Newborn babies don't shiver. It turns out, they just don't need to.
Older children and adults shiver when they're cold as a way to create heat. Shivering causes muscles to expand and contract really quickly, which in turn, creates warmth. But babies have more of a type of fat that keeps them warm instead.
This brown adipose tissue, a.k.a. brown fat, burns calories and releases heat. Babies have this fat around their necks, chests, backs, and butts and it acts kind of like a big scarf to keep them warm.
While babies lose most of their brown fat within their first few years, adults still have some brown fat. One study published in the New England Journal of Medicine found that thinner people tend to have more brown fat than heavier people do. Unlike white fat, which stories calories, brown fat burns calories and turns them into heat.
Hibernating mammals also use brown fat to keep themselves warm.
But this brown fat isn't enought to keep babies warm, of course. They still need to be bundled up when it's cold.
The American Academy of Pediatrics recommends dressing babies in one more layer than an adult would wear when venturing out in frigid temps, and in a wearable blanket or warm one-piece sleeper at night.
