The author doesn’t suggest that the tortoise height contest is used by only a few species. Perhaps it’s used by many species. Even if it is used by only a few species, the purpose still isn’t related to this point. The purpose is to show an example of the behavior described by the species-specific model.

At this point in the passage, the author has not raised the idea that repetitive use of the same visual and vocal displays might not be used by all species. Although the author raises this point in P2 when he brings up A. aperta, the tortoise example doesn’t demonstrate this point.

The author doesn’t try to support the claim that fighting behavior doesn’t vary greatly for most species. Rather, the author is merely giving us an example of the traditional theory of animal conflict behavior — the species-specific model. The author doesn’t try to provide evidence to support this model and its application to species; he simply describes the model and an example to help the reader understand the model.

The author doesn’t suggest that the tortoise’s height contest is “unique” to Galapagos tortoises. Maybe other animals use the same kind of dispute resolution behavior. If you like (D) because you think “species-specific” implies behavior that is unique to species, you’re making an unwarranted assumption. There’s nothing in the name “species-specific” that must imply that each species has its own unique style of dispute resolution.

The best captures the purpose of the reference to tortoises. The author wants to provide an example to help us understand the traditional theory of animal behavior. According to this theory, animals use repetitive visual or vocal displays and avoid escalated fighting. The tortoise height contest is an example of this kind of behavior.