How much does size matter? And is it generally males or females who are the giants? Sue Broom scours the animal kingdom for the biggest size differences between males and females and asks how they got that way.
Back in the 19th century, Charles Darwin's explanation for the obvious differences between so many male and female animals was what he called sexual selection.
Males, he said, were often bigger in order to fight off rivals or better looking to beat off the competition.
But modern science shows that things are a little more complex than that.
Bigger isn't always better and the heavyweights are more often females than males.