Skip to content
← Theoretical A

Q38 - Attractant vs. Repellent Pheromones in Pharaoh Ants

Theoretical A Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license
Animal Communication

The following experiment was used to determine whether Pharaoh ants (Monomorium pharaonis) use attractive or repulsive chemical signals to mark their foraging routes. The researchers used two ant colonies.

The first colony was used as the trail-laying colony in a paper Y-bifurcation apparatus (left side), where they were able to leave pheromone trails. The researchers put a feeder with sucrose at the end of one branch (feeder branch) and left the other branch’s end empty (non-feeder branch). After foraging, they removed five sections (S, Fb, Nb, Fe, Ne) and a control from the apparatus and put them, alternately, in different unbranched apparatuses (right side):

  • S: 1 mm before the bifurcation
  • Fb: 3 mm after the bifurcation, in the feeder branch
  • Nb: 3 mm after the bifurcation, in the non-feeder branch
  • Fe: 60 mm after the bifurcation, in the feeder branch
  • Ne: 60 mm after the bifurcation, in the non-feeder branch
  • Control: no ant trail pheromones used

Diagram of the Y-bifurcation apparatus (left, “Title 1”) with sections Ne and Fe marked near the fork tips, Nb and Fb marked just below the fork, and S marked at the base, plus a small “control” section shown separately; and an unbranched test apparatus (right, “Title 2”) with a single test section placed midway along a 180mm tube (90mm marked on each side), 10mm wide. Fig.1. Experimental setup. Title 1 = Trail sections. Title 2 = Test position of section. L1 = feeder. L2 = From nest.

The researchers tested each section with 50 ants from another colony, and counted how many times the ants made U-turns.

Bar chart showing number of U-turns relative to control for each section: S is strongly negative (-14, P<0.001); Fb is mildly negative (-7, P<0.05); Fe is strongly negative (-15.5, P<0.001); Nb is strongly positive (+16, P<0.001); Ne is near zero, not significant (NS). Fig.2. The result of the experiment: number of U-turns compared to control. P<0.001 and P<0.05 indicate the significance level of the difference compared to control (the lower the value, the stronger the significance). NS means no significant difference compared to control. Axis = Number of U-turns ants made relative to control.

Based on the significant results of the experiment, determine which type of pheromone the ants used in each of the five sections.

  • A – attractant pheromone
  • R – repellent pheromone
  • Q – no trail pheromone used
Q38.1 (Ne). Pharaoh ants laid pheromone trails on a Y-bifurcation apparatus (feeder on one branch, empty on the other). Five sections were cut out and each tested on a fresh unbranched apparatus with 50 new ants, measuring U-turns relative to a no-pheromone control: S (near bifurcation), Fb/Nb (just past the fork, feeder/non-feeder branch), Fe/Ne (far past the fork, feeder/non-feeder branch). Ne showed no significant difference in U-turns vs. control. What type of pheromone signal does this indicate?
Q38.1 (Nb). Nb (just past the fork, non-feeder branch) showed significantly MORE U-turns than control (P<0.001). What type of pheromone signal does this indicate?
Q38.1 (Fe). Fe (far past the fork, feeder branch) showed significantly FEWER U-turns than control (P<0.001). What type of pheromone signal does this indicate?
Q38.1 (Fb). Fb (just past the fork, feeder branch) showed significantly fewer U-turns than control (P<0.05). What type of pheromone signal does this indicate?
Q38.1 (S). S (near the bifurcation, before the branches split) showed significantly fewer U-turns than control (P<0.001). What type of pheromone signal does this indicate?
← Previous: Q37 Back to Theoretical A →

Question reproduced from IBO 2019, Theoretical Exam A, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Official answer key