Interestingly, they apparently are counted when showing total number of discussion comments:

> 20 comments

But there's only 20 if counting the dead spam comments too.

So an idea: One can use them, to make one's own submissions look more interesting (more people discussing).

Does that impact page ranking on HN? And what purpose would that serve for OP or the study about rat social structures?

That being said, the poster of this study does submit quite a bit. So maybe there's something to that.

Oh, it's just that I went to this discussion because there were (seemed to be) many comments, so I thought it was something interesting. Social proof. And I suppose others a bit look at the comment count too. But this time, turns out it's all just comments about comments :-)