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← Theoretical 1

Q35 — Ninespine Stickleback — Sexual Dimorphism and Sneaker Males

Theoretical 1 Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license
Mating Systems & Sexual Selection

Sexual dimorphism is characteristic of the Ninespine stickleback Pungitius pungitius. Males, in contrast to females, acquire a dark color during the breeding season, build nests, and aggressively exclude other dark-colored males. They invite females to lay eggs in the nest, which the male then fertilizes. However, some mature males don’t differ much in color from females and are tolerated in the same schools by dark-colored males.

Figure 1. A dark-colored male Ninespine stickleback during the breeding season. Figure 1. Dark colored male of Ninespine stickleback.

Figure 2. A Ninespine stickleback female; some adult males also share this coloration. Figure 2. Ninespine stickleback female. Some adult males also have this coloration.

Using the information and images, determine whether the following statements are true or false:

A. Males resembling females during the breeding season may be an adaptation for tricking females into complacency before they fertilize the eggs that the females have laid in the dark-colored male's nest.
B. Males that do not differ in appearance from females are not yet mature, have not acquired a typical coloration of males and cannot yet reproduce.
C. The absence of sexual dimorphism between the light-colored males and females helps individual males to save energy and participate in reproduction.
D. Dark-colored males are able to distinguish males that are colored like females and drive them away from their nests.
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Question reproduced from IBO 2022, Theoretical Paper 1, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)