Why is cannibalism not an evolutionary stable situation?

Question

In the 'the selfish gene' Dawkins writes (page 109): "The reason lions do not hunt lions is that it would not be an ESS (evolutionary stable situation) for them to do so. A cannibal strategy would be unstable for the same reason as the hawk strategy in the earlier example."

To quickly summarise this: we consider an interaction between two animals where one possibly stands something to gain. Dawkins then defines 'hawk' behaviour as attacking behaviour, whilst 'dove' is described as peaceful and retreating behaviour. He then argues that a population of purely hawk or purely dove is unstable, since in either of the situations the other kind of behaviour is more favourable. As it turns out, the stable situation is 7:5 for hawk:dove.

Turning to cannibalism, we can regard the hawk behaviour as 'engaging in cannibalism' and dove to be the refraining party. Why, then, don't we see similar behaviour when considering cannibalism, i.e. why don't we see a reasonably large amount of lions engaging in cannibalism whilst the others refrain from it? If I am not mistaken we see in fact the situation more closely described by completely dove? Any insights are appreciated!

Answer 1

Cannibalism is incredibly common in nature. There are more species that eat their own kind than do not. Chimps eat other chimps. Lions eat other lions. Wolves eat other wolves. Prairie dogs go into the burrows of other prairie dogs and eat their babies to get the protein-rich meals needed to produce milk for their own litter. The only animals that don't regularly engage in cannibalism are some birds, and that is more because they physically cannot fit other members of their species down their throat than any moral qualms.

However, cannibalism is typically a bad evolutionary strategy, long term. For several reasons...

1. in a dioecious species (i.e., one with two sexes), cannibalism means a 50% chance of eating a potential mate and losing the opportunity to pass on your genes
2. diseases are more likely to transfer between species the more closely related they are. If you are a cannibal you are eating a species with a 99% similarity to you and are more likely to get diseases or parasites. Like kuru/mad cow/chronic wasting disease.
3. it precludes social behavior and all its benefits. Why engage in mutually profitable behavior when you can just eat those around you? This is thought to be one reason why pack-hunting is rare in some groups of vertebrates like monitor lizards.
4. on a broad-scale evolutionary perspective, cannibalism means removing members of your own population from the gene pool. It may be good for the individual but it's bad for the species as whole
5. cannibalism means you are feeding on an animal that is potentially your own size. That's a bad strategy in general. Most cannibalism in nature tends to be adults eating infants or adults scavenging carrion of their own species. By contrast predators are normally larger than their prey, with a few exceptions like pack-hunting wolves and lions.