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I'm playing around with an idea in my head for a speculative species and I'd appreciate any help with a problem I've encountered trying to make them plausible.

Let's say I have a species of animal, X. They have two distinct forms they go through in the course of life. X are born as small, bipedal babies a mere 17 cm long. As they grow they have a growth spurt and reach 61 cm tall, or about the size of a raven, before slowing down. At this stage, their organs are fully developed and their brains are almost as advanced as humans, however they are unable to reproduce yet. They are still juveniles. Young X live omnivorous lifestyles eating bugs, fruit, and small mammals while living in scattered Neolithic societies, and can stay in this adolescent state for decades.

Then puberty hits. Around the time they become sexually viable X suddenly become solitary and territorial towards their once-peers, leaving the community to strike it out in the wilderness. They lose the ability to think critically and preserve only the instincts needed to survive. At the same time their physiology changes: they become herbivorous, they pack on fat stores, and most importantly they grow very, very big. A well-fed and aged X can grow up to 3 meters tall. These adults are the only ones that can breed, and once pregnancy is over they will give birth near a juvenile settlement and leave their children to be raised by the youth.

My question is: how could this plausibly evolve? How could a creature have two distinct life stages that occupy different niches and have different levels of intelligence? I know T. Rexes have distinctly different juveniles and adults, and certain invertebrates lose neurons as they mature (none that I can recall the name of), but I'm having difficulty putting the two together.

Feel free to toy with the specifics of the species if that helps you answer. They can live in any climate and at any time period- I'm more focused on the plausibility of their biology rather than solidifying their world for now.

Thanks!

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    $\begingroup$ How could a creature have two distinct life stages that occupy different niches and have different levels of intelligence? (1) Caterpillars and butterflies. (2) Humans have very different levels of intelligence at age 2 and at age 22. $\endgroup$
    – AlexP
    Jan 31, 2022 at 3:54
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    $\begingroup$ Intelligence is not the be all and end all. There is a wasp that puts its brain turbo mode only occasionally to count the bugs in its larder, then turns it off again. Birds don't evolve to identify cuckoos in their nest because the extra brain power means extra weight. It is not worth it. The adult form in your case presumably has a reproductive advantage that probably stems from a time when the juveniles were less intelligent, but intelligence in the juveniles was worth it for some reason. Perhaps competition between juveniles. Many options. $\endgroup$
    – Bruce
    Jan 31, 2022 at 12:12
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    $\begingroup$ @Bruce "There is a wasp that puts its brain turbo mode only occasionally to count the bugs in its larder, then turns it off again" - I did not know that! Could you possibly post the name of that wasp species, and link to the research paper describing the measurement of brain activity? $\endgroup$
    – AJM
    Jan 31, 2022 at 12:26
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    $\begingroup$ @AJM My statement was prompted by something I read in "the extended phenotype" page 49, "constraints on perfection" *Dawkins) work by Baerends (1941). His conclusion was based on noting that if he removed prey from the burrow in the morning, the Ammophila Campestris would notice - and replace the food, but not in the afternoon. $\endgroup$
    – Bruce
    Feb 1, 2022 at 7:21
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    $\begingroup$ The sea squirt has a brain and swims around until it finds a rock it likes the look of. Then it glues itself to the rock permanently and, having no further use for the metabolic load of the brain, promptly digests it. If the adult reproductive process doesn't require a large brain, then doing away with it means more energy for making babies. In humans the brain consumes 20% of our calorie intake, so it can make a pretty big difference. $\endgroup$
    – Perkins
    Feb 1, 2022 at 20:17

14 Answers 14

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In order to have a species with a high intelligence as a juvenile and a low intelligence as an adult, it would be necessary to have a metamorphosis as an intermediate step.

During metamorphosis, the individual's body changes radically, potentially to the point that an adult doesn't look anything like a juvenile. The adult's adaptations can be completely different to those of the juvenile, and may be mediated by different genes.

So, the juveniles may be adapted to being intelligent. However, the intelligence of the adults may depend upon a different set of genes... and in the general rebuilding of the body, much of the brain may be repurposed or may simply be discarded.

Sure, it seems like a waste, but that's evolution for you. The intelligence of the adults will just have to evolve separately.

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    $\begingroup$ 'In order to have a species with a high intelligence as a juvenile and a low intelligence as an adult, it would be necessary to have a metamorphosis as an intermediate step.' This isn't necessary. Many humans tend to loose cognitive ability as they grow older, especially after they became retirees. It is suggested that it may be caused by the lack of stimuli. Is it possible that the species become aggressive because of hormones and they loose ability to think because they do not need to? That they have created habits that take care most daily activity?(As youths they had to actively learn) $\endgroup$
    – Bartors
    Jan 31, 2022 at 8:17
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    $\begingroup$ When humans grow older, they not only loose mental abilities but also muscular, sexual, digestive, physical abilities also. But here it is opposite. So they need a metamorphosis to get a sudden large body size, change in eating habits, aroused sexual desire with mental degradation. $\endgroup$
    – imtaar
    Jan 31, 2022 at 11:29
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    $\begingroup$ Why were ghouls in the fallout universe the first thing that came to my mind reading the question...? $\endgroup$ Jan 31, 2022 at 13:24
  • $\begingroup$ Yes the second set of genes is different, clearly. Their realms are different. But I don't think many things become a waste in evolution. Energy considerations may play a role in this second set of genes.. there's always a reason. I upvoted this answer, my answer could be regarded as a supplement to this, with a plausible (and science-based) reason for the little one needing a brain and for the big one loosing its brainpower. $\endgroup$
    – Goodies
    Jan 31, 2022 at 21:34
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Challenge: What evolutionary benefit would it provide to go the extremely expensive way of developing intelligence to the point of sapience, to only throw it all away when the specimen enters an age when "providing your knowledge to others" would be most beneficial to the group?

Also, the fact that your species X enters a territorial and solitary lifestyle before they become fertile seems very implausible to me.

No, i doubt that a lifecycle like this could could arise naturally within one species.

But wait - What about the surroundings? What about a parasite that lays dormant for a long period of time that, when becoming active, results in a deterioration of mental faculties? That's not something unheard of on our planet.
It could also explain the shift to a herbivore lifestyle, by introducing food intolerances - like the Lone Star Tick.

However, infertility before reaching that point, as well as the sudden substancial growth spurt while switching to a less energy dense diet would be hard to explain based on this.

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  • $\begingroup$ "less energy dense diet" <- this is not necessarily true. Grains tend to have a higher calorie density than meat. They are also available in much higher volumes. $\endgroup$
    – Nosajimiki
    Jan 31, 2022 at 15:27
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    $\begingroup$ "higher volume" has no impact on density, and cultivated grain (in terms of "being planted" and "being selected for yield") doesn't seem applicable to the intellectually reduced, solitary adult form that's reduced to instinct. good point though, should be kept in mind. maybe the juvenile forms are agriculturally advanced enough to feed themselves and the adults at some point? $\endgroup$ Jan 31, 2022 at 15:41
  • $\begingroup$ Yes, domestic grains like wheat and rye average around 3,500 cal/kg whereas wild grasses average 1780 to 2740 cal/kg. However, even wild grasses have higher average calorie yield per kilogram than most meat. Raw meat averages about 1410 to 2,690 cal/kg. $\endgroup$
    – Nosajimiki
    Jan 31, 2022 at 16:39
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No Large Predators.

Your species has always had a metamorphosis occur at puberty. The juvenile form is small and weedy and vulnerable to all sorts of predators. They are designed to exist in vast numbers, most of which are eaten, and run and hide, and live in the treetops. . .

enter image description here

. . . until it grows large enough to have no predators.

enter image description here

At this point it metamorphoses into a larger form, moves to the ground, and lays thousands of eggs. The large form is useful because it means more eggs and more young for the next generation.

Being herbivorous is good because grass is available in abundance on the ground. Being solitary means they are inclined to spread out and lay their eggs over a wider area. There is no advantage to living in groups since they don't need protection anymore. Likewise a big brain is not needed. Dipolodocus did fine with its pea brain:

enter image description here

Since the two forms fit separate niches they are free to evolve separately. The small form evolved to be smart since it makes it easier to make tools and work together to stay alive. Note that unlike humans these guys only develop smartness as they get older. In the earliest stages of life they rely on their tinyness and vast numbers to ensure enough make it to the next generation.

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I'll try to give short, general explanations for all the distinctions and how they may have developed:

Your creature always went through the tiny, small, and huge stages. That part itself doesn't require special explanation, frogs do the exact same thing (and don't go through a full body meltdown like insects)

The huge stage evolved towards pure herbivores in an over-compensating development of evolving away from accidentally eating their own juvenile counterparts.

The small ones evolved towards sapience for the same reason we humans did: Because being medium-sized and stupid is hard.

The huge ones are aggressive and territorial because that works quite well for big herbivores - Rhinos do it. There is also practically no need for big brain time if you're the undisputed owner of the grass you eat. Why waste energy on brainpower when you can use it to bash predators into the ground instead?
And they have very good reason to be very defensive - since the small, infertile(!) stage lasts quite long, the survival of the fertile stage is paramount for the survival of the species.

That said, it has probably already become a spiral: The small stage is evolving to last longer and longer, since that is beneficial when you're sapient.

What I can't explain is the exact neurological process that takes away their brainpower. But a huge shift in priorities on how blood and energy are distributed throughout the fast-growing body seems one plausible explanation.

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The reasons could be the same as on Earth

Q: "how could this plausibly evolve? How could a creature have two distinct life stages that occupy different niches and have different levels of intelligence?"

Earth science sais

1. Metamorphosis makes sure caterpillars and butterflies don't compete

For different niches, the answer is yes. Many insects do the trick.. During your life, you'll have two realms. According to Scientific American, the primary incentive for the adaptation in Earth's nature is preventing competition between the young and the old in one species. I'll add a link about that.. https://www.scientificamerican.com/article/insect-metamorphosis-evolution/

2. Intelligence makes us adapt to new environment quicker

There's hard-science support for this,

"Here, we test this hypothesis for a major animal group (birds) by examining whether large-brained species show higher survival than small-brained species when introduced to nonnative locations. Using a global database documenting the outcome of >600 introduction events, we confirm that avian species with larger brains, relative to their body mass, tend to be more successful at establishing themselves in novel environments."

https://pubmed.ncbi.nlm.nih.gov/15784743/

Your world is a different world..

These Earth explanations need not hold for your planet. It could be any thinkable adaptation. As an example, see the answer Daron put. It gives a plausible reason. The young animal is smaller and certain predators may profit from that. The young would need to anticipate.. hide.. be smart..

Assume the reason would be the same as on Earth..

So ad 1) the old and young of this species don't share the same environment

And ad 2) a larger brain allows quicker adaptation to new circumstances.

Suppose.. the young ones roam on land, the old ones live in the water.

.. Maybe they are egg-layers ? Sea-turtle behaviour: dig in your egg on the beach and return to the sea. On your planet, the circumstances on land ar much more complicated and change much quicker than sea and sea life. The young would be born with legs and a tail.. suitable for land.

The egg is put on land. The young will have to cope with e.g. instable weather, various predators, lots of competition.. they could develop nomadic behaviour, roaming around, to find a place suitable for their transition stage. They encounter all kinds of species, find new food, or find new land.. they profit from their relatively large brain. They were born with it.

Now it enters the metamorphosis stage. They'll hatch underwater. Reason: When your big one is full-grown, it has become whale-like, far too big to move on land. It will remain in the ocean, which provides it lots of food. They enter the grazing phase of their life.

Why would this animal keep its advanced brain, after the metamorphosis ? Energy-wise, it would be more effective to replace it.

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Not sure if realistic but... one thing I have heard is that elephants, despite their larger brains, aren't that smart because so much of their brainpower is used to control their enormous bodies (and complex digestive system). The bigger the body, the more brain matter is required to control it. Similarly, dolphins are pretty smart but a large part of their brainpower manages their sonar capability. They might be as smart as people if not for that. Point being, brain power isn't just a factor of brain size, but what, precisely, the brain has to spend time working on. Humans can be smart because we have big brains for our mass and we aren't that complex to run.

So my thinking is that this creature's brain stops growing early, but the body mass never does. They just get bigger and bigger. And the brain adapts to handling the larger and larger mass basically by sacrificing intelligence -- repurposing neurons. Makes me think of something like a dragon. They just never stop growing. And they're incredibly smart when they're young but the older they get, the bigger they get, the dumber they get until eventually they are just big angry lizards.

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  • $\begingroup$ As a dragon, I object to that characterization. But the basic idea is interesting $\endgroup$
    – Dragonel
    Jan 31, 2022 at 19:17
  • $\begingroup$ Although the reason is implied to be more supernatural than physical, you might be interested in the case of the sea gods from Deeplight, by Francis Hardinge. Except in some unusual cases, the larger they grow, the less intelligent they become. This is explicitly causal: cutting off portions of the gods' bodies prevents them from growing so large and unintelligent that they go on rampages and cause catastrophes. $\endgroup$
    – Obie 2.0
    Feb 2, 2022 at 1:09
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It's not unheard of for humans to lose out on brainpower and acquire more physically aggressive tendencies as they age: we call it dementia. Fortunately, its typical onset late in life tends to balance out the aggression with the physical weakness common to aging. If your species were afflicted with a tendency to develop dementia around puberty rather than at the twilight of their lives, that would be a very scary creature indeed! (And it opens the possibility of some rare individuals managing to not develop dementia as they grow, which could lead to any number of different possible storylines for them...)

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Juveniles Live in Safe Places; Adults Journey into Danger:

Your juvenile species is smart to stay alive and develop food resources in areas that are relatively safe, like protected valleys and oases. They hunt, engage in simple agriculture, and nurture the young that turn up or are dropped off by the adults. They live, in other words, like simple hunter-gatherers.

But these safe areas are few and far between. The rest of the world is deadly in some stripe - either full of hazards, predators, or just inhospitable. Adults that retained their intelligence led to significant inbreeding. After all, you'd have to be stupid to leave a safe enclave for the almost suicidal outside world.

But biology is cruel. A relatively tiny number of successful adult reproductive acts allow large numbers of genetically diverse offspring to thrive in the more protective regions. So adults who's intelligence declines become overwhelmed by instinct and throw caution to the wind. They build up reserves they'll need to survive in the hostile outer areas, and venture forth to seek mates. Many die, but the females who manage to get back to the fertile areas birth (or lay eggs) many offspring. They don't need to be smart, because the juveniles take care of the young. Then the remaining females can either die or venture back out to seek mates once again, for as long as they survive.

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The adult form is caused by the accumulation of toxins or prions in the brain. As a result, neurons are damaged and the area of ​​the brain responsible for the secretion of growth hormone is stimulated. This triggers the growth and sexual maturation phases. As the individual grows, toxins / prions accumulate in the brain, which lead to impaired thought functions and increased hormone emissions, and so on until the individual dies - most likely as a result of cancer.

Evolution: initially the phases were short and the adult units were smaller and fell easily prey. The middle phase was intelligent and eliminated the predators - the longer phase resulted in more predators being eliminated which leads to longer life for the adults and more offspring. At one point, adults began to die of old age, not as victims. Since feeding is related to the accumulation of toxins / prions, the intelligence of the middle phase has allowed the elimination of the most harmful foods. This overlapped with his innate immunity. Once they discovered the basics of medicine, the avalanche could not be stopped.

Consider fast maturing as a result of eating the brain of an adult.

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That way of functioning doesn't have to have an evolutionary advantage.

You can exploit the weaknesses of evolution for this one.

Namely, evolution does not have backtracking.

Do you know why our eyes are so complex and yet primitive. Because eyes evolved before life left the oceans. The new eyes had to adapt to land-based life - so we evolved various mechanisms to keep them wet and the lenses evolved to focus light without being submerged. But, still eyes completely evolved on land would have been much better. Evolution however never starts all over - especially with important organs that must always work.

So if your species evolved from another primitive species that worked the same way - without having very advanced intelligence - then your more advanced species will keep this. Especially if it is linked to some other basic function that evolution cannot easily replace - like digesting food destroys your nervous system.

But you still have to find an evolutionary advantage for the intelligence - this species must be somehow better at surviving than the more primitive one.

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  • $\begingroup$ That's an interesting take. Maybe the primitive species used metamorphosis in its development, and the genes active in the first stage evolved to develop intelligence because it was beneficial for the species, but at the later stage (which uses an independent set of genes), the creature is so big that it didn't need intelligence, so it never evolved (or even reduced over time because it was a waste of energy), $\endgroup$
    – Llewellyn
    Feb 1, 2022 at 21:18
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Elaborating on @fectin's answer (which I didn't manage to spot before I posted my own):

This may not constitute a complete answer, but Ambulance Ship by James White features a species that exhibits some similar characteristics. The reasoning he gives could apply here, at least for some parts of it.

The planet in question is a severely competitive environment, so the physical strength, durability and violence of the "adults" evolved. They also gain awareness and intelligence (and telepathy, but that's irrelevant here) before birth (because they needed to be born ready to survive). The ability to think, however, turns out to be an evolutionary disadvantage outside of the womb, since it impedes the fast reflexes necessary for the species' survival in their fast-moving, competitive swamp biome; so it gets lost during the birthing process.

You could easily translate this to your scenario. Adults have to have the fast reflexes and durability to survive dangerous species of plant and animal in your environment. They might initially have the imperative to protect their more vulnerable young, who develop awareness (but not physical strength) at birth, and eventually find each other and begin forming societies. Since these societies have higher survival rates than children alone in the forest with their mindless parents, evolution begins to favour the instinct to bring newborns to existing settlements, and the impulse to keep their young nearby is lost.

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When James White wrote a similar species, they had a very, very long gestation period and were telepathic. To keep them safe in the womb, they were paralyzed. Part of the birth process included release of an enzyme that countered the paralysis, but also stripped their sentience ("to be born is to die"). Adults were tough and violent.

Closer to home, octopi and some squids are quite smart, but essentially give up and starve to death after sex.

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They Don't Stop Growing

Perhaps this species never stops growing. It grows a brain large enough to control a large body, but while the body is small, it has excess brainpower that can be used for language, culture, and perhaps even science.

But as they continue to (slowly) grow, they need to get more aggressive (their earlier passive lifestyle no longer supplies enough calories) and more and more of that brain power is repurposed into controlling their body. This later form needs to be successful and dominant enough that there is no evolutionary pressure into maintaining the smaller form.

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Toxins in some of the critters the juveniles eat, collect in the body. How long it takes to trigger the "adult" phase depends on what critters are available - but eventually enough of these toxins get collected and kick the dormant growth and testosterone system kicked into overdrive. Could also be more of a alumnium-toxicosis due to some of the bugs providing it and leading to dementia / alzheimer - therefor reducing intelligence.

Growth spurts and more aggressive behaviour develops. They also somehow recognize where the toxins come from and avoid their former omnivourous diet and start munching plants.


Then there is this brain-slug - you get it by happenchance and if the stars align correctly (or in-correctly) it passes through your ear into your brain and starts devouring it. Its a slow process ... but you'll get dumber and dumber and eventually start eating plants that you wouldn't have touched with sane mind - those plant hormones let you grow and get fertile.

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