As a biologist, I don't think it is irrelevant. As far as we know, your thoughts and rationality do not have an independent existence. They are, generally speaking, the products of your physical body which is built from a blueprint, your DNA. This blueprint has been naturally selected for and is a product of billions of years of evolution. Your sense of morale, your fondness or not for cooperation, how easily you anger when someone steals from you, what you want for dinner, all that and more is flavoured by your biology. You say we're rational. I agree, but to understand how rational we are, you have to understand where that rationality comes from. Is it rational to send money to a charity supporting starving children in Africa after watching a moving infomercial on the telly? Is it rational to get jealous if your girlfriend flirts with another guy? To make fully sense of that and more, you have to include knowledge of human biology in past and present.
Furthermore, we have learned stuff from watching animals which is or should be relevant to discussions on anarchy. This is something I posted earlier :
The point here was that although anarchy might sound good to an idealist (like everyone sharing food might), I believe it would be likely to create an environment which would reward exploiters.
Animals evolve in ways that create instinctive behaviours that create social environments where exploitation may get punished (see for example
reciprocal altruism). They do so through natural selection. We have the luxury to do so also through politics and how we choose to construct our societies. Those may look like completely different solutions, but the problem may be the same, generally speaking.
As I wrote earlier, what I presented was a point made against a very specific argument, one which did not come from you. If I was to argue a case against anarchy in general, then I wouldn't start with the social mammal argument. However, it is, as I mentioned, just another level from which you can debate (against) anarchy.