Millions of years ago, humans lived out on the open plains and tundra. We hunted and gathered and braved the elements. Despite the use of shelters and fire, winter life was a lot colder back then. Because of this, humans packed on a few extra pounds when winter approached. A little excess fat reserve was necessary for us to keep warm and get us through leaner months when food was scarcer. All these years later, it's still natural for humans to gain a little weight in the winter. Our bodies need to generate more heat to keep warm, forcing our cells to operate on overdrive. Humans supply the fuel by eating a little more. This is why you may notice being hungrier when you're consistently out in cold weather. Although we don't hibernate, it's a similar concept that many other mammals experience during the winter in order to survive the elements.