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I've got a D&D-inspired setting, and one of the races present are yuan-ti. They're sentient snake people with a variety of forms, ranging from mostly human-looking to basically just a giant talking snake, plus a bunch of weird forms like a person with snakes for arms. Status is based on how snake-like they are.

I was thinking, they'd probably have snake-like eggs. And it occurred to me that they might be able to use candling (placing a light behind the egg to reveal the contents) to figure out what kind of child they're having. Anyone who knows reptile care better than me able to weigh in?

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    $\begingroup$ You need to define what aspects they are looking for (a finite list) , otherwise the question is so broad that it could require an "infinite list" which would be off topic. $\endgroup$ – Confounded by beige fish. Jun 23 at 23:32
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    $\begingroup$ Please note that in some reptiles (e.g. alligators, crocodiles, most turtles) sex is not determined genetically but rather environmentally -- incubate the eggs at one temperature to get girl reptiles, incubate them at another temperature to get boy reptiles. $\endgroup$ – AlexP Jun 24 at 1:42
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    $\begingroup$ Ugh. She has her father's snout. Poor kid. $\endgroup$ – user535733 Jun 24 at 6:26
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    $\begingroup$ @AlexP That's worthy of an answer in its own right. Especially given that because of ambient temperature variations, reptile DNA codes for different developmental hormones/proteins depending on temperature, so a sophisticated reptile race could well have a complex sequence of optimal temperatures, or perhaps even different optimal temperature regimes to maximise strength/intelligence/whatever. You got there first, so you can take it if you want. $\endgroup$ – Graham Jun 24 at 8:51
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    $\begingroup$ Not all snakes lay eggs. In fact about 30% are viviparous (give birth to live young). $\endgroup$ – Martin Bonner Jun 25 at 11:56
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Developing comments between @AlexP and me into an answer...

As other answers have said, candling of reptilian eggs is possible. For in ovo testing of chicken sex, if the male and female have different pigmentation or other easily-distinguishable features then this can be highly reliable. There is no reason to think this would be different for reptiles.

However, there is an important frame challenge required to this question. The question assumes that genetic randomisation is the dominant feature for reptilian development, and that parents would want to screen their developing eggs for positive/negative features. This is largely true for mammals and birds, because the parent's body keeps the developing embryo at a constant temperature.

Reptiles develop differently though, because the majority of reptiles produce eggs which in nature are left to develop on their own. (Exceptions do exist such as the midwife toad, some frogs or some snakes, of course, but the OP's question assumes a species which does not birth live young.) Since the ambient temperature will naturally vary, reptile DNA contains a large amount of "countermeasures" to vary the embryo's developmental pathways and gene expression depending on temperature. The epigenetic effect of ambient temperature is therefore highly significant, to the extent that the embryo's sex can be dependent primarily on the temperature at which the egg is kept.

For an intelligent species such as the yuan-ti, this has important consequences. It is entirely possible that the physical characteristics of an individual can be predetermined solely by managing the temperature of the egg during development. Humans naturally have not put a great deal of effort into investigating how far this goes, but is likely that intelligent reptiles would have a very long history of discovering what temperature changes at what times during the embryo's development produce what effects - gender as a start, of course, but also physical size and strength, quality of eyesight or hearing, intelligence, or many other possibilities. Candling could be used to screen for developmental abnormalities (embryos which do not look "snaky" enough), but this would be a backup to a much more systematic selection of characteristics for your offspring.

As with pre-medicine human midwifery, of course it is quite likely that many of these would be some kind of tribal knowledge. Some may work, some may only work partially, and some may not work at all. But there would still be a body of knowledge which would be called on, and there would likely be individuals who would fulfil a similar advisory role to human midwives, except on a more continuing basis because eggs need longer-term monitoring.

So returning to the frame challenge, your yuan-ti parents would not just use candling to "discover what kind of child they're going to have" - they would actively choose what kind of child they wanted, and candling would then just be confirmation that development was proceeding as they expected. The implications of this difference for the parents and for the society in general are immense, because this introduces the concept of "designer babies" to a pre-industrial society.

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    $\begingroup$ This is amazing. The noble houses each have a closely guarded secret sequence of temperatures that reliably yield whatever attribute their family line is known for. $\endgroup$ – Luke Jun 24 at 20:10
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    $\begingroup$ This is an excellent answer. It not only answers the question as asked, but points out the secondary traits of relevance (sex selection by temperature being a notable one) and how these could lead to unexpected results. For the record, I suspect a caste system (loose or strict is up to debate) is the most probable outcome. As @Luke points out, noble houses would definitely set up specially prepared incubation chambers, and plenty of intrigue around how the secrets of creating their specific lines would be guarded from the other houses... now I want to read this story! $\endgroup$ – Palarran Jun 24 at 22:44
  • $\begingroup$ @Palarran Thank you. :) AlexP was responsible for key points in this - definitely a case of the two of us getting a better answer than either could have done individually. It would be nice to be able to link another user's rep to an answer where they've contributed significantly, but I doubt that's a feature SE plan to add. And yes, I'd like to see where the OP goes with this if they pick it up! $\endgroup$ – Graham Jun 25 at 0:11
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    $\begingroup$ As an additional comment, the nests of crocadillians are remarkably stable, temperature-wise. $\endgroup$ – Michael Richardson Jun 25 at 14:27
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    $\begingroup$ Note there are plenty of reptiles that use what we consider "normal" chromosomal determination of sex. $\endgroup$ – John Jun 26 at 3:12
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This is pretty much a let me google that for you kind of Q&A:

Reptile eggs can be candled, just like bird eggs. (Probably because, you know, birds are dinosaurs, etc.)

Reptile keepers can certainly determine if an egg is viable or not. As with humans and ultrasounds, your sophont reptiles could paint or photograph the resulting images as Junior grows and develops in the egg!

And naturally, these pictures can be brought out by a proud Mama when Junior starts courting, much to his own mortification and the eternal delight of his sweetie pie!

Reptile egg candling.

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Let's put some imagination to work

  • The shape of the embryo, possibly determining biological sex, but also determining physical abnormalities such as deformed or missing limbs and stunted growth.

  • Density variations in the shell, possibly showing illness or condition of the mother that can have a consequence for the child or issues that might keep the child from properly hatching.

  • Density variations in the fluid, possibly showing the development of fungus or bacteria indicating disease or abnormalities in embryonic development.

  • Color variations in the fluid, possibly showing the presence of blood.

Several of those could also be used to identify the presence of parasites, even insects that have bored into the egg and now threaten the child.

This kind of information can also lead to diagnoses that include uneven heating during gestation, the need to turn the egg (or not turn it), premature development (egg must be manually hatched early to save the child).

Frankly, there's a lot you can do (or could believably do) with candling an egg.

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Screening for monsters

This is a fantasy. Let us make it fantastic. There are a variety of acceptable Yuan-ti phenotypes. But occasionally something is produced that is really weird; monstrous - unacceptable. Perhaps a Dunwich horror type line with vestiges of other races or even demigods. Shambling many-headed horrors. I envision the candling, and an eye within the egg swivels to look through the shell.

Civilized Yuan-ti want no part of such things, and these abominations can prove difficult or dangerous to dispose of after they emerge from the egg. Most are born with abilities that help ensure that they survive. Catching them in the egg before they emerge is a much surer way to prevent their existence. There are correct ways to destroy such an egg; creatures are kept by the Yuan-Ti for this purpose.

There may also be those who are interested in hatching and raising these aberrant progeny...

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They could tell if the egg was fertilized and developing normally. They could tell if the embryo died during development, and they'd get a sense when they could no longer see through the egg how long until it hatched.

I can't imagine they could tell anything once the embryo fully develops since they'd just fill the egg with their non-transparent body.

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