Scientists have counted the number of heartbeats a mouse has in a lifetime and found a direct relationship with its size and weight. They then did the same with an elephant and found the relationship/ratio between the number of heartbeats and the size of the elephant to be the same as that of a mouse.
This means that all creatures have a number of given heartbeats in a lifetime commensurate with their weight and size.
A mouse being small and fast-running has a very fast heart beat. He therefore uses his allotted number of heartbeats quickly and dies in a matter of a few months.