FACULTY NOTES

This exercise is appropriate for upper-level biology students with some knowledge of experimental design and the ecology of the rainforest, but may be challenging for less-experienced students.

Before working on this exercise, students should get some introduction to the biology and ecology of army ants, either presented by the instructor or by reading the background material. In particular, students will be better able to design an appropriate experiment if they can visualize the habitat and phenomenon being studied. Some nice pictures that support the background material are available in Gotwald (1995), Willis and Oniki (1978), and the web references listed in the Resources section includes links to general information.

In addition, students need some knowledge of the elements of a well-designed experiment. The Resources include links to rubrics for judging the quality of students’ experimental designs. Many teachers give students rubrics like this ahead of time to so that their students clearly understand evaluation criteria. For this activity, a rubric could be given to each group for students to discuss before they develop their experimental designs.

A class discussion of the experiments designed by the student groups (as described in the Student Instructions) with comments encouraged from other groups can be very useful. Careful management will be important here. Students will need clear instructions for reporting (e.g., select one person in the group to be the reporter, know there is limited time available for reporting per group, etc). Depending on students' level of experience, each group could preview another's design before the discussion and present their critique as a way to start the discussion. The instructor and students should help point out good points and flaws of different groups' experiments, helping to introduce or reinforce key elements of experimental design. Each group can turn in a written description of their experiment for assessment. The Army Ant Experimental Design [PDF] form will help students organize this assignment. Students can fill the form out together in class and hand it in as a group or do this individually as homework. Students can fill the form out together in class and hand it in as a group or do this individually as homework.

Wrege et al. (2005) includes the following description of their design:

The experiment consisted of paired, 10-min trials, during which we quantified the capture success of the army ant swarm in the presence, and then in the absence, of attendant birds. We could usually exclude birds from the experimental area simply by standing immediately adjacent to the area and gesticulating with our arms. The occasional persistent bird was discouraged with a shot of water from a squirt gun, or by throwing a fragment of bark into the leaf litter nearby when the bird began to approach too closely…We initiated trials when we expected the ant swarm to traverse relatively homogeneous habitat, and when the attending ant-followers were active and generally numerous (>3 birds).

E. burchellii aggregate on prey items just after capture when prey are larger than can be handled by a single worker. These groups of ants then subdue and dismember the prey (Rettenmeyer 1963, Schneirla 1971, Gotwald 1995). As prey size increases, the number of workers involved in subduing and processing increases, and "aggregations" on prey larger than ∼0.8 cm persist sufficiently long to be counted as a measure of foraging success. In 2001, we presented crickets, or parts of crickets, to foraging E. burchellii and monitored the persistence of the aggregations for the purpose of estimating the size ranges of prey likely to be counted, beginning 20 min after the ants passed through an area.

As an alternative, students could be given this experimental design to critique and to explain in a step-by-step fashion.