Put alongside mice or elephants, human beings live as much as four times longer than they should, given their size and pace of life. There is, it seems, a correlation in the wild between body size, metabolic rate and lifespan. A mouse scurries about the undergrowth and, if not eaten, can expect to survive for up to three years. Elephants are more sedate, consume energy more slowly and may live to sixty. Humans should die sometime between the two.