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In my fantasy world, I have the suchians, a race of reptilian humanoids that live for 200-300 years and keep growing throughout their lives. I am wondering what a reasonable "growth curve" is.

My working assumption is that their mass grows in a linear fashion. I.e., a 30-year-old is about as heavy as a man, 70 kg or so, whereas as a 200-year-old averages 400-500 kg. I don't know how "realistic" this is.

I am writing fantasy, not hard science fiction or speculative evolution, so I can of course do whatever I want. But I want to take some hints from real biology before I decide.

Do you know of some good examples of real animals that keep growing (e.g. crocodilians), and how their growth curves look?

The "tree of life" of my fantasy world is largely earthlike, but with more reptiles and fewer mammals. I see my suchians as a kind of archosaurs. They have crocodile-like traits and dinosaur-like traits; I haven't decided exactly how they fit into the phylogenetic tree. But they definitely have a warm-blooded metabolism like birds, NOT a cold-blooded metabolism like crocodiles.

Here is a picture of one (drawn by Avanciia).

Suchian with a gun

Thanks in advance!

EDIT 1: Someone asked about suchian senescence. That is a good question. The average suchian remains sharp and in good health for at least 150 years. Some time typically around 200, senescence gently begins to set in, and the body slowly becomes more susceptible to disease and slower to heal. But even then, most remain in good health for some decades. Perhaps they senesce at an "exponential" rate, meaning that age-related weakness slowly grows over several decades. At some point, usually between 220 and 260, the body starts to feel really old - organs start to fail, age-related illnesses appear. The average suchian is dead by 250, but some exceptional individuals live to 300 or rarely even 350.

EDIT 2: The suchians are highly communal and live in large families/clans of 30-70 individuals which band together in larger tribes or nations. So infirm individuals will be cared for by their kin.

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    $\begingroup$ I was halfway through an answer when some problems arose. What does it mean for your creatures to die? Here on Earth, bones get brittle. Muscles atrophy. The immune system breaks down. In a word, things decay. But you're creating a condition of continued growth (new bone material, new muscle mass, a flourishing immune system) right up to the moment of death. So, could you edit your Q and explain exactly what it means for a creature that's continually growing to actually die? $\endgroup$
    – JBH
    Commented Jul 29, 2022 at 9:25
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    $\begingroup$ By the way, if you'll forgive me offering advice. Ignore how "realistic" anything is. Folks on this Stack often get too wound up in things being "realistic." That shouldn't be your goal. What you want is a set of world rules that are consistent. We humans who are exposed to your fantasy world are being asked to suspend our disbelief. We're actually very willing to do so! Where it becomes problematic is when the world rules aren't consistent. So, throw "realistic" out the window and focus on creating creatures that live their lives consistent with the world around them. $\endgroup$
    – JBH
    Commented Jul 29, 2022 at 9:28
  • $\begingroup$ Depending on the social/cultural aspects of this species you could have their grow curve be whatever level of shallow or steep as you want, as a low amount of elderly support would only fit with a shallow curve and a high amount of elderly support would allow steeper curves(You don't have to risk breaking bones, a heart attack from exertion, etc, if you can just sit around and become fatter as you're hand-fed by those younger than you, is what I'm saying) $\endgroup$
    – Lemming
    Commented Jul 29, 2022 at 9:28
  • $\begingroup$ Paragraphs added about senescence and social structure. Good questions! $\endgroup$ Commented Jul 29, 2022 at 9:46
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    $\begingroup$ Why, it is simple. it is turtles. Turtles, all the way down. Or rather tortoises. $\endgroup$ Commented Jul 29, 2022 at 12:44

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Animals that continue to grow through their lives grow quickly at first and slower and slower as they age. this is true for lizards, crocodiles, sharks, and turtles. The main reason is a body shape only works in a certain size range you can't just scale up an animal and have it still function. So as an animals gets larger its proportions need to change. Also keep in mind you need a proportionally larger increase in mass to get the same increase in size. A doubling in size means you need an increase in mass of 7-8 times.

enter image description here

enter image description here

https://www.researchgate.net/figure/Bull-Shark-growth-data-based-on-vertebral-band-counts-Growth-curves-have-been-fitted-to_fig5_262605712

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    $\begingroup$ …Is the implication here that male lizards live twice as long as females? Or is there some other reason the data cuts off at that point? $\endgroup$ Commented Jul 29, 2022 at 20:01
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    $\begingroup$ One story I read is about a dragon species that never stops growing. After a certain size, the amount of energy they spend hunting is more than what they gain by eating, and they slowly starve to death even while undergoing a feeding frenzy. They often attack inhabited places in hope of being slain before losing all sanity to hunger. $\endgroup$ Commented Jul 29, 2022 at 20:03
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    $\begingroup$ @DarrelHoffman in the case of that particular study on komodo dragons, yes male live a lot longer than females. researchgate.net/figure/… $\endgroup$
    – John
    Commented Jul 30, 2022 at 18:05
  • $\begingroup$ Kind off surprised this answer didn't mention the square-cube law $\endgroup$
    – DeepSpace
    Commented Jul 30, 2022 at 20:35
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You could use turtle growth as a basis for your calculations. They can live for hundreds of years. They grow fastest when young and then it gradually slows once they reach maturity.

So the curve is steep until maturity then increasingly shallow until death.

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I'd think it most plausible if the growth rate tapers off over time. Maybe it never quite goes flat, but approaches some maximum asymptotically.

If they grow at an increasing rate, eventually you're going to run into square/cube problems: Surface area increases at a square rate while volume increases at a cube rate. Lots of biological functions require some balance between surface and volume.

Like, a creature tends to produce heat proportional to its volume. Every cell is working and producing some heat. But they mostly lose heat through the skin, and the amount of heat lost is proportional to the surface area. So a creature that's too small would lose too much heat. I read a discussion once saying the shrew is about as small as a mammal can be, that at its size it has to eat almost constantly to get enough calories to make up for the heat loss. A creature that's too big would lose too little heat and overheat.

Or, strength of bones is basically proportional to the area of a cross section. So if a creature gets too big, its bones won't be strong enough to support its weight.

It's easy to make a movie with a man 600 feet tall. But in practice, if you grew a man to 600 feet and kept all his body proportions the same, he'd quickly die. Like, a giraffe's neck is not just like a horse's neck only longer. The giraffe has to have an extra "pump" in there to get blood up that long neck to the brain. And then what happens when the giraffe tilts its neck down to, say, drink from a pool? All the blood would go rushing to its head and it would explode, were it not that it has some extra valves in there that clamp down to restrict the blood flow.

So as the creature grows, it either has to stay within the range that can be supported by its basic body design, or the body design has to change as it gets bigger. Proportions have to change. A human can grow from a baby who is maybe a foot tall to an adult who might be six feet tall. But a human can't grow to be 50 feet. What's the limit? I don't know, it's not like we can perform the experiment. Perhaps someone has calculated it.

Now all that said, as @JBH says in a comment, I wouldn't worry too much about "realistic". Science fiction and fantasy routinely hand wave and just ask the reader to accept that some new invention or alien biology is possible. If I'm reading a story about, say, time travel, I rarely scream at the book that the author has failed to explain the physics of how his time machine works. I presume the author has no idea. If he did, he'd build a real time machine. (Don't try to make a career out of time travel. There's no future in it.) He just tells us that the hero built a time machine and we accept it for the sake of the story.

I think you want to maintain some level of plausibility. But "this sounds like it might be possible if we assume that ..." I'd avoid introducing anything crazier than is necessary to make the story work. But I wouldn't get bogged down in trying to design the DNA for each creature mentioned in your story.

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  • $\begingroup$ Valid points. I think I'll rule that their growth starts to taper off in the early 200s, to a maximum weight of 600 kg for an average individual or up to twice that for an individual who has always been exceptionally large. If a suchian lives to 300, it is a safe bet that he has stopped growing by then. $\endgroup$ Commented Jul 29, 2022 at 17:24
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    $\begingroup$ The tallest people ever were all just shy of 9 feet (~270 cm), all of whom died in their twenties. A quick scroll of wiki shows the tallest person to live more than 55 years was 8 ft (242), but there's still a lot of early deaths in there. A limit to a healthy life is probably somewhere in the upper 7 ft (220+) range. $\endgroup$ Commented Jul 29, 2022 at 17:59
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Growth spurts

sraph

source

Like large birds and mammals, but unlike living reptiles, tyrannosaurids probably experienced extremely high neonate mortality, followed by few deaths after 2 years of age (presumably a release from predation), and then increased mortality at mid-life (probably from the rigors of reproduction), so that few individuals had a long reproductive life span...

You could have slow and steady growth as the comparison animals in this graph did. It might be interesting to have a prolonged juvenile phase and then a huge growth spurt to the adult reproductive phase. That was apparently the case for Tyrannosaurus Rex.

Paleontologists are unraveling the mysteries of young T. rexes. Creatures they thought were 2 species turned out to be kids and adults.

young Trex

It makes sense. At different life stages the same creature has different hunting styles and different prey, so they do not compete with each other. You could take this a step further and have the adult males be substantially bigger than the females, because they must fight other adult males.

Also that juvenile T rex is a cutie pie.

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Most reptiles grow fast at beginning then slow down. Tyranosaurus rex bone analisis give fast growth for 10 years then slow down to almost no grow at 20.

Here You have study of crocodile:

https://www.dgip.unach.mx/images/pdf-REVISTA-QUEHACERCIENTIFICO/Revista12_2/2.-Growth-rates-of-the-American-crocodile.pdf

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  • $\begingroup$ Thanks! Interesting stuff. I only just paged through it, but in the right hand graph on page 12, the highest curve seems to indicate that these crocodiles' length grows in proportion to the square root of the age. If mass is proportional to length cubed, then the mass actually grows faster than linear (in the highest curve). That is very interesting. 🙂 $\endgroup$ Commented Jul 29, 2022 at 12:36
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    $\begingroup$ T-Rex isn't a reptile though... $\endgroup$
    – stix
    Commented Jul 29, 2022 at 16:31
  • $\begingroup$ "TRex not a reptile" independent.co.uk/news/science/… "the giant predator is closely related to chickens and ostriches" A TRex as an ostrich? lol TIL $\endgroup$
    – WernerCD
    Commented Jul 29, 2022 at 22:59
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I think the only thing you need to make this feel realistic is for the world around them to be full of things they could eat in order to maintain their weight/growth. This may also include minor details as to their digestive system, like if they have scavenger adaptations that allow them to eat older meat, or if they had snake-like metabolisms designed to swallow a lot of food all at once and digest it slowly over a long time (Crocodilians also do this).

If this world has even just a few truly dinosaur sized animals that the Suchians are capable of hunting, and they can safely consume the meat of those animals for some time after a kill, I would have no further questions about continued upwards growth as you have described it.

Of course, your story seems to take place in a more developed society, past its hunter-gatherer phase, so this may be a more cultural phenomenon - older Suchians need more food, being larger. Does this lead to a 'honor your elders' kind of culture where the young are expected to crunch in order to feed their parents/grandparents? If older Suchians naturally rise to important ranks, it may appear more as a tax or offering.

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