The leeches were eating the toadlets! This was something we really wanted to film for the series, but it only happens over a very short period each year, so it was full steam ahead to get set to film.

The baby leeches offer hope of boosting fragmented wild populations. There was even a 1959 black-and-white horror film, called "Attack of the Giant Leeches," playing on our fear of them.

In this short film Dr Michael Mosley interviews a surgeon who is pioneering the use of leeches in medical research. He allows a leech to attach to his arm and suck his blood, the mouth of the ...