312

Through selective breeding and misplaced import laws, we only ever get to see Russian dolls that are "aesthetically pleasing". In the wild, it is relatively common to get a "double-yolker" - that is, sometimes when the outer layer dies, there are 2, or perhaps 3 dolls inside. Since these dolls are often a little misshapen when compared to the outer layer, ...


126

In the deepest, darkest, hidden places in the forest dwell the Queen Dolls. Rarely seen, these creatures resemble a normal Russian Doll only at the tops of their vast, misshapen bodies - their lower halves more closely resemble a nightmarish wooden bee hive. Riddled with thousands of cells, they disgorge the normal Russian Dolls commonly found in the ...


96

Actually, there is an organism on earth that is vaguely similar to the Russian dolls that you describe. The volvox: Individual volvox cells, a kind of freshwater algae, reproduce in the conventional way (more or less), however, they also collect into spherical colonies (called volvocates, I think) with a tiny opening at one end. These spheres can reproduce ...


83

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. ...


76

Last time I looked into this I was amazed these experiments (mammalian conception to delivery in orbit) had not been done. I figured I just had not found it and so I dug in this time. For mammals, they have not been done. Pregnant rats have gone to space and come home and delivered so microgravity is not immediately lethal to a fetus. Rats mated in space ...


73

The different causes of death are the biggest issue here. Humans will lose a lot of children to disease, elves don't seem to suffer from this Humans lose fertility early then die fairly young, again not a problem for elves Most of the death rate for elves appears to be wars, in a time of war they'd need a birthrate approximating that of humans just to ...


68

That's not too hard. There are plenty of real-life animals that want to be eaten. They're either parasites, or the hosts of parasites, whose behavior is modified by the parasite that they carry. Of course, since you want the interaction to be mutually beneficial, your animals won't technically be parasites, but their ancestors could have been. It's also ...


62

The concept of a menopause is actually unusual, it's known to exist in the wild in 5 species, Humans, Orcas, Belugas, Narwhals, Short Finned Pilot Whales. While some other species exhibit menopause in captivity, others are definitely known not to e.g. cats and dogs. In all other species the females are believed to remain fertile for their entire lives. ...


58

Its been observed that jet-pilots tend to have more girls than boys. Its even something of an inside joke among aviation types. Its been given some scientific credence lately when it was theorised that higher exposure to cosmic radiation or potentially even the high G-forces could be damaging their Y chromosome. Due to a quirk in chemistry the composition of ...


54

Low natality rates. They would live long, but procreating a child is a very rare event for all couples. That should balance the lack of aging, if deaths due to other reasons are not too frequent.


52

To get pregnant again, you must finish your prior pregnancy. The woman cannot have another pregnancy because she has never finished the prior one. It is not because she has been physically damaged in any way. It is because the mystical unborn has not been born. The connection between mother and fetus remains intact; from the OP "the unborn is bound to ...


51

Some quick thoughts for you: Social Taboo: yes, they could mate with each other, but that's not socially acceptable. If this taboo is strong enough, it'll result in species differentiation. (Um, I mean "maintain a pure breed..."). I think (American) society is much more accepting than it used to be for this sort of thing. Courting between races may be ...


50

You can go through with a normal cycle, menstrual cycle runs as normal, menopause runs as normal at a certain age, after which no more periods. You can say that the immortality means that the normal menstrual cycle keeps running forever. All these things have hormonal consequences, and as such consequences for the body, if you want to keep people "young" ...


46

I would think along the lines of Ligers, Mules and other mixed species mating. They produce offspring but their offspring are infertile. The number of "ligers" in general population would be unlikely to reach large numbers due to inter-species cultural differences and possibly more importantly, inter-species attraction. I can't see myself going for a ...


46

Here are two creatures of the same species: If you and most of your kin looked like the first picture, you would probably consider the second picture--10 times larger, with an un-naturally slim face, threatening ears, loose jowls, and weirdly short and dark hair--to be a monster. Very similar underlying genetics can account for quite significant variance ...


42

Regenerate the top part. The splitting doesn't actually kills the doll - just its bottom half. The bottom half becomes a nest for the little doll that was gestating on the inside of the "mother doll" for a bit, until it can go around hunting for cookie crumbles for itself. The upper part, on the other hand, shrinks a bit in size and regenerates. After ...


39

Self produced testosterone antibodies. As soon as the male embryo starts producing testosterone, it gets attacked by the mother's antibodies and as a result cannot complete its development.


38

Budding and out-growing. Budding: each outer doll, once in a while, produces a bud, which in this case is a tiny Russian doll. Once it detaches from the mother body, it starts living on its own. Out-growing: once the doll has eaten enough, it grows, on its outer, a new shell. Think of it like the trees growing each year adding a layer to their bark. At the ...


37

For raising children. Let's say their world was rather hostile. Early in their evolution they could be attacked at any time. Having two parents to protect the young was therefore safer for the children. That is, parents who stay together will more successfully raise children. Once they've raised children that bond is no longer required, but there is no ...


36

It is not only possible, apparently it actually exists on earth. The abstract of the linked article reads: Two recently discovered cases of genetic caste determination in social insects might provide the first example of a major evolutionary transition from two to more than two sexes. I argue here that the system can be interpreted as comprising primarily ...


35

You haven't nailed down the process, which is rather important as to how it will affect the population. Let's consider this solely from the perspective of the magic tree and blood... This process has some messy issues. What stops the local Duke, age 90, from hauling out the local pretty girl, age 6 and nowhere near birthing age, from combining blood to ...


32

Based on some handy data from 2011, we can estimate worldwide deaths at around 55 million per year. At that rate, for 700 million people to die, you’d only have about 12.65 years. Now, this annual death toll is definitely going to see some yearly variance based on current world conflicts, epidemics, natural disasters, etc. In general, I would not expect ...


30

Note: as it currently stands, this answer does not meet all the OP's requirements. (The OP's question requires that a cambion be able to produce offspring when mating with a demon. I had missed this requirement when I composed this answer, but it was brought to my attention in comments. I'm waiting to hear back from the OP on a certain point, after ...


30

Aging is not necessary to maintain their low population. No population can grow without limit, and in the animal kingdom, most animals die before they would otherwise die of extreme old age. These are the things that limit population growth. Food stocks. Regardless of your will to reproduce, if you don't have enough food, you can't grow new humans. ...


29

I'm by no means sure of this, biology is not my wheelhouse, but I would think that the most likely "solution" in this case is that the Cockatrice reproduces by introducing a live-born predatory infant into the eggs of prey species. This tiny creature then burrows into the shell, attaches itself to the inside to seal the egg and halt evaporation and then ...


28

Evolution should already ensure that species will not want to mate with other species often. This is due to a principle of Koinophilia. On the most basic level The odds of a hybrid species being healthier then non-hybrid is unlikely and thus we all have an instinctual aversion to mating with someone from another species because it's likely our children ...


28

Well, human women have all their eggs pre-made -- they are born with a finite number of eggs and that's what they will ever have. If you want immortal women to have an infinite number of periods you will have to alter their physiology significantly; as a consequence, since you have already altered their physiology, you may as well introduce a patch which ...


27

This is an extreme case of the so-called Mother's curse: In biology, mother's curse is an evolutionary effect that males inherit deleterious mitochondrial genome (mtDNA) mutations from their mother, while those mutations are beneficial, neutral or less deleterious to females. There are known cases where an mtDNA defect largely or solely affects males of ...


27

Have you hear of r/K Selection Theory? (Wikipedia) Basically, when reproducing, there is a trade-off between quantity (r) and quality(K). You can make 5,000 eggs and lay them... if they are "low-quality," so low investment in energy into them, and low chance of survival per egg. If you only have one offspring at a time, you want to make sure that it survives,...


26

Plants naturally produce edible fruit because they can't move, and animals can - animals eat the fruit, move the seeds, then defecate them out. The seeds then grow in a potentially uncolonised location (with bonus fertiliser). Egg fruit You can easily imagine a sessile animal (like a sea anemone) that uses the same process. It lays eggs whose embryo is ...


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