The classic line "Why the long face?" is usually asked to someone who seems to have a bad day. But beyond that, a new scientific review, titled "Facing the facts: adaptive trade-offs along body size ranges determine mammalian craniofacial scaling" published in Biological Reviews, explains that horses and other large mammals have elongated faces due to evolutionary advantages.
The team of researchers from Flinders University and the University of Adelaide explore this phenomenon as it plays a significant role in mammal evolution studies, as extended facial features often correlate with larger body sizes within specific animal groups.
Unraveling Craniofacial Evolution Across Mammalian Groups
The phenomenon termed "craniofacial evolutionary allometry" (CREA), observed across diverse mammalian groups like cats, rodents, deer, kangaroos, and some monkeys, manifests in the relative lengthening of faces as body size increases. Instances of CREA become apparent when comparing different animals, such as a sheep's face to a cow's or a small deer's to a gigantic moose's.
Despite its prevalence, the explanation for CREA remains elusive. One proposed idea suggests that the CREA pattern might be an inherent aspect of skull development, wherein a mammal's face automatically elongates as it grows. However, this explanation faces challenges, as instances of CREA pattern absence or reversal exist.
Animals like Tasmanian devils, sea otters, and orcas, larger than their relatives, possess shorter faces, while small-sized long-nosed potoroos, honey possums, and nectar-feeding bats have elongated faces.
The researchers suggest that the biomechanics of how species use their faces for eating provides a plausible explanation for CREA's commonality. Closely related animals with similar diets exhibit similar craniofacial patterns.
For instance, sheep and cows both consume grass, but due to their smaller size, sheep need to exert more force when biting, favoring shorter faces that enhance biting efficiency.
Larger mammals, benefiting from more substantial muscles, can afford longer faces, advantageous for herbivores to reach more leaves or take larger mouthfuls, and for carnivores to accommodate larger fangs or facilitate faster jaw closure.
Exceptions and Dietary Influences in CREA
The proposed explanation for CREA offers insights into its exceptions, often linked to significant shifts in diet. A prime illustration is evident in the canine family, housing both small-prey hunters like foxes and large-prey hunters like wolves. While both categories adhere to CREA within their dietary groups, larger individuals within a fox group manifest longer faces.
Interestingly, even though wolves generally surpass foxes in size, their faces are comparatively shorter than the largest foxes, underscoring the influence of hunting larger prey on facial morphology.
Exceptions to CREA are anticipated in species where facial structures play a minimal role in food acquisition or breakdown. Notably, humans stand out as an example, possessing relatively short faces in comparison to a substantial braincase.
This adaptation aligns with the fact that humans don't rely on facial features for obtaining sustenance; instead, our hands, tools, and cooking capabilities serve this purpose.
The findings not only provide a novel framework for comprehending variations in face length across mammalian groups but also open avenues for understanding the feeding habits of extinct megafauna, particularly in Australia.
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