Spiny rats produce bony plates called osteoderms under the skin of their tails, which fall off when the animal is attacked, allowing it to quickly flee. Credit: Edward Stanley
Unlike crocodiles, turtles, lizards, dinosaurs, and fish, which have bony plates and scales, mammals have long since replaced the carapace of their ancestors with an insulating layer of hair.
The armadillo, which has an overlapping and defensive bony shell, is thought to be the only surviving anomaly. However, a new study was published in the journal iScience It was unexpectedly shown that the African spiny rat produces similar structures under the skin of its tail, most of which have not been discovered so far.
The discovery was made during routine CT scans of museum specimens for OpenVertebrate, an initiative to provide 3D models of vertebrate organisms to researchers, educators, and artists.
“I was scanning a rat specimen from the Yale Peabody Museum, and the tail appeared abnormally dark,” said co-author Edward Stanley, director of the Digital Imaging Lab at the Florida Museum of Natural History.
Suppose that initially the color change was caused by a defect that occurred during the preservation of the sample. But when he analyzed the X-rays a few days later, Stanley saw features he was all too familiar with.
“My entire PhD is focused on the development of the ostoderm in lizards. After scanning the sample was processed, the tail was clearly covered in osteoderms.”
Spiny rats have been observed at least once before and were noted by German biologist Jochen Niethammer, who compared their architecture to medieval stone buildings in an article. Published in 1975. Niethammer correctly interpreted the plates as some kind of bone but never followed up on his initial observations, and the group was largely ignored for decades—until scientists discovered another oddity apparently unrelated to spiny rats.
A Learn from 2012 The spiny rat was shown to actually be able to regenerate injured tissue without scarring, a capability common in reptiles and invertebrates but not previously known in mammals. Their skin is also extremely fragile, tearing with almost a quarter of the force needed to injure the skin of an ordinary mouse. But the spiny rat can recover twice as fast as its close relative.
The researchers, hoping to find a model for human tissue regeneration, set out to map the genetic pathways that give spiny mice their extraordinary healing abilities. One of these researchers, Malcolm Madden, happened to own a laboratory in the building across from Stanley’s office.
“Spiny mice can regenerate skin, muscle, nerves, spinal cord, and possibly even heart tissue, so we saved a colony of these rare creatures for research,” said Madden, a professor of biology at the University of California. University of Florida Lead author of the study.
Madden and colleagues analyzed the development of spiny mouse osteoderms, confirming that they are in fact similar to armadillos but most likely evolved independently. Osteoderma also differs from pangolin scales or the fur of porcupines and porcupines, which are composed of keratin, the same tissue that hair, skin and nails are made of.
There are four genera of spiny rats, all of which belong to the subfamily Deomyinae. However, despite the similarities in DNA and possibly the shape of their teeth, scientists have not been able to find anything in common in this group that sets them apart from other rodents.
Suspecting that their differences may only be profound, Stanley surveyed additional museum specimens from all four breeds. In all, the spiny rat tails were found to be covered by the same bony sheath. Deomyinae’s closest relatives – the gerbils – lack osteoderms, meaning the trait only evolved once, in a different earlier spiny mouse ancestor.
The ubiquitous presence of osteoderms in the cluster indicates that they perform an important protective function. But what that function may not be immediately clear, given another odd characteristic of the spiny rat: Its tail is uncharacteristically detachable. Loss of the tail is so common in some species of spiny shrews that approximately half of the individuals in any given group do not have it in the wild.
“It was a real head-scratcher,” said Stanley. Spiny rats are notorious for removing their tails, meaning the outer layer of skin peels off, leaving behind muscle and bone. Individuals often chew the remainder of their tail while this is happening.
Despite its ability to regenerate, tail wagging is a trick that spiny rats can only do once. Unlike some lizards, they cannot regrow their tails, and not all parts of the tail fall off easily.
To find out why a seemingly ambivalent rodent guarding its tail has trouble covering it with armor, the authors turned to a group of gecko fish from Madagascar. Most geckos lack bony skin, but as the name suggests, the fishtail gecko is covered in thin, overlapping plates and, like the spiny mouse, has very fragile skin that peels off at the slightest provocation.
According to Stanley, the osteoderms in gecko fish and spiny mice likely act as a sort of escape mechanism.
“If a predator bites its tail, the shield can prevent the teeth from sinking into the underlying tissue, which doesn’t separate,” he said. The outer skin and bony appendages pull away from the tail when attacked, allowing the rat to make a quick escape.
Reference: “Osteosteoclasts in Spiny Mouse Mammal Acomys and Skin Shield Independent Evolution” By Malcolm Madden, Trey Polvador, Aroud Polanco, W. Brad Barbazok, and Edward Stanley, 24 May 2023, Available Here. iScience.
DOI: 10.1016/j.isci.2023.106779
2023-07-24 04:08:26
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