Kirsten published the manuscript from her MSc research on size variation in mammals, and show how, well, variable it is. Is there any rule? Can we use the present to predict the future response of mammals species to climate warming? I was a strong advocate of this ever since graduate school, and Link and I published a review on this 15 years ago in the journal Ecology Letters. We are back this time with a large dataset Kirsten collected from museum collections for 17 species in North America. The more we look into it, the more exceptions to the rules we find. Van Valen in 1973 wrote that "we do not understand why mammals have the size they do". I should have listened better!