Rat Study: ‘If the Skull Ain’t Broken, Don’t Fix It’

Associate Professor Vera Weisbecker holding a rodent skull at Flinders University paleontology lab. Flinders UniversityIn general, the more diverse a species becomes, the better chance it has to adapt to different environments. That’s why a team of researchers were surprised to find that rat skulls haven’t changed all that much in the last 10 million years.
Diversity Rules—or Does it?
Biological diversity is a critical component of life on earth. Through evolution, animals gain adaptations to help them thrive in specific environments and lose them to succeed in others. These adaptations can be as simple as a color change or as complex as a reworked skeleton.
Considering that rats have been living in Australia for around four million years, the researchers assumed that gave them plenty of time to adapt biologically to different surroundings.
Morphometrics to the Rescue!
So, scientists from Australia’s Flinders University and The University of Queensland used 3-D surface scanners to scan the skulls of hundreds of rodents from museums. Thirty-eight species in all were represented. Then they used a process called geometric morphometrics—a combination of geometry and statistics—to analyze the scans. They found that skulls varied by size, but not much else; all the skulls were basically identical if you removed size as a factor.
“Because well-adapted skulls are key to the survival of mammals, we expected to find a lot of locally adapted skull shapes,” said Dr. Ariel Marcy, from The University of Queensland. “What we found was the opposite of what we expected: there was low variation in the skull shape of rodents, and body size explained most of it. Native rodents just scale from being a small ‘mouse’ shape to being a bigger ‘rat’ shape!
“And this relationship between skull shape and size is at least ten million years old, because invasive rodents—like the house mouse and Norway rat—share this pattern, too,” she added, referring to rat species that have been around for a long, long time.
Marcy and Flinders University Associate Professor Vera Weisbecker, who supervised the study,  believe the lack of change is because the rat’s skull just works, especially in terms of its jaw.
“Rodent skulls and jaws have a complicated yet highly versatile arrangement that seems to work well in a multitude of conditions,” said Marcy. “We think that this discourages evolutionary change. We saw unusual skull shapes only in extreme cases of ecological adaptation, for example, in the water mouse or rakali, which is a very unusual meat-eating predatory rodent.”
If it Ain’t Broke
Put another way, says Marcy:
“It seems intuitive that a group of animals that displays a wide variety of shapes should be more successful in evolution. However, Australian rodents demonstrate that shape diversity doesn’t always mean evolutionary success. So it really does show if the skull ain’t broke, don’t fix it.”
The research, which helps clarify the evolutionary biology question of why some species are diverse, and others aren’t, has been accepted for publication in the peer-reviewed journal, American Naturalist.

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