Hedgehogs may be one of the oldest mammal groups, having remained relatively unchanged for around 15 million years. Their bones structure is just like a dog. Their most distinguishable characteristic is their spines, which are modified hair made out of keratin, the same material as your hair and nails. Hedgehogs have 5,000 to 7,000 spines all over their body, with the exception of their underside, legs and face. Hedgehog anatomy and structure is basic, resembling primitive mammals, apart from specialisation of their integument. Both sexes have five pairs of nipples. In males the prepuce is some distance from the anus, near the navel, while in females the vulva is much closer to the anus. The testes are intra-abdominal.