To Amphibian and Beyond! What Actually are Frogs and Who are Their Amphibian Friends?

I’ve written about amphibians before but what actually is an amphibian? What makes a salamander an amphibian and a lizard not an amphibian? Lizards are reptiles, by the way.

The Naming System

Humans have created a naming and categorizing system for all living things. It is called scientific nomenclature.  It starts with a very broad category called the Kingdom, there are 4 of these: Animals, Plants, Fungi, and Chromista (think algae).  Each of these is broken down into smaller and smaller categories all the way until you get a species.  Many species are what we have common names for.  For example a racoon (Procyon lotor) is a specific species of animal, a ponderosa pine tree (Pinus ponderosa) is a specific species of plant, and a lion’s mane mushroom (Hericium erinaceus) is a specific species of fungi. 

Overall, there are 7 main categories to scientific nomenclature.  I remember the order of them with a mnemonic device where the first letter of every word in a silly saying matches to the first letter of the category. So: King Phillip Came Over For Great Sex = Kingdom, Phylum, Class, Order, Family, Genus, Species.

Back to Amphibians.  Amphibians are animals with a backbone (i.e. like us humans or a puppy, but not like a butterfly).  Being an animal with a backbone puts them in the Animal Kingdom and Chordata Phylum.  Amphibia is the name of the actual Class, and then there are three Orders of Amphibians: 1. Anura (frog/toads), 2. Caudata (salamanders/newts), and 3. Gymnophiona (caecilians).  From there on they are broken down into different types of frogs, different types of salamanders, or different types of caecilians (1).

What Makes an Amphibian Different From an Earthworm or a Lizard, etc.

During part of my undergraduate days I cared for a rescued American Alligator (Alligator mississippiensis).  This alligator lived on campus and its home had windows looking out on a lounge (we referred to it as the “lizard lounge”).  I remember one day I was sitting in the lounge working on homework when a campus tour came through and someone saw the alligator and said: “Is that a salamander?!”  No… no it was not.

Put simply, things like worms and slugs are invertebrates, which means they do not have bones.  Amphibians are vertebrates so they, like us, have bones.  Animals like snakes, alligators/crocodiles, turtles, and lizards/skinks are reptiles and not amphibians.  Reptiles have scales over their body.  Amphibians, like frogs, salamanders, and caecilians, do not have scales.  Amphibians have thin, often damp, skin that they can actually breathe through.  

Although amphibians and reptiles are different from each other, they often get lumped together.  In fact, herpetology refers to the study of amphibians and reptiles (1).  I personally am not a fan of the term herpetology.  I do not consider myself a herpetologist as I know less about reptiles than I do about amphibians.  All of that said, amphibians and reptiles do have things in common.  For example, amphibians and reptiles are the only land-dwelling vertebrates that cannot produce their own body heat (1).  When an animal cannot produce its own body heat they are called ectothermic because they get their body heat from outside of their body.  Some people call this: cold-blooded, but this is not accurate as they are not cold, especially if they live somewhere hot!

Caecilians

I will be the first to admit that I know very little about caecilians. I have only ever seen aquatic ones at zoos/aquariums, but there are many that live underground in the tropics and only emerge during/after a rain to breed. They do not have visible arms or legs and kind of look like giant worms, except they have teeth and backbones.

Frogs/Toads

As the name of my website can probably give away, frogs were always my favorite, although I do love all amphibians and most living things.  Frogs and toads have 4 limbs (arms/legs) and often a squat body.  Anuran actually means “tailless” so as you can guess, the majority of frogs and toads do not have a tail (1).

Frogs versus Toads

What makes a toad different from a frog?  Some may think it is the toad’s bumpier “warty” skin and that they live on land more than water.  And that frogs have smooth skin and spend more time in water.  However, some toads can have smooth skin and prefer living in the water, such as the Eastern Narrow-Mouthed toad (Gastrophryne carolinensis).  While some frogs can have bumpier skins like the Vietnamese Mossy Frog (Theloderma corticale).  

The Parotoid Glands

When I was an undergraduate student I believed the difference between frogs and toads was the parotoid glands.  I had thought that only toads had parotoid glands and frogs did not.  The parotoid glands are found behind the toad’s eye: one on each side.  These glands hold a concentration of poison that the animal will release when it feels incredibly threatened.  All amphibians do secrete small amounts of poison through their skin, but the parotoid glands are a concentrated amount of this poison.  As it turns out not only do toads have parotoid glands but some frogs do too (2)!

So what actually is the difference?

Really “frog” and “toad” are just common names people use to refer to the 4 legged, tailless amphibian.  The general characteristics above do tend to define what a person will call a toad and what they will call a frog.  So, although there really aren’t scientific differences, what people commonly call toads do generally have drier skin, they do generally have “warty” skin, and they do tend to have bigger parotoid glands.

Salamanders/Newts

Salamanders and newts, like frogs, have four limbs.  Their body tends to be longer and they also have tails, unlike frogs (1).  Some species of salamanders will never leave the water and will keep the appearance of their younger larval selves.  When an adult looks like its larva it is called neoteny.  The most common example of this is the axolotl also known as the Mexican salamander (Ambystoma mexicanum).  I think they look like Falkor from The Neverending Story (p.s. If you haven’t read the book, it is way better than the movie).

Salamanders versus Newts

So what is the difference? Is it just the name like with frogs and toads? There is actually a difference.  Salamanders tend to mate on land while newts tend to mate in the water (3).  An exception to this is the sweet neotenic salamanders I mentioned above.  Generally speaking, adult newts also tend to have rougher skin, while salamanders tend to have smoother skin.  The skin is not a scientific difference though.

But Don’t Newts have an Eft Stage?

Some newts have an even more complex life cycle than other amphibians.  Most amphibians start out as aquatic larvae (i.e. babies) then they metamorph into land dwelling juveniles (i.e. teenagers) and adults.  There are some salamanders that will metamorph into land dwelling teenagers, like a typical amphibian, but then when the time comes to adult, they will change again and return to the water.  They will still retain their four limbs, but their skin becomes smoother and their tail becomes broader to propel them in the water.  The time they spend on land as juveniles is called the eft stage.  This eft stage can last for up to 14 years in the Eatern Red Spotted Newt (Notophthalmus viridescens) (4).  So, yes some newts have an eft stage but not all newts have this type of life cycle.

Citations

1. Pough, F.H., R.M. Andrews, M.L. Crump, A.H. Savitzky, K.D. Wells, and M.C. Brandley (2016). Chapter 1: Why Study Herpetology?. Herpetology: 4th edition, pp 379-405. Sunderland, MA: Sinauer Association Inc.

2. Antoniazzi, M.M., P.R. Neves, P.L. Mailho-Fontana, M.T. Rodrigues, and C. Jared (2013). Morphology of the parotoid macroglands in Phyllomedusa leaf frogs. Journal of Zoology 291: 42-50.

3. Kieren, S., M. Sparreboom, A. Hochkirch, and M. Veith (2018). A biogeographic and ecological perspective to the evolution of reproductive behaviour in the family Salamandridae. Molecular Phylogenetics and Evolution 121: 98-109.

4. Pough, F.H., R.M. Andrews, M.L. Crump, A.H. Savitzky, K.D. Wells, and M.C. Brandley (2016). Chapter 3: Systematics and Diversity of Amphibians. Herpetology: 4th edition, pp 379-405. Sunderland, MA: Sinauer Association Inc.