Ben Garrod, primatologist and master skeleton builder, shares his unique passion for bones. He embarks on a very personal journey through the remarkable and surprising story of how a single, universal body plan - the skeleton - has shaped the animal kingdom. There are over 62,000 species of vertebrate of every size and shape from squirrels to sperm whales and aardvarks to anacondas. Each skeleton differs in small, but critical ways and, in Ben's hands, those differences are decoded to reveal an animal’s complete life story - not only how it moves, where it lives and what it eats, but also its entire evolutionary journey. Secrets of Bones brings the remarkable world of bones to life in a unique way. By conducting fascinating experiments, meeting expert contributors, using integrated graphics and a wealth of archive from the BBC's Natural History Unit, Ben unlocks a fascinating and hidden world we never get to see. In this episode, Ben Garrod seeks out the big part that bones can play in reproduction. Through sexual selection, the skeleton has adapted to aid courtship, competition and even copulation. On his travels, Ben meets baseball players, drops a 10kg weight on a sheep's skull and finds out that by not having a penis bone humans are very much in the minority.