While animals fall into many different classifications, you can more broadly classify them as either vertebrates or invertebrates with vertebrates being animals with a backbone and invertebrates being animals without a backbone.
Frogs and other amphibians are vertebrates, so frogs do have backbones. But unlike other vertebrates, they have a very unique skeletal structure that forms from cartilage to bone as they go through their metamorphosis.
A Frog’s Backbone Explained
Tadpoles, or frogs in their larval stage, do not have a vertebral column (backbone) because their bodies are made primarily of cartilage. As they grow into adult frogs, they start to grow a skeleton. That means that frogs may not be born with a vertebral column (instead, they are born with what is known as a notochord), but once they develop into adults, they will have a fully formed one with 10 vertebrae or fewer, depending on the species. Unlike other vertebrates, frogs have very short backbones and large hip bones to support their muscular legs.
The topmost vertebra is the atlas, which is the only cervical vertebra in the frog’s anatomy. Following the atlas, several more vertebrae are of similar shape, called typical vertebrae. The amphicoelous vertebra follows the typical vertebrae and is concave at the anterior and posterior ends and is found in most fish and certain amphibians.
After the amphicoelous vertebra is the sacral vertebra and then the urostyle; a rod-like fusion of the sacral vertebrae that runs in parallel with the extended iliac blades of the pelvis. The urostyle allows for a strong, shock-absorbing pelvic floor that supports leaping.
The 4 Phases of Frog Development
There are several thousand species of frog worldwide and they can be found on every continent except for Antarctica. Because of how widely distributed they are, various species are adapted to live in a wide variety of climates and habitats across planet Earth.
Though some species do not go through a tadpole stage, many of the species we are familiar with will hatch as tadpoles and transform into frogs over time. Female frogs will lay their eggs in ponds, streams, and other calm or stagnant bodies of water. They will lay large numbers of eggs at a time to increase the chance of survival for the young, as they will be vulnerable to predation through all life stages. While in the larval stage, the tadpole will develop and eventually eat the yolk of the egg and hatch into the water.
Rather than bones, tadpoles consist of primarily cartilage and soft tissue which will transform as they grow and develop. Within the first 4 weeks of life, they will freely swim and live solely underwater feasting on algae and organic matter. Teeth will begin to develop allowing them to consume a wider variety of foods.
2. Tadpole With Legs
After about 5 to 9 weeks, hormones will begin the process of metamorphosis and the tadpoles will start to form legs as their bodies continue to grow and develop. The hind legs will begin to form first, followed by the front legs. During this time, the tail will begin to disappear, and the head will become more froglike. As this occurs, the cartilage begins to be replaced with bone.
When the tadpole reaches the froglet stage, it is close to being considered an adult. At this point in development, the gills have disappeared, and the lungs have developed, allowing them to leave the water. The once prominent tail of the tadpole will only be a small stub.
Once the tail has completely disappeared, the frog has reached adulthood and will now be sexually mature and able to reproduce. The teeth they once had as a tadpole will be long gone, and their mouths and digestive system will now be fully prepared for their carnivorous diet. Their unique skeletons will have fully formed to suit their natural behaviors for the remainder of their lives.
All species of frogs are considered vertebrates, meaning they have a backbone. They first have to go through the process of metamorphosis for their skeleton to develop. But their backbone and the rest of their skeleton are very important, as they support their unique movements and protect vital organs.