American bison in Yellowstone National Park, Wyoming

© Steve Gettle/Minden Picture

Bundle up, bison. American bison

For an American bison, winter is the season to slow down, plump up, and let your hair grow out. Relatable, sure, but these massive mammals get there without the aid of beer, chips, and a couch.

In fact, even when looking to put on a few pounds, bison still order the salad. Grazers of grass and other wild brush, bison are ruminants with specialized digestive systems that process and metabolize food extremely slowly, squeezing as many calories as possible out of a few bites of rabbit food. If extra fat is hard to come by, they can also rely on their extra-thick winter fur—often seen frosted with snow, since even these giants' body heat can't escape the hardy, woolly coat.