses of Acromyrmex versicolor display an interesting and unusual behavior, where one of the unrelated co-founding queens becomes a foraging specialist, and takes all the risks of foraging above-ground for the group (Rissing et al 1989)! The queen assigned to do this hazardous job is seemingly not picked based on size or reproductive capabilities (as measured by ovary size). In cases where the assigned queen refuses to do her job, the entire group usually is doomed since the group delays or does not replace the specialist forager, and the fungus garden dies (Pollock et al 2004).

The path to maturity for a young nest is fraught with peril at every turn. Predation on queens both during the nuptial flight and after landing is very high. Birds, armadillos, rodents, and especially other ants prey heavily on the plump and juicy female reproductives. Even after workers emerge and the colony starts growing quickly, survivorship of these incipient colonies is very low. One study found that only 2.5% of young A. sexdens colonies were still alive after 3 months. Excluding major predators like armadillos, rodents, and other ants still resulted in very low survivorship, with at most 1.9-6.4% of the colonies surviving after 3 months. Thus, most deaths of young colonies are probably due to the failure of the fungus garden, the spread of pathogens, or even because the queen was not fertilized.

The Leafcutter nests
Leaf cutter nest.
A huge nest mound (c) Univ of Georgia

The leafcutter nests, especially of the Atta spp, are marvels of engineering. Young colonies typically have only a few nest mounds, with foraging holes opening several meters away from the mounds, but the numerous (up to hundreds of nest openings) nest mounds of large mature colonies sometimes fuse together to form one huge mound. In this case, the central nest mound may be 30 m in diameter, have numerous 0.3 m diameter feeder mounds extending outwards to a radius of 80 m, and may occupy 30 to 600 square meters! More amazing, the underground chambers may extend downwards to more than 6 meters in depth!

The immense size of Atta nests, and the very large populations of the colony, can cause problems in terms of creating enough ventilation throughout the nest to prevent the high build-up of CO2 and loss of oxygen. There are two mechnisms by which leafcutter ants may prevent from suffocating.

In the first method, called thermal convection, the hundreds of nest openings in mature Atta nests create a ventilation system that continuously circulates air throughout the nest. Because the central chambers tend to have the brood chambers and gardens, these areas become somewhat warmer than the peripheral chambers. The warm air moves up and out through the central hole openings, drawing colder air from the outer chambers into the central area. This in turn draws fresh air from the outside into the nest holes on the periphery.

The second method is called wind-induced ventilation, and this was elucidated by Kleineidam, Ernest, and Roces (2001). In this case, air is drawn out of the central nest mound openings by wind blowing over the nest openings. This outflow in turn causes air to rush into the nest via the outlying nest holes and tunnels, thus ensuring the rapid circulation of air throughout the nest chambers.

Leafcutter ant waste management systems

Because of the peculiarities of their fungus-growing lifestyle, leafcutter ants may have one of the most sophisticated waste management systems in nature (Hart and Ratnieks 2002). Like human cities, leafcutter nests produce copious amounts of waste product (including spent fungal substrate, dead ants, and the like), which could cause diseases not only in the ants, but in their fungus cultures.

The first line of defence is the deliberate segregation of waste products away from the main nest chambers. These "dump" or waste "heap" areas may be internal in some species (for example, A. cephalotes has large dump chambers situated on the nest periphery), or it may be located in external piles away from the nest mounds (in A. mexicana and A. colombica, the waste dumps may reach a meter of more in height in large colonies!).

The second line of defence is the creation of "waste workers" who specialize in working constantly with the waste. These ants can be separated into two groups: (1) transporters who carry waste products from the sources and unload them close to, or at the periphery of, the waste dump; and (2) heap workers who live exclusively in the waste heap and work to aerate and turn it over constantly. The second type in particular is strongly ostracized from the nest proper by other colony members, who may kill them if they persist in trying to venture too far away from the heap.

The third line of defense is behavioral, with the overall aim always being to minimize exposure of the main nest and workers to any waste products. For example, researchers found that in A. colombica (which has external dumps), the location of the waste heaps in large colonies is influenced by the environment, such that heaps are placed lower than the nest mounds if there is a slope. The result of this is that no waste matter will be inadvertantly introduced into the mounds during rainstorms. In addition, foraging lines always bypass the heap area.

Through these methods, leafcutter ants ensure the continued health of the colony and its fungus culture.

Leafcutter ants and human beings
Leaf cutter nest in Puerto Vallarta.
Large nest mounds sprouting from Nuevo Vallarta, Mexico sidewalks (c) ASJ

Leafcutter ants have benefitted greatly from the spread of humanity. Nest densities of some of the leafcutters reach very high levels in man-made areas (San Juan and Li, 2004b), including urbanized locations such as small towns and cities. Because of the ubiquity and ecological dominance of leafcutters, local people in the areas where they are present recognize and know these types of ants, and are able to distinguish them easily from the many other ants.

The dominance of leafcutters in urban areas may be due to several factors. Firstly, many leafcutter species seem to be adapted for open areas, so the destruction of primary and secondary forests by man has opened up new land for the exploitation of these ants. Secondly, whereas leafcutters do well even in highly urbanized locations (e.g. there were some large leafcutter nests in the middle of downtown San Jose, Costa Rica), the few predators that can breach their nest defences (some vertebrates(?), Nomamyrmex army ants) are not as adaptable and are almost wholly absent in these areas. Thirdly, ants which deterred predation on certain plants (e.g. Azteca ants nesting in Cecropia trees) may not exist in urbanized and other man-created habitats, thus allowing leafcutters to exploit these new food sources. Finally, the presence of human beings has altered the biological landscape to a degree that is favorable for leafcutters. For example, many of the exotic and crop plants favored by humans are entirely deficient in the protective defenses of native trees and plants in the local region.

The economic impact of leafcutter ants on people is quite pronounced. Each leafcutter colony can consume as much leaf matter in a day as a full grown cow, and leafcutter ants are by far the dominant "herbivores" in the neotropics, with the amount of vegetable matter being cut estimated at an astonishing 12-17% of annual leaf production! Damage (indirect and direct) has been estimated to be in the billions (USA dollars) annually, and the grass cutting species reduces the carrying capacity of pasturelands in the area by as much as 10%!

To be continued.

Literature Cited

Hart, A. G. and Ratnieks, F. L. W. 2002. Waste Management in the leafcutting ant Atta colombica. Behavioral Ecology 13: 224-231.

Kleineidam C, Ernst R, Roces F Wind-induced ventilation in the giant nests of the leaf-cutting ant Atta vollenweideri Naturwissenschaften, 88: 301-305 (2001)

Linksvayer TA, McCall AC, Jensen RM, Marshall CM, Miner JW, and Mark J. McKone (2002). The Function of Hitchhiking Behavior in the Leaf-cutting Ant Atta cephalotes. BIOTROPICA: Vol. 34, No. 1, pp. 93–100.

Mintzer, A. 1990. Foundress female weight and cooperative foundation in Atta leaf-cutting ants. Pp. 180–183 in R. K. Vander Meer, K. Jaffe, and A. Cedeno, eds. Applied myrmecology: a world perspective. Westview Press, San Francisco, CA.

Pollock GB, Cabrales A, and SW Rissing (2004), On Suicidal punishment among "Acromyrmex Versicolor" Cofoundresses: the Disadvantage in Personal Advantage. Evolutionary Ecology Research, in press.

Rissing SW, Pollock GB, Higgins MR, Hagen RH, and DR Smith (1989). Foraging specialization without relatedness or dominance among co-founding ant queens. Nature 338, 420 - 422

San Juan A and LI Hongmei (2004a) Behavioral plasticity in soldiers of Atta mexicana and its adaptive significance in urban environments. Submitted to Notes from Underground.

San Juan A and LI Hongmei (2004b) Atta mexicana in the resort community of Nuevo Vallarta, Mexico. Submitted to Notes from Underground.

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