All natlangs have both consonants and vowels, but it's not immediately obvious why a human language couldn't be made from only vowels.

Has anyone theorised about why natlangs always have consonants?

Have there been any attempts to create conlangs with only vowels, and were they considerably harder to learn than languages with both vowels and consonants?

A vowel-only language is surely constructable (and I think, learnable, too), but I am afraid that it will be instable against evolutionary pressure.

Vowel sequences like /aua/ or /aia/ tend to develop into glides /awa/ and /aja/ giving raise to the first consonants in the language, and at the hiat between two vowel a third consonant, the glottal stop, may materialise.

Efficiency of the coding is also a concern, maybe tones can take over the role of consonants in creating more different syllables from the vowels.

There are generally a lot fewer vowels than consonants in the phoneme inventory of human languages. That means, with fewer sounds you need to make the words a lot longer if you want to have a decent-sized vocabulary.

Also, vowel pronunciation is more varied. There aren't many different ways to pronounce /t/ or /k/, but any regional dialect will change vowels. Partly because there are no fixed places of articulation: vowels are distinguished by the opening of the jaw and the position of the tongue. Both of these are almost infinitely variable compared to closing your lips and releasing them (as you'd do for a /p/ or a /b/).

And finally, why would you disregard most potential sounds that the human vocal apparatus can produce? It's just not an efficient use of your anatomy. A bit like hopping on one leg instead of walking.

So the two main arguments against a vowel-only language are size of phoneme inventory and variability of vowel pronunciation.

Well, there is always Solresol, which has several isomorphic representations, and some of those could be considered vowel-only (depending on the instrument used).

For natlangs, there are whistled languages, with a very reduced consonant inventory. They could fit your criteria especially if you consider tones to be a feature of vowels, and look at the phonetic level - consonants are realized mostly by glides and occlusion (phonemically, consonants are copied from the spoken language & reduced).

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