Q21 — Cannibalism Probability and Injury Bias in Japanese Mitten Crabs

Theoretical B Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license

Japanese mitten crabs, Eriocheir japonica, can display cannibalistic behavior. The absence of other food resources can greatly facilitate cannibalism. As a result of this, some crabs might be injured or killed. Injury level is not limited to the number of lost extremities but also includes any other forms of it. Figure 1 (A,B) displays the probability of cannibalism versus Factor S:

$$\text{Factor } S = 1 - \frac{\text{prey size}}{\text{predator size}}$$

Figure 1-D shows hypothetical results.

Panel A: a sigmoid curve of cannibalism probability vs. Factor S for males, reaching 50% at S=0.43. Panel B: a similar curve for females, reaching 50% at S=0.79. Panel C: a bar chart of mean Factor S for killed vs. injured crabs. Panel D: a bar chart of injured-crab frequency against number of lost legs. *Figure 1. A,B Cannibalism among only male (graph A) and only female (graph B) crabs, n-number of crabs. C Y – mean (+SD) of Factor S among killed and injured groups, p<0.05. D Injured crabs’ occurrence frequencies.

On your answer sheet, indicate “T” for true statements and “F” for false ones.

A. Among male crabs, the cannibalism probability is 50% when predator to prey size ratio is about 1.75.

B. Females have a higher eagerness to cannibalism than males

C. In a cannibalistic environment, the occurrence of injured crabs will resemble the frequencies displayed in Figure 1D.

D. The difference in size affects whether the crab is killed or merely injured

← Previous: Q20 Next: Q22 →

Question reproduced from IBO 2024, Theoretical Exam Part B, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF