American bison in Yellowstone National Park, Wyoming, USA

Bundle up, bison. American bison

For an American bison, like these in Yellowstone National Park in the USA, winter is the season to slow down, plump up and let your hair grow long. But even when they are actively looking to put on a few pounds, bison still go for the salad. Grazers of grass and other wild vegetation, bison are ruminants, hoofed herbivores with special digestive systems that process and metabolise food extremely slowly, squeezing out as many calories as possible. If extra fat is hard to come by, they can rely on their extra-thick winter fur, often seen frosted with snow, as even these giants' body heat can't escape their hardy, woolly coat.