A bat’s life cycle is very much dictated by the weather. Along with dormice and hedgehogs, they are the only British mammals to hibernate, typically emerging in early spring when they move between different roost sites. Although they mate in the autumn, females can store the sperm over winter, and in May start to form maternity colonies. Unlike other small mammals, bats are long-lived, with a lifespan of 30 years not uncommon. This is one of the reasons they produce just one pup, unlike a mouse, for instance, which breeds prolifically to compensate for its short life. Males spend most of their time living in their own smaller, bachelor roosts until the mating season begins. Over autumn bats build up their fat reserves before finding a good site to over-winter when temperatures drop. As well as churches and old buildings, caves and mine shafts are popular.