Body heat is lost across the surface, and the surface-to-mass ratio rises steeply as an animal shrinks. At around 1.8 grams, an Etruscan shrew is close to the practical lower limit for a warm-blooded land mammal. Everything about its physiology is scaled to compensate: breathing rates in the hundreds per minute, a heart recorded at up to about 1,511 beats a minute, and heart muscle making up roughly 1.2 per cent of body mass.
The consequence is an appetite out of proportion to size. Etruscan shrews eat about one and a half to two times their own body weight in invertebrates each day, and cannot go long without food. They hunt in short bouts around the clock rather than keeping a day or night schedule.
Which mammal is smallest depends on the measure used. The Etruscan shrew is the lightest by mass, while Kitti's hog-nosed bat is smaller by skull size and body length, so both hold a claim. Peak heart rates are also hard to pin down, since the highest figures come from handled or rewarming animals and vary between methods.