Osteoderms are ossicles located in the skin of some vertebrates. They have a protective function and are found in many reptiles, such as crocodiles and turtles. However, among modern mammals, they have so far been known only in armadillos. But recently, a team of zoologists led by Malcolm Maden from the University of Florida discovered osteoderms in spiny mice and representatives of three other closely related rodent genera.
Protective function of osteoderms
Needle mice, hard-coated mice, Congolese mice and large-toothed mice in the subfamily Deomyinae are common in Africa, the Middle East and the Mediterranean islands. After studying museum specimens of these rodents using computed tomography, Maden and his co-authors discovered unusual structures in their tails – overlapping ossicles in the dermis, shaped like a tile. They encompass the entire tail, forming rows of rings surrounding the vertebrae and spinal cord, each containing eight to eleven separate ossicles.
The ossicles located near the body are as dense as the bones, while those located closer to the end of the tail are much less dense. In shape and structure, these structures are very similar to the osteoderms on the backs of armadillos and on the tails of Yelgarian lizards. Having compared all the obtained data, the authors concluded that the ossifications located in the tails of rodents of subfamily Deomyinae can be considered as osteoderms.
Zoologists have found that osteoderms serve rodents for protection. If a predator grabs a puffer mouse by the tail, it will shed its skin like a stocking and run away. The ossifications in the skin make this strategy more effective, ensuring that the enemy cannot sink its teeth into the deeper, non-weaving tissues.
Structure of the osteoderms
The osteoderms of adult spiny mice have lacunae within which osteocytes reside. In addition, neurovascular canals with capillaries in them pass through the ossicles. The dorsal surface of each osteoderm adjoins the epidermis, from which it is separated by a layer of dermis three or four cells thick. The ventral surface of the ossicles is immersed in the connective tissue or in contact with the fatty tissue. There are hair follicles between the osteoderms, and three hairs emerge from under each lamina.
Interesting facts about osteoderms
– Osteoderms have occurred independently in different evolutionary lineages of amniotes at least nineteen times.
– Osteoderms are particularly common in reptiles. For example, osteoderms underlie the horny flaps of crocodiles and turtles and the scales of some lizards. At the same time, birds do not have such structures.
– Among the living mammals, only armadillos possess them, in which osteoderms form the carapace (some authors also refer to osteoderms as the bony base of horns of cloven-hoofed mammals).
Iliac mice and other rodents of the subfamily Deomyinae were added to the list of mammals with osteoderms. This is further evidence that evolution can lead to similar adaptations in different animal species. The study of osteoderms helps to understand how animals protect themselves from predators and how these defense mechanisms evolved.