American Society of Naturalists

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“Australian rodents reveal conserved Cranial Evolutionary Allometry across 10 million years of murid evolution”

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Ariel E. Marcy, Thomas Guillerme, Emma Sherratt, Kevin C. Rowe, Matthew J. Phillips, and Vera Weisbecker (Dec 2020)

A size-shape pattern going back ~10 million years in Aus rodents: give me the mouse size, I’ll give you the skull shape!

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Australia’s smallest rodent, the molinipi (<i>Pseudomys delicatulus</i>), considers one of Australia’s largest rodents, the otter-like rakali (<i>Hydromys chrysogaster</i>). They share a skull shape gradient that goes back further than either species’ arrival to their shared continent.<br/>(Illustration by Alison K. Carlisle, Papadore Illustrations)
Australia’s smallest rodent, the molinipi (Pseudomys delicatulus), considers one of Australia’s largest rodents, the otter-like rakali (Hydromys chrysogaster). They share a skull shape gradient that goes back further than either species’ arrival to their shared continent.
(Illustration by Alison K. Carlisle, Papadore Illustrations)

Rodents (mice and rats) are the only land-based placental mammals in Australia, which could have given them an opportunity to evolve unique body shapes compared to relatives on other continents. To investigate this, Dr. Ariel E. Marcy and colleagues 3D-scanned skulls for 38 species with a range of diets, habitats, and body sizes (6 grams to 1 kilogram). Despite their ecological diversity, Australian rodents all scaled their skull shapes along the same size-shape gradient. In other words, the shape of any skull in the study can be closely predicted by the rodent’s body size. This is also true for invasive species, like the house mouse, which has been separated from native rodents by over 10 million years of evolution.

As Australian rodents scale along their size-shape gradient, they follow a newly proposed shape pattern rule for mammals: larger “rat-like” species have longer snouts and smaller braincases compared to smaller “mouse-like” species. The authors suggest that Australian rodents follow this rule (called CREA) to keep the skull proportions they need for gnawing. If this is the case, then stabilizing selection on this adaptation likely limits the diversity of skull shapes in rodents in Australia and elsewhere in the world. This strict size-shape gradient makes rodents an ideal case to explore what causes the “CREA” shape pattern and how these causes compare to other groups of mammals with greater shape diversity. Australian rodents could help us approach one of the oldest evolutionary questions: why are some animal groups more diverse than others?


Among vertebrates, placental mammals are particularly variable in the covariance between cranial shape and body size (allometry), with rodents a major exception. Australian murid rodents allow an assessment of the cause of this anomaly because they radiated on an ecologically diverse continent notably lacking other terrestrial placentals. Here we use 3D geometric morphometrics to quantify species-level and evolutionary allometries in 38 species (317 crania) from all Australian murid genera. We ask if ecological opportunity resulted in greater allometric diversity compared to other rodents, or if conserved allometry suggests intrinsic constraints and/or stabilizing selection. We also assess whether cranial shape variation follows the proposed “rule of craniofacial evolutionary allometry” (CREA), whereby larger species have relatively longer snouts and smaller braincases. To ensure we could differentiate parallel versus non-parallel species-level allometric slopes, we compared the slopes of rarefied samples across all clades. We found exceedingly conserved allometry and CREA-like patterns across the 10 million year split between Mus and Australian murids. This could support both intrinsic constraints and stabilizing selection hypotheses for conserved allometry. Large-bodied frugivores evolved faster than other species along the allometric trajectory, which could suggest stabilizing selection on the shape of the masticatory apparatus as body size changes.