The morphlings versus the axolotls (how frogs have warped tadpoles into new shapes and sizes), part 2

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The best examples of morphlings are to be found in true toads (Bufonidae), and particularly the largest species of toads. While some of the African and Asian toads are fairly large as frogs go, the true giants of the bufonid family occur in the Neotropics. Below I show the distribution ranges of two of the largest spp., namely Rhinella marina and R. diptycha. There is some overlap between these two species in the southeastern Amazon basin, but essentially the former is found to the north and the latter to the south. Rhinella marina extends from southernmost Texas all the way to the Amazon. Rhinella diptycha takes over in the caatinga, cerrado, chaco, Atlantic forest, and Pantanal, reaching northern Argentina although it does not extend as far south as the Pampas.

What this means is that, collectively, these two spp. of large toads cover most of South and central America. Even if morphlings were restricted to just these two spp. and no other frogs, their existence would be noteworthy, not so?

Rhinella marina:

Rhinella diptycha:

See 'Cannibalism is Common' in

This was written by Rick Shine or his colleagues, and explains how cannibalistic the large toad Rhinella marina (introduced into Australia and now a major pest) can be. As he points out, it’s not the fully mature individuals that are most cannibalistic, it’s the young adults. These young adults semi-specialise on eating the morphlings of their own species (which Rick Shine of course does not call morphlings, calling them ‘metamorphs’ instead. But the point is that the life history strategy of this large species of true toad, which is presumably typical of Bufonidae generally, involves a level of cannibalism that is systematic rather than being an occasional aberration. Each individual of this species has to survive a veritable gauntlet of cannibalism during the course of its life, in the morphling stage which is like a second infancy after metamorphosis.

(See my other Post about cannibalism in amphibians,


I have been unsure whether the incidence of morphlings is essentially a scaling phenomenon within Bufonidae, or a phenomenon that will remain after corrections for fully mature body size. I’m leaning towards the latter based on a small species of North American toad, namely Anaxyrus debilis.

Anaxyus debilis ( is so small that I would not expect it to have morphlings. However, its metamorphs are larger than I expected, and far larger than those of enormous toads such as Rhinella marina. This makes it clear that A. debilis falls into a different pattern of development, as opposed to just being a 'scaled-down cane toad' as it were, in which the morphlings cease to be remarkably small relative to the adults.

Breeding females of A. debilis have snout-vent length 4.6-5.4 cm, while metamorphs have snout-vent lengths 1.9-2 cm. Please bear in mind that the morphlings of R. marina have snout-vent ca 1 cm. Having metamorphs even smaller than in R. marina is what one would expect from the small species A. debilis if the two toads shared a pattern in common. Instead, the truth is that in A. debilis, compared with R. marina, the adults are smaller and the metamorphs are larger. So the pattern is broken. This suggests that morphlings occur in some, but certainly not all, bufonids. It’s still possible that morphlings occur in all large bufonids, but if so it won’t be just because they grow large.

Adding to the pile of sometimes inconsistent information on the actual sizes of adults and morphlings in true toads (Bufonidae):

I have before me a source that states that in Rhinella marina, the mature toads are male 14 cm and body mass 1 kg, and female up to about 23 cm and up to 1.5 kg. The morphlings are as follows: “In spite of the enormous size of its parents, a newly-formed cane toad is no more than 6-7 mm long.” This refers no doubt to snout-vent length.

Adults of Phrynobatrachus (, with snout-vent length about 2 cm, have body mass ca 0.5 g. I deduce that the body mass of the morphlings of R. marina is usually 0.1 g or less.

Putting the idea behind the invention of the word ‘morphlings’ as simply as possible:

Bufo bufo, the typical true toad, has a mature female body mass ca 100 g. The following value needs checking, but, assuming that the freshly metamorphosed toad has body mass ca 0.1 g (= ten to a paper clip!), these ‘babies’ are only 0.1% of maternal body mass. Yet these metamorphs are widely and unquestioningly called ‘adults’.

The central problem: how can a toad possibly be called ‘adult’ at only 0.1% of mature body mass? Fact is, true toads have BOTH LARVAE AND INFANTS, and ‘morphling’ is the name I suggest for these infants.

A problem with 'toadlet' is that morphlings are not peculiar to toads, and toads are not clearly defined anyway.

The morphlings are, remarkably, proportionately similar to the fully mature stage, i.e. morphlings are as TINY relative to the fully mature stage as infants would be, but are shaped like mature animals, not like infants.

Putting this another way: in true toads (particularly the largest spp.), the life history is divided into two completely different processes. During the larval stage, there is little change in body size but immense change in body form. After metamorphosis, the situation is quite different: there is immense change in body size but minimal change in body form. Of course this is the basic pattern in amphibians in general, but there is extreme polarisation in bufonids.

The trouble with the word ‘infant’ in this context is that it is too vague. Infant can mean any kind of baby. Instead of increasing understanding by boosting precision, it detracts from understanding by boosting vagueness. Putting this another way, imagine a scientific tradition in which frog tadpoles (which are particularly different from adults, even relative to salamander larvae) had no particular name but were just called ‘larvae’ or, worse still, ‘juveniles’. There’s absolutely nothing incorrect about calling frog tadpoles juveniles, because that is indeed what they are. The trouble is that, while correct, this is too vague.

I only discovered at age 63, after a lifetime of particular interest in and study of frogs, that true toads have exceptionally tiny ‘adults’, because this fact, although known for many centuries, has been hidden by the lack of an apt term.

Another problem with calling these juveniles of true toads ‘infants’ is that this would introduce unnecessary confusion between the ‘adult’ infants and the larval infants. If the morphling is really an infant, then why is the tadpole not also an infant? For example, a kangaroo neonate is just as much an infant as a zebra neonate, obscuring the enormous difference in degrees of development between extremely altricial and extremely precocial neonates among mammals. If one said ‘frog infant’ to most people, I suspect that they would imagine tadpoles. For that matter, why don’t we call foetuses in mammals ‘infants’? In Science, the more precise and specific the word used to describe something, the better.

It seems basic to the definition of morphlings that they would belong mainly, or only, to large species of frogs. This is because what ‘morphling’ describes is a ‘second babyhood’ after metamorphosis, which makes most sense where the eventual fully mature body size is far greater than that at metamorphosis. Of course, fully mature body size is not the only operative variable; also important is the maximum size of the tadpoles. For example, the huge toad Rhinella marina has both large mature body size and extremely small tadpoles at full development of its larval stage.

Based on this thinking, it seems sure that another example of a frog lineage with morphlings is the Conrauidae of Africa. This family contains the largest of all frogs, Conraua goliath (, which can reach 3.6 kg. A frog that large seems likely to qualify for morphlings just on mature body size alone, but as it happens C. goliath also qualifies in terms of its larva: the tadpoles are of unremarkable size compared with other frogs, reaching only 5 cm long before metamorphosing. Those would be large tadpoles for a bufonid, but they are more or less the same size as those of large ranids.

For comparison, the largest frog in North America apart from Rhinella marina is Lithobates catesbeianus (Ranidae), which reaches a maximum body mass of 0.8 kg and has tadpoles 5-7.5 cm long and up to 18 cm long. The African and North American giant frogs are directly comparable because both are among the more aquatic of frogs worldwide.

The West African giant has tadpoles less than a third the length of the North American giant despite having fully mature mass four-fold greater. Morphlings for sure?

This is rather nice, because what it would mean is that both the largest aquatic frog on Earth (C. goliath) and the largest terrestrial frog (R. marina) on Earth have morphlings.


To North Americans, leopard frogs (Ranidae: Lithobates pipiens and related spp.) are bog-standard frogs, similar to the closest thing to a bog-standard frog in the southwestern Cape of South Africa, namely Amietia fuscigula (Pyxicephalidae). The mature frogs are >10 cm snout-vent length, and the freshly metamorphosed juveniles have a snout-vent length of about 2.5 cm, which is a quarter of the mature dimension. The juveniles grow for a further three years before reaching sexual maturity, and then after that keep growing to some extent to full maturity.

These relative sizes are illustrated below. It may help to know the dimensions of my hand: 10 cm wide at the palm, with the last section (phalanx) of middle finger being about 2.5 cm long.

As you can see, the freshly metamorphosed juvenile frogs are not particularly small relative to the human hand. They are about 2.5-fold longer than the morphlings of toads of comparable mature body size, which means that they presumably weigh an order of magnitude more than morphling toads, likewise freshly metamorphosed from the tadpole stage.

The following photo shows the mature size of Lithobates pipiens or a closely related species. If the palm is 10 cm wide, you can see that this frog exceeds 10 cm in snout-vent length.

The following is another photo of the same type of frog, again showing similar mature dimensions.

I’m not sure that the following juvenile is freshly metamorphosed, but it must be close. As you can see, its snout-vent length is about one inch. It’s certainly a ‘baby’, but not nearly as diminutive as morphling toads (Bufonidae). These relative sizes are quite ordinary for juvenile vs mature vertebrates and there’s no need for a special term for these juveniles. My guess is that, although the fully mature stage is comparable in body mass between leopard frogs and toads, the freshly metamorphosed stage is an order of magnitude different in body mass. That’s why I feel that the word ‘morphling’ is useful for the small juveniles. The morphling toad would weigh about as much as the visible section of upper hindleg of the frog in the photo below. following photo shows a handful of the freshly metamorphosed juveniles. Again, as you can see the body size is about one inch long snout-vent, with a body volume similar to that of the last phalanx of the middle finger of a man. Considering that a small standard paper clip weighs about 1 g, it seems safe to assume that the distal phalanx of the middle finger weighs more than a gram, and that in turn the freshly metamorphosed frog also weighs somewhat more than 1 g. Compare this with the minimum size of morphlings in that paper by Shine and co-authors, in which I remarked that it took 20 morphlings to weigh as much as a paper clip.


Perhaps the most famous of all frogs w.r.t. paternal (fatherly) care, namely Darwin’s frog (Rhinodermatidae: Rhinoderma darwinii, may have been hiding in plain sight as an example of what we call the ‘morphling’ phenomenon.

Nobody with any broader knowledge of frogs can have failed to hear of this species of frog, because the male does something so bizarre, with no parallel in any other animal: it ‘gestates’ the tadpoles in its vocal sac.

By the way, the description ‘vocal sacs’ is rather misleading in the same sense as ‘cheek-pouch’ is misleading for the extensive compartments into which certain hamsters stuff food while foraging. The sacs in question, in R. darwinii, extend from the throat all the way on the ventral surface of the male frog, to the groin and on the flanks almost to the back. Entrance to this modified and extended vocal sac, which creates a space between the skin and the muscles of the body, is gained through a pair of slits inside the mouth. What we’re talking about is a huge and newly-invented cavity, effectively sealed off, into which offspring can be inserted for the purposes of parental care. And from which there is a process of ‘giving birth’ because of the sphincters involved.

The male (snout-vent length 2.2-2.8 cm, slightly less than the female which reaches 3.1 cm) guards the large (diam. 4 mm) eggs until they are nearly hatched (which takes 3-4 weeks), and a noteworthy possibility is that the eggs he chooses are not necessarily the ones he’s fathered because the females lay up to 40 eggs, far more than the male can actually gestate. Just before they hatch, the male takes up to 19 eggs into his mouth and gets them to pass through the paired slits into the ‘vocal sacs’. The eggs hatch about three days later in this body cavity of the male (which may or may not be the father) body and, provided with enough yolk by the mother at the time of hatching, they continue to develop as larvae in this cavity, for 50-70 days. Goicoechea et al. (1986) have shown that the tadpoles nourish themselves partly on secretions inside the male’s sac, which would be an even more remarkable case of male gestation. The male ‘gives birth’ to the offspring when they have partly metamorphosed, with just a stump of the tail remaining and a length (presumably snout to ‘tail’ tip) of about 1cm (which is about the size of a morphling toad).

Please see this video clip of the male ‘giving birth’'s_Frog#p0074thp . You can see from this footage that the ‘neonates’ are small relative to the size of the male, which makes mechanical sense because he has to fit up to 19 into a cavity under his skin. If the adult male is 2.5 cm long and his newborns about 1 cm long, this may not seem like a big difference in length. However, because of overall scaling principles one would not expect the newly metamorphosed frogs to be as small, relative to mature size, as we find in large toads. Considering how tiny the adults are in R. darwinii, I’d suggest that the newborns are small enough to be called ‘morphlings’, which makes sense because a lot of them have to be accommodated in the ‘vocal sac’.

A different way of putting this: morphlings may be consistently about 1 cm long in all species, regardless of the great variation in adult body sizes, because of an allometric exponent. What makes them morphlings is that the babies are small relative to those of other frogs of similar adult sizes.

What constitutes a morphling inevitably depends on the body size of the frog species in question. But the bottom line is that any one inch-long adult male that accommodates more than a dozen newly metamorphosed offspring in a single cavity of his body is almost self-evidently accommodating morphlings, i.e. unusually small metamorphs.

If so, I suspect that many or most frog lineages with external development (eggs laid out of the water, and development of the larvae within the egg capsules based on yolk) will turn out to have morphlings as well. This would include, for example, Arthroleptis ( in Africa and Eleutherodactylus ( in the Neotropics. This needs confirmation, though.

Rhinoderma darwinii newly born with adult male. This ‘baby’ may not look particularly tiny next to an adult male but please bear in mind that even the adult is only one inch long:

Newborn Rhinoderma darwinii; this individual looks less than 1 cm long to me:

Here are two more examples relevant to morphlings, heterochrony, and the flexibility of development in frogs.

We’ve seen that true toads feature morphlings, which are extremely small ‘adults’. The superficially toad-like Pelobatidae, which occur in Europe and spend much of their lives underground but breed in seasonal pools, turn out to be different, and more like paradoxical frogs in their life history.

Paradoxical frogs (Pseudis) have tadpoles that grow up to 25 cm long (taking four months to grow this large), then metamorphose into adults of only a bit more than 7 cm snout-vent length. Well, in the case of pelobatids such as Pelobates fuscus, the tadpoles can grow to 8-10 cm long or even up to 15-20 cm long in some cases (compared with only about 3 cm long for the tadpoles of even the largest true toads such as Rhinella marina). After metamorphosis the snout-vent length is a mere 2-4 cm, which means shrinkage even if one allows for the fact that the external tail has been lost.

I infer that this aspect of the development of pelobatids differs from their Nearctic counterparts the Scaphiopodidae (, which is perhaps one of the reasons why the spadefoot toads of the Northern Hemisphere, previously all lumped into one family, have now been split into a North American family and a different Eurasian family.

Secondly, in the Hyperoliidae, an African family that includes apparently annual reed-frogs in the genus Hyperolius:
the South African Kassina maculata grows to 6 cm snout-vent length as an adult, but its tadpoles reach up to 13 cm before metamorphosing (the larval phase takes up to 10 months). Here again, there must be shrinkage even allowing for the loss of the external tail.

So European Pelobates and African Kassina seem to be further examples of the phenomenon episomised by South American pseudids.

Again, note the difference:
Sclerophrys pantherina stays small as a tadpole, and fully metamorphoses into a tiny version of the adult, but elsewhere in South Africa (as far south as Zululand) we have the hyperoliid Kassina which does the opposite, growing into a tadpole so large that, even in full maturity, the metamorphosed frog never regains such length in its head and body.

These are extremely different patterns but previously hidden by a lack of suitable terms. I dare say there are naturalists in South Africa who know much about frogs but don’t appreciate this axis of difference, because the literature has not brought it out for what it is.

to be continued in

Posted on November 24, 2023 12:00 AM by milewski milewski


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