These spiny mice have a highly unusual feature among living mammals: tails armored with bony plates
IMAGE: OSTEODERMS ON THE SPINY MOUSE view more
CREDIT: EDWARD STANLEY
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
iScience (@iScience_CP) is an open-access journal from Cell Press that provides a platform for original research and interdisciplinary thinking in the life, physical, and earth sciences. The primary criterion for publication in iScience is a significant contribution to a relevant field combined with robust results and underlying methodology. Visit http://www.cell.com/iscience. To receive Cell Press media alerts, contact press@cell.com.
JOURNAL
iScience
METHOD OF RESEARCH
Experimental study
SUBJECT OF RESEARCH
Animals
ARTICLE TITLE
Osteoderms in a mammal the spiny mouse Acomys and the independent evolution of dermal armor.
ARTICLE PUBLICATION DATE
24-May-2023
Move over, armadillos. There’s a new bone-plated mammal in town
Armadillos, with their protective and flexible shell of imbricated bone, are considered the only living exception. But a new study, published in the journal iScience, unexpectedly shows that African spiny mice produce the same structures beneath the skin of their tails, which until now had gone largely undetected.
The discovery was made during routine CT scanning of museum specimens for the openVertebrate program, 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,” said co-author Edward Stanley, director of the Florida Museum of Natural History’s digital imaging laboratory.
He initially assumed the discoloration was caused by an imperfection introduced during the specimen’s preservation. But when he analyzed the X-Rays several days later, Stanley observed an unmistakable feature he was intimately familiar with.
“My entire PhD was focused on osteoderm development in lizards. Once the specimen scans had been processed, the tail was very clearly covered in osteoderms.”
Spiny mice osteoderms have been observed at least once before and were noted by German biologist Jochen Niethammer, who compared their architecture to medieval stonework in an article published in 1975. Niethammer correctly interpreted the plates as being 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 peculiarity of spiny mice.
A study from 2012 demonstrated 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 at roughly one-fourth the amount of force required to injure the skin of a common mouse. But spiny mice can heal twice as fast as their relatives.
Researchers hoping to find a model for human tissue regeneration have begun mapping the genetic pathways that give spiny mice their extraordinary powers of healing. One such researcher, Malcolm Maden, just so happened to have a lab in the building across from Stanley’s office.
“Spiny mice can regenerate skin, muscle, nerves, spinal cord and perhaps even cardiac tissue, so we maintain a colony of these rare creatures for research,” said Maden, a biology professor at the University of Florida and lead author on the study.
Maden and his colleagues analyzed the development of spiny mice osteoderms, confirming they were in fact similar to those of armadillos but had most likely evolved independently. Osteoderms are also distinct from the scales of pangolins or the quills of hedgehogs and porcupines, which are composed of keratin, the same tissue that makes up hair, skin and nails.
There are four genera of spiny mice, which all belong to the subfamily Deomyinae. However, aside from similarities in their DNA and potentially the shape of their teeth, scientists have been unable to find a single feature shared among species of this group that distinguishes them from other rodents.
Stanley, suspecting their differences might only be skin deep, scanned additional museum specimens from all four genera. In each, he found spiny mice tails were covered in the same sheath of bone. The closest relatives of Deomyinae — gerbils — lacked osteoderms, meaning the trait had likely evolved just once, in the ancestor of erstwhile disparate spiny mice.
The ubiquity of osteoderms in the group indicate they serve an important protective function. Just what that function might be wasn’t immediately apparent, however, due to yet another peculiar attribute of spiny mice: Their tails are uncharacteristically detachable. Tail loss is so common in some spiny mouse species that nearly half the individuals of a given population have been shown to lack them in the wild.
“This was a real head-scratcher,” Stanley said. “Spiny mice are famously able to deglove their tails, meaning the outer layer of skin comes off, leaving behind the muscle and bone. Individuals will often chew off the remainder of the tail when this happens.”
Despite their powers of regeneration, tail shedding is a trick that spiny mice can only perform once. Unlike some lizards, they can’t regrow their tails, and not every part of the tail separates easily.
To find out why rodents that seem ambivalent about keeping their tails would go through the trouble of covering them in armor, the authors turned to a group of similarly odd fish-tale geckos from Madagascar. Most geckos lack osteoderms, but as their name implies, fish-tale geckos are covered in thin, overlapping plates, and just like spiny mice, they have incredibly fragile skin that sheds at the slightest provocation.
According to Stanley, the osteoderms in fish-tale geckos and spiny mice possibly function like a type of escape mechanism.
“If a predator bites down on the tail, the armor might keep the teeth from sinking into the tissue beneath, which doesn’t detach,” he said. The outer skin and its complement of bone plating pull away from the tail when attacked, affording the mouse a quick escape.
JOURNAL
iScience
ARTICLE TITLE
Osteoderms in a mammal the spiny mouse Acomys and the independent evolution of dermal armor.
ARTICLE PUBLICATION DATE
24-May-2023
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