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Skull evolution: Why some mammals have long faces

Why do large mammals tend to have much longer faces compared to smaller closely related species? A new study suggests that this phenomenon, known as Craniofacial Evolutionary Allometry (CREA), can be explained by feeding adaptations.

The research, led by Dr. Rex Mitchell from Flinders University, represents a major step in understanding the evolution of mammal skulls. 

Skull shape evolution 

“The patterns behind the diversity of mammalian morphology have been a core interest of western science for centuries. Of particular interest for most of this venerable history has been the evolution of the mammalian cranium (the skull without the lower jaw), which reflects the diversity of mammals like no other part of the skeleton,” wrote the researchers.

“Crania incorporate the brain and the majority of an animals’ sensory apparatus, such as the eyes, nose, and ears. Perhaps more importantly, their role in the ingestion of food makes the functional adaptation of the cranium a matter of day-to-day survival, so that cranial morphology offers insights into a given mammal’s feeding ecology and behavior.”

“The patterns of mammalian cranial shape evolution have thus become a shorthand for understanding how mammalian diversity has evolved.” 

Mammalian cranial diversity

The researchers explained that recent advances in technology have revolutionized our capacity to characterize mammalian cranial diversity patterns. 

In a comprehensive review of mammalian skull evolution, the team discovered that feeding adaptations are a key driver behind the varying face lengths observed in different mammal species. 

The CREA pattern 

“The CREA pattern is observed in a diverse range of mammals, from cats to mice, antelopes to baboons, and deer to kangaroos, yet its cause remained elusive until now,” said Dr. Mitchell.

Study senior author Professor Vera Weisbecker noted the complexity of this pattern, citing exceptions among certain mammals. 

“Confusingly, there are also many mammals where the rule does not hold up or is even reversed,” said Professor Weisbecker. “Australian carnivorous marsupials are a great example. We found that their largest living member, the Tasmanian devil, has an exceptionally short face compared to other species.”

Naturally strong bite

Dr. Emma Sherratt from The University of Adelaide said that the team went back to basic “lever mechanics” to explain both the rule of the long face and its exceptions.

The analysis revealed that larger mammals generally possess a naturally stronger bite due to their larger muscles. This strength reduces their reliance on short faces for effective feeding. 

By contrast, smaller mammals benefit from shorter faces, which enable them to exert more bite force, crucial for consuming the same types of food as their larger counterparts.

“Closely related mammals tend to eat similar foods. For example, different kangaroos species all eat grass or leaves and different species of wolves and foxes all eat meat. However, bigger animals of a group have a naturally stronger bite because their skull, teeth and jaw muscles are simply bigger,” explained Dr. Sherratt.

Drastic dietary shifts 

However, the team also noticed significant deviations from this trend linked to diet changes. In cases where smaller mammals have longer faces than their larger relatives, a drastic shift in dietary habits is often observed. 

“Our analyses of 22 mammal families show that there is almost always a radical change in diet when smaller mammals have longer faces than their larger relatives,” said Professor Weisbecker.

“For example, Tasmanian devils are known to crush bones, and the short-snouted orcas feed on much larger prey than their fish-eating dolphin relatives. These behaviors need higher bite forces.”

“But this also works the other way around. For example, the tiny honey possum has a much longer snout than would be expected for any possum, but it also mostly licks nectar from deep inside flowers and does not need to bite hard.”

Humans are the main exception

The researchers agree that humans are probably an exception among exceptions when it comes to cranium evolution.

“Humans do not use their snouts as a primary means of obtaining food. They use their hands and tools to manipulate can soften their food through cooking,” said Dr. Mitchel. “This means that the evolution of the human face is probably more influenced by other things than their ability to bite.”

The research is published in the journal Biological Reviews.

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