A jaw-dropping conundrum: Why do mammals have a stiff lower jaw?

June 23, 2023

From the 20-foot-long jawbones of the filter-feeding blue whale to the short, but bone-crushing, jaws of the hyena and the delicate chin bones of a human, the pair of lower jawbones characteristic of mammals have evolved with amazing variation.

But at first glance, having a single bone on each side of the head — which creates a stiff lower jaw, or mandible — doesn’t appear to give mammals an advantage over other vertebrates, which have at least two and as many as 11 bones comprising each side of the lower jaw.

Crocodiles, for example, have an edge over hyenas when it comes to their bite strength relative to size, despite having around five bones on each side of the jaw. Snakes, which have an articulated lower jaw with around four bones, are able to open their mouths larger for their size than baleen whales and actually dislocate their jaws to ingest prey larger than their heads. Even extinct hadrosaurs, or duckbill dinosaurs, with six bones in their jaw, could masticate plants using oral movements that were more complex than those of today’s cows.

So what advantage, if any, did two single jawbones — which in humans and other primates are fused at the chin into one solid mandible — give mammals?

UC Berkeley doctoral student Sergio Garcia Lara, a co-author of the paper, stands next to a gray whale jawbone while holding a glass vial containing a bat jaw. The comparison highlights the variety of jaw sizes and shapes that have evolved in mammals, despite the fact that they reduced the number of bones in the lower jaw to one per side, unlike most other vertebrates. (Photo credit: Jack Tseng)

That question motivated paleontologist Jack Tseng, assistant professor of integrative biology at the University of California, Berkeley, to construct a database of more than 1,000 vertebrate jaws — a small fraction of the approximately 66,000 living jawed vertebrate species on Earth — to systematically study whether mammalian jaws were a big advance over the multiply-boned jaws of fish, lizards, snakes and other non-mammals. He even printed 3D models of the lower jaws of many to test their sturdiness.

Berkeley News