A hedgehog is any of the spiny mammals of the subfamily Erinaceinae and the order Erinaceomorpha. There are 17 species of hedgehog in five genera, found through parts of Europe, Asia, Africa, and New Zealand (by introduction). There are no hedgehogs native to Australia, and no living species native to the Americas (although fossils of at least four extinct species were discovered in South America). Hedgehogs share distant ancestry with shrews (order Soricidae), with gymnures possibly being the intermediate link, and have changed little over the last 15 million years. Like many of the first mammals they have adapted to a nocturnal, insectivorous way of life.
The name 'hedgehog' came into use around the year 1450, derived from the Middle English 'heyghoge', from 'heyg', 'hegge' = hedge, because it frequents hedgerows, and 'hoge', 'hogge' = hog, from its piglike snout. Other names include 'urchin', 'hedgepig' and 'furze-pig' .
Hedgehogs are easily recognized by their spines, which are hollow hairs made stiff with keratin. Their spines are not poisonous or barbed and, unlike the quills of a porcupine, cannot easily be removed from the hedgehog. However, spines normally come out when a hedgehog sheds baby spines and replaces them with adult spines. This is called "quilling." When under extreme stress or during sickness, a hedgehog can also lose spines.
A defense that all species of hedgehogs possess is the ability to roll into a tight ball, causing all of the spines to point outwards. However, its effectiveness depends on the number of spines, and since some of the desert hedgehogs evolved to carry less weight, they are much more likely to try to run away and sometimes even attack the intruder, trying to ram into the intruder with its spines, and rolling as a last resort. This results in a different number of predators for different species: while forest hedgehogs have relatively few, primarily birds (especially owls) and ferrets, smaller species like the Long-eared Hedgehog are preyed on by foxes, wolves and mongooses.
Hedgehogs are primarily nocturnal, although, depending on the species, they may be more or less active during the day. The hedgehog sleeps for a large portion of the daytime either under cover of bush, grass, rock or in a hole in the ground. Again, different species can have slightly different habits, but in general hedgehogs dig dens for shelter. All wild hedgehogs can hibernate, although not all do; hibernation depends on temperature, species, and abundance of food.
