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

Newswise — Spiky rats in the Acomys genus appear similar to well-known Mus house rats, yet their fur possesses rigid guard hairs, which grant them their moniker. However, in a recent publication in the journal iScience on May 24, scientists have unveiled an extraordinary characteristic of spiky rats, one that is remarkably rare in mammals but more prevalent in reptiles: their tails possess ossified scales known as osteoderms.

Prior to this revelation, armadillos were deemed the exclusive mammalian species harboring osteoderms. As per the researchers' report, the spiny mice's findings indicate that osteoderms have emerged and disappeared repeatedly in vertebrates throughout the evolutionary timeline. This occurrence is likely facilitated by a cluster of genes that can be activated or deactivated.

"Within this sub-family of rodents, osteoderms are exclusively found and not observed in any other living mammal except armadillos," stated Malcolm Maden from the University of Florida. He further explained, "They are absent in avian species, but commonly present in reptiles, such as dinosaurs and crocodiles, albeit infrequent in frogs. This implies that osteoderms can be lost and re-emerge multiple times throughout the animal kingdom, and this phenomenon has occurred at least 19 instances."

Maden elucidated that what sets osteoderms apart from other skin structures is their composition of bone. Additionally, they are situated deep within the lower dermis layer of the skin, rather than being present on the surface. This distinguishes them from scales found on the epidermis of various animals, such as pangolins and birds, whose feet are composed of keratin.

The revelation occurred through a fortuitous instance of scientific serendipity when Edward Stanley, one of the co-authors, was engaged in an "openVertebrate" project (https://www.floridamuseum.ufl.edu/overt/). This initiative aimed to CT scan a vast collection of 20,000 museum specimens from various locations across the United States, with the objective of compiling detailed anatomical data for over 80 percent of vertebrate genera at a high resolution.

Malcolm Maden remarked, "I had provided Ed with some of my spiny mice (Acomys) for scanning as part of his project, and astonishingly, we discovered the presence of extremely uncommon bony plates within the skin of their tails—a feature previously observed solely in armadillos among living mammals." He further explained, "I was studying spiny mice due to their remarkable regenerative abilities, as they can regenerate skin, muscle, nerves, spinal cord, and potentially even cardiac muscle. Consequently, we had a colony of these exceptional creatures readily available. This finding was a classic case of serendipity, where two individuals happened to be in the same location, working on different projects, and made this unexpected discovery."

Inspired by the revelation in Malcolm Maden's spiny mice, Edward Stanley embarked on a search for museum specimens belonging to closely related species. Through CT scans of these specimens, osteoderms were also detected in the other three genera of the same sub-family. Notably, these osteoderms were absent in other relatives, such as gerbils, underscoring their unique presence within this specific group of rodents.

Subsequent investigations unveiled that osteoderms in spiny mice originate in the proximal tail skin. The development of these bony plates initiates during the early stages and progresses over time. Remarkably, the process of osteoderm formation reaches completion approximately six weeks after birth in spiny mice.

Maden's research team employed RNA sequencing techniques to identify the specific genes and gene networks responsible for the formation of osteoderms. Their analysis revealed a significant downregulation of keratin genes accompanied by the activation of osteoblast genes. Additionally, Maden expressed his astonishment not only at the presence of osteoderms but also at their striking resemblance in shape and structure to the fossilized osteoderms found in extinct sloths. This similarity suggests that osteoderms have recurred in mammalian evolutionary history, indicating that they have a precedent in mammalian species that lived in the past.

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