Life on the metabolic edge
The Etruscan shrew (Suncus etruscus) operates at the extreme biological limits for a mammal. Weighing an average of 1.8 grams—less than a dime—it holds the title for the smallest mammal by mass. Its tiny body, about 4 centimeters long excluding the tail, has a very high surface-area-to-volume ratio, causing it to lose heat at a dangerous rate. To counteract constant hypothermia, the shrew evolved an exceptionally fast metabolism, the highest of any mammal relative to its mass.
This metabolic furnace demands a constant supply of fuel. An Etruscan shrew must consume 1.5 to 2 times its own body weight in food every day. Its diet consists mainly of insects like grasshoppers and crickets, as well as larvae, earthworms, and even small lizards or rodents nearly its own size. It hunts relentlessly, mostly at night, using its sensitive whiskers to detect prey in the leaf litter rather than its poor eyesight. The periods between meals are short, as the animal is always moving when awake. This endless hunt is a matter of immediate survival; starvation can occur in just a few hours.
An anatomy built for speed
The physiology of the Etruscan shrew is full of extremes. Its heart, which makes up a considerable 1.2% of its body weight, beats at a resting rate of around 835 times per minute, but this can accelerate to an astonishing 1,511 beats per minute (about 25 beats per second) during activity. This is one of the fastest heart rates ever recorded in an endothermic animal. Its respiratory rate is equally extreme, reaching up to 900 breaths per minute.
To support this high-speed existence, the shrew's skeletal muscles are composed entirely of fast-twitch fibers, allowing for incredibly rapid contractions for running, breathing, and shivering. When food is scarce or temperatures drop, the shrew can enter a state of torpor, a form of temporary hibernation. It can lower its body temperature to as low as 12°C (54°F) to conserve energy. The rewarming process is itself an athletic feat, with shivering at a frequency of 58 muscle contractions per second and the heart rate ramping up from 100 to over 800 beats per minute. This strategy allows the shrew to survive brief periods of hardship in its otherwise fast-paced, two-year lifespan.