Salamanders are intriguing creatures that attract the curiosity of many people. However, there are also many misconceptions about them.
Like all amphibians, salamanders have bones. Their internal skeletons are made up of many interconnected bones and cartilage. Salamanders’ bones are light and hollow, which helps them move efficiently on the ground and in the water.
That said, salamander bones differ from those of other amphibians, due to their ability to regenerate when lost.
For example, a salamander that loses a leg can regenerate another leg that closely mimics the original, bone, nerve cells, and all.
This unique ability to regenerate lost limbs has attracted interest from scientists who are studying them with the hope of using this knowledge to improve healthcare.
The Skeletal Structure of Salamanders
The skeletons of salamanders are adapted to their form of walking.
That said, the exact skeletal structure varies considerably among different species of salamanders.
Additionally, the sequence of bone formation varies among species, mainly due to the different environments and reproductive strategies.
For example, in axolotls, ossification in limbs starts in juveniles that are close to sexual maturity. On the other hand, in many newt species, it starts in the late larval stages, shortly after the completion of limb development.
Despite some variation among species, all salamanders share the same axial skeletal structure.
1. Skull
Adult salamanders generally have weak skulls made up of paired and unpaired bones. These bones may fuse or be lost in different species.
Cartilage also plays an important role in the skulls of salamanders. Some species such as mushroom-tongued salamanders (Bolitoglossa) even have cartilaginous tongue skeletons.
The lighter, simplified skull is an adaptation that gives salamanders greater survival adaptability in their various habitats.
2. Vertebral Column
Salamanders have a backbone made up of many tiny interconnected bones called vertebrae. The vertebrae form the spinal column that runs from the base of the salamander’s head to its tail.
Depending on the species, salamanders may have anywhere from 11 to 60 dorsal vertebrae (the part of the spine that runs from the bottom of the neck to the tail).
However, most salamanders have 14 to 20 dorsal vertebrae.
The spinal column is completed by about 20 to more than 100 tail vertebrae.
3. Ribs
Salamanders have ribs that help protect their internal organs and provide support for their body structure.
These ribs vary in number and size among different salamander species and are attached to the dorsal vertebrae. In most species, all but the last 1 or 2 dorsal vertebrae usually bear ribs.
Appendicular Skeleton
The appendicular skeleton is made of bones of the upper and lower limbs. It also includes the pectoral girdle, which attaches the upper limbs to the body, and the pelvic girdle which attaches the lower limbs to the body.
In most salamanders, the pectoral girdle is relatively reduced, and the fused parts are mostly cartilage.
The forelimbs and hind limbs are the same size and the humerus and femur are held horizontally to the body.
Most salamanders have four digits on the front feet and five on the hind feet, with no claws on either. However, some species may have fewer digits.
Some aquatic salamanders such as amphiumas (Amphiumidae) have short tiny vestigial legs. Sirens (Sirenidae) have stumpy forelimbs and no hind limbs at all.
Salamanders Have Bones Specialized for Their Lifestyle
Salamander bones are highly specialized for efficient locomotion, both on land and in the water.
Their bones are generally thinner and lighter than the bones of many other animals, aiding their survival and adaptability.
Additionally, the structure and arrangement of salamander bones give these amphibians greater agility, depending on the habitat.
Salamanders Often Lose Their Tails, Bones, and All
Salamanders can lose part of most of their tails a self-defense mechanism to distract predators. This process is known as autotomy (meaning “self-severing” in Greek), or self-amputation.
Salamanders are born with a line of weakness in their tail, called a fracture plane, which is the spot along the tail that is meant to break and release.
If a salamander is grasped by a predator, (particularly if grabbed by the tail), the muscles along the fracture plane pull away from one another in a reflex muscle spasm.
The pulling apart of the muscles causes the tail vertebrae to break off at the fracture plane.
Salamander tail autotomy is so efficient that when the tail breaks off, the salamander suffers little to no blood loss.
Salamanders Can Regenerate Their Bones
There are over 700 species of Salamander around the world and all of them have a certain degree of regeneration ability.
The range of tissues these animals can regenerate varies from full limbs to parts of the brain or heart, depending on the species.
When a salamander loses a limb, it can regenerate a replacement that closely mimics the original, bone, nerve cells, and all.
This ability to regenerate bone and even nerve cells sets salamanders apart from other animals.
How Exactly Do Salamanders Regenerate Their Limbs?
When a salamander loses a limb, stem cells in the spinal cord enable it to regenerate replacement.
Within the first few hours after tail loss, the salamander’s epidermal cells in the area migrate to cover the open flesh.
This layer slowly gets thicker over the next few days, forming something called an apical epithelial cap.
Cells within the salamander’s body, called ‘fibroblasts‘ also gather under that epidermal covering.
These Fibroblasts play a central role in wound healing and tissue repair and are undifferentiated.
This means that they’re free to become multiple types of cells, depending on which body part needs replacing.
After that initial process, the blastema develops from the mass of fibroblasts. Over time, the blastema will develop into the replacement tail.
Since salamanders lose not only flesh but also nerves when they lose limbs – nerve axon regeneration happens at the same time as tissue, bone, and muscle regeneration.
Once regenerated, the new appendage is fully functional.
Conclusion
All salamanders have bones, although the skeletal structure and exact number of bones can vary widely among different species.
These bones are lightweight, enabling easy movement through various environments.
Apart from locomotion, salamander bones also help protect delicate nerves and organs.
Sources:
Ross A. Alford, Stephen J. Richards, Keith R. McDonald, Amphibians, Biodiversity of, Editor(s): Simon A Levin, Encyclopedia of Biodiversity (Second Edition), Academic Press, 2013, Pages 169-178, ISBN 9780123847201, https://doi.org/10.1016/B978-0-12-384719-5.00254-9.
Riquelme-Guzmán C, Sandoval-Guzmán T. The salamander limb: a perfect model to understand imperfect integration during skeletal regeneration. Biol Open. 2024 Feb 15;13(2):bio060152. doi: 10.1242/bio.060152. Epub 2024 Feb 6. PMID: 38319134; PMCID: PMC10868587.
Elad Bassat, Elly M. Tanaka, The cellular and signaling dynamics of salamander limb regeneration, Current Opinion in Cell Biology, Volume 73, 2021, Pages 117-123, ISSN 0955-0674, https://doi.org/10.1016/j.ceb.2021.07.010.