Phenotypes of the Living Dead

Andy Wood answers the perennial question: "Why don't flesh-eating zombies ever turn on each other?"

It's not as if these creatures are particularly hygenic or well known for their personal grooming habits. What is it that gives them such a discerning palate that they must seek out fresh meat for sustenance, when meat, a few days dead, may be much closer to hand? Normally, when you see them, they hang around in large, frightening mobs to hunt down the living. You would think that some of them would spare themselves the bother and eat one of their own kind, yet you hardly ever see this happen.

The answer, of course lies in evolutionary biology. The most compelling interpretation of the Theory of Natural Selection is the neo-Darwinian synthesis, famously popularised by Richard Dawkins in his book The Selfish Gene. According to this view, Natural Selection acts upon the replicating units, meaning those entities which are copied from one generation to the next, namely the genes. Consequently, when attempting to understand an evolutionary adaptation, such as an organ or a pattern of behaviour, it is a mistake to consider the effect that it would have on the interests of the organism. Instead, what should be considered, is the effect it has on the reproductive success of the genes which are correlated with the adaptation. A heritable trait, which allows someone to live a long and happy life, will not survive many generations if most of the carriers of the corresponding genes are also rendered infertile. Conversely, the self-destructive sexual impluses of the male mantis may not serve the mantis very well, but they ensure that his genes are copied into the next generation.

Applying this to the problem in question, we need to identify the genes associated with the zombies' behaviour pattern. According to The Zombie Survival Guide, zombies are carries of the solanum virus, which spreads via infected bodily fluids entering the bloodstream of a victim, such as by a bite. Thus, we should consider a zombie's taste for living flesh not as a source of energy, but as an adaptation, which aids the replicatation of the genes of the solanum virus. Seen in this light, it should be clear why zombies shun the meat of their fellows and seek out meat of the living: a zombie which tried to eat other zombies would be doing nothing to reproduce the solanum virus, as it would be trying to infect a victim that is already infected. Therefore, mutations of the virus which caused their carriers to eat other zombies would be out-reproduced by mutations which induced them to seek out uninfected victims. Hence, we tend not to see such mutations, and their associated behaviour patterns, in nature. Q.E.D.

This raises another question. Rabies is a similar virus, which spreads by inducing its carriers to bite other animals. Presumably the same logic applies. So, is there any tendency for rabid animals to preferentially bite uninfected victims? If we were to put some rabid animals in a cage with non-rabid animals, would the rabid animals leave each other alone and attack the non-rabid ones?

Go and read The Extended Phenotype by Richard Dawkins.