I want to know the answer to the following question:
Why is it necessary for smaller animals to have an insulative covering?
All the background I'm provided with is that "Modern monotremes have lower body temperatures and more variable metabolic rates than marsupials and placentals, but there is evidence that some of their ancestors, perhaps including ancestors of the therians, may have had body temperatures like those of modern therians.". Is that enough to answer the question?