New discovery challenges our understanding of mammal evolution

Mammals have a unique characteristic when it comes to their skeletal structure. Unlike crocodiles, turtles, lizards, dinosaurs, and fish that possess bony plates and scales, mammals evolved to trade their ancestral armor for a layer of insulating hair.

While armadillos are considered the only living exception with their protective and flexible shell made of imbricated bone, a recent study published in the journal iScience has brought to light an unexpected discovery. It turns out that African spiny mice possess similar structures beneath the skin of their tails, which had gone largely unnoticed until now.

The finding came about during routine CT scanning of museum specimens for the openVertebrate program, an initiative aimed at providing 3D models of vertebrate organisms for researchers, educators, and artists.

Edward Stanley, the director of the Florida Museum of Natural History’s digital imaging laboratory and a co-author of the study, stumbled upon the discovery while scanning a mouse specimen from the Yale Peabody Museum. The tails of the spiny mice appeared abnormally dark during the scanning process. Initially assuming it to be a preservation flaw, Stanley later analyzed the X-rays and found a familiar feature.

As a researcher who had focused his entire Ph.D. on osteoderm development in lizards, Stanley recognized the unmistakable presence of osteoderms on the scanned mouse specimen’s tail. Osteoderms are bony plates commonly found in reptiles, but their existence in spiny mice was largely overlooked until now.

Notably, spiny mouse osteoderms had been observed once before by German biologist Jochen Niethammer in 1975. Niethammer described their structure as reminiscent of medieval stonework, correctly identifying them as a type of bone. However, he did not further investigate this phenomenon, and the significance of the osteoderms in spiny mice remained largely ignored for several decades.

In addition to the discovery of osteoderms, scientists have previously uncovered another intriguing aspect of spiny mice: their remarkable ability to regenerate injured tissue without scarring, a characteristic commonly observed in reptiles and invertebrates but previously unknown in mammals. Spiny mice also have particularly delicate skin that tears at only a fraction of the force required to injure the skin of a typical mouse. However, they can heal twice as quickly as their non-spiny counterparts.

These findings shed new light on the unique features and capabilities of spiny mice, challenging our understanding of mammalian biology and opening up avenues for further research into tissue regeneration and the development of protective structures in mammals.

Scientists have embarked on a quest to unravel the genetic pathways responsible for the exceptional tissue regeneration abilities observed in spiny mice, hoping to find insights applicable to human tissue regeneration. Among these researchers is Malcolm Maden, a biology professor at the University of Florida, who conveniently had his lab situated across from Edward Stanley’s office.

Maden, the lead author of the study, explained that spiny mice possess the remarkable capability to regenerate various types of tissue, including skin, muscles, nerves, spinal cord, and potentially even cardiac tissue. To facilitate research on these extraordinary creatures, Maden and his team maintain a colony of spiny mice.

During their investigation, Maden and his colleagues examined the development of osteoderms in spiny mice and confirmed their similarity to those found in armadillos. However, they concluded that the osteoderms in spiny mice most likely evolved independently. These osteoderms differ from the scales of pangolins or the quills of hedgehogs and porcupines, as they are composed of a different tissue called keratin.

Spiny mice are categorized into four genera within the subfamily Deomyinae. Despite some similarities in their DNA and potentially the shape of their teeth, scientists have struggled to identify a distinct feature shared among species of this group that sets them apart from other rodents.

Suspecting that the differences might be superficial, Stanley expanded his analysis to include additional museum specimens from all four genera. To his surprise, he discovered that spiny mouse tails across all species were covered in the same layer of bone. Gerbils, the closest relatives of the Deomyinae subfamily, lacked osteoderms, indicating that this trait likely evolved only once in the ancestor of the previously disparate spiny mice.

The prevalence of osteoderms in spiny mice suggests that they serve an essential protective function. However, the exact purpose was not immediately apparent due to another peculiar attribute of these mice—their tails are unusually detachable. Tail loss is so common in certain spiny mouse species that nearly half of the individuals in a given population lack tails in the wild.

Stanley expressed his intrigue, saying, “This was a real puzzle. Spiny mice have the ability to deglove their tails, which means the outer layer of skin comes off, leaving behind the muscle and bone. When this occurs, individuals often chew off the remaining tail.”

Although spiny mice possess regenerative abilities, tail shedding is a one-time event for them. Unlike some lizards, they cannot regrow their tails, and not every part of the tail separates easily.

To understand why these rodents, seemingly indifferent about keeping their tails, would invest in armoring them, the researchers turned to a group of similarly unique fish-tail geckos from Madagascar. Most geckos lack osteoderms, but fish-tail geckos are covered in thin, overlapping plates, resembling spiny mice. Additionally, both species have incredibly delicate skin that sheds easily.

Stanley proposes that the osteoderms in both fish-tail geckos and spiny mice may function as a type of escape mechanism. He suggests that if a predator bites down on the tail, the armor could prevent the teeth from penetrating the underlying tissue, which does not detach. When attacked, the outer skin and its bone plating separate from the tail, allowing the mouse to swiftly escape.

These discoveries provide valuable insights into the fascinating adaptations and behaviors of spiny mice, shedding light on the evolution of protective structures and mechanisms in mammals.

Source: Florida Museum of Natural History

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