Warm-bloodedness is a key mammal trait, but it's been a mystery when our ancestors evolved it. A new study points to an unlikely source for telling a fossil animal's body temperature: the size of tiny structures in their inner ears. The fluid in our ears becomes runnier at higher temperatures, so animals with warm bodies don't need as big of canals for it to flow through. Turns out, mammal ancestors became warm-blooded nearly 20 million years later than previously thought.