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 here.

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.

  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 students.  

Using the data below, you should be able to answer these questions:

• How much biomass is moved in one hour?
• How much biomass is moved in one day?
• How much biomass is moved in one year?

• 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 estimate now?

Note: There are several ways to make these estimations, so be sure to explain your reasoning!

1. The plant biomass of one hectare of Costa Rican rainforest.
2. The number of Atta colonies per hectare of Costa Rican rainforest.

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!

Data

The files below contain the raw data needed to do the assignments listed above.  Your instructor may specify which files to use, so pay attention.

Videos

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. 

<-- Nest       Tree -->

Small File - 17 Mb

Video 1

<-- Nest       Tree -->

Large File - 58 Mb

Video 2

Video 3

<-- Tree       Nest -->

Video 4

Leaf Fragment Weights

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 them.

Click on Image for Larger View

Image 1

Ant Size

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 individuals.

The figure to the right shows the head capsule width plotted against the length for 3 castes of leafcutters.

Figure 1

Figure 2

The figure to the left shows average weights for 3 castes of leafcutters.

Data - Leafcutter ant measurements

Graph: Leafcutter Ant measurements

This site has some interesting observations on insect strength.

  Some other interesting data can be found here.