Why do hedgehogs roll into balls?

Answer: They tuck in soft parts and point sharp spines outward to deter predators.

They stretch their back muscles by curling to keep their bodies limber. — Wrong. Curling can look like a stretch, but it is not exercise for the back muscles. The same tight ball is a defensive posture that brings the spines between the hedgehog and a predator. A relaxed stretch would not hide the soft belly or face.

They tuck in soft parts and point sharp spines outward to deter predators. ✓ — Correct! A hedgehog rolls into a tight ball when threatened, using special muscles to pull its body inward. The soft belly and face stay protected while the sharp spines cover the outside in every direction. The spines are modified hairs, and predators such as badgers and foxes must learn how to deal with them.

They curl tightly to hold in body heat when the weather turns cold. — Wrong. Hedgehogs do curl up when they hibernate, so cold-weather curling is not imaginary. But the threatened ball is mainly anti-predator defense, not a way to control body temperature. It puts the spines outward and keeps the soft belly and face inside.