The morphlings versus the axolotls (how frogs have warped tadpoles into new shapes and sizes), part 1
In the process of metamorphosis typical of class Amphibia, different parts of the body can be transformed at different stages.
This phenomenon, called heterochrony, helps to explain the evolutionary diversification of this class of vertebrates.
It is well-known, for example, that all of the ten families of salamanders (https://en.wikipedia.org/wiki/Salamander) retain juvenile features as the body grows to full size. In all of these families, paedomorphosis (retention of juvenile features during development, https://en.wikipedia.org/wiki/Heterochrony) has previously been recorded.
In particular, the larval gills are kept beyond sexual maturity in various salamanders loosely termed axolotls (https://en.wikipedia.org/wiki/Axolotl).
Narrowly defined, ‘axolotl’ refers only to Ambystoma mexicanum (Ambystomatidae, https://www.inaturalist.org/taxa/26777-Ambystoma-mexicanum), which reaches sexual maturity and full body size as an aquatic larva retaining gills. However, this term can be loosely applied to other paedomorphic salamanders in different genera and families.
In the case of frogs (https://en.wikipedia.org/wiki/Frog), by contrast, such flexibility (i.e. heterochrony) in the metamorphic sequence is not generally recognised.
The order Anura contains 53 families worldwide.
However, paedomorphosis has not previously been recorded, in the literature, in any of them. Virtually no species of frog postpones the loss of juvenile features such as gills.
Such disparity between the two orders of amphibians would be understandable. This is because typical tadpoles are so different from adult frogs that it seems implausible that their features could be usefully retained in frogs in the ways seen among salamanders.
However:
On closer scrutiny, the metamorphic sequence is in fact somewhat flexible in many lineages of frogs worldwide. The retention of various juvenile features - other than gills - in adult frogs tends to have been overlooked/underinterpreted.
Most intriguingly:
True toads (Bufonidae, https://www.inaturalist.org/observations?taxon_id=21359) show the opposite pattern: namely the early loss of gills and the external tail when the body is still remarkably small.
The opposite pattern from paedomorphosis is peramorphosis (https://en.wikipedia.org/wiki/Heterochrony). Although paedomorphosis is well-known in amphibians, peramorphosis remains to be studied, as such, in frogs.
Only salamanders show much developmental flexibility within a species. However, the following observations show the considerable variation among species, genera and families of frogs, in
- precociality, or
- retention of juvenile features.
Please note the following:
Firstly, paradoxical frogs resemble axolotls in postponing the loss of larval features. The result is that the fully-grown tadpoles are larger than the fully mature frogs, with metamorphosis involving a permanent shrinkage.
The extreme examples of paradoxical frogs are Pseudidae. However, a similar pattern occurs in
- European spadefoot toads (Pelobatidae, https://www.inaturalist.org/observations?taxon_id=25308),
- midwife toads (Alytidae: Alytes, https://www.inaturalist.org/observations?taxon_id=24472), and
- African running frogs (Hyperoliidae: Kassina, https://www.inaturalist.org/observations?taxon_id=23499).
Secondly, glass frogs (Centrolenidae, https://en.wikipedia.org/wiki/Glass_frog) and Indian frogs (Ranixalidae: Indirana, https://www.inaturalist.org/observations?taxon_id=26137) lose the external tail so long after losing the gills that even the terrestrial stage of these frogs initially has a tail longer than the head and body.
New Zealand frogs (Leiopelmatidae, https://www.inaturalist.org/observations?taxon_id=23269) show a similar pattern although the tail is already short just after metamorphosis.
Thirdly, large species of true toads tend to metamorphose (acquiring lungs and completely losing their external tail) when still diminutive, with body masses two orders of magnitude less than those at sexual maturity.
Glass frogs temporarily retain a long tail beyond the point of metamorphosis from an aquatic tadpole to a terrestrial ‘adult’. This shows that
- the distinction in body form between frogs and salamanders is not absolute, and
- the potential remains for frogs to converge with salamanders in evolutionary terms.
The tail disappears before sexual maturity is reached. However, it is likely that the skin of the tail is shed and eaten at least once in the life of glass frogs.
This is significant because skin-shedding and -eating does not occur in tadpoles.
Here we have an overlooked example of the retention of juvenile features in frogs, because the larval tail has been retained past the point of conversion from an aquatic tadpole (using gills) to a terrestrial ‘adult’ (using lungs).
In large salamanders, the main pattern is the retention of larval features at adult body sizes. In large true toads, the main pattern is the acquisition of mature features at larval body sizes. Conversely, in several types of frogs, metamorphosis is delayed to the point that the larvae reach – and even exceed – the fully mature body size of the adults.
It is in the Americas that developmental flexibility has produced the greatest range of body forms in both frogs and salamanders.
For example:
- In certain parts of tropical South America, there is sympatry among toads, paradoxical frogs and glass frogs.
- In North America, sexually mature salamanders vary from forms respiring by means of gills (e.g. axolotls among Ambystomatidae) to forms lacking both gills and lungs (e.g. adult Plethodontidae, https://www.inaturalist.org/observations?taxon_id=26909), as well as normal forms possessing gills as larvae and lungs as adults (e.g. Salamandridae, https://www.inaturalist.org/observations?taxon_id=27701).
What emerges is that developmental flexibility, although different from the narrowly-defined heterochrony seen in salamanders, has been underrated in the evolution of frogs.
And just as the existing name ‘axolotl’ is useful in describing salamanders with adult sizes but larval body forms, so a new name, ‘morphling’, would be useful in describing toads and other frogs which attain adult body forms at larval body sizes.
to be continued in https://www.inaturalist.org/journal/milewski/87002-the-morphlings-versus-the-axolotls-how-frogs-have-warped-tadpoles-into-new-shapes-and-sizes-part-2#...
Comments
morphling’9, :: for Western Leopard Toads, these are known as "toadlets" and are the dispersive phase from the tadpole wetlands, mass emigration taking place unpredictably during aseasonal summer rains in December-January. [eggs are laid during mass breeding events over a few days in August-Sept; so after about 4 months; toadlets more than double in size by the next breeding season, but are still juvenile and dont breed for a few years]
The hylid Litoria caerulea (https://www.inaturalist.org/taxa/517075-Ranoidea-caerulea) is a large frog. Its tadpole reaches about 5 cm long including the tail. The freshly metamorphosed (tailless) frog has a snout-vent length of about 1.6 cm, which means that it is initially about the same size as a man’s thumbnail. That means about the size of those big dung beetles one sees in elephant faeces. It can live >20 years, and over that time it grows to a snout-vent length of 10 cm.
Compare this with bufonids. Although the mature cane toad (Rhinella marina) is bigger than Litoria caerulea, its tadpole is smaller, reaching only 3.1 cm long including the tail. The freshly metamorphosed (tailless) stage is about 0.7 cm long snout-vent, about the size of a blowfly. This then goes on to grow even more than is the case in Litoria caerulea.
So there is a difference between this hylid and this bufonid, in relative size of tadpole and freshly metamorphosed frog. The difference in body mass is about an order of magnitude, in the case of both the fully-grown tadpole and the freshly metamorphosed ‘adult’ (tailless for the first time).
There is certainly a difference in life history strategy here. It remains debatable whether this difference justifies the introduction of a new term, viz ‘morphling’ (which needs objective criteria), for the extremely small freshly metamorphosed stage of the bufonid, which is more fecund than the hylid.
The value of making this distinction would be more apparent in a comparison with Pseudis (https://www.inaturalist.org/observations?taxon_id=24406), which has a much larger tadpole again and does not seem to grow much after metamorphosis.
Incidentally, Hyperolius (https://www.inaturalist.org/observations?taxon_id=23276) has a tadpole that is larger, relative to the mature size of the frog, than is the case for Litoria caerulea. This is partly because the fully mature Hyperolius is so small. No matter how we define ‘morphling’, Hyperolius would certainly not qualify as possessing such a stage in its life history. The whole concept of a ‘morphling’ probably only matters in large frogs, i.e. frog species in which the fully mature stage has a snout-vent length of say >5cm.
The morphlings of Anaxyrus terrestris (https://www.inaturalist.org/taxa/64988-Anaxyrus-terrestris), which I take to have snout-vent length of ca 8 mm, can be as small as 0.055g according to the figures in http://onlinelibrary.wiley.com/doi/10.1046/j.1365-2435.2000.00386.x/pdf
For comparison, a house fly (length 6mm) has body mass of 0.012g. This means that even the smallest morphlings of Anaxyrus terrestris are about five-fold heavier than the average house fly.
Fully mature Anaxyrus terrestris reaches snout-vent 8 cm or up to 9.2 cm, which is large for a frog.
So the morphlings are small but certainly much larger than fruit fly size.
This doesn’t necessarily apply to Sclerophrys pantherina (https://www.inaturalist.org/observations?taxon_id=517449), but toads in general are remarkably fecund. They represent the side of the amphibian spectrum that specialises on sheer fecundity, as opposed to parental care. Toads (Bufonidae) typically lay huge clutches of thousands of eggs, and in the largest spp. this runs to tens of thousands.
This means that there are probably thousands of morphlings emerging from the water at a time in many environments around the world that have toads, quite possibly including the marshes of the southwestern Cape of South Africa.
In its original state, Tokai (https://en.wikipedia.org/wiki/Tokai,_Cape_Town) would have had times of the year when large numbers of morphlings of the western leopard toad would have emerged from the water and foraged in fynbos. These morphlings may perhaps have remained close to the water and vlei vegetation for a time, as opposed to roaming far and wide in sparser vegetation.
However, the morphlings would indeed have been plentiful where they occurred, because the life history strategy of toads is to produce extreme numbers of eggs, somewhat reduced numbers of tadpoles, somewhat reduced numbers of morphlings (but still great numbers compared with those frogs that lack the morphling stage), and then a gradual loss of numbers through to full maturity (partly via cannibalism).
It is a numbers game with toads, whereas in many other frog lineages around the world the emphasis is instead on parental care of a relatively small clutch, and even extended parental care of the tadpoles themselves.
What I have not heard of yet is any frog species that has parental care of metamorphs, whether morphlings or not. Once metamorphosis occurs, all of the nearly five thousand spp. of frogs in the world seem to be on their own.
This, incidentally, is what is so significant about centrolenid frogs: at the time of metamorphosis they retain a tail as long as the body + head (= snout-vent length).
Please consider that metamorphosis is the closest thing in the life histories of frogs to a standard point of reference. No matter what the life history strategy, all frogs metamorphose, and beyond this point there is no parental care in any frog.
For centrolenids (and possibly certain other families) to transgress this standard-point in development by retaining such a long tail after leaving the water is an overlooked aspect of amphibian biology.
Putting the two groups, i.e. bufonids and centrolenids, together:
Bufonids metamorphose when they are so small that it would not be surprising if they retained a long tail. Yet they do not: they are perfectly formed ‘little adults’. By contrast, centrolenids are not diminutive at the tadpole or metamorph stage, and yet they do retain a long tail.
What has been overlooked is that centrolenids are like salamanders in showing paedomorphosis. Whereas the heterochrony in salamanders (in which paedomorphosis is well-recognised) is centred on the gills, the heterochrony in centrolenids is centred on the tail.
So the upshot seems to be that within the frogs there are two diverging life history strategies: some frogs such as bufonids show peramorphic heterochrony, losing their tail at a remarkably small size relative to full maturity. Other frogs such as centrolenids show paedomorphic heterochrony, retaining their tail at a remarkably large size relative to full maturity.
Note that the metamorphs of a large species of toad and a typical centrolenid are actually of similar absolute size, with snout-vent lengths about 0.7 cm. But one is quite tailless whereas the other has a tail as long as its head + body.
Large toads have tadpoles no longer than about 3 cm, which metamorphose into small ‘adults’ with snout-vent length not much more than 1.5 cm (and in some cases, I gather, as little as a third of this).
By contrast, the American bullfrog (Lithobates catesbeianus, https://www.inaturalist.org/taxa/65979-Lithobates-catesbeianus), which is about the size of a large toad in maturity and is probably the largest non-toad frog in North America, has tadpoles up to 17.7 cm long. I have not found data on snout-vent length at metamorphosis but if the tadpole is >15 cm then presumably the frog is initially at least 5 cm long, severalfold the corresponding size in toads.
The metamorphs of L. catesbeianus can weigh 5 g, compared to <0.05 g in the case of bufonids with similar mature sizes. That’s a difference of two orders of magnitude in body mass at the stage of metamorphosis.
I think it’s safe to say that in most comparisons of bufonids with ranids of similar mature body mass, we can expect a difference of an order of magnitude in body mass at metamorphosis.
Furthermore, ranids such as Lithobates seem to be a bit like centrolenids, in retaining more of the tail at metamorphosis (when the animal leaves the water) than is true of most families of frogs. Bufonids don’t seem to go in for the retention of a residual tail at all.
So typical toads and typical frogs (ranids, so familiar in Europe and North America, although absent from South Africa where their place is taken by pyxicephalids) are similar in their commonness and fecundity, but differ in their development: bufonids have morphlings whereas ranids do not.
Western Leopard Toads do not return to water after dispersal. They stay in moist environments, but during the dispersal events toadlets permanently leave the ponds and move away to "adult" 'dry'land habitats. For the bulk of the rest of summer the toadlets are in aestivation under stones, logs and possibly underground, although in watered gardens they may remain active.
The paper below, Cohen & Alford (1993), gives data on the body sizes of morphlings for Rhinella marina.
I infer that a morphling can be defined in this species as having snout-vent length less than 3 cm.
The smallest morphlings seem to have snout-vent length of 9 mm, which is bigger than I thought, at least in the population these authors studied. I can’t understand how a morphling of 0.9 cm could possibly weigh as little as 0.025 g (which if memory serves is the minimum body mass given a paper by Shine). A morphling of length 0.9 cm is certainly at least blowfly size, not housefly size and certainly not fruitfly size. But I would still classify these as morphlings, because even at ca 1 cm they are still tiny relative to fully mature body sizes. So tiny that an additional, even tinier, larval stage seems ‘over the top’.
The significance of the morphling stage in toads is that:
this stage combines larval body size with adult form;
toads have essentially two consecutive ‘baby’ stages in their life history, viz tadpole and morphling;
morphlings are subject to cannibalism by juveniles regardless of whether they are also cannibalised by adults;
morphlings are part of a life history strategy of extreme fecundity (enormous clutches of eggs, up to 30,000 by a single mother), in which parental care is replaced by parental ‘hyperinvestment’.
http://www.publish.csiro.au/paper/WR9930001.htm
I assume that the lungs of toads only develop at or after metamorphosis (?known as long ago as 1931 when Noble wrote his book about amphibians).
However, it’s worth noting that in many other families of frogs the lungs start to develop well before metamorphosis, along with the developing legs.
This means that the later stages of the tadpole already possess, and use, lungs in many frogs as well as in the salamanders.
Toads seem to be an exception, which makes sense to me in view of the tiny size of the toad tadpoles.
So in toads the appearance of the lungs coincides with the loss of the external tail, whereas in salamander larvae there is no such coincidence because lungs are already present in the larva, and the tail is not lost; while in most tadpoles there is no such coincidence because the lungs develop before the tail is lost.
The following reference invokes peramorphosis in frogs of the family Ceratophryidae (https://www.inaturalist.org/observations?taxon_id=64727). From the abstract it’s not clear what the basis of peramorphosis is.
The ‘early onset of metamorphic transformations’ mentioned in the abstract is indeed what peramorphosis is all about, but the features concerned in this case seem too subtle or obscure to be specified in the abstract. So I’ll have to delve into the body of the paper itself.
http://onlinelibrary.wiley.com/doi/10.1111/j.1096-3642.2008.00420.x/abstract;jsessionid=0FB2DFDE4A06FDA17DBD9C02A397C7C9.f03t02?deniedAccessCustomisedMessage=&userIsAuthenticated=false
The following illustrations show the body sizes relative to human figures for scale.
The species illustrated is the natterjack toad (Bufonidae: Epidalea calamita, https://www.inaturalist.org/taxa/65450-Epidalea-calamita) of Europe, but the body sizes and shapes are typical of many bufonids including Sclerophrys pantherina.
The natterjack toad is about average size for a toad, usually about 7 cm snout-vent length in maturity. The tadpoles, which complete their growth within two months, are small and this toad metamorphoses at about 0.7 cm snout-vent length.
Initially the morphlings (which are diurnal, presumably to avoid being cannibalised) are easily mistaken for invertebrates in the poolside herbage. The morphlings are so small relative to the fully mature animal that they take 3-4 years to reach sexual maturity.
The point of all of this is that toads typically metamorphose at remarkably small body size, which means that their life history can best be understood by dividing it into not just the three stages normally described for amphibians, viz eggs, larvae, and adults, but rather into four stages: eggs, tadpoles, morphlings, and adults.
The morphling can be thought of as an adult at larval size, and the development of toads can be interpreted, in a sense, as peramorphic – although I’ve never seen this suggested in any literature or anywhere on the internet.
Bufonidae: Epidalea calamita: mature individual:
http://i1.rgstatic.net/i/profile/60b7d54391f1a0ec3a_l_c69d9.jpg
Epidalea calamita morphling:
http://www.denbighshirecountryside.org.uk/files/Natterjack%20toadlet%202012%201.jpg
Epidalea calamita morphling:
http://farm7.staticflickr.com/6094/6349420673_f129805899_z.jpg
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 (https://www.inaturalist.org/observations?taxon_id=26688), 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.
@olvr_a @alexanderr @calebcam @tom-kirschey @herping_with_berks @tyroneping @m_burger @wernerconradie @simontonge
Perhaps the most famous of all frogs w.r.t. paternal (fatherly) care, namely Darwin’s frog (Rhinodermatidae: Rhinoderma darwinii http://amphibiaweb.org/cgi-bin/amphib_query?where-genus=Rhinoderma&where-species=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’ http://www.bbc.co.uk/nature/life/Darwin'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 (https://www.inaturalist.org/observations?taxon_id=24581) in Africa and Eleutherodactylus (https://www.inaturalist.org/observations?taxon_id=22086) 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:
http://ichef.bbci.co.uk/naturelibrary/images/ic/credit/640x395/d/da/darwins_frog/darwins_frog_1.jpg
Newborn Rhinoderma darwinii; this individual looks less than 1 cm long to me:
https://frogmatters.files.wordpress.com/2010/02/baby-rhinoderma.jpg
To North Americans, leopard frogs (Ranidae: Lithobates pipiens https://www.inaturalist.org/taxa/66003-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 https://www.inaturalist.org/observations?taxon_id=922252 (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.
http://wyomingnaturalist.com/images/herps/A_FROG_Northern_Leopard_Frog_20.jpg
The following is another photo of the same type of frog, again showing similar mature dimensions.
http://cache1.asset-cache.net/gc/160009510-leopard-frog-rana-pipiens-in-a-small-childs-gettyimages.jpg?v=1&c=IWSAsset&k=2&d=9K5noEkSGKgRk%2BspTItEKPJjIb4IufRdEtLuAbvq3xXsAA1wCgoOHtdTkteLQbpK
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.
http://i15.photobucket.com/albums/a368/Pareeeee/LeopardFrogJuvenile1.jpg
The 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.
http://www.rinr.fsu.edu/fall2004/images/hands1.jpg
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The following photos illustrate the diminutive size of morphlings well. Both species shown are common toads in Europe.
Bufonidae: Epidalea calamita morphling. The adults reach 7 cm snout-vent length and I’ve previously emailed you the information that the snout-vent length of the morphlings is about 0.7 cm immediately after metamorphosis. My middle finger is 2 cm wide, and by that standard the morphling in this photo would indeed be about 1 cm long. You can get an idea of the relative size of the fully mature adult by considering that the palm in this photo is probably 9 cm wide, 2 cm in excess of the snout-vent length of the fully mature toad (7 cm). As you can imagine, this means that this is a small ‘baby’ as babies go.
https://farm6.staticflickr.com/5269/5674811787_9812b21909.jpg
Bufonidae: Bufo viridis morphling. Adults reach 7-8.5 cm snout-vent length (which is larger than the E. calamita) and a maximum body mass of about 66 g. This morphling is about the same size as that of E. calamita, shown above.
http://farm3.static.flickr.com/2159/5799820149_caf703400d_m.jpg
Bufonidae: Bufo viridis growth curve. The data on maximum snout-vent lengths are real but the point of intersection of the y axis is extrapolated, in my opinion incorrectly. The graph suggests that this toad species metamorphoses at a snout-vent length of >1.7 cm, which I think is nearly double the real value. As you can infer, B. viridis remains a morphling for only a short period (<6 months), i.e. the morphling stage is confined to the first growth season in this rather typical toad. The tadpoles are presumably hatched in spring, metamorphosing in summer, and the morphlings have already grown to juveniles by the time winter interrupts their growth (this seasonality is of course smoothed out in the graph below).
http://www.medwelljournals.com/fulltext/?doi=javaa.2011.1469.1472
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 (https://www.inaturalist.org/taxa/25737-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.
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: https://en.wikipedia.org/wiki/Cane_toad#/media/File:Bufo_marinus_distribution.png
Rhinella diptycha: https://en.wikipedia.org/wiki/Rhinella_diptycha#/media/File:Distribution_map_of_Rhinella_schneideri.svg
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.
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 (https://www.inaturalist.org/observations?taxon_id=25337), 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.
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 (https://www.inaturalist.org/taxa/64975-Anaxyrus-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.
See 'Cannibalism is Common' in http://www.canetoadsinoz.com/babytoads.html.
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, https://www.inaturalist.org/journal/milewski/67274-why-are-amphibians-so-cannibalistic#.)
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