Biology, Linguistics, and the Folly of Two

As a child, I had a book that described the differences between numerous common animal categories. It provided two-page spreads, lavishly illustrated, narrating the differences between frogs and toads, salamanders and newts, snakes and lizards, turtles and tortoises, whales and dolphins, whales and fish, and more. Each was authoritative, each was simple, and each, I would later learn, was either only superficially correct or outright wrong. Deeper exploration of these animal groups shows how these basic dualities offer little explanatory power or understanding and reveal more about us than they do about the world. Biology does not deal in dualities, and classification in particular finds them quaint and obsolete.

So, let’s dive into a few of them.

Frogs and Toads

Like most of these terms, “frog” and “toad” as the two definitive categories of hopping amphibian got its start in Europe. Europe’s fauna, particularly of amphibians, is impoverished compared to the far greater diversity found in tropical climates and lent itself easily to dividing its hoppers between terrestrial, warty “toads” and semiaquatic, smooth “frogs.” Even here, arboreal “treefrogs” add a wrinkle to the story. And even now, the two most common families of these animals in Europe, the Ranidae and Bufonidae, get called the “true” frogs and “true” toads.

Northern leopard frog, a green frog with dark brown spots.
Northern leopard frog, Lithobates_pipiens, a “true” frog. By Brian Gratwicke – https://www.flickr.com/photos/briangratwicke/5898747773/, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=15848126

When one ventures into the tropics, one sees what a mistake it was to define two categories this way. The neotropics alone hand us the drab, fully aquatic Pipa; the terrestrial, smooth Dendrobates; the warty, arboreal Osteopilus; and more, and yet more variations appear in Africa, Asia, and Australia. What’s worse, the traits that supposedly distinguish frogs and toads appear and disappear numerous times in every combination throughout the anuran family tree, with the family of “true toads” including the smooth, brightly colored Atelopus genus from Central America and the warty, largely terrestrial Pyxicephalus turning out to be close relatives of the “true frogs.” Even Europe’s own lineages hardly fit into this dichotomy, with the warty, usually drab, semiaquatic genus Bombina being variously referred to as “fire-bellied frogs” and “fire-bellied toads.” “Frog” and “toad” had, at best, shown themselves to be deeply flawed names for broad ecotypes, and at worst become completely inapplicable outside their original home.

So English fudged, and the results are a tangled, inconsistent mess that puts lie to the tidy dichotomy I was taught as a child.

Pipa became the “Surinam toad” in English, despite its only fit into the “toad” archetype being its brown coloration. Kaloula pulchra, a ground-dwelling, slightly warty, brown species not closely related to “true toads,” is the “Malaysian painted frog.” Osteopilus is a genus of treefrogs, but the similarly sucker-fingered, brightly colored Rentapia hosii is the “Asian yellow-spotted climbing toad.”

Amidst all of this linguistic foolishness, “frog” slowly emerged as a shorthand for the entire group, sometimes including “toads” and sometimes not, with most people hedging their bets by saying “frogs and toads” as though either term carried any real weight.

They’re both, and they’re neither. They are fascinating, cute, and so far beyond the primitive dichotomy that created these names as to make them useless. But the names persist anyway.

Salamanders and Newts

This one is more forgivable.

“Salamander” and “newt” emerged as opposites much like “frog” and “toad” did. All urodelans have aquatic larvae with visible gills, and the difference between salamanders and newts, in the colloquial sense, is based on what happens after they metamorphose out of this life stage. “Salamander” referred to urodelans with terrestrial adults, whereas “newt” referred to those with long residence times in both habitats after metamorphosis. Newts are characterized by distinct terrestrial (“eft”) and aquatic (“newt”) forms that they take as part of this migration. North American newts typically remain aquatic for the rest of their lives after exiting the eft stage, whereas European newts become aquatic for their breeding season and resume living on land for the rest of the year, changing shape seasonally to suit each habitat.

Taricha torosa, a red-orange newt in its terrestrial
Taricha torosa, a newt in its terrestrial “eft” phase, photographed by Connor Long.

This peculiar life history meant that, unlike “toad,” “newt” was always a specific term, even if people using it often simply meant “warty-looking terrestrial slime-lizard.” It thus turned out that the creatures known as newts formed a monophyletic group, with a single common ancestor and shared genetic inheritance. What is interesting, however, is that this group, the Pleurodelinae, nests entirely within the core salamander family Salamandridae. Newts, then, are closer to Europe’s archetypal salamander, the fire salamander Salamandra salamandra, than they are to the rest of their order. Any discussion of salamanders, then, would have to include newts to carry any biological weight.

With newthood confined to a small number of species entirely within the core salamander family, “salamander” would emerge as the umbrella term for this whole order of amphibians. Species in other, more distantly related families, whether they were terrestrial or aquatic as adults and regardless of whether they metamorphosed at all, would all be called “salamanders.” “Newt” would remain reserved for the species with distinct habitat phases as adults, all grouped together in the single subfamily Pleurodelinae.

The “Salamanders and newts” qualifier that discussions of the Urodela in general often receive is, thereby, redundant. We don’t say “and mudpuppies, olms, amphiumas, and sirens,” so “and newts” is a relic of a long-gone time. “Newt” retains some utility as a name for the peculiar life history of one subfamily of salamandrids, but it is in no way the “opposite” of the others. Biology does not deal in dualities.

Snakes and Lizards

One would think the distinction between lizards and snakes, at least, would hold special reality, and for a while, it seemed to. Snakes have long existed as a natural sort of opposite to lizards, clearly related to one another but also clearly distinct and having enough species around the world to warrant their own word. Modern research has revealed, however, that this is not because snakes are distantly related to other squamates (the group containing lizards and snakes), but because snakes are highly specialized. Their distinctive highly distensible jaws and elongated, flexible skeletons mark them, not as an early-branching distant cousin, but as highly derived group nested deeply within the broader arrangement of lizards. Specifically, snakes are within Toxicofera, a lineage within Squamata that contains snakes and the large lizard groups Anguimorpha (including monitors and Gila monsters) and Iguania (including iguanas and chameleons). Toxicofera, in turn, does not include geckos, skinks, tegus, and numerous other lizard species.

In effect, much like newts, snakes are a clear taxonomic group, a single evolutionary lineage, but lizards are incomplete unless “lizard” is taken to include snakes.

Interestingly, snakes are not even close to the only legless group within this grand assemblage of lizards. Other notable legless lizards include the anguid slow worm, numerous pygopodid geckos, and most amphisbaenians, themselves a group once held to be “neither lizard nor snake” and now found to be definitely lizards, just as snakes are. There are even more examples of lizards with highly reduced legs, including many skinks whose legs are barely large enough to support their weight. And then there are the visible leg traces on some snakes, more pronounced than those found on some non-snake legless lizards.

Lialis burtonis, a light brown legless gecko with a snakelike body, licking its eye in very gecko fashion.
Lialis burtonis, Burton’s legless lizard, a kind of gecko that has lost its legs. Not a snake, definitely a lizard.

This reality leaves us with the terminological silliness of having multiple lineages of “it’s not a snake, it’s a legless lizard” elsewhere in the squamate family tree, in positions little different from the one snakes hold. Tidying this mess requires expunging the idea of “legless lizard” as a concept that excludes snakes and accepting that snakes, too, are highly specialized lizards, a particularly extreme and successful expression of the common lizard motif of reducing legs.

Turtles and Tortoises

One would expect the distinction between turtles and tortoises to be obvious and natural. Tortoises live on land, have highly domed shells, and are herbivores, and turtles live underwater, have flatter shells, and have other diets. Until these European terms encountered Asian and North American testudines, this simplicity could reign largely unchallenged. But North America and Asia both have domed, terrestrial testudines with other diets (the box turtles), unrelated to each other and to the tortoise lineage. The Americas also have the Kinosternidae, a family of dome-shelled aquatic turtles, albeit not as domed as most terrestrial turtles, and Africa has the pancake tortoise, a rock-dwelling member of the tortoise family that is flatter than most aquatic turtles. Clearly, the old terms would not do.

Cuora flavomarginata, the Chinese box turtle. A dome-shelled, dark-colored turtle with burnt orange spots.
Cuora flavomarginata, the Chinese box turtle. Not a tortoise. By Torsten Blanck.

Surprisingly, the solution common English found was not to make a frog/toad mess of these terms. Instead, “tortoise” was reserved for the Testudinidae, whose domed members were the first recipients of this name, and the others were left to be called turtles regardless of diet or habitat. In the US, the indigenous word “terrapin” is sometimes used for aquatic turtles as well. “Turtle” was permitted a degree of primacy as the umbrella term for the whole order, but many still insist on tacking “and tortoises” onto it in general settings. Meanwhile, Spanish and French lack this distinction entirely, using tortuga and tortue, respectively, throughout.

Conclusion

Linguistics and biology are not fields of study that spend a lot of time together, and days like this, it’s clear why. The distinctions that humans latch onto when naming the world around them rarely reflect organisms’ evolutionary history, nor should we expect them to. These common words instead reflect far more basic, practical, immediately observable distinctions that are often specific to the time, place, and society that created those names. But we are no longer those people, and we are not beholden to the thoughts that held them when they were coining our terms. Etymology tells us much about the relationship to nature of the people who named things, and biology tells us how much those people did not see. One reveals the past and the other, hopefully, shows us the future.

So here’s to frogs, salamanders, lizards, and turtles, sensu lato, in their grandest and most diverse profusion.

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Biology, Linguistics, and the Folly of Two
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