The Fact That Shouldn’t Be a Fact
A horse’s face is longer than its bite actually requires, and so is a baboon’s, a red deer’s, a kangaroo’s, and a big cat’s, each one measured against its own smaller relatives. Within almost any group of closely related mammals, the larger species tends to have a proportionally longer face and a smaller braincase than its smaller cousins. Biologists call this Craniofacial Evolutionary Allometry, or CREA: a scaling pattern, meaning a trait that changes predictably with body size. Dr. Rex Mitchell led a team with Associate Professor Vera Weisbecker, both at Flinders University, and Dr. Emma Sherratt of the University of Adelaide, publishing a review of the pattern in Biological Reviews in 2023. What made CREA strange before this paper wasn’t that it existed. It was that it showed up in animals with almost nothing else in common, from mice to antelopes to kangaroos, and nobody had explained why.
The Barbecue Tongs
A short face is a mechanical shortcut to a hard bite. Picture barbecue tongs: grip them near the hinge and you can crush a stubborn piece of charcoal; grip them near the tips and the same squeeze barely dents a marshmallow. A jaw works the same way. The jaw muscles sit near the hinge, and the teeth do their work out at the far end of that lever arm, the distance between the pivot point and where the force lands. Shorten the face and you shorten that arm, so the same muscle effort turns into more force at the teeth. That’s why a short-faced animal can out-bite a long-faced one of similar size.
But size itself changes the equation. As a species gets bigger, its skull, teeth, and jaw muscles all scale up together, and bite force climbs along with them, no lever trick required. A big animal gets a strong bite for free. A small one doesn’t have that luxury: if it needs to crack the same tough food as its larger relative, a short face is one of the few ways it can get there.
What the Extra Face Is Actually For
Once an animal is big enough that bite force stops being a problem, the face is free to do other jobs. Mitchell, Weisbecker, and Sherratt’s review describes a longer snout as an advantage in its own right for large herbivores, letting them reach further into tree canopies or gather a bigger mouthful of vegetation in a single bite, exactly the kind of face a horse carries. For large predators, the same extra length can mean a faster jaw closure, useful for snapping up small, quick prey before it escapes.
Small relatives don’t get offered this deal. They still need every bit of bite-force advantage a short face provides just to process the same food, so they stay compact regardless of what a longer snout might otherwise let them reach or catch. The long face, in other words, isn’t a design flourish. It’s what happens the moment a constraint lifts and an animal is finally big enough to spend the surplus on something else.
The Exceptions That Prove It’s Not Magic
CREA is a default, not a law, and the exceptions make that clear. The Tasmanian devil (Sarcophilus harrisii), the largest living member of its carnivorous marsupial family, has an unusually short face for its size. Weisbecker’s team points to diet: devils crush bone, and bone-crushing demands the bite force a short face provides, size advantage or not. Orcas (Orcinus orca) tell the same story from the ocean: they have noticeably shorter snouts than their fish-eating dolphin relatives, because taking down much larger prey calls for a harder bite than snapping up fish does. Then there’s the honey possum (Tarsipes rostratus), tiny by any measure, carrying a long snout anyway. It feeds by licking nectar, so it never needed a strong bite in the first place, and the usual small-animal rule simply doesn’t apply.
The researchers found that across the 22 mammal families they examined, exceptions to CREA line up almost every time with a real shift in diet. That kills the idea, floated before this review, that long faces in big mammals are just an automatic side effect of growing larger, a passive rule with no mechanism behind it. The cases where CREA fails to show up are exactly the cases where the mechanical logic of feeding points somewhere else.
The One Species the Rule Goes Quiet On
Humans, the authors note, are probably an exception among exceptions. We don’t use a snout to get food in the first place; we use hands, tools, and cooking to do the work a jaw and a strong bite would otherwise have to do. That removes the whole mechanical pressure the rest of CREA runs on. What actually did shape the length of the human face, if bite mechanics mostly stepped out of the picture, is a question the review doesn’t answer. That gap is left for future work to fill.