An African rodent keeps ancient armor on its tail

While uncommon in mammals, osteoderms are widespread in other groups of vertebrates, including reptiles, fish, and even amphibians.

While uncommon in mammals, osteoderms are widespread in other groups of vertebrates, including reptiles, fish, and even amphibians. -EDWARD STANLEY

May 24. (EUROPE PRESS) –

African spiny mice maintain an armor-like structure under the skin of their tails, something that was considered unique to the armadillo among living mammal species.

The unexpected discovery was made during a routine CT scan of museum specimens for the program openVertebrate, an initiative to provide 3D models of vertebrate organisms for researchers, educators, and artists.

“I was scanning a mouse specimen from the Yale Peabody Museum, and the tails looked abnormally dark,” he said. it’s a statement Co-author Edward Stanley, director of the Digital Imaging Laboratory at the Florida Museum of Natural History.

He initially assumed this was caused by an imperfection introduced during the specimen’s preservation. But when he reviewed the X-rays several days later, Stanley noted a distinctive feature with which he was intimately familiar.

“My entire PhD was focused on the development of osteoderms in lizards. Once the sample scans were processed, the tail was clearly covered with osteoderms.”

Spiny mouse osteoderms have been observed at least once before and were noted by German biologist Jochen Niethammer, who compared their architecture to medieval stonework in a paper published in 1975. Niethammer correctly interpreted plates as a type of bone, but never followed up on his initial observations, and the group was largely overlooked for several decades, until scientists discovered another seemingly unrelated quirk of spiny mice.

A 2012 study showed that spiny mice can completely regenerate injured tissue without scarring, an ability common in reptiles and invertebrates but previously unknown in mammals. Their skin is also particularly fragile, tearing about a quarter of the amount of force required to injure the skin of an ordinary mouse. But spiny mice can heal twice as fast as their relatives.

Researchers hoping to find a model for human tissue regeneration have begun to map the genetic pathways that give spiny mice their extraordinary healing powers. One of those researchers, Malcolm Maden, had a lab in the building across from Stanley’s office.

“Spiny mice can regenerate skin, muscles, nerves, spinal cord and perhaps even heart tissue, so we maintain a colony of these rare creatures for research,” said Maden, a professor of biology at the University of Florida and lead author of the study.

Maden and his colleagues analyzed the development of spiny mouse osteoderms and confirmed that they were indeed similar to those of armadillos, but that they had probably evolved independently. Osteoderms are also different from the scales of pangolins or the quills of hedgehogs and porcupines, which are made of keratin, the same tissue that makes up hair, skin, and nails.

There are four genera of spiny mice, all belonging to the Deomyinae subfamily. However, aside from the similarities in their DNA and potentially in the shape of their teeth, scientists have not been able to find a single characteristic shared among the species in this group that distinguishes them from other rodents.

Stanley, suspecting that their differences might only be superficial, scanned additional museum specimens of all four genera. In each, he found spiny mouse tails covered in the same bone sheath. The Deomyinae’s closest relatives, the gerbils, lacked osteoderms, meaning the trait had likely evolved only once, in the ancestor of the ancient disparate spiny mice.

The ubiquity of osteoderms in the group indicates that they fulfill an important protective function. It wasn’t immediately apparent what that function might be, however, due to another peculiar attribute of spiny mice: Their tails are unusually detachable. Tail loss is so common in some species of spiny mice that nearly half of the individuals in a given population have been shown to lack them in nature.

“This was a real head scratcher,” Stanley said. “Spiny mice are notorious for removing their gloves from their tails, which means the outer layer of skin is shed, leaving behind muscle and bone. People often chew the rest of the tail when this happens.”

Despite its regenerative powers, moulting its tail is a trick spiny mice can only perform once. Unlike some lizards, they cannot regrow their tails, and not all tail parts detach easily.

To find out why rodents that seem ambivalent about keeping their tails would go to the trouble of covering them in armor, the authors turned to a group of equally bizarre fishtail geckos from Madagascar.

Most geckos lack osteoderms, but as the name implies, fishtail geckos are covered in thin overlapping plates and, like spiny mice, they have incredibly fragile skin that sheds at the slightest provocation.

According to Stanley, the osteoderms in fishtail geckos and spiny mice possibly function as a type of escape mechanism.

“If a predator bites the tail, the armor could prevent the teeth from sinking into the tissue underneath, which doesn’t come off,” he said. The outer skin and its complement of bony plates detach from the tail when attacked, allowing the mouse to quickly escape.

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Written by Editor TLN

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