Tailed frogs are one of the world’s oldest frog species. Their nine vertebrae are evidence of their evolutionary relationship with salamanders – modern frogs only have five to eight. The males’ ‘tail stump’, on the other hand, is actually an extension of the cloaca and is used for procreation, because they live in fast-flowing streams. Their sperm would just be swept away if they tried to secrete it into the water like other frogs. Tailed frogs are the only frogs to have developed this kind of internal fertilisation.