The Canon and Broad Interpretation
Eridians live 689 Earth years, around 4.6 6^4 Erid years. Since they appear to pair bond to life and lay small 5 egg clutches I'd guess they are k-selected (have few young and care for them well). They at least have some affection for young as Andy Weir says they have incubators for eggs
Of course there was also the "500 years old and has 300 kids" comment for the character director and puppeteer characterizing Rocky. Obviously deeply exaggerated since Rocky is 291, but it does imply he has a bunch of kids he never even mentions. So maybe they are r-selected (bunch of kids most don't make it to reproducing) a bit?
Their incubation time of 42.33 days (one Eridian year) implies they could be mature and independent very quickly, especially because Grace's students at the end of the book are all 30 Earth years or younger, which for humans would be a 3.5yo. Rocky and Adrian have been together 186.3y. Since Rocky is 291 that would mean they got together when Rocky was around 104.7, which would be like a 12 year old
Ok. What's Nitty Gritty of Growing Up?
Jame Ortiz imagined them as a traditional arranged marriage gone right, so who knows what the courtship looked like, what Rocky considers the start of their relationship (knowing Rocky it's the first introduction but maybe not), or when the traditional maturity for such an arrangement is. Maybe for them a 12 year old (100 year old) is a mature professional. Maybe they arrange couples at puberty so they get to know each other as they grow up together. We have no idea
We do know that due to their very social nature the kids have to be raised with a lot of care and community. It's essential for developing that community. It's just not clear if that's coming directly from their parents. Maybe all the kids of a town live together and adults just visit or have shifts. Oh wait. If that were the case Rocky could not have any kids, he just lived in a community where all the kids were kind of "his"
I'm officially modelling Eridians off of Saddleback Tortoises. At least for growth and population. Not for social dynamics or hunting or behavior. For obvious reasons. They live a median 49 years in captivity, and can live up to 3x that. They lay about 2-7 eggs in sand. Eridians hatch in less than half the time... which is not really long at all. Definitely doesn't feel long enough for the development an adult Eridian structure would require
Molting would just be too dangerous. Their pathogens are aggressive and their immune function is heating their only opening. That doesn't work if you open up the rest of your body. So the rocky exterior has to be replaced in small ready made bits or just never come off. Turtle scutes or Tortoise layers
The 2nd implies every panel should have the smaller previous panel on top. Which kind of works for the top of his head? The vents? They have a protruding shape and would probably be dangerous to chip off. We don't see distinct layers anywhere else. We see sharp edges, which the kids also have. So maybe Eridian worker cells bind new scutes so tightly it looks like the new face was always there and attached to the old ones at an angle, creating a new sharp edge
With how dangerous being exposed I feel like growing would take a long time. Maybe they have a long "baby form" like the nymphs or pupa of insects which focus on getting as big as possible over many years, not really developing any other way or function except crawling out of the sand. Once they are big enough however I imagine development comes at Rocket speed due to high calories of Erid. Lots of energy to grow with, it's just a question of getting it.
With the colony/cluster structure of Eridian communities this could be the most intensive part of their childrearing. Constant care. Never alone, always watched, constantly fed. Maybe there are a bunch of parents walking around Erid with a rock larva strapped to their back and 4 more scattered among relatives and friends for years on end
The largest young are probably too big to carry. These could be left with professionals to feed and watch, who could also monitor for deformities and assist in any shedding. Or make community "volunteers" do it
From here I imagine an "adolescent" phase. Now they can learn and grow and scurry about as they please. The 30 year olds Grace teach are probably in this phase. Eridians aren't eusocial despite being compared to eusocial insects like ants and bees. So they probably take a while to sexually mature once in adult form because it isn't their only purpose. They have jobs and regular Eridians have mates. They can take their time
It's possible reproduction happens late in life. With 5 kids per coupling only half of all Eridians would need to reproduce, and the kids seem to develop very quickly and exist within very connected communities there's no need to worry about trying to share the max decades possible within your lifespan. Have them by 500 and you'll have a good 289 years together, and they'll be in an arranged marriage for 189 years of that.
So reproduction clearly isn't the point of marriage. It would make sense if it's mostly to make sure you always have a person. Watching them sleep is probably in the marriage vows. And if you do like each other enough to have kids that's just peachy
