Why Evolution is True is a blog written by Jerry Coyne, centered on evolution and biology but also dealing with diverse topics like politics, culture, and cats.
It’s World Frog Day! Give some love to our slimy green friends! World Frog Day celebrates all anurans, so toads are included. Here’s a nice big American Toad (Bufo americanus) from Will County, Illinois.
American Toad (Bufo americanus), front, Will County, IL, July 13, 2023.
American Toad (Bufo americanus), back, Will County, IL, July 13, 2023.
American Toad (Bufo americanus), habitat, Will County, IL, July 13, 2023.
I’ve mentioned in previous posts how I periodically engage in turtle or frog and toad “rescues”, taking animals that had fallen into human made traps, such as window wells and stairwells, and releasing them, sometimes after feeding them for a while in captivity to fatten them up prior to release. A couple of days ago I decided to stop and check a stairwell on my campus, the University of Wisconsin-Parkside, where I’ve previously found toads and a turtle, and sure enough I found a young American toad (Bufo americanus), about 30 mm in snout-vent length, hunkered down in some leaf litter at the bottom of the stairs. I took the little fellow’s picture with a lady bug, the type of beetle made famous by Jerry’s academic grandfather Theodosius Dobzhansky.
American Toad with ladybug in stairwell, University of Wisconsin-Parkside, Somers, Wisconsin, 16 August 2016.
The beetle of course was not trapped, and could just fly away when it wanted to. I checked the same stairwell again the next day. It had rained in the general area the previous night, which might encourage toads to be moving about– and thus fall down the stairs– but I wasn’t sure if it had rained on campus. There were two more American toads. These were smaller, about 18 mm snout-vent length. (A penny is about 19 mm in diameter.) These two were hopping about— they had just fallen in, and were in good shape. The toad from the previous day, although it looked good, may have been stuck in the stairwell for some days during a generally dry period, and was not active, but rather hiding in the leaf litter.
American Toads from stairwell, University of Wisconsin-Parkside, Somers, Wisconsin, 17 August 2016.
Here’s the stairwell, on the northern side of the Communication Arts building, in which the toads (and last year a painted turtle) got trapped. This year’s larger toad was under the leaves on the far right. Once they go down a step, they cannot climb back up, and they get ratcheted to the bottom.
Stairwell at UW-Parkside, NE corner of Comm Arts extension, 16 August, 2016.
I released these toads immediately after photographing them in Greenquist Woods, shown in the photo below, approximately under the large basswood leaves visible at the right. You can see how the ground slopes down to the left– just behind that screen of bushes is Greenquist Pond, which is where the toads breed, and the painted turtles live.
Greenquist Woods, University of Wisconsin-Parkside, Somers, Wisconsin, 16 August 2016.
Here’s Greenquist Pond looking north, with Greenquist Woods to the north and east, a lawn area (not well seen) behind bushes to the west, with a sidewalk and lawn edging to the south (from where the photo was taken).
Greenquist Pond, University of Wisconsin-Parkside, Somers, Wisconsin, 16 August 2016.
The smaller toads were recent transformlets from tadpoles this season. The 30 mm toad was a bit puzzling. Either it’s a transformlet from earlier this year which has grown quite a bit, or it’s a one year old from last year’s brood. It seems too small, based on my experience of toad growth in captivity, to be a year old, yet it seems odd to have in just one breeding season such a wide size range in the season’s transformlets (18 to 30 mm). I’ve not quite worked out the breeding phenology of the toads– perhaps I should figure this out.
After releasing the 30 mm toad in the woods the first day, I stopped at the Pond with the colleague who accompanied me, and there we found many small frogs that jumped in the water. At the size of those we saw, you need to get a good look at them to tell bull frogs (Rana catesbeiana) from Green frogs (Rana clamitans)– both species occur in the Pond. They all were diving quickly in the water, and we had no binoculars to get a close look at those that surfaced in the water, but one large individual sat still and let me approach. It was a large adult male green frog: a green frog, because the dorsolateral ridge extends from the eye over the ear and along the side toward the groin (in bull frogs, the ridge curls round the ear); and a male, because the ear is larger in diameter than the eye.
Large male Green Frog in Greenquist Pond, University of Wisconsin-Parkside, Somers, Wisconsin, 16 August 2016.
JAC: Instead of “Readers’ Wildlife” today, we’ll have a report on frogs by Greg Mayer, who’s just visited Costa Rica.
by Greg Mayer
Although quite cryptic on the forest floors it calls home, the smoky jungle frog (Leptodactylus pentadactylus) in the photo below is too obvious to be a candidate for “spot the frog”. This large species of frog (reaching over a 150 mm in snout-vent length) sports what in the military would be called “defense in depth”– a series of defensive behaviors and adaptations that help the frog avoid becoming someone else’s meal. The one in the photo below I encountered at the Lapa Rios Ecolodge, near the tip of the Osa peninsula, on my recent trip to Costa Rica.
Smoky Jungle Frog (Leptodactlyus pentadactylus), Lapa Rios Ecolodge, Osa, Costa Rica
It was a large individual (well over 100 mm), and we found it at night in the rainforest. Its first line of defense is that it’s quite hard to see against the variegated mixture of brownish leaves, twigs, and mud of the forest floor. (The red shine of the eye is more noticeable, but fortunately for the frog, natural predators don’t carry flashlights!) When first seen, the frog was sitting up at attention, but when we approached, it pushed itself down flat against the substrate, and as I moved around in front of it for a picture, it really pushed its face into the ground, making itself less noticeable.
Smoky Jungle Frog (Leptodactlyus pentadactylus), Lapa Rios Ecolodge, Osa, Costa Rica
Since all we wanted was pictures, the frog did not move to its further lines of defense. Had we provoked it, it would have assumed an elevated defense posture, with the back raised, also inflating its body and expelling air to make a hissing sound, similar to what is seen in some toads (Leptodactylus is not a true toad). I haven’t seen (or at least can’t recall seeing) this in Leptodactylus— the behavior was described in this species by Jaime Villa (1969)– but I have seen it in giant toads.
Elevated defensive posture of Leptodactylus pentadactylus (Villa, 1969: Fig. 6).
This of course draws a potential predator’s attention to the frog– having hidden, why would it now face up to its foe? This is where the next lines of defense come in. First, the frog is big, and this behavior makes it look even bigger. For some predators, the frog is a mouthful too far. Next, if the frog is touched, it exudes a copious and toxic mucus. This mucus induces a strong allergic response in humans, and presumably others, at least mammals if not all other vertebrates- intense sneezing, watery and itching eyes– the unpleasantness of which I can attest to from personal experience. It is said that people merely in the vicinity, who have not touched the frog, can, through aerial transmission of toxin droplets, get the same symptoms. The mucus can irritate the skin, and cause pain to any scratches or open wounds (which I fortunately did not have when catching the frogs). And the frog will also emit a loud, piercing shriek, which might well startle a predator into releasing its grip. Norm Scott reported that caimans were attracted to this cry, and even speculated that that was its function– to attract caimans to dispatch the frog’s predator– sort of like a bugle call to the cavalry!
More straight forward than the multi-layered defenses of the smoky jungle frog is the defense of poison dart frogs– aposematic, or bright, warning coloration, accompanied by very toxic skin secretions. We encountered two species at Lapa Rios. Phyllobates vittatus, with bright orange stripes, is a member of the genus which contains the three species of the poison dart frog family, Dendrobatidae, that are actually used by Indians to make poison darts.
Poison Dart Frog (Phyllobates vittatus), Lapa Rios Ecolodge, Osa, Costa Rica
We found three of them, during the day, along the Rio Carbonero. We also found three Dendrobates auratus along the paths at the Lodge itself, wandering about during the day, bold as brass, as is their wont. I’ve seen them quite abundant in other parts of Costa Rica, but we saw only three during 4.5 days at Lapa Rios. For neither species of dart frog was I able to get a very good picture; there’s a better picture of auratus in an earlier post, and, in another earlier post, more details and references on poison dart frogs. BBC Earth has a nice explainer on poison dart frogs, with links to interesting papers
Poison Dart Frog (Dendrobates auratus), Lapa Rios Ecolodge, Osa, Costa Rica
Savage, J.M. 2002. The Amphibians and Reptiles of Costa Rica: A Herpetofauna between Two Continents, between Two Seas. University of Chicago Press, Chicago
Scott, N.J. 1983. in D.H. Janzen, ed. Costa Rican Natural History. University of Chicago Press, Chicago
Villa, J. 1969. Comportamiento defensivo de la “Rana Ternero”, Leptodactylus pentadactylus. Revista de Biología Tropical 15:323-329. pdf
Following up on Matthew’s linguistic investigation of larval amphibians, I’d like to address another amphibian linguistic conundrum: the English words for adult members of the order Anura. Just as we have two standard words for a larval anuran in English, we have two standard words for the adults: frog and toad. But this linguistic duality comes nowhere near encompassing the biodiversity of anurans. There are, by many estimates, over 40 families of anurans. I myself consider this taxonomy a bit oversplit, but even a conservative taxonomy would have more than two dozen families. There are thus many more sorts of anurans than there are English words to name them. Why is this so?
The answer, I believe, is simple. In Great Britain, where the language developed, there are four native species: two frogs (Rana temporaria and Rana lessonae), and two toads (Bufo bufo and Bufo calamita). So in England, there are indeed only two sorts of anurans. Here’s one of the frogs, the common frog (North American readers will note the resemblance to our wood frog, Rana sylvatica, which also has a tympanic dark spot and dorsolateral folds):
The Common Frog (Rana temporaria), near Bad Kohlgrub, Bavaria. Photo by Richard Bartz (Wikimedia).
And here’s one of the toads, the common toad (the green flecks are duckweed or some other plant):
Common Toad (Bufo bufo), Broomscroft, Kent. Photo by Peter K. Moore.
We can distinguish frogs from toads, because frogs are more aquatic, with long hind limbs for jumping, webbed toes (easily seen above), and moist, smoother skin. Toads are more terrestrial, squat with short legs for hopping, and have dry, warty skin. And this distinction works for the anurans of Britain– the frogs are members of the family of “true frogs”– Ranidae, while the toads are members of the family of “true toads”– Bufonidae.
As the English encountered more kinds of anurans around the world, each new anuran was shoe-horned into being either a frog or a toad. Thus the long limbed, arboreal, jumping anurans of the family Hylidae (which English nobility would have encountered in their Continental estates) were called, aptly enough, tree frogs. And in the North American colonies, the squat, warty burrowing members of the family Pelobatidae were called spadefoot toads. But with dozens of families of anurans, and a great diversity of ecological habits and body forms, the distinction breaks down, and our English common names wind up forcing an exuberant diversity into just two names.
I wonder to what extent the biodiversity of a language’s native land influences the language’s naming diversity. In the only other language I (sort of) speak, Spanish, I know three words– rana (for frogs), sapo (for toads), and maco. The latter is a word I learned in the Dominican Republic, and it has a very different meaning in standard Castilian, as given by the Real Academia Española: it means ‘knave’ or ‘rogue’ if converted to a noun (in Castilian it is an adjective). It’s possible that the Dominican word is of Taino or West African origin, rather than Spanish.
So, I’d like to ask our non-Anglophone readers, how many words for kinds of adult anurans are there in your language? And how does this compare to the biological diversity?
[The title of the post refers of course to a story about demons in the Gospels. A Roman legion had 6,000 men (and there are about 6000 species of anurans). If each family of anurans should have a common name, there names would not be quite legion, but there would be a lot more than two!]
In a previous post here at WEIT, I’d reported on some toads and a painted turtle that I’d rescued from stair and window wells, and then released back into the wild last spring. I’d mentioned at the time that I periodically check these places, especially a deep (ca. 20 feet down) window well on the west side of the building my office is in, because it faces a pond and woods, and animals coming out of the woods regularly fall down into it. So at the beginning of the semester in early September, I took my vertebrate zoology class out during our first lab period, and we investigated the window well. There was a pretty good ‘crop’ this fall– eleven American toads (Bufo americanus), and 23 green frogs (Rana clamitans).
American toads just outside Greenquist Woods, Kenosha, Wisconsin, 17.ix.2015. There are clearly six of them, the same ones as in “Spot the toads“.
The toads fell into two size classes: medium (in picture above), about 55 mm snout-vent length, and small, about 40 mm. The green frogs were all about the same size– 35 mm. These latter were probably all a single age class, having metamorphosed from tadpoles earlier in the summer, and then hitting the building and falling in the window well as they began to disperse away from their natal pond. Bullfrogs (Rana catesbeiana) are also common in the pond, but we’ve never found them in the window well– they must have different dispersal behavior.
Chris Noto helps to release American toads in Greenquist Woods, 17.ix.2015.
Frogs and toads are collectively known as anurans— it means “not having a tail”– and adult frogs and toads do, of course, lack tails. The anurans we rescued from the window well had been there varying lengths of time, but most were in at least decent shape, though some were thin and dehydrated. We kept them in the lab for a couple of weeks, feeding them and rehydrating them. We then released them on two warm days just as autumn was about to begin. My colleague Chris Noto was teaching a lab on a floor looking out over the woods, and he saw me encumbered with toads as I attempted to take their pictures and carry them back out to the woods. He came down and helped with the pictures and the release (which were featured in two “Spot the __” posts, on frogs and toads).
Small American toad, about to be released into Greenquist Woods, 17.ix.2015.
The green frogs were released a few days later.
Green frogs anxiously await their transfer to the pond, 20.ix.2015.One last picture.Well, maybe just one more.And then it’s time to re-enter the pond.Green frogs queue up to enter Greenquist Pond, 20.ix.2015.
[JAC: a video of the release. I have to commend Greg for both taking the time to rescue these frogs and also calling them to our attention. Frogs are not only underappreciated animals, but are harbingers of human damage to the environment, climatic and otherwise. And I’ve always said that if frogs hadn’t evolved, we simply wouldn’t be able to imagine them!]
There are at least a dozen or more different sorts of anurans around the world that are worthy of their own vernacular name, but because only two sorts occur in England, we are stuck with calling them all either “frog” or “toad” in English. The American toad and the green frog do, however, correspond to the two sorts found in England (what are sometimes called the “true toads” and the “true frogs”, respectively).
Anurans are amphibians, and like most amphibians, have a complex life cycle. The word “amphibian” alludes to this– it means “both lives”, because a typical amphibian lives both on the land and in the water. Reptiles and their descendants (the amniotes, including birds and mammals), do not have this dual life cycle. One of the former candidates for the title of “first reptile” was the 270 million year old Seymouria, which has reptile-like features; but when it was found that its close relatives had aquatic larvae with gills, it was clear they were not reptiles, but rather led the “both lives” of an amphibian.
“Both lives” does not seem to adequately summarize the life of a typical amphibian, such as the American toad. They begin life as eggs in water, hatch out as tailed, gilled, tadpoles, that then swim about, eventually losing their tails and gills and sprouting legs to transform into toadlets, which then move onto land. After sexually maturing, they return to the pond each spring, to resume an amphibious existence, there to mate and reproduce. The adults then leave the pond for the summer to live wholly on land, while their eggs begin the complex cycling again. To paraphrase The Who, “Amphibians? They’re bleeding Quadrophibians.”
As a preview of an upcoming post on frogs and toads, a chance to practice your spotting skills. No real difficulty here, but you need to spot all of them.
Green frogs (Rana clamitans) entering Greenquist Pond, Kenosha, WI, 20.ix.2015.
As we’ll see in the upcoming post, these are “rescue frogs”.
Well, it’s not that hard to spot, but you can see how the wood frog (Rana sylvatica) is aptly named.
Wood frog, near Lake Superior, Minnesota, 10 June 2014.
My Minnesota correspondent found this fellow along Caribou Trail (a road) and Jonvick Creek near Lutsen, Cook County, Minnesota, about a half mile from the north shore of Lake Superior, on 10 June 2014. The region is mixed spruce and maple forest; the frog was in a “mapley” area. The great herpetologist Robert C. Stebbins thought the species’ distribution tracked, for much of its range, pretty closely to the distribution of spruce.
The distribution of wood frogs is interesting for at least two reasons. First, they are the most northerly distributed of any North American amphibian (or reptile, for that matter), and extremes are always interesting. They can survive for weeks at temperatures below freezing, in part through elevated levels of blood glucose acting as an “anti-freeze”.
Range of the wood frog (Rana sylvatica), from USGS via Wikipedia.
They’re not immune to freezing though—I once found, during an early spring field trip near Northampton, Massachusetts, a dead female who had laid her eggs in a small pond. She was perfectly intact, and I suspected she had frozen, as contact with ice crystals (from the pond) makes them more vulnerable to freezing.
Second, there are a number of outlying populations to the south of the main range (which, as shown above, crosses northern North America from the Bering Strait to the North Atlantic, descending into eastern North America along the Appalachians). In particular, note the outliers in Colorado and Wyoming. These are almost certainly relicts from cooler glacial times when the frog occurred further south in the Rockies; it moved northward as the glaciers retreated, leaving behind populations in some favorable southern localities. The isolated Colorado-Wyoming population was named as a distinct species (maslini), but currently it is not recognized, not even as a subspecies.
Wood frogs are are also famous for another “non-subspecies”: cantabrigensis, a short-legged form from the northwestern part of the range, versus the longer legged ones to the east. While the variation in leg size is real, there is a gradual cross-continental gradient (a cline, in technical terminology), with no break in leg size, and most systematists do not distinguish such clinal patterns of geographic variation with nomenclatural recognition. So cantabrigensis is not recognized either, and the wood frog has become a classic case of clinal variation.
Bagdonas, K.R. and D. Pettus. 1976. Genetic compatibility in wood frogs (Amphibia, Anura, Ranidae). Journal of Herpetology 10:105-112 (jstor)
Costanzo, J.P., M.C.F. do Amaral, A.J. Rosendale and R.E. Lee. 2013. Hibernation physiology, freezing adaptation and extreme freeze tolerance in a northern population of the wood frog. Journal of Experimental Biology 216:3461-3473. (pdf)
Dodd, C.K. 2013. Frogs of the United States and Canada. Johns Hopkins University Press, Baltimore (publisher) (Google books)
Porter, K.R. 1969. Evolutionary status of the Rocky Mountain population of wood frogs. Evolution 23:163-170. (jstor)
Stebbins, R.C. 2003. A Field Guide to Western Reptiles and Amphibians. 3rd ed. Houghton Mifflin, Boston. (publisher)
