How common is cannibalism in the animal kingdom?
Some kind of control mechanism probably. Some species eat their weak and unhealthy brood.
Cannibalism is quite common among animals. Lions eat other lions, bears eat other bears, and chimpanzees eat other chimpanzees. This is not necessarily the result of a food shortage. Such incidents occur even when there is a plentiful supply of food from other sources. In the case of lions, for example, a male lion sometimes kills and eats the cubs of a female lion without a partner to protect her. The male lion does this because the female lion will not mate as long as she has cubs, but she will mate after the cubs are dead. For some species, cannibalism is extremely common. Approximately one fifth of the diet of a tiger salamander, for example, consists of other tiger salamanders.
The point to be learned from this is that we should not be basing our moral code on the behavior of other animals, but instead strive for something better. If we were to believe that eating meat is OK simply because other animals did it as well, then this would imply that there is also nothing wrong with cannibalism.
Cannibalism is quite common among animals. Some animals eat the same of their kind for their own reason, some for food other for mating and some for protection.