Hedgehogs are known for their formidable armour – a densely packed set of keratin spines across their backs. When they feel threatened they curl up into a spiky ball to protect their vulnerable head and stomach from predators.
Each of their ~5,000 spines lasts for about a year before it drops out and is replaced by a new one. The spines are hollow and connected to muscles which help to hold them straight, but they can also bend at the base to protect the hedgehog from impacts without snapping, like shock absorbers.
Baby hedgehogs – called hoglets – initially have soft spines under their skin (to protect their mother during the birth), before a second set emerges a few days later.
Hedgehogs are known to coat their spines in a foamy saliva as part of a peculiar self-anointing habit. This behaviour can be triggered by strong smells, new foods or the presence of other hedgehogs. It is not known why they do this, but theories include scent camouflage, protection against toxins or to attract potential breeding partners.