Images of how much fat a grass-fed bison has (carcass photos):
Tribe of Five: Hunting for Good Food and Roaming Bison
Grass-fed/wild meats do have less intra-muscular marbling, but we need to remember that HG groups don't eat just the muscle meat, unlike many North Americans. They use most of the animal, including rendering the fats, so although the muscle meat itself is leaner, the animals still store a good lot of fat. This is especially true for game animals in cold climates--they're lean in the spring, but by fall, the deer are nice and fat for winter, as are the rabbits, squirrels, and so on. Also, anyone who's cooked more traditionally cut meats will discover that a grass-fed cow still has enough fat to make a nice cushion of fat on top of the roast to keep it moist when you cook it, as long as your butcher doesn't trim all that good stuff off.