Ch'i

The simple answer is this: each different animal has its own different weaknesses and strengths. Throughout the course of their evolution through natural selection, animals will begin refining methods of eluding or confusing predators, and capturing their own prey through survival traits inherited from generation to generation. This octopus, for example, because of its vulnerability (i.e. soft body, lack of defensive capability) has over time developed methods of camouflaging itself and mimicking other animals as its defense. In contrast, the crab you see attacking said octopus has developed a hardened outer shell to protect its slow moving body along with two strong claws to clamp down on elusive prey. In conclusion, an animal's unique traits and abilities are a result of which traits kept them alive, and which ones didn't. Its all about adapting to an environment via survival. Can they learn? Some better than others, but the key lies in what they are handed down by their predecessors.