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
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
Castes Among Leafcutter
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.
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
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
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.
white material in the picture probably is the fungus Leucocoprinus
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
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!
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.
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
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.
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.