"The origin of cuddling ... gave some beasts an evolutionary edge. Living in tree holes and burrows, out of sight of hungry reptiles, brought mammals into close physical contact. That shared warmth was a boon to offspring that often have difficulty staying warm on their own. Staying safe led to staying warm, locked in through natural selection as a behavioral trait that has made mammals more mindful parents than some of the terrible reptiles roaming the world. Affection and survival have become intertwined."
-- Riley Black, The Last Days of the Dinosaurs