A horse walks into a bar: an explanation for the long face

It is a common observation that most larger (over around 5–10 kg) herbivorous mammals have longer faces than carnivorous ones. A horse has a relatively longer face than a lion, or even than a wolf. This difference in face length is not the case for smaller mammals: rabbits and ferrets have similarly short faces. A shorter face bestows a mechanical advantage at the front of the jaw, advantageous for cropping forage as well as killing prey. Why is this ability sacrificed in long-faced larger herbivores? We propose that longer faces relate to the ability to use the forelimbs in feeding; for reasons of allometric scaling and locomotor specialization, forelimb mobility becomes restricted in larger animals, and a longer face becomes important for food prehension. We test this hypothesis with a large dataset of face lengths and anatomical assessment of forelimb mobility in extant mammals (including a few extinct equids).