Hedgehog

It is their main and often only defence. A hedgehog cannot run fast, climb well, or fight effectively, so when it cannot escape it relies on its spines. A large circular muscle runs around the edge of the spiny skin on its back, like the drawstring of a bag, and when the hedgehog contracts it the loose spiny skin is pulled down and inward over the head, flanks, and rear, enclosing the vulnerable face, belly, and legs inside a sphere bristling with points. Most predators, including foxes, dogs, and birds of prey, give up on a tightly rolled hedgehog. The main exceptions are badgers, which have the strength and long claws to force one open.

Yes, the temperate species are among the relatively few mammals that undergo true hibernation. As autumn temperatures fall and food becomes scarce, a hedgehog that has built up enough fat retreats to an insulated nest of leaves, called a hibernaculum, and enters a state of torpor in which its body temperature drops to within a degree or two of its surroundings, its breathing becomes slow and irregular, and its heart rate falls dramatically. It stays like this for weeks at a time, rousing briefly every so often, sometimes to move nest, and lives entirely on stored fat. Underweight juveniles born late in the year often cannot survive the winter without help.

They are useful, eating large numbers of slugs, snails, caterpillars, and beetles, though not enough to be a complete pest control. To help them, the single most important thing is to link gardens with a hedgehog-sized gap, about 13 by 13 centimetres, in fences and walls, since a hedgehog roams over many gardens in a night and solid boundaries trap it. Beyond that, leave wild corners and leaf piles for nesting, check long grass before strimming, avoid slug pellets and pesticides, cover drains and pond edges with an escape route, and put out meaty cat or dog food and fresh water, but never bread or milk, which hedgehogs cannot digest.

No, they are only very distantly related, and their spines evolved separately. Hedgehogs belong to the family Erinaceidae in the order Eulipotyphla, the group that also contains shrews and moles, and they are small insect-eaters. Porcupines are large rodents, in the same order as rats, beavers, and squirrels, and there are two only distantly related porcupine families, one in the Old World and one in the New. Both hedgehogs and porcupines have converted body hairs into defensive spines, but this is convergent evolution, the same solution reached independently, in the same way that unrelated animals have repeatedly evolved wings or camouflage.