Have tads really lost their pole? (de-tailed amphibians conceal a hiptail of unknown potential)
@tonyrebelo @jeremygilmore @ludwig_muller @olvr_a @alexanderr @calebcam @tom-kirschey @herping_with_berks @tyroneping @m_burger @wernerconradie @simontonge @graytreefrog @danieleseglie @syrherp @benjaminmb @sandboa
Frogs (class Amphibia, order Anura, https://en.wikipedia.org/wiki/Frog) seem consistently tailless, despite varying greatly in other respects and having diversified into some 50 families and five thousand species.
However, there is more to this than meets the eye.
Frogs are ancient, so the loss of the tail seems profound. The first fossil anurans appeared in the Triassic, around the start of the dinosaur era.
Does this preclude the potential to re-evolve a tail if there were adaptive advantages in doing so? For example, in Africa and southeast Asia there are no counterparts for the salamanders of equatorial South America (https://en.wikipedia.org/wiki/Salamander#/media/File:Cypron-Range_Caudata.svg). Why have certain frogs here not come to resemble salamanders as a result of natural selection?
The two main orders of amphibians seem to be aptly named from the ancient Greek word ‘URA’, meaning tail:
- Anura, the order of frogs, are tailless whereas
- Urodela, the order of salamanders (https://en.wikipedia.org/wiki/Salamander), consistently possess a tail.
The derivation of Urodela is tail (uro-), and evident or conspicuous (-dela). An alternative name for the order Urodela is Caudata, which likewise means possessing a tail.
So here we seem to have one of the clearest-cut phylogenetic splits among vertebrates:
The anurans categorically lack a tail, and the salamanders categorically retain the tail that is ancestral to all amphibians.
However, the split between frogs and salamanders is not absolute. The apparent taillessness of frogs is misleading for two main reasons.
Firstly, all larval frogs retain a tail – a structure familiar in the typical tadpole and one which remains after the growing legs outsize those typical of adult salamanders (https://www.alamy.com/frog-from-peru-looks-incredibly-odd-and-silly-while-part-of-the-way-through-with-metamorphosis-soon-it-will-lose-the-tail-and-look-more-froglike-image177833537.html and https://www.alamy.com/stock-photo-common-frog-rana-temporaria-metamorphosis-nearly-completed-tadpole-49971138.html and https://www.sciencephoto.com/media/128972/view/american-bullfrog-tadpole-with-legs).
This indicates that potential remains for the retention of the caudal vertebrae in fully-grown frogs. This would be by similar means to the retention of gills in the axolotl (Amphibia: Urodela: Ambystomatidae: Ambystoma mexicanum, https://www.inaturalist.org/taxa/26777-Ambystoma-mexicanum) and various other sexually mature salamanders. (Paedomorphosis involving the gills is known to occur in all ten families of Urodela.)
Furthermore, a hypothetical re-emergence of the ancestral tail in adult frogs is easy to envisage given that metamorphosis in amphibians – unlike that in butterflies – is gradual and flexible, allowing certain larval organs to persist to full maturity. (This flexible system is called heterochrony, https://en.wikipedia.org/wiki/Heterochrony.)
Secondly, all adult frogs already retain a modified caudal skeleton, the disappearance of the tail during development being illusory.
What appears to be absorption of the tail by the metamorphosing tadpole is really subsumption into the posterior part of the torso in the form of a hidden but substantial strut – formed of fused caudal vertebrae – called the urostyle (https://en.wiktionary.org/wiki/urostyle). This results in the whole posterior slope of the convex back of the typical frog being reinforced by the modified caudal section of the vertebral column (http://www.ikonet.com/en/visualdictionary/animal-kingdom/amphibians/frog/skeleton-of-a-frog.php).
The urostyle, a skeletal rod formed from fused caudal vertebrae, is wholly internal: it is located completely in front of the cloaca, within an abdomen of novel configuration. It serves as a spine for the elongated pelvic girdle typical of frogs (https://blogs.scientificamerican.com/tetrapod-zoology/the-remarkably-weird-skeletons-of-frogs/ and https://atanatomy.weebly.com/v41.html and https://www.pixtastock.com/illustration/68957990).
In most vertebrates the tail skeleton begins where the body ends. By contrast, in adult frogs the cloaca has been shifted in a posterior direction with the lengthening of the ilia, thus enveloping the tail.
Although the modified tail lies invisibly within the body, it remains – in all fully mature frogs – of comparable length with the head or thoracic spine.
The urostyle can be explained as part of the morphological specialisation for hopping and leaping. However, many lineages of frogs retain the urostyle despite having lost the typical mode of locomotion of frogs, instead crawling much as salamanders do.
It seems factually true that the modification of the pelvic girdle in frogs includes a buried tail. However, the potential adaptive implications that have been overlooked.
Owing to the fact that tail bones are in fact retained in both larvae and adults of all amphibians, natural selection for an adult tail – similar to that consistently present in adult salamanders – remains plausible in frogs.
Indeed, evolution should e able to reinstate the external tail far more easily in adult frogs than in other ‘tailless’ animals such as great apes. This is because all that would be needed is a chronological shuffling of various organs along the metamorphic sequence, amounting to an indefinite delay in the process of subsumption of the tail.
And all of this suggests that the real reason why no frog has undergone evolution converged with salamanders is that there are - for unknown reasons - no ecological niches for salamanders in many of the regions of the world inhabited solely by frogs.
Comments
https://twitter.com/rebeccasbones/status/1085195559475048448?lang=zh-Hant
https://egyankosh.ac.in/bitstream/123456789/57537/1/Exercise%201%20Frog%20Skeleton.pdf
https://toadsnfrogs.com/do-frogs-have-tails/
https://www.natureinstitute.org/article/craig-holdrege/do-frogs-come-from-tadpoles
What would be the advantages of a tail for a frog? I can't think of any great ones. Frogs have specialised their limbs and feet to do what salamanders do with their tail, often better? Like climbing and swimming. Also, in areas with no salamanders other species may have already occupied the suitable niches. Lizards have a very similar body plan and have a few advantages. For example, in West Africa you have Cophoscincopus (https://www.inaturalist.org/taxa/37184-Cophoscincopus) that live much like some salamanders do, on plants next to stream and also under rocks alongside the rivers.
@alexanderr Perhaps I can answer your question in the context of mediterranean-type climates, which have warm, dry summers and cool, wet winters. This climatic category occurs in California, Chile, southwestern Australia, southwestern South Africa, and the Mediterranean Basin.
The climates in these disjunct areas are similar enough that one might expect at least gross convergence in the life-forms that have evolved here, or that have been recruited to these areas from nearby biomes.
The distinction between frogs and salamanders is gross enough that one might expect at least some similarity in the representation of the order Anura and the order Urodela in the various areas of mediterranean-type climate in the world.
Yet such is not the case. California has a diverse fauna of salamanders (http://www.californiaherps.com/salamanders/salamanders.html#:~:text=%20%20%20Scientific%20Name%20%20%20Common,%20%20Endemic%20%2026%20more%20rows%20) as well as frogs. The Mediterranean Basin also features both orders (with some puzzling absences such as Sicily, Cyprus, and Crete). Yet salamanders, or salamander-like anurans, are absent from the other areas - including Chile, despite the fact that salamanders have deeply penetrated South America (to about the latitude of Harare in Zimbabwe).
To the question of how such faunal disparities have arisen, most naturalists might answer 'salamanders simply did not reach Australia and southern Africa; and in the case of Chile there were also significant geographical barriers.'
This may be true, but it seems to defy evolutionary principles. This is because the forces of natural selection should, theoretically, have produced forms at least approximately similar to salamanders in the absence of this group.
The puzzle is all the more because, in a sense, it is the salamanders that have the more generalised/versatile body form among amphibians. Because frogs have specialised body form in their adult taillessness, large heads, and enlarged hind limbs, one might expect it to be frogs that show the greater restriction in occurrence in the world.
Furthermore, the adaptive radiation among frogs, worldwide, seems greater than that seen in salamanders. This evolutionary diversification is so great, from goliath frog to micro-frogs, and from hopping forms through crawling forms and climbing forms (including even a gliding form) to burrowing forms, that the conservatism w.r.t. the absence of an external tail in adults/morphlings seems incongruous.
Does this help to explain? Your further thoughts?
Yeah I agree, the fact that they are not in most of Africa seems to be due to geographic barriers. I'm sure they would do quite well.
I do think frogs are more versatile than salamanders, and their specialisations (especially in hindlimb and pelvic girdle) I think prevent them from benefitting from retaining tails. The interesting exception here are frogs that only walk (as you mention), but do these species have a walk that is distinct from salamanders? Perhaps the tail would still hinder them. Another point of interest is that no frogs (to my knowledge) show paedomorphism, if frogs could benefit from a tail I would expect them to stay in some larval stage and reproduce at that stage, similar to the axolotls?
@tonyrebelo @alexanderr
Some true tadpoles are not aquatic, and some sexually immature frogs do retain tails after all.
Several lineages of frogs, such as the family Ranixalidae of India, actually have terrestrial tadpoles. These do not grow on yolk; they actually forage for themselves, presumably on algae scraped from e.g. bark.
Moreover, it hints at convergence between frogs and salamanders that these tadpoles can actually jump in the form of 'tailed frogs'.
When you look at https://en.wikipedia.org/wiki/Indirana_semipalmata#/media/File:Tadpole_iruppu.jpg, would you call that a ‘tadpole’? I would not, but that is what it is called. This stage of development a) is terrestrial not aquatic, b) still has a tail longer than its snout-vent length, and c) hops like a frog, yet the amphibian lexicon is so limited that it is ineptly described as a tadpole. Since the creature is already out of water, foraging for itself (still presumably on algae, although for all I know it has already switched partly to insectivory), and breathing with lungs instead of gills, in what way is it a tadpole?
The point is that, in Indirana semipalmata (https://en.wikipedia.org/wiki/Indirana_semipalmata), we have what is effectively a tailed frog – albeit tailed only at a sexually immature stage - in all but name. Is this not the anuran equivalent of an axolotl, in a way?
On the other hand it does not have forelimbs! So it is a tadpole with hindlegs, rather than an immature with a tail. Presumably there is unexplored potential in the transition from tadpole to immature frogs?
Very interesting, but I don't think it is equivalent. All tadpoles go through a metamorphic phase where they start to grow legs and the tail is slowly reabsorbed (some frogs do the metamorphosis in the egg). There is a massive handicap for froglets with tail for jumping (which has been well documented). If the frog kept the tail through adulthood that would be another matter. I wonder what the benefit of the tail is in that species, because it appears to retain it even once the hindlegs are well developed? Would be interesting to see a video of the movement it uses from a young age (does it have legs then)? Our ghost frogs (Heleophrynidae) have massive sucking mouthparts and can move out of water on wet rock surfaces to graze. But they do not require any limbs to do this.
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