These spiny mice have a highly unusual feature among living mammals: tails armored with bony plates

Spiny mice in the genus Acomys look much like more familiar house mice in the genus Mus, although their coats do have stiff guard hairs that give them their name. But now, researchers reporting in the journal iScience on May 24 have discovered that spiny mice have another feature that’s highly unusual among mammals but more common in reptiles: the skin of their tails is armored with bony plates called osteoderms.

Before this discovery, armadillos were thought to be the only living mammal with osteoderms. The findings in spiny mice show that osteoderms have come and gone multiple times in vertebrates over the course of evolution, the researchers report, presumably thanks to a set of genes that can be switched on and off.

“Osteoderms are present in this sub-family of rodents and nowhere else in living mammals except armadillos,” said Malcolm Maden of University of Florida. “They are absent in birds, frequent in reptiles—think of dinosaurs and crocodiles—and infrequent in frogs. This means that they can be lost and re-evolved time and time again in animals, and this has happened at least 19 times.”

What distinguishes osteoderms from other appendages of the skin is that they’re made of bone, Maden explained. They’re also found deep in the lower dermis layer of the skin, not on the surface. That’s in contrast to scales on the epidermis of many animals, including pangolins and birds, whose feet are made of keratin.

The discovery came about as a classic case of scientific serendipity when study co-author Edward Stanley was working on an “openVertebrate” project (https://www.floridamuseum.ufl.edu/overt/). The effort involved CT scanning 20,000 museum specimens from all around the United States to gather high-resolution anatomical data for more than 80 percent of vertebrate genera.

“I had given Ed some of my spiny mice (Acomys) to scan as part of his project and, lo and behold, they had very rare bony plates in the skin of their tails—only seen before in living mammals in armadillos,” Maden said. “I was working on spiny mice because of their amazing powers of regeneration for a mammal; they can regenerate skin, muscle, nerves, spinal cord, and perhaps cardiac muscle, so we had a colony of these rare creatures available. It was a classical serendipitous finding of two people in the same place working on different things.”

Prompted by the discovery in Maden’s spiny mice, Stanley looked for museum specimens of close relatives. The CT scans revealed osteoderms in the other three genera of the sub-family as well and not in any other relatives, such as gerbils.

Further study found that osteoderms develop in spiny mouse tails starting in the proximal tail skin. The bony plates finish developing six weeks after birth.

Maden’s team used RNA sequencing to identify the underlying genes and gene networks involved in their formation. They found a widespread down regulation of keratin genes as osteoblast genes switch on. Maden said beyond the existence of osteoderms, he also was surprised by “how similar they are in shape and structure to extinct sloths, whose osteoderms are fossilized, so they have ‘been here before’ in mammals.”

Maden notes that the new finding is especially notable because spiny mice can be studied in ways that armadillos and most other animals can’t. As a result, they can now continue to study the underlying molecular evolution to understand why and how these structural novelties appear in evolution. He reports that they want to learn more about the regulatory genes responsible for switching keratin “off” and osteoblasts “on” in the dermis “so that we can eventually make an armor-plated lab mouse.”

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This work was supported by the National Institutes of Health and the National Science Foundation.

iScience, Maden et al. “Osteoderms in a mammal the spiny mouse Acomys and the independent evolution of dermal armor” https://www.cell.com/iscience/fulltext/S2589-0042(23)00856-8

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