Quite a Tail: A mouse has been hiding its armour all this time

NEW YORK: At first, it looks like a slightly more hairy rodent. But the spiny mouse’s body is full of secrets. Found in rock outcrops throughout Africa and Europe, its back is full of porcupine-like quills made of stiffened fur. It has soft, easily torn skin and a remarkable ability to regenerate, like a species of desert gecko.

Now, researchers have revealed another surprise in the journal iScience on Wednesday: Their tails are lined with osteoderms, or bony plates, making them only the second group of living mammals known to be equipped with underskin armour like an armadillo.

“Although spiny mice are widely known and commonly used in all sorts of lab experiments, nobody had ever noticed they had these,” said Edward Stanley, a biologist at the Florida Museum of Natural History and an author of the study.

The discovery came when he was CT scanning specimens for the openVertebrate Project, an effort to build a public online database of 20,000 vertebrate specimens from museum collections across the United States. X-rays of the mouse’s tail gave him pause: They reminded him of the lizards he had worked on for his Ph.D. But the only living mammals with known osteoderms were armadillos.

“I know enough about osteoderms that it’s a fairly unknown thing for rodents to have them,” Dr Stanley said.

The discovery was serendipitous, said Malcolm Maden, a biologist with the University of Florida and an author of the study. Dr. Maden already had a longstanding research project built around spiny mice, centered around their remarkable ability to regenerate skin, muscle, nerves, and parts of their spinal cord. The researchers joined forces, studying how the osteoderms developed over a mouse’s lifespan and sequencing the species’ RNA in an attempt to identify the genetic switches responsible for the bone armour’s development.

Dr. Stanley also scanned specimens of the spiny mouse’s closest relatives — the link rat, brush-furred mouse, and Rudd’s mouse. He found that all three also had armoured tails, while more distant relatives did not. The discovery suggested that a common ancestor of all four species possessed the trait. The purpose of the osteoderms isn’t clear. Spiny mice may use them to shield themselves from predators while burrowed in crevices, Dr. Stanley said. Another possibility: While the mice’s skin tears easily, the armour might help protect the inner tail structure, like wearing chainmail under an easily-removed glove. Osteoderms have re-evolved at least 19 times in different lineages of animals, Dr. Maden said. They are often found in reptiles such as lizards, crocodiles, and non-bird dinosaurs. They have also been found in a few extinct mammal groups like immense armadillo relatives called glyptodons and giant ground sloths — whose skin armour the spiny mouse closely resembles. Finding osteoderms in a fast-breeding, easily maintained animal like a mouse could help unlock how and why the forces of evolution have continually produced underskin bone armour, Dr. Maden said. Now that they have narrowed down a list of genes that might be responsible for this trait, they can try to produce osteoderms in lab studies. “I want to work out what genes are responsible for making osteoderms and then make a lab mouse with armour plating,” Dr. Maden said.