We’re all familiar with the outer appearance of frogs. They’re typically small, have smooth skin, and webbed feet. But what does the inside of a frog look like? Do they have a backbone?
Animal species are classified as either vertebrates or invertebrates. Vertebrates are those having a vertebral column (backbone). Invertebrates, as you already guessed, do not have a backbone. So, are frogs vertebrates?
It may surprise you to learn that Anurans (frogs and toads) are vertebrates. More specifically, they’re Tetrapod Vertebrates, which is a fancy way of saying they have four legs and a backbone.
A frog’s backbone is quite interesting. The reason it’s interesting is due to the number of vertebrae and a thin, rod-like bone attached to the end. Continue reading to learn more!
About A Frog’s Backbone (Vertebral Column)
There are over 5,000 species of frogs in the world, all of which are vertebrates. You’ll recall from the opening paragraphs that vertebrates are animals having a vertebral column (a backbone). Among these 5,000+ frogs, there are many, many differences.
The differences range from shape and size to the number of vertebrae in their backbone. Despite a few outliers, most frogs have 10 vertebrae . To give you a quick comparison – adult humans have 24 vertebrae.
The Parts of a Frog’s Backbone
The Atlas is the topmost vertebra in the backbone. It’s a frog’s only cervical vertebra. Following the Atlas, there are seven more vertebrae of similar shape and structure. Due to their similarities, the 2nd – 7th vertebrae are called “typical vertebra”.
Amphicoelous vertebrae are found in amphibians and fish. It has concave surfaces on both sides . Directly behind it is the Sacral Vertebra, which is the last in the spinal column.
The Sacral Vertebra is the last vertebra in the frog’s backbone. It is positioned between the Amphicoelous and Urostyle, as shown in the diagram above.
I go into more detail on the frog skeleton page so. Should you want to learn more about that, feel free to check that out after you finish this page!
Frogs aren’t born with a vertebral column, however. It’s important to remember that Anurans begin life as larvae – tiny tadpoles inside an egg.
Metamorphosis: From Cartilage to Bone
Frogs begin their life in the larval stage and do not have bones yet. Adult female frogs lay their eggs in ponds, streams, and water puddles. The growing tadpoles eat the yolk of the egg and eventually hatch into the surrounding water.
Tadpoles primarily have cartilage and soft tissue during this stage of their life. That changes as they metamorphose into juvenile frogs.
Over the following weeks and months, the tadpoles grow in size. The back legs grow first and then the front legs. As the tadpole nears the froglet stage, they begin absorbing their tail. During this stage, a tadpole will eat very little or nothing at all because the tail acts as a source of nutrition. Lungs develop, the tail is absorbed, and what was once water-bound is now capable of walking (hopping) on land.
Somewhere in this process, parts of the tadpole’s cartilage and soft tissue turn into bone. This process is called ossification. Ossification is the formation of bone tissue; soft tissues becoming calcified or hardened.
Notochord – The beginning of a backbone
As you’ve just learned, tadpoles don’t begin with bones. They have a cartilaginous skeleton that eventually turns to bone in the process of ossification. Tadpoles start with a “Notochord”.
The Notochord is a flexible rod made of cartilage-like material. It runs from front to back on all animal species in the Chordata phylum.
Eventually, the Notochord will develop into the spinal column but in this stage, it provides signals during tissue development as a place for muscle attachment .
Animal Classification (Taxonomy)
Classification is the process of sorting living organisms into groups or categories based on their characteristics. You may recall learning about the main taxonomic ranks in biology class during school. It looks like this:
Describing the details of each rank is beyond the scope of this article so I’ll briefly touch on the main points.
Getting to Amphibia
Domain is the highest taxonomic rank. So far, there are three in this group: Archaea, Bacteria, and Eukarya. Amphibians are traced through the Eukarya domain, along with other Animalia in the Kingdom rank.
Kingdom is the second-highest rank and it can be divided into five kingdoms: Animalia, Fungi, Plantae, Protista, and Monera. Amphibians, as you might have guessed, are Animalia.
While there are many classifications to acknowledge, animals are easily divided into two groups:
- Invertebrates – Animals without a backbone.
- Vertebrates – Animals with a backbone.
Phylum is the third-highest rank. There are 35 groups of Phyla. Frogs and toads are characterized as being within the Chordata Phylum.
Members of the Chordata Phylum are called “chordates”. Remember that Notochord described in the previous section? That’s part of where this term comes from.
Class is fourth in the taxonomic rank and precisely where amphibians fit in. Amphibia is the class containing all amphibians.
Amphibia to Anura
As you now know, amphibians are a class in the taxon. Frogs and toads are characterized one rank deeper. But first, let’s look at the three members of Amphibia.
- Anura – frogs and toads
- Caudata – salamanders
- Caecilians (Gymnophiona) – limbless, worm-like amphibians
As you can see from the list above, frogs (and toads) are within the Anura order – they’re “Anurans”. You can continue to characterize all frogs and toads beginning with this order. So, Anurans are vertebrates because they have a backbone. Also, they’re tetrapods which means they have four legs!
The words “frogs” and “toads” are used informally. Most people know frogs to be water-dependent, having smooth skin, and webbed feet. Toads are known to have warty skin with short, stumpy limbs. Regardless, they’re both within the Anura order.
TLDR; Do Frogs Have Backbones?
Yes, adult frogs have backbones. Put simply, species of animals are broadly classified as vertebrates or invertebrates. Vertebrates are those having a vertebral column or “backbone”. Frogs are vertebrates because they have a backbone.
Frogs don’t begin their life with bones, however. Remember, they are born into the world in a larval stage. They’re laid in a clutch of eggs, in small pools of water, or sometimes attached to plants and/or leaves directly above the water.
Tadpoles feed on the egg yolk and eventually hatch into the surrounding water. At this point in their life, a tadpole eats algae, detritus, mosquito larvae, small bugs, and other organic plant material in order to grow.
Meanwhile, tadpoles have a “notochord”. A Notochord is found in all animal species within the Chordata phylum. This makes tadpoles (frogs) “Chordates”. Think of the notochord as the beginnings of a spinal column.
Metamorphosis ensues and the little tadpoles sprout back legs first and then front legs. They absorb their tail, develop lungs, and parts of their cartilaginous skeleton turn into bones in the process of ossification.
So, as described above, frogs begin their lives as tadpoles, having a cartilaginous skeleton that eventually turns into bone via the process of ossification. The notochord turns into the spinal column.
Adult frogs have 10 vertebrae in their back backbone, with few exceptions.
- Hofrichter, Robert. “The Encyclopedia of Amphibians.” The Encyclopedia of Amphibians, Gardners Books, 2000, p. 66.
- “Amphicoelous Vertebra.” Paleontica, Fossiel.net Team, www.paleontica.org/information/article.php?id=130&/Amphicoeel&language=en. Accessed 23 July 2021.
- Stemple, Derek. “Structure and Function of the Notochord: An Essential Organ for Chordate Development | Development | The Company of Biologists.” The Company of Biologists, 1 June 2005, journals.biologists.com/dev/article/132/11/2503/52334/Structure-and-function-of-the-notochord-an.