Why are there no mammals smaller than a shrew, and why shouldn’t you give dogs human chocolate?

We explore the curious questions that science can answer

Shrews need to eat a prodigious amount of food to stay alive (Getty/iStock)

Why are there no mammals smaller than a shrew?

The answer to this question comes down to heat loss. Shrews are mammals, so in common with other mammals, they maintain a constant body temperature that’s nearly always above that of their surroundings. As a result, they lose heat from their bodies to their surroundings. The source of the heat is the energy that the mammal gets from food. The more heat is lost, the more food is needed to replace this energy loss.

Heat loss is a bigger problem for smaller animals because they have a high surface-area-to-volume ratio: they have a lot of surface area to lose heat through. And, as the smallest mammals, shrews suffer from heat loss like no others. As a result, they need to eat a prodigious amount of food to stay alive.