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

Newswise — The incredible diversity in the structure of mammalian jaws, ranging from the immense jawbones of filter-feeding blue whales to the formidable jaws of hyenas, as well as the delicate chin bones of humans, highlights the remarkable evolution of this characteristic feature.

At first glance, the presence of a single bone on each side of the head, forming a rigid lower jaw or mandible in mammals, may not seem advantageous compared to other vertebrates, which possess two to eleven bones constituting each side of the lower jaw.

For instance, crocodiles exhibit superior bite strength relative to their size compared to hyenas, despite having approximately five bones on each side of their jaws. Snakes, with an articulated lower jaw consisting of about four bones, can open their mouths wider relative to their size than baleen whales and even dislocate their jaws to consume prey larger than their heads. Even the extinct duckbill dinosaurs, known as hadrosaurs, had six jaw bones and could masticate plants with more complex oral movements than present-day cows.

So, what advantage, if any, does the presence of two single jawbones provide to mammals? In the case of humans and other primates, these two jawbones fuse at the chin, forming a single solid mandible. This intriguing question spurred paleontologist Jack Tseng, an assistant professor of integrative biology at the University of California, Berkeley, to compile a database of over 1,000 vertebrate jaws—representing a small fraction of the approximately 66,000 living jawed vertebrate species on Earth—to systematically investigate whether mammalian jaws offered a significant advantage over the multi-boned jaws of fish, lizards, snakes, and other non-mammals. To test the strength of these jaws, he even created 3D models of many lower jaw specimens.

Surprisingly, the findings indicate that a lone lower jawbone on each side does not confer a substantial advantage over a jaw composed of multiple bones.

Tseng argued against the notion that the evolution of the mammalian jaw was solely an adaptation that contributed to the success of mammals after the extinction of dinosaurs. According to Tseng, the advantage of having multiple bones in the jaw lies in the increased biting capability of an animal. These bones work in tandem, providing flexibility and speed. Conversely, mammals, with only a single bone per side, seemingly have restricted evolutionary options. However, this limitation hasn't hindered mammals from adapting to a diverse range of diets, rivaling the dietary variety of vertebrates with multiple jaw bones.

Tseng explained the trade-off between the additional flexibility and speed offered by multiple jaw bones and the increased stiffness and bite strength associated with a single bone in the jaw. This contrast between mammals and non-mammals supposedly enabled mammals to become versatile eaters.

Nevertheless, this idea had never been rigorously tested until now, Tseng noted. No previous attempts had been made to integrate information from various groups of jawed vertebrates in order to examine the general relationship between jaw shape and function.

In conclusion, Tseng stated that the structure of the jaw in vertebrates is less directly linked to its function than commonly believed.

Tseng highlighted the contrasting characteristics between mammal jaws and non-mammal jaws, emphasizing their shape diversity and biomechanical constraints. According to Tseng, mammal jaws exhibit a wider array of shapes compared to non-mammal jaws, but these varied shapes have a narrower range of mechanical properties.

This observation represents a novel insight that could potentially revolutionize the study of mammal jaw biomechanics. Tseng explained that the main discovery was that mammal jaws, despite their single-boned lower structure, consistently possess significantly higher strength or stiffness in comparison to any non-mammal jaw. This finding holds true across all types of mammals, irrespective of their dietary preferences as carnivores or herbivores. Stiffness, therefore, emerges as a defining trait of mammalian jaws rather than being exclusive to predators or herbivores.

Tseng's study was published recently in the journal Philosophical Transactions of the Royal Society B, as part of a series exploring the evolutionary aspects of the mammalian skull.

Shifting our focus from the jaw to the ear

So why did mammals undergo a reduction in the number of bones in their lower jaw? Well, they didn't exactly lose those extra bones. Instead, the additional bones that were present in the lower jaw of vertebrates, concentrated around the hinge connecting the lower and upper jaw, evolved into the intricate inner ear of mammals. This evolutionary transition may have granted mammals superior hearing capabilities compared to their vertebrate relatives.

According to Tseng, the development of a solid and stiff jaw in mammals is believed to be a byproduct of establishing a distinctively mammalian auditory system.

As these jaw bones were co-opted and repurposed for the ear, mammals ended up with only one bone in the lower jaw per side. This resulted in a rigid jaw structure that provided certain advantages in terms of stiffness, enabling mammals to, for instance, crack bones. However, it also imposed limitations on their descendants, confining their jaw variations to modifications of a single bone, even when a stiff lower jaw was not necessary for consuming soft food. An illustrative example is the anteater, which evolved a downward-curving jaw that serves as a slot for their elongated tongue to slide through.

Until now, the focus of research on the major evolutionary transition in mammals has predominantly revolved around the complex inner ear rather than the jaw. However, Tseng, known for studying bone-crushing creatures like the hyena, aimed to approach the question from the perspective of the jaw and an engineering standpoint. In order to achieve this, he digitally analyzed two-dimensional jaw shapes from over 1,000 vertebrate species, identified the crucial characteristics of vertebrate jaws, and then simulated the mechanical performance of various jaw shapes. This included both natural and hypothetical shapes, with the aim of assessing the strength and function of mammals and non-mammals across the entire spectrum of potential jaw configurations. Tseng discovered that both groups encompassed a wide range, indicating their ability to adapt to similar levels of strength and function. Nevertheless, mammalian jaws tended to cluster more around rigid shapes compared to non-mammalian jaws.

Tseng intends to expand his database by incorporating more species of vertebrates, while also integrating 3D scans of jawbones to enhance the biomechanical evaluation of stiffness and strength. Additionally, he hopes that other researchers will delve into the genetic mechanisms underlying the transition in mammals towards a complex ear structure and a simpler bone structure in the lower jaw. Understanding the implications of this transition for mammalian evolution, as well as why this particular jaw trait appears to be entrenched in mammals, is of great interest. Tseng expressed his desire for the findings to inspire investigations into the genetic foundations responsible for this unidirectional progression. Furthermore, he highlighted the importance of comprehending how the detachment of structure and function in mammalian jaws facilitated their adaptation to new environments during significant geological periods, such as the extinction of non-avian dinosaurs and the emergence of land bridges connecting continents, which facilitated the mixing of diverse ecological communities.