Why do mammals curl up during hibernation?

Mammals hibernate to survive the lack of food in winter and extreme cold. For this, the animal prepares in advance a warm, wind-protected place where it will comfortably spend the winter. When hibernation occurs, the animal curls up into a ball to protect the most important organ from hypothermia – the heart, because mammals have thicker and warmer fur on their backs, but the stomach remains less warmed. Therefore, the animal, at the level of instincts, curls up into a ball and protects its weak point.