The head and body of adult hedgehogs range from 5 to 12 inches (13 to 30 centimeters) long and their tail can add 1 to 2 inches (3-5 centimeters).
When hedgehogs are born—up to seven in a litter—their spines are soft and short. Soon after birth, their spines harden, becoming stiffer, sharper, and longer. Babies stay in the nest until they're about three weeks old. By that time, their eyes are open, their spines are effective, and they can safely follow their mother outside the nest as she looks for food.
Hedgehogs sometimes add extra protection to their spines by "self-anointing." Immune to poisons in some plants, hedgehogs sometimes eat those plants and then make a frothy saliva in their mouths. The hedgehogs then lick their spines, spreading the saliva with the plant's poison all over the spikes. Though hedgehogs mainly stay on the ground, they swim quite well and even climb trees.