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So basically the elves of the setting have varying appearances depending on the season they're born in, but they're entirely biological and while the setting does have magic I don't want there to be a mystical or magical component to their appearances.

For the question's simplicity's sake I'll be focusing on hair colour, where if an elf was born in summer, they'll be born with either blonde or green hair, autumn, red or brown, winter, black or grey or white, and spring, a random colouration to their hair within the visible light spectrum(with hidden ultra-violet highlights).

I have the basic idea and how it affects their culture but I don't really have an explanation for it, and so I've come to ask this question: How can I tie an individual's general appearance with the season they're born in?

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    $\begingroup$ Assuming gestation remains static, it's really about the season they are conceived in. Access to particular nutrients within a few days of conception then unalterably change the expression of genes in early development. Appearance ends up fixed. Would want to develop a explanation for changes in the mother's diet throughout the year. $\endgroup$
    – John O
    Aug 22, 2022 at 13:09
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    $\begingroup$ Building on @JohnO's comment, different foods are available during different seasons. Don't let our modern world fool you, where food is shipped all over the world, in the "good old days" one could only get certain foods in their season. $\endgroup$
    – JBH
    Aug 22, 2022 at 14:41
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    $\begingroup$ @JBH Certain game can only be hunted in one part of the year. Certain fruits only ripen in another. The grain's not ready to harvest and grind before Autumn, and it's all gone by mid-winter. Lots of possibilities here, depending on how he wants the story to go. $\endgroup$
    – John O
    Aug 22, 2022 at 15:26
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    $\begingroup$ Building even more on @JBH! Wealthy elves would be able to buy non-seasonal food to influence the hair colour of their children. Want a blonde baby in the winter, then you'll have to import some summerberries from the southern kingdom at vast expense. $\endgroup$
    – David258
    Aug 24, 2022 at 15:42
  • $\begingroup$ Like all spring-born Gluugs that Space Captain Jones had seen, Rokzthak The Mighty had pale green tentacles. $\endgroup$
    – Michael Harvey
    Aug 25, 2022 at 13:51

11 Answers 11

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Epigenetics

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Epigenetics is the study of how your behaviors and environment can cause changes that affect the way your genes work. Epigenetic changes are modifications to DNA that regulate whether genes are turned on or off. These modifications are attached to DNA and do not change the sequence of DNA building blocks. It is known that maternal factors, including diet, induce epigenetic change. Environmental chemicals during pregnancy can also induce epigenetic change.

For your elves, prenatal conditions like a mother's diet, exposure to pollen, the strength of sunlight (which produces vitamin-D) and other factors turn various genes off and on that regulate hair color and possibly other traits like pigmentation, tolerance of heat and cold and even amount of body fat (especially in children). This could have played a role in pre-civilized survival, where children born during winter may have baby fat for insulation, pale skin to absorb more sunlight/vitamin-D, and dark, curly hair for greater heat absorption and retention, while children born during summer have less baby fat, darker skin (or greater ease of getting tanned) and straight, airy, fair hair. Spring and autumn children may have green and red hair, respectively, for better camouflage.

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    $\begingroup$ Epigenetics are a fascinating and underestimated factor in genetic switching! The vitamin D and diet components could be a bit shaky in a more advanced civilization, but pollen exposure in particular is a brilliant idea! $\endgroup$
    – Doktor J
    Aug 22, 2022 at 17:30
  • $\begingroup$ Diet IS a factor in epigenetics today - though mainly poor diet for the pregnant mother. Diet will vary by season in non-modern societies (e.g. fruit, nuts and mushrooms during autumn) and could conceivably be a factor in elven epigenetics. $\endgroup$
    – Klaus Æ. Mogensen
    Aug 22, 2022 at 18:12
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    $\begingroup$ Hormones from other nearby elves can also shift with season (length of day triggers different sleep patterns, for example) and trigger epigenetic changes. $\endgroup$
    – SRM
    Aug 22, 2022 at 19:27
  • $\begingroup$ Oh I'm not saying it's not a factor... just saying that over hundreds or thousands of years, diet and nutrition would change, so some of these "seasonal effects" could be drastically altered by the course of civilization. $\endgroup$
    – Doktor J
    Aug 22, 2022 at 22:13
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    $\begingroup$ If I recall, it is temperature sensitive epigenetics that gives siamese cats black fur in their face, tail and paws. If they are shaved for sugery on the body, the fur grows back black before turning white again. I see no biological reason something like this couldn't "lock in" shortly after birth in other breeds, not to mention other species, depending on temperature, sunlight exposure, diet, or a number of other seasonal factors. $\endgroup$
    – Arthur
    Aug 23, 2022 at 6:57
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In some reptiles the sex of the to-be-born embryo is determined by the incubation temperature of the egg.

You can have a similar mechanism in place, in which the phenotype is determined by the temperature present during the period before birth.

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    $\begingroup$ This answer is good because it's believable and easily explained. Compared to epigenetics in the other answer, this one keeps it simple. $\endgroup$
    – fredsbend
    Aug 23, 2022 at 16:11
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    $\begingroup$ I recently read that, because of warmth, one population of sea turtles has no new males this year. :( $\endgroup$
    – Anton Sherwood
    Aug 24, 2022 at 2:52
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    $\begingroup$ @frеdsbend this answer is a specific example of epigenetics $\endgroup$
    – james turner
    Aug 24, 2022 at 19:38
  • $\begingroup$ @jamesturner I understand that, but "because epigenetics" is a far more complicated explanation for readers than "like how the temperature affects some lizard's sex". Depends on the audience, but most fantasy readers aren't super interested in detailed scientific explanations right in the middle of the story. $\endgroup$
    – fredsbend
    Aug 24, 2022 at 20:05
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They nurse on Plants

These elves, when they are born, take their nutrients from the sap of a deciduous plant. Their appearance, as in other species, should be closely tied to the nutrients availible in the beginning of their life (which in the elves' case is bound to the seasonal cycle of the plants)

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    $\begingroup$ Plants also produce hormones (such as "rooting compound"), so perhaps the blend of hormones could be affecting those elves? en.wikipedia.org/wiki/Plant_hormone $\endgroup$
    – Tangurena
    Aug 23, 2022 at 14:36
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The simplest method would be a dead-end genetic trait that changes the baby's hair color based on the average temperature and sun intensity the body receives.

The hormones in our body change with the seasons. A simple example would be vitamin D and the sun, but with medieval living the food availability will also change based on the season so the composition of the food would also change along with the seasons. This could be enough of an effect on the DNA of the child before birth and during his early life when receiving milk from the mother to set the hair color.

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    $\begingroup$ I'm... a little stuck on why you think it would be a dead-end trait, since they aren't made infertile by this feature of theirs. $\endgroup$
    – Lemming
    Aug 22, 2022 at 10:06
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    $\begingroup$ @Lemming it serves no real purpose from an evolutionary standpoint (unless the traits you link to it do). Its not about if it will cause the species harm in the long term. $\endgroup$
    – Demigan
    Aug 22, 2022 at 10:18
  • $\begingroup$ Ah, alrighty then. $\endgroup$
    – Lemming
    Aug 22, 2022 at 10:23
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    $\begingroup$ @Demigan Depends on what the colour is; white hair to disguise as snow in winter, orange to match the fallen leaves in autumn, green for the foliage in summer, etc, could help to protect the infant during its early months? $\endgroup$
    – Chronocidal
    Aug 23, 2022 at 9:02
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Procreation is not random.

The elves want people to have the correct appearance for the season of their birth. Elf gestation like human gestation is fairly predictable. Blonde summer people therefore conceive babies in the fall and their blonde summer babies are born in summer. Red autumn people conceive babies in the winter and have their autumn babies in the autumn.

Autumn people and summer people do not make babies together although different season people could certainly have sex for reasons other than procreation.

This is selective breeding and timing of breeding. No magic needed.

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  • $\begingroup$ It doesn't even have to be selective. People might only be physically capable of conceiving in the window of time that would cause their child to be born at the same general time of year as they were. If hair color is a dominant gene, then it would appear that hair color and seasons were linked. $\endgroup$
    – bta
    Aug 25, 2022 at 0:58
  • $\begingroup$ If elves of different hair colors cannot interbreed, wouldn't that in effect make them different species? $\endgroup$
    – Falco
    Aug 25, 2022 at 9:38
  • $\begingroup$ @Falco - they can interbreed. They are just not supposed to. The elves seasonal thing according to appearance is a cultural project that is only a few thousand years old. That is a second in evolutionary time. $\endgroup$
    – Willk
    Aug 25, 2022 at 12:30
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Mothers who gave birth during different seasons have differences in their breast milk. Thus babies are fed differently and this affects their hair color.

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Estrous Cycle

In the animal kingdom we have the "estrous cycle" (when females can get pregnant) and the "rutting period" (male mating season). So here's my thinking:

Elves are almost like 4 separate species, represented by 4 different estrous cycles. The "species" is revealed by the hair color. This fertility trait is a very dominant trait carried from the mother and coupled tightly with hair color. So blond elven women are only fertile during a time of the year that means they will always bear their children in summer, and their children will be blond and (if female) have the same estrous cycle as the mother.

I kinda like this idea because I could see it developing as a form of population control. Elves are long lived, so birthing rates are a big deal and overpopulation could be a serious problem so nature's solution (perhaps aided by the elves themselves via some careful breeding over thousands of years) resulted in this pattern where only one-quarter of the female population is ever fertile all at once, and even then only during limited parts of the year. (e.g., assuming an earth-like cycle, summer babies are born in July, with a 9 month gestation period, meaning the blond women are only fertile in November. That's it. Just November.)

Being fertile year round lead to overpopulation. Being fertile only part of the year makes things too brittle -- "oh population is good now, no need for mating season this year. ... Oh no a disaster happened, we need to re-engage population growth but we have to wait a full year now". Having 4 mating seasons with 25% of the women being fertile per season let the elves have good population control but also better flexibility than once a year.

Or if you want it based off the men then do the same thing but, er, "rutting period", with the dominant gene coming from the men. But I think it makes way more sense to do an "estrous cycle" because while one man can make many females pregnant, this estrous cycles is a hard limit on population growth rates, so you have something that is (I think) genetically feasible and also has a bit of world building sense.

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Star signs

Everybody knows that the primary influence in shaping a newborn’s life is the gravitational pull of celestial bodies at unfathomable distances. As the planet revolves through the year, the position of the Sun at the moment of birth changes relative to the zodiac (or your planet’s equivalent). Hair colour is only one of the many features that are affected: their temperament, luck, and fate all draw from this astrological influence. And the Sun is not the only player in the game: you’ll have other planets, moon(s) and distant constellations too. They may influence smaller details, such as the specific hue of highlights.

Astrology has been followed, in one form or another, by pretty much all cultures we know of, and it definitely matches well with a yearly calendar. In your world, it also works.

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Temperature

Real-world fact. Siamese kittens are born white. As they age, their paws and ears start to turn coloured. As the adult cat ages, the colour gradually creeps up their legs and with an elderly cat even the body can start to darken.

The colour that their fur grows is determined by the temperature of the blood in the hair follicle. Kittens have higher body temperature overall. Legs and ears get cooled below core body temperature.

So let's say elves do not have such precise body temperature regulation as mammals. In cold weather their core temperature drops somewhat to save energy. Let's also say that this affects the development of a foetus in an evolutionarily harmless way. It causes particular hair-color-generation cells in hair follicles to proliferate at the expense of others. Maybe also eye-colour. You might even think of reasons why evolution selected for this, but traits which do not harm a creature's chance of survival are not discriminated against, and hair coloration may be a neutral trait with respect to survival.

Do you need to explain? Maybe, it just is. Heredity is modulated by the seasons, and an elf can tell at a glance what season another elf was born in. (And perhaps, how hard that winter was. Could be fun banter! )

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Camouflage.

The elves evolved from a weaker and less intelligent species that relied on camouflage of newborns to help hide them from apex predators until more mature and able to run or defend themselves.

At some point, they lost the ability of the hair to change color with the seasons so they are stuck with whatever color they are born with. Perhaps the hair and/or skin coloring differences are now far more subtle, but the remnants of the coloring still persist.

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Chlorophyll

Real world, plants are coloured based on distance to the sun and the light waves that hit them, if your elves had Chlorophyll(or similar light absorbing abilities like human skin does) in their hair (and or skin pigments) that are 'set' in the first 3 months of their birth, (and or mature at a specific age) their hair could range from a deep greenish blue, through green-red-browns to orange to yellow, like leaves do around the world.

The body would 'know' how much energy its getting from the suns rays, and set itself to that colour, more likely as a birth sun-energy ratio.

giving you nice hooks into children born into the dark and kept in the dark longer so they have the blond hair (low energy) vs children taken outside into the sun for as much absorbtion, to have the deep green colour.. maybe if you have caste systems, some parent would try to give false light so their hair is darker/lighter.

p.s. I use the same in my own worldbuilding, elves in the equatorial regions have much greener hair, while those in the icy regions with no sun have blond / white, initially I based it on humans brown->blond for the same reasons, but to get the greens, and reds, went for plant blood, yet I think this mechanic is generic enough to use in anything.

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