Encyclopedias should now be updated: The armadillo is no longer the only known mammal with bony plates in its skin. Researchers have also identified so-called osteoderms in the tails of spiny mice. The brick-like bone structures apparently serve a sophisticated concept that can help the rodents to escape from attackers, say the scientists.
Normally, only the skeleton of vertebrates is made up of hard bone material. But some representatives have also produced bone structures in the course of evolution, which are housed in the dermis. For example, some dinosaur species armored themselves with these so-called osteoderms, and this concept can also be found quite frequently in today’s reptiles. In birds, however, the bony platelets are not documented and as far as modern mammals are concerned, there seems to be only one exception: the armor of armadillos is made up of osteoderms. On the other hand, other hard structures in birds and mammals, such as scales, horns and claws, consist of the horn building material keratin.
The discovery of what is now the second example of the special bone structures in mammals is based on an incidental finding as part of a major scientific project: researchers used computer tomography to scan 20,000 museum specimens of various animal species in order to collect high-resolution anatomical data. Senior author Edward Stanley from the University of Florida in Gainesville also x-rayed preserved spiny mice. It is a group of rodents from Africa that resemble normal mice. However, they have unusually stiff hair structures, which has given them their name.
Chance find in museum specimens
“When I scanned spiny mouse specimens from the Yale Peabody Museum, I noticed that the tails looked unusually dark,” reports Stanley. At first he assumed that the coloring was caused by the preservation. However, when he analyzed the X-ray images more closely a few days later, structures that were well known to him were revealed. “I have long studied the development of osteoderms in reptiles. Now the scans of these mouse tails showed very clearly that they are also covered with these structures,” says the researcher. They encase the appendages of the mice like a kind of brick layer beneath the surface of the skin. The structural features clearly resembled those of armadillo osteoderms. Subsequent research also documented that all representatives of the spiny mice are equipped in this way.
The discovery is surprising, because this group of rodents is already quite prominent – due to a bizarre property: their skin detaches from the body with about a quarter of the force required to injure the skin of an ordinary mouse. This concept is used by the rodents to escape from predators. Sometimes only scraps of skin get stuck in their claws – but the spiny mouse can escape. What’s also exciting is that the wounds in the spiny mice heal twice as fast as in their relatives. Researchers are currently investigating the basics of this healing ability. “Prickly mice can regenerate skin, muscles, nerves, spinal cord and maybe even heart tissue. That’s why we keep them for research purposes,” says first author Malcolm Maden of the University of Florida.
Part of a whimsical survival strategy
This circumstance has now also enabled detailed examinations of the osteoderm findings: the research team used RNA sequencing to investigate which genes underlie the formation of the mouse tail bone platelets. It was shown that keratin genes are downregulated in the relevant tissue layers, while bone-forming osteoblast genes are switched on. Apparently, the formation of bone structures in the skin can be established relatively easily in vertebrates – if there is a purpose for it. It is becoming apparent that the adaptation in different animal groups developed independently several times and could subsequently be lost again, say the scientists.
The question remains what purpose the osteoderms serve in the case of spiny mouse tails. As the scientists explain, they apparently serve to easily detach the skin on the tails of these rodents – a strategy intended to improve their chances of escaping from predator attacks: “If a predator bites its tail, the shell could prevent its teeth from digging into what’s underneath to drill lying tissue that doesn’t detach,” explains Stanley. In conclusion, the scientists write: “Future comparisons with reptilian osteoderms could help us understand how these structures evolved and why they are so rare in mammals.”
Source: Florida Museum of Natural History, Article: iScience, doi: 10.1016/j.isci.2023.106779