Longer necks for eating high leaves did evolve in other species! Think of all the sauropod dinosaurs, or more recently, of the giant herbivorous long-necked bir

Longer necks for eating high leaves did evolve in other species! Think of all the sauropod dinosaurs, or more recently, of the giant herbivorous long-necked birds (elephant birds, moas) that humanity drove to extinction. So the question is rather why so few mammals evolved long necks. Maybe there's something about mammalian physiology that makes long neck mutations usually unviable?