The closest living relatives of hedgehogs are gymnures, small animals that resemble hedgehogs but lack the protective spines. Moles and shrews are more distant relatives, while the hedgehog tenrec actually isn't a hedgehog at all and is not very closely related.
Hedgehogs have inhabited the earth for about 15 million years and appear to have changed very little during that time. They are small animals ranging from the size of a mouse to that of a rabbit. There are 16 living species distributed throughout Europe, Asia and Africa. They have been introduced to New Zealand and are sometimes kept as pets in the United States and Canada. Wild hedgehogs are beneficial to humans, as they eat many pests, including insects, snails, slugs, mice and poisonous snakes.
Hedgehogs' characteristic spikes are made of keratin, the same substance that forms human fingernails and hair. These spikes are fairly soft in babies but hard in adults. When the animal is threatened, it curls up into a ball so that only the spines are exposed to a predator. While this behavior is similar to that of some porcupines, a hedgehog's spines are not barbed like a porcupine's, nor can they be easily detached from the animal's skin.