Experimental design, tools, and procedures — ACT practice questions

Isopods are small land-dwelling crustaceans that often occur beneath logs. Students tested whether isopods choose a chamber mainly because of its humidity or its light level. A choice arena had 2 connected chambers. Twenty isopods were released at the center, and after 10 min the students recorded the percentage in each chamber. Each condition was repeated 5 times. Chamber positions were switched between trials, and the arena was cleaned and dried before the next group was tested.

In Experiment 1, one chamber was dark and humid while the other was bright and dry. An average of 78% of the isopods occupied the dark, humid chamber. In Experiment 2, the dark chamber was dry and the bright chamber was humid. An average of 64% occupied the bright, humid chamber. In Experiment 3, both chambers were humid, but one was dark and the other was bright. An average of 72% occupied the dark chamber. In Experiment 4, both chambers were dry; 60% occupied the dark chamber.

Student 1 argued that humidity is the direct cue. According to Student 1, isopods choose the more humid chamber whenever humidity differs, and any apparent preference for darkness occurs because dark shelters usually retain moisture.

Student 2 argued that darkness is the direct cue. According to Student 2, dry conditions merely make isopods move faster, so they leave a dry chamber more often; when movement rates are equal, isopods choose darkness. Both students agreed that the isopods should be allowed to move freely between chambers. Relative humidity was checked with sensors immediately before each trial: humid chambers were maintained near 90%, and dry chambers near 35%. Identical mesh covers equalized airflow while allowing the light treatments to differ.