Leafcutter Ants

Leafcutter ants move a tremendous amount of plant material in the forest.  They remove living tissue from the leaves and carry it underground to feed to a special fungus that they grow.  Later, they feed on the fungus.  This massive movement of materials from the canopy to underground has profound ecological implications. 

Any visitor to the neoptropics. even a casual visitor, will no doubt encounter leafcutter ants - we even saw them on the streets of the capitol, San Jose.  The colonies are massive, often filling small clearings.  Hölldobler and Wilson (1990, 597) report on a metastudy which concluded that between 12 and 17% of leaf production in the forests adjacent to these nests is ultimately consumed by the ants.  If the nest is located next to an agricultural field, the ants can be major pests.  

Here, a group of our students observe a foraging column of leafcutter ants. We are assuming that all of the leafcutters we observed in Costa Rica are the common species Atta cephalotes; however two other species of Atta (A. columbica and A. sexdens) and 4 species of another genus (Acromyrmex) are found in Costa Rica.  The close-up images shown here of specimens obtained at the La Suerte field station appear to be A. cephalotes, but the images of foraging colonies may be of any of the species.
Here is an ant's eye view of the column as it crosses a path behind the beach at Cahuita National Park on the Caribbean coast.  We were amazed to find the ants at several beachfront locations where it would appear that groundwater would limit the development of their fabled underground nests.

Castes Among Leafcutter Ants

Like many other Hymenoptera (the insect group including bees, ants and wasps), the leafcutters are social animals.  This sociality is aided by a polymorphism in the species; this polymorphism (the presence of several different body types) in hymenopterans is typically referred to as a caste system, with each morph (body type) representing one of the castes.  The castes divide the labor of the hive between them, and this facilitates the development of a social system.  Among Hymenoptera, the typical castes are queens, workers and drones.  The drones are males; they are only produced when it is time to reproduce the colony, and they do no other work in the colony.  In most Hymenoptera the drones are haploid, that is they carry only one set of chromosomes.  In most animals, males are diploid; this means they have two sets of chromosomes and a special type of cell division, meiosis, is needed to produce sperm with only one set of chromosomes. Ant drones, being haploid, produce sperm without meiosis, and all of the sperm from an individual drone carries the exact same genetic information.  Since the drone's purpose is reproduction and the transfer of genetic information between nests, they have wings and can fly.

Go Girls!

The other ant castes are female - the vast majority of ants, wasps and bees are female, and if you see any of these doping anything useful like cutting leaves, pollinating flowers, building a hive, etc. then you know you are looking at a female.  The remaining ant castes are the queen and the workers.  The queen, after establishing the colony, does not do much work other than lay eggs.  She (and any other queens in the colony) are the only fertile ants.  A new queen has wings; when she leaves the nest she mates with several drones (who soon die) and then comes back to the ground, where she removes her wings.  Their job in dispersal is done.  The queen - like all of the workers, the third caste, remains wingless for the rest of her several-year long lifespan.

The workers comprise the majority of the individuals (millions of ants in a large nest).  The leafcutters are interesting in that the worker caste is further subdivided into subcastes - majors, medias and minimas - each with specific tasks to perform in the nest.  The 3 subcastes can easily be distinguished by size, as the image above shows.

 
The largest subcaste, the maximas (also called soldiers) are large indeed.  Their heads are much more massive than the entire body of the minima.  The maximas weigh more than 30 times what the minimas do, and are about 7 times more massive than the medias (also called workers).

The maximas are only produced when the colony reaches a certain size - around 10,000 workers (Hölldobler and Wilson 1994, 121). Their role is not to cut and carry leaves, but, as the name soldier implies, to defend the colony from other insects and even from vertebrate predators.  Their massive jaws and a sting (the ovipositor of these sterile females) provide the means of defense, although all of the workers can bite and sting as well.  Soldiers also use their massive bodies to move objects that the smaller medias and minimas cannot. 
The medias are often called workers simply because they are the ones that seem to be doing all the work (technically, maximas and minimas are also workers).  To be fair, much of the minimas' work takes place out of sight, underground.  However, it is the medias which are carrying all those leaves back to the nest in the huge columns crossing the forest floor.  They are considerably larger than the minimas, albeit much smaller than the maximas.

The chief tools of the media are their mandibles (below).  These strong mouthparts, driven by huge jaw muscles (the majority of the inside of  the head capsule of an ant (or a caterpillar) is filled with these muscles), are able to cut into the leaves and slice them up into manageable portions.  In contrast, the even larger jaws of  the maxima (powered by even larger muscles)  are able to bite through leather, and the small mandibles of the minimas are used for delicate tasks such as gardening fungi and grooming other ants.
This ventral (bottom) view of a media shows the pointed mandibles, the large head capsule, and the slender body of the ant.  Leafcutter ants, to my eye - seem to have extra-long legs, perhaps the better to facilitate carrying leaves over uneven terrain.
Back in dorsal (top) view, the media is still much larger than the minima.  My measurements of the head capsule width show that the media have head capsules ranging from 1.5 to 3 mm in width.  Hölldobler and Wilson (1994, 120) state that a worker's head capsule must be at least 1.6 mm in size if it is to forage successfully.

 

Gathering the Leaves

Up in the canopy, the job of cutting off leaf fragments falls to the media workers, those with head capsule widths greater than 1.6mm.  The workers anchor their legs around the area to be excised, and proceed to cut through the leaf.  In my observations, the work seems to be done with a sawing movement of one of the mandibles; any kind of shearing action is reserved for cutting through the thicker veins of the leaf, as the ant tot he right is doing.

In most cases, the ants endeavor to hold onto the leaf fragment; after it is cut free they then carry it down the trunk.  On the other hand, Hölldobler and Wilson (1994, 116) report that some workers will drop the cut fragments to the ground for other workers to pick up.  This would certainly be more efficient.

 

Video of ants cutting off leaf fragments.  Filmed outside the Cheese Factory in Monteverde, audio from La Selva as the original audio includes traffic and other "monkey" (Homo sapiens) noises.

 

  

 Leafcutters on the trail bringing leaves back to the nest.
One way or another, the leaf fragments find themselves on the forest floor being carried by the media workers.    Our research shows the average size leaf fragment to be about 0.03 grams in weight, while the average media worker is 0.01 grams.  Thus, each worker ends up carrying a weight about 3 times its own body weight, a far cry from the 50x weight differential heralded by popular television shows about the ants (of course there is a difference between what the ants CAN and DO carry).

 

It's not only leaves that are cut and carried; flower petals and bits of stems may be brought to the nest as well (below)
Larger fragments may be carried by teams of workers; and there may be some confusion when they reach the nest with a piece too big to fit.
It's not unusual to see one or more minima workers riding on the fragments as they move to the nest.  According  to a pamphlet produced by Amy Mertl of the Butterfly Garden in Monteverde, the minimas are there to rid the leaf of any phorid fly eggs. Other sources, however, such as this one, suggest that the flies lay their eggs directly on the workers.  The minima would be too small to support a parasitoid, so they are not vulnerable themselves and are free to chase away the parasitic flies.

 In addition, the minimas are getting a head start on their job of raising fungus on the leaves; at this stage they are cleaning off any potential competing fungal spores so they don't get into the nest.  

This image shows the foraging workers of all 3 subcastes.
The ants try to keep their main trails clean.  This facilitates the movement of workers with the leaves.  In extreme cases, they can wear a groove into the soil; the "superhighway" above was found near a large nest and was several inches wide and deep - and situated in such a way to suggest it was created by the ants themselves, not by another agent such as water.  Below, a "logjam" created by an obstacle in the trail near the beach at Cahuita National Park.

 

 

Video of Leafcutter Ants at Cahuita National Park.

 

 

Back at the Nest

Of course, the purpose behind all the leafcutting is to transport the leaves back to the nest where they can become food for the fungi.  These two images show  what the area around the nests look like - bare of most vegetation, as one might expect.  From these low mounds trails radiate out in several directions; the trails may extend for hundreds of meters.  A large nest can have thousands of chambers, some of these may be a foot or so in diameter.  Some chambers are used for brood, others for the fungal gardens (more on those in a minute) , and, in some species, there are chambers used for trash.  Hölldobler and Wilson (1994, 115) discuss a number of studies which illustrate the size of the colonies; the largest was reported to have involved the movement of about 44 tons of earth.  Scientists studying the nests have used bulldozers to uncover them (Hölldobler and Wilson (1994, 115); this too should give you an indication of how big they are. 

This tremendous churning of the soil - bringing soil to the surface and taking vast quantities of organic material underground, no doubt has huge implications for the fertility of the soil.  The colony also has hundreds of entrances, no doubt to help control the climate within.
The colony above (from La Suerte) is almost certainly Atta cephalotes as ants gathered there key out to that species.  The colony to the left, at La Selva, on the edge of a clearing, may be A. cephalotes, but could also be another species.

 

Why is there any vegetation near the nest?  Scientists are divided on this point.  In some cases, the ants may bypass nearby plants because they don't meet the nutritional needs of the fungus.  More likely, certain plants may produce substances toxic to the ants or the fungi (obviously the ants not only need to know what will kill them, but what will kill the fungi as well).  Plants with higher concentrations of such chemicals will be skipped.  Since some plants raise the amount of these defensive chemicals once they are attacked, it may be that the ants will partially defoliate a plant, stop feeding on it as it responds with increased defensive chemicals, and return to it later after it has "forgotten" the attack and the concentration of defensive chemicals is lowered.  Do the ants avoid certain plants so as not to kill all the food within easy reach and thus allow certain plants to recover?  Unknown.
In the nest, the relative importance of the size of the different worker castes undergoes a dramatic shift.  On the outside, it is crucial that a worker be big enough to cut and carry the leaves.  Inside the nest, smaller size is the important thing.  Only small ants can crawl through the tiny openings in the fungus garden, tend to the fungus, chew leaf particles to the small size needed for optimal fungal growth (this is done by an assembly-line chain of progressively smaller workers producing progressively smaller leaf bits),  pull out strands of fungus to inoculate the new leaf material, and weed out invading fungi of other species.  These jobs largely fall to the minima, the smallest of the workers, whose head capsules may be less than 1mm wide.  The large workers to the right are medias; the smaller workers minimas.

The white material in the picture probably is the fungus Leucocoprinus gongylophorus )(Hölldobler and Wilson, 1990, 599), however, little is known about the fungus (and some recent papers even refrain fron naming the fungus because of the confusion).  It seems that under the ants' care the fungus has no need for the fruiting structures (such as mushrooms) that most fungi use (and which most mycologists use to classify the fungi).  Grown by the ants, the fungus takes on a peculiar shape with enlarged structures full of nutrients that the ants can bite off and consume.  The minima workers ensure that the fungus is placed on newly arrived leaf bits; add their own feces as a fertilizer (recycling it in the process - smart!); and when it becomes too dense they either move it to newer leaf bits or feed it to other ants.
It turns out that the story does not end here.  Another fungus, Escovopsis, can invade the nest and overgrow the fungus that the ants are growing.  This second fungus is not good for the ants to eat, and the colony could die if it is not controlled.  Since    Hölldobler and Wilson wrote their books in the early 90's, another player has been discovered.  It turns out that in addition to the fungi, the ants "grow" another organisms - a species of the bacterium Streptomyces.

 

Streptomyces are common in the soil, and most of them produce potent antibiotics to kill their competitors.  The Streptomyces in the ant nests is no exception, and the workers apparently use the excretions from the bacteria to kill any of the invading fungus that escapes the normal housekeeping of the ants.  Certain areas of the ant worker's bodies have been found to be be modified, presumably to enhance the growth of the Streptomyces, and in fact the Streptomyces has been cultured from those patches on the ants.  Thus, the leafcutters are not only fungal "farmers", but sophisticated pharmacists as well, producing and using antibiotics.  And we though we were smart!

 
Any close association between two species is called a symbiosis (particularly so when one species cannot live without the other).  A symbiosis where one species benefits and the other is harmed is called parasitism, and an arrangement whereby both species benefit  is called a mutualism.  The association between the ants and their gardened fungi has long been held out as a prime example of a mutualistic symbiosis.  What is interesting here is that the  Escovopis parasitizes the ant-fungus symbiosis (as opposed to parasitizing an individual organism) as both the cultivated fungus and the ant lose out to the benefit of the Escovopis  in this case.  On the other hand, the Streptomyces seems to be another, third member of the mutualistic symbiosis with the ant and the fungus since all three of these species benefit from the association.

These 3 articles discuss this symbiosis: 

http://www.botany.utoronto.ca/H-paper_2/398701A0.pdf - The paper which first found the Streptomyces connection.

http://www.biosci.utexas.edu/IB/faculty/mueller/pubs/resolve.pdf - An interesting technical paper on the intricacies of such mutualisms.

http://nationalzoo.si.edu/Publications/ZooGoer/2004/4/antfarmers.cfm - A popular article on the bacteria and the two fungi.

You might wonder - how does the fungus get to the nest in the first place?  It turns out that when a queen ant leaves the nest where she was born she takes a bit of the fungus with her.  She even has a special pouch near her mouth to carry it.  It has also been determined that she takes the Streptomyces  bacterium with her as well.  After mating, she removes her wings and digs a hole down into the ground; at the base of the hole she excavates a nest cavity.  She never goes above ground again.  She lays some eggs and begins to rear young.  She also removes the fungus and grows it by feeding it her feces.  The young she raises by feeding them unfertilized eggs; she gets the resources to produce these eggs by breaking down the large flight muscles she no longer needs.  She regulates the size of her developing young by feeding them carefully; the more she feeds them the larger they get, and she is aiming for nothing larger than the media with the 1.6mm wide head capsules.  These are the smallest workers that can go out and cut leaves.  Once her first workers reach adulthood, they open up the nest and begin foraging; inside the nest they take over care of the fungi and the remaining young.  The queen's duties are now relegated to laying eggs.

 
The other big activity in the nest is the rearing of new ants.  Again, this is a task left largely to the smaller workers.  They tend to the young, first moving the eggs from place to place, later moving and feeding the larvae, and finally moving the pupae as needed (the pupae, unlike many hymenopterans, are not encased in a silk cocoon).  

The young ants also have to be groomed to eliminate parasites such as mites and other problematic organisms such as disease-causing fungi.  The small minimas are well-suited to these tasks.

In the figure to the right, you can see a number of media workers, some minimas, and a number of immature ants (recognizable by their pale whitish appearance).  Among the young are grub-like larvae and the pupae.  The pupae look more like the adult ants since they have legs and antennae (those these are held folded together against the body).  You can also see the dark pigments of the eyes present in the pupae.

A video view of a leafcutter ant nest under glass. This nest is maintained at the Butterfly Garden in Monteverde.

 In the video to the right you can get a rare glimpse into the world of the leafcutter ant nest.  This particular nest is maintained under glass at the Butterfly Garden in Monteverde (despite its name, the Butterfly Garden hosts, exhibits and educates about a variety of insects other than butterflies).  This clip shows several views of the nest.  If you look closely you can see various castes of workers, developing ants, and even the fungal gardens.  There is some distortion where water droplets condensing on the glass obscure the view, and some parts of the video were made with near-infrared light, which eliminates colors and leaves the ants (as well as the larvae and pupae) a ghostly white.
The fungal garden:  Right, a maxima (soldier) dwarfs the media and minima workers on the surface of the fungal garden honeycomb.  The honeycomb, fashined by the mimima workers from bits of chewed leaves, is permeated by tunnels that the mimimas patrol, tending to the fungus.

 

Below:  A close-up of the fungal filaments and an out-of-focus minima.  The fungus is the main source of food for the colony, although no doubt the minima extract a lot of nutrition from the juices produced as they chew up leaf fragments for inclusion into the garden.

References:

In addition to the web references listed above, these two books are excellent!

Hölldobler B, Wilson EO.  1990.  The ants.  Harvard University Press.  732 pp.

Hölldobler B, Wilson EO.  1994.  Journey to the ants.  Harvard University Press.  224 pp.

and this website isn't bad, either:

http://www.blueboard.com/leafcutters/index.htm

Finally, you can access more leafcutter videos and a host of data we collected in the field, as well as questions suitable for assignments on the Leafcutter Ant Assignment Page  linked below.

 

Leafcutter Ant Assignment Page

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