I came up with an alien species that has the following reproductive life:

  • They hatch out of eggs, tended by their mothers, and by the end of the first year (about two Earth years long) they are ready to mate as males.
  • After mating they go into a metamorphosis cave; by spring they will come out as slightly larger females.
  • They are still not mature, and need a few adults to supervise them through this second year. In fall, the cool temperatures again trigger mating urges, this time they mate as females, with males who are a year younger. Then it's off to some caves where they go into a semi-hibernating state, keeping their eggs warm, hatching them, and feeding the the young through a lactation tube.
  • These third-year females tend the males all year, then shepherd them to the mating arena, and on to the male-to-female metamorphosis cave--then they are finished with motherhood, and free to go to the cave where their second metamorphosis will take place, leaving them as (still somewhat small) adults.
  • For this species, sex is for kids. When they need to limit their numbers, they can manipulate conditions so that males, or females, can change directly into adults without mating (though they will be smaller than normal).

My question is--is it illogical for evolution to deliver a species that has a metamorphosis AFTER breeding? Earth creatures do their breeding in the final state. Perhaps it serves no "purpose" to have a major change after one's reproduction is completed. But I can see one precedent, in human menopause; the thesis I've seen is that older women can assist their daughters in successfully raising their grandchildren, thus improving their DNA's success rate better than by having more children themselves when they're not young and spry. The second metamorphosis of the aliens could have come about as a "mistake" that was retained because it was useful. What do you think?

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    $\begingroup$ Sounds a lot like the Pak from Larry Niven's Ringworld universe. First stage is a normal sexually immature child. Second stage is normal sexually mature breeder, whose main purpose is to produce and care for offspring. Third stage, with the help of a virus found in Tree of Life root, changes the breeder into a Protector. An asexual, super intelligent being who's only job is to ensure the survival of it's decedents. If the Protectors decedents die, the Protector either loses the will to live, or finds a way to help the species as a whole. $\endgroup$
    – AndyD273
    Dec 22, 2015 at 15:56
  • $\begingroup$ This isn't illogical at all if you ask me - I like it quite well $\endgroup$
    – BlueWizard
    Jan 1, 2016 at 0:52
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    $\begingroup$ Have you considered what happens if all the breeders die off since your question presumes a linear progression of the species reproducing methods? For example, what happens if all the males are wiped out one year? Can you halt the annual metamorphosis to ensure there will be sufficient breeding candidates the next year? $\endgroup$
    – Marion
    Apr 2, 2016 at 18:41

5 Answers 5


The effect on a species of having a caste or metamorphosis that cannot ever reproduce, while still being physically capable is that such individuals are - in an evolutionary sense - disposable. This can lead to them being the species' soldiers.

Since reproduction for the soldiers is a non-issue, their deaths are not individually significant, and are only significant in terms of the advantage they give to their reproductive relatives. Thus, we have soldiers who are more inclined to sacrifice themselves if necessary to protect their younger reproductive relatives.

Since the end-of-life phase of these metamorphosing creatures is to a non-reproductive form, it is quite likely that it would evolve to be as nasty a fighter as possible, since its only purpose is to protect - and educate - younger members of its species. Self-sacrifice in the name of defending the breeders would be relatively common.

Humans would also appear to follow this pattern of reproductive behaviour to some extent. The presence of menopause could be considered a limited metamorphosis. It is hypothesised that menopause evolved in humans since giving birth was historically a risky activity (far more so than today), and any dependent children would be likely to die if the mother died. An older woman would probably have children who were no longer completely dependent, and increasing the probability of the woman's survival to a greater age by eliminating reproduction would allow her to concentrate on increasing the survivability of her earlier children and their children, by providing both physical aid and knowledge. This is known as the Grandmother Hypothesis.

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    $\begingroup$ You may want to consider adding ideas from the grandmother hypothesis to improve your answer. $\endgroup$
    – PipperChip
    Dec 22, 2015 at 16:06
  • $\begingroup$ @PipperChip, good point. Done. $\endgroup$
    – Monty Wild
    Dec 22, 2015 at 22:34

The big thing would be that having a sex after it isn't 'needed' would be some kind of 'drain' on the individual, and on top of that, their survival past sexual reproduction improves the likelihood that their offspring live to reproduce successfully as well.

So maybe during the metamorphosis to asexual adults, they become smarter and are not distracted by sexual urges (or something) allowing for better care and direction of the young. The 'grandparents' watch over the mother and children protecting them.


Such a creature's development would just take multiple steps and serendipity. But that is quite literally true for any creature when looking over its development through hindsight.

A potential line of development:

Original creature is hermaphrodite that goes through several stages of growth, originally sexually mature in their final stage.

Due to a violent inter-group conflicts, those more combat capable are selected for. However, due to increased musculature, shells, horns, combat hormones, or whatever, these forms actually became less fertile and/or less physically capable of mating.

Fortuitously the creature mutated in such a manner that they became sexually mature during their second stage. Over time the second stage form actually became more fertile and likely to bear young than the final stage, which allowed the final stage to develop in such a way as to become completely non-breeding.

The premature maturation mutation also mutated itself and hyper-matured the male sex organs so that they developed during the first stage of the creature. However, this hyper-maturation of the male sex organs was also detrimental to those organs and they became non-functional during the change to the second stage form.

This development would also encourage a more socially tight knit group which could then develop greater intelligence and sentience.


I think the big problem here is how something as radical as a post-sexual metamorphosis could evolve. The only way I can imagine is if the final stage had once been the one that bred, and that sexual potency in the immature forms was a recent evolutionary development.

The next question is why evolution has not disposed of the sterile old ones? That's easier. Either simply insufficient time has passed, or they are still important. They're the parents. They safeguard and educate the youngsters. Their role is very similar to grandparents looking after orphans.

One pattern you'll see repeated in nature is that if a species is long lived compared to other closely related species, chances are high that it's a social species. Older individuals lack vigour but have greater experience. If they can pass on their knowledge then that's a survival advantage for the younger generation and for the species.

So: this species once metamorphosed multiple times and it was the final stage that bred. Then sex determination went awry (genetically that's a particularly common form of mutation because of the peculiar inheritance pattern of the sex chromosomes). The species gains the ability to breed younger and faster but keeps the final generatoon because it's intelligent and social.

Reading that last again I think it may imply that there are some really nasty predators or parasites preying on them. The needed to breed before they could grow up (now, or in recent evolutionary past).


The thing is - as far has evolution is concerned - everything that happens after reproduction is completely irrelevant. After that point It's not being passed on to the next generation.

Natural selection can be summed up as, who gets to breed? If those young are not undergoing some kind of trials, the species is not evolving.

Now I think this idea can work, you just have you lengthen the period of time spent in each phase, and make them more independent rather then under constant watch like we treat our children.

Lets say each phase rather then a year, takes a decade. Lets say the males have wings and leave their mothers to travel the planet spreading diversity. This phase will likely see the greats droop in numbers. The surviving males then seed a female perhaps staying with them as they change. The females loss their wings and become much larger. It is the females who do most of the work in the society. I would also suggest some means by which the females must earn the right to lay eggs. Lastly the females that are no longer able to lay eggs undergo their final change. Their form will mostly likely be made for combat, but these grand elders could have whatever social role you want them too.

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    $\begingroup$ "The thing is has far has evolution is concerned everything that happens after reproduction is completely irrelevant. After that point It's not being passed on to the next generation." — that's not completely true: If the grandparents help their children to find a good mating partner and/or to successfully raise the grandchildren, then they still increase the likelihood that their genes will survive and spread, since the children are carriers of their genes, and pass them on to the grandchildren. $\endgroup$
    – celtschk
    Dec 22, 2015 at 12:45
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    $\begingroup$ Another point where that wouldn't be true is if there is conflict between different families of the same species. Let's say the last phase are "warriors" - the better warriors will keep their young alive where the others will not. Unfortunately, the inherent delay involved means that entire "branches" of the species will be getting pruned off the "evolution tree", rather than just a couple "leaves" - but with a massive "tree" it won't matter overall. $\endgroup$ Dec 22, 2015 at 19:11
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    $\begingroup$ Another counter-example to this is hive-based insects like bees. Almost all bees are non-reproducing, but are absolutely essential to the success of the hive. $\endgroup$ Apr 2, 2016 at 14:57

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