Hedgehogs have changed little in the past 15 million years. If threatened they curl up into a ball and, for futher protection, they can raise or lower the five to seven thousand spines they have on their back.
Baby hedgehogs are called hoglets and the collective noun for hedgehogs is an ‘array’.
They hibernate in winter, are lactose intolerant and have very bad eyesight but their sense of smell and hearing are impeccable.
Dangers to their population include less natural habitat due to over-manicured gardens as well as insecticides and bonfires.
In addition fences, walls and roads create artificial barriers making it much harder for them to easily traverse  the range of ten to twenty hectares they need in order to survive.  
Indeed as nocturnal animals they are known to roam over two kilometres a night in search of food.