It's rare that a straightforward biology book (with a fair amount of palaeontology thrown in) really grabs my attention, but this one did. Liam Drew really piles in the surprising facts (often surprising to him too) and draws us a wonderful picture of the various aspects of mammals that make them different from other animals. More on this in a moment, but I ought to mention the introduction, as you have to get past it to get to the rest, and it might put you off. I'm not sure why many books have an introduction - they often just get in the way of the writing, and this one seemed to go on for ever . So bear with it before you get to the good stuff, starting with the strange puzzle of why some mammals have external testes. It seems bizarre to have such an important thing for passing on the genes so precariously posed - and it's not that they have to be, as it's not the case with all mammals. Drew mixes his own attempts to think through this intriguing issue with th