On two field trips to Costa
Rica, one to La Selva Biological station in May, 2005 and the second
to La Suerte Biological Station in 2007, students from Marietta
College observed leafcutter ants as they foraged. We also
videotaped the ants, collected the leaves they were carrying, and
measured the ants themselves. That data is presented for you
To the left, you can see the
basic setup in 2005. The observations were made at night -
around 9PM. The pen marks the point on the trail where the
counting occurred. The nest was to the right in this picture;
the tree was to the left. Each time an ant crossed the point
indicated by the pen with a leaf, a tally was recorded. The
number of leaves carried in 5 minutes was thus determined.
In addition to counting the
leaves, we also took a number of images. Future world-renowned
biological photojournalist Tyler Snell is shown at work here.
Note the wide trail through the grass; the ants clip the grass
blades to make it easier for them to return to the nest with the
The trail we were studying came
from a single tree, the crown of which is shown to the
As you can see, in addition to the
leaves of the tree itself, there were numerous epiphytes in place
for the leafcutters to harvest. Thus, the leaf fragments
collected are not all from a single species of plant.
After counting the leaves for 5 minutes,
the students removed all of the leaves from ants returning to the
nest for a period of 1 minute. These leaves were counted,
bagged, weighed, dried and reweighed. An image of the
bag is shown here:
In 2007, we visited another
field station, La Suerte, also in the Caribbean lowland rainforest
of eastern Costa Rica. We carried out a very similar
experiment on an ant colony there (Important note: we are
assuming that both colonies were of Atta cephalotes, given the
habitat. The ants have not been keyed out).
Lessons learned from the
2005 trip included placing a ruler in front of the video camera for
scale. We carried out these observations in daylight, almost 2
years to the day from the previous ones.
New lesson learned in
2007: There was no drying cabinet at La Suerte, so our leaves
got moldy as we tried to dry them out and they had to be
discarded. Next time, we will take desiccant!
In 2007, several trails were
active at the same time, and the video below shows one of these
trails. The point at which the students were taking
measurements was "downstream" (closer to the nest), and
another stream of foragers merged with the stream being videotaped
between the video camera and the sampling site for the
Using the data below, you
should be able to answer these questions:
1. Play back one of the
videos (videos 1-4) and count the number of leaves being returned to
2. Use the data in image 1
to calculate the average weight of a leaf fragment.
3. Use your answers to
questions 1 and 2 to estimate the biomass (weight of all the leaves)
moved by the ants in a ONE MINUTE period.
How much biomass is moved in one hour?
How much biomass is moved in one day?
How much biomass is moved in one year?
4. Choose one of these
1 and 3
2 and 3
1 and 4
2 and 4
3 and 4
Answer questions 1-3 for each of
the videos, then answer this question - in which observation were
the ants moving the greatest biomass per minute?
5. Using video 1 or 2, determine the average
speed of ants carrying leaves. Determine the speed of at least
10 individuals, then take an average.
6. Assuming that no ant crosses the video
screen twice (that is, goes off the screen, drops its load, then
walks back through from the other side), what is the MINIMUM number
of ants in the nest. Explain your calculations.
Assume the trail is one of 2, and that each trail is 100 m
long and has ants the entire length. What is your minimum
Note: There are several ways to make these estimations, so be
sure to explain your reasoning!
7. Watch the video and tally the number of
minimas, medias and maximas (soldiers) that you see. Use this
information to calculate the percentage of the foragers made up by
each sub-caste (note that these percentages are not good for the
colony as a whole since the workers remaining in the nest may be
apportiuoned differently from those out doing the foraging).
8. Use your answers from #6 and #7 and the
data from figure 2 to estimate the biomass of the entire ant colony.
9. In a one day period, how does the plant
biomass moved by the ants compare to the biomass of their coloby?
10. Using the internet and other
The plant biomass of one hectare of Costa Rican rainforest.
The number of Atta colonies per hectare of Costa
With those numbers in hand, what percentage of the biomass in
the rainforest is consumed by the ants AT A MINIMUM (remember, we
don't have data on how many trails the ants have going
simultaneously; alternately, we don't know if the foraging activity
on tape here is sustained 24 hours/day).
Don't forget to cite your sources!
11. How much weight can an ant move relative
to its body size? Use Video 5 to obtain your answer, and
compare this to at least 3 sources on the web. Don't forget to
cite your sources!
12. Open the Excel file
near the bottom of this page. Applying the appropriate
statistical techniques, determine if there is a SIGNIFICANT
difference in head capsule width between each of the 3 groups.
Likewise, look for SIGNIFICANT differences in body length.
The files below
contain the raw data needed to do the assignments listed
above. Your instructor may specify which files to use, so pay
Photo of the video setup for
the 2007 observation. Some of the video in the files below is
the same as seen on the screen of the camcorder here.
The files above
were taken from video captured on May 25th, 2007 at the La Suerte
Biological Station in Costa Rica. Both files are large and may
take a few minutes to load. There is about 5
minutes of video. The files are identical except for size; it
may be more practical to use the smaller file over a low-speed
internet connection. The tree (source of leaves) is to the
right of the frame. There were several trails active at the
time of filming.
The video to the left was taken about 9pm
at the La Selva Biological Station on May 26th, 2005. It is
about 16 minutes long and about 210 Mb in size. Note that the
direction of the tree and the nest are reversed in this video; the
tree is now to the left and the nest is to the right. If you
decide to extract data from this video, you might want to randomly
choose a place to start in the video and then count for 5 minutes.
The video to the right was taken near dawn at the La Selva Biological Station on May
25th, 2005. It is
about 2.5 minutes long and about 5 Mb in size. The
tree is to the left and the nest is to the right.
You can estimate the weight of the leaf
fragments the ants are carrying using the information in the picture
to the right (click on picture for larger view). These are
leaf fragments collected from every worker passing a certain point
during the May, 2005 experiment. The leaves for the 2007
observations molded rather than dried and we don't have data for
Click on Image for
These measurements were taken on Leafcutter Ants at the La Suerte Biological Field Station in Costa Rica. In the course of studying ant foraging on May 24th, 2007, I gathered a number of the foraging ants for measurement. Aside from a conscious effort to get specimens from all 3 of the foraging castes (soldiers,
workers (media), minima), the ants were gathered as randomly as possible, however, the numbers gathered (9 soldiers, 35 workers, 18 minima) are not representative of the actual ratios of these castes among the foragers. In particular, the soldiers and minima are over represented in this sample. The specimens were preserved in 70% ethanol. Specimens were visually sorted to caste at 7x magnification based on relative size. They were then measured using a millimeter rule under a dissecting scope at 7x magnification. Head capsules were measured across the dorsal back side of the head capsule (the widest point). Length was measured by gently stretching the specimens to extend the abdomen; in most specimens the head was tucked under the body so the anterior point of measurement was the frons. All measurements were to the nearest 1/2 mm. Weights were measured on an electronic digital scale. Average weights were determined by weighing all individuals of each caste (after drying on tissues) and then dividing by the total number of
The figure to the right shows
the head capsule width plotted against the length for 3 castes of
The figure to the left shows average weights for 3
castes of leafcutters.
The files below link to the data
used to generate these graphs.
There are a number of sources
which claim to "know" how strong an ant is and they usually
express it as "an ant can lift x times its own body
The data above show the average
weights for different caste sizes of one group of ants we collected.
The video below shows a plastic ruler being moved by a single
worker. The ruler was weighed and found to weigh 2.41 grams.
The average soldier (of 9 we collected, including the one shown moving the
ruler) weighed 0.072 grams. Now, this ant was not picking the ruler
entirely off the ground, but it does work for a first order estimate of
how strong an ant is.
site has some interesting observations on insect strength.