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Dragons are flying reptile like animals the size of quetzalcoatlus but slightly more voluminous since gravity on this planet is between 0.75 g and 0.79 g.

They can shapeshift the same way caterpillars and larvae transform into flying insects. But dragons transform into humans, biologically identical to humans until they reach old age. To avoid death by old age they transform again into dragons, small child dragons who have to eat a lot of food to grow up again.

Dragons can repeat this as many times as they want to avoid death until they are killed or decide to let themselves age to death.

I suppose Dragons do not need genitals, regardless of gravity, it is a redundant weight for a flying animal who can already morph into something else to reproduce.

So is it plausible that on a distant continent where dragons were isolated and didn't need their dragon form anymore they slowly evolved to be only humans?

Do we know of animals on earth which lost their transforming abilities?

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    $\begingroup$ Well, the axolotl loses transformation capacity, but this actually helps with regeneration. $\endgroup$ Apr 11 at 15:12
  • $\begingroup$ What do you mean by "plausible" and how much hard-science would a good answer have? I have no idea how to meaningfully answer this question -- The soft-answer is "Of course it's plausible. It's your world, Do what you want!" and the hard science answer is "The idea of dragons transforming into humans transforming into dragons. . . . is already wildly implausible." $\endgroup$
    – Daron
    Apr 11 at 18:13
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Reverse metamorphosis is a real thing.

https://en.wikipedia.org/wiki/Turritopsis_dohrnii

Like most other hydrozoans, T. dohrnii begin their life as tiny, free-swimming larvae known as planulae. As a planula settles down, it gives rise to a colony of polyps that are attached to the sea-floor... Jellyfish, also known as medusae, then bud off these polyps and continue their life in a free-swimming form, eventually becoming sexually mature...If the T. dohrnii jellyfish is exposed to environmental stress, physical assault, or is sick or old, it can revert to the polyp stage, forming a new polyp colony... Theoretically, this process can go on indefinitely, effectively rendering the jellyfish biologically immortal, although in practice individuals can still die.

Your dragon / human life stages seem biologically unlikely and if I were reading your fiction I would be watching for some sort of metaphorical application to the real world - perhaps adults reverting to childhood roles in their mom's basement after life stressors.

Back to biology: in your world, dragons are the polyp (juvenile) analog and humans are the adult (jellyfish) analog. You ask if there are creatures that lose the ability to do metamorphosis. Reverse metamorphosis as your creatures (and Turritopsos) does is unusual and I could imagine a mutation could block the ability to revert. These creatures would be juveniles, then adults, then old adults, then die as most creatures do. If you wanted (and I dont know why you would) to stick to strict biology you could have a telomere-like cap on the number of cycles, or otherwise incur damage to the organs used to revert.

I assume in this scenario "humans" give birth to baby dragons. Perhaps a human pregnancy goes on longer than usual and instead of a baby dragon, a baby human is born. This is sort of like the mammal trick of having developmental stages inside the mother rather inside an egg as our ancestors did. The new type carries out the dragon developmental stage in utero. These humans cannot change back to dragons because they never really were dragons.

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Living forever is bad from an evolutionary standpoint.

As I said in a comment, it would help to give detail for what counts as "plausible" for the purpose of this question, since the setting (dragons transforming into humans transforming into dragons. . . . ) is a lot wilder than the question topic (losing metamorphosis).

That said, I'll just point out that natural selection favors a species that reproduces and dies, rather than one that lives forever and doesn't reproduce. This is because each new generation is better adapted than the previous. For example if the climate changes or a new virus appears, most of the population dies, and is repopulated by the reproducers rather than the immortals. The babies are better adapted than their parents and carry the reproducer genes.

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    $\begingroup$ An animal which can reproduce indefinitely and can birth dozens of generations in a single life time is indeed preferred in some niches, hence why on our planet some immortal animals do actually exist. $\endgroup$
    – user84798
    Apr 11 at 18:41

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