The name "Hedgehog" came into use around the year 1450, derived from the Middle English word "heyghog" = hedge, because it frequents hedgerows, and "hoge" - hog, from its piglike snout. Hedgehogs 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. Hedgehogs are easily recognized by their spines, which are hollow hairs made stiff by keratin. Their spines are not poisonous or barbed and, unlike the porcupine, cannot be easily removed from the hedgehog. However, when under extreme stress or during sickness, a hedgehog can lose spines. There are about 5,000 to 7,000 quills on the average hedgehog, these are durable on the outside, while being filled with air pockets on the inside. A defense that all species of Hedgehogs possess is the ability to roll into a tight ball, causing all the spines to point outward. However, the effectiveness depends on the number of spines. All Hedgehogs possess the stamina to run, many can make 4.5 miles in an hour. Like mice and moles, they have some natural immunity to snake venom. Hedgehogs are primarily nocturnal, although, depending on the species they may be more active during the day. They sleep a large portion of the day under cover of a bush, grass, rock, or hole in the ground. All wild Hedgehogs can hibernate, but not all of them do; hibernation depends on temperature, species, and abundance of food. They mainly feed on insects, snails, frogs, snakes, eggs, grass roots, and berries.