[chub500]

I think the frustrating thing about this article is that every single observation has obvious exceptions. The thesis "mammals are unique" could just as easily be antithesized by the chosen evidence.

This taxonomical difficulty has always fascinated me. I give biologists the benefit of the doubt that they have solid evidence for their categorization but in the back of my mind I've always been curious about this. Why would these unique features exist in other lineages? Perhaps the space of diversity in behaviors is smallish to the point that random overlap occurs? Maybe it's purely definitional and the actual behaviors are distinct between say mammals and birds?

Or maybe my problem is that I failed my 7th grade taxonomy exam catastrophically and have never quite gotten over that :)

[lopmotr]

There's not much solid about taxonomy of animals. Nature doesn't really have clearly defined classes, just rough clusters of similar organisms. It's the humans that like classifying things because it's easier for us to think about them and make predictions when we see them as classes rather than as individuals or even the whole global ecosystem as a single integrated super-organism. The more common features we can find in a class, the less thinking we have to do but it's OK if there are exceptions as long as it's not so many that it makes the classes useless.