Why do some tadpoles never turn into frogs?
In: Biology
Some amphibians, most famously axolotls, are “[neotenic](https://www.sciencedaily.com/terms/neoteny.htm)”, which in this case means that they reach sexual maturity while still in the larval stage and do not continue from there. As you might imagine, this probably arose as a mutation in a small number of individuals, but there would likely have been immediate barriers to reproduction between these sexually mature juveniles and adults for behavioural and/or physiological reasons.
As far as I’m aware, there are no analogous species in frogs where all animals never progress past a mature larval stage. However, there are recorded observations of *individuals* that do seem to be stuck at this point, which may be somewhat similar to how the first axolotls evolved. Thanks to experiments from [over 100 years ago](https://zenodo.org/record/1639358#.XPUkV497mUk) involving the feeding of various organ extracts to amphibian larvae (and lots of subsequent research too), we know that metamorphosis in amphibians is primarily controlled by hormones from the thyroid gland. So, changes in developmental timing (or a halting of development entirely) are likely due to changes in the structure, concentration, and/or timing of the production of these hormones.
Unsurprisingly, this is exactly what has been seen in some experimental studies of these permanent tadpoles. [This paper](http://jeb.biologists.org/content/207/12/2133) by Rot-Nikcevic and Wassersug studied such tadpoles in [African clawed frogs](https://media.eurekalert.org/multimedia_prod/pub/web/125300_web.jpg), which are a commonly used study organism in labs. They found some [humongous tadpoles](http://jeb.biologists.org/content/jexbio/207/12/2133/F1.large.jpg?width=800&height=600&carousel=1) which actually lacked thyroid glands entirely, but also discovered they could induce partial metamorphosis by giving them the thyroid hormones they were missing (though unfortunately they died before managing to turn into giant frogs).
Interestingly, this same study also found that the giant tadpoles had much more well-developed eggs and sperm than normal tadpoles. Though they were still never observed to be capable of reproduction, it’s still pretty easy to see how a population of these individuals could potentially become a new neotenic species like axolotls if they managed to overcome the last few hurdles of reproductive viability.
A number of factors can alter how long it takes a tadpole to turn into a frog, including temperature, availability of food, and presence of other tadpoles (e.g. competition for resources), and not enough space. Eventually, they should turn into frogs.
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