Feasibility of pregnant birth: xenobiological

[onomatomanic]

Hello forum,

I'm world-building for a SF-story and would like some feedback on a xenobiological idea which occurred to me recently.

The setting is a planet on which rather peculiar astronomical and geological factors ultimately produce physical conditions and an ecosystem which closely resemble those found on Earth, for the most part. However, for various reasons I'd like to tie the mating cycle of all mammals (or mammal-analogs, strictly speaking) to one of those peculiar factors, namely a periodically occurring spike in brightness of one of the planet's suns. The in-universe scientific justification for this link is that mating at that time is evolutionarily beneficial, that is, it statistically results in more helpful and less harmful genetic variation in the resulting offspring. Why that is so remains to be seen. Potentially, it could be this advantage which allowed my mammal-analogs to outcompete my dinosaur-analogs, though I don't currently have any plans to incorporate that idea into the story. As time progressed, the link was reinforced by natural selection to the point at which all extant mammalian species go into a single-minded sexual frenzy once said astronomical event occurs (which period is thus called "The Rut"), while being completely infertile in the interim.

That's a rough outline of the background - I can fill in more details as needed, of course.

Now, the time gap between two Ruts is 18 (Earth-)months, and I'm trying to figure out what impact that would have on the various species, assuming that their sizes and aging rates resemble those of Earth's mammals.

The largest land-species is the "dreadnought", so called because it has no natural predators, and closely resembling a woolly mammoth. Elephants have a gestation period of close to two (Earth-)years, apparently. So it seems very plausible that dreadnoughts would have the longest mammalian gestation period on my planet, and that this would be just short of 18 months. They give birth just before the next Rut and, if the calf is stillborn or immediately weaned, the cows can become receptive again almost right away.

As on Earth, smaller animals would then have shorter gestation periods. "Altlings", the human-analogs and sentient protagonists of the story, could have the familiar 9-month term and give birth right at the half-way point between two Ruts.

The problem, which you may have spotted right away, presents itself for the smallest animals whose longevity is on the same order as or even below those 18 months. It took me some googling, but from what I found, there are indeed some (though not many) species of Earth mammals which commonly live for less than that, such as some opossums and voles. The first and obvious solution which occurred to me was that their analogs on my planet could either be somewhat bigger and/or longer-lived, or not exist at all. In the latter case, their biological niches would simply have to be filled by non-mammalian creatures instead.

However, a second, more complicated but rather appealing alternative came to mind, and that's where the subject heading comes into play and what I mainly would like comments on. Namely, that there might be a way to have several generations between Ruts without either additional matings or adopting some form of parthenogenesis: Tribbles!

The way I imagine this working is that a female mates with several males during The Rut, each of which fertilizes one or more eggs. The fertilized eggs are then subdivided into groups of a certain size. In each group, one begins to develop into an embryo, while the others remain inert and are ultimately encompassed by the developing one in such a manner that they end up lodged within its reproductive tract. Some time later, the offspring is born in the usual way, except that all females are "born pregnant", give or take. When they are well-grown, one of the eggs begins to develop in turn, again incorporating any remaining dormant eggs, and this results in their giving virgin birth. To a sibling, in genetic terms. How cool is that. The process can repeat for as many generations as there were eggs in the initial grouping, and ends with the generation which lives to see the next Rut.

Is this feasible, or did I overlook major problems in this chain of events, from either a reproductive or evolutionary point of view? If so, I also have a slightly less appealing alternative, somewhat similar to the way in which insect hive queens procreate, if I'm not mistaken - except that here it is spread out over several generations: The original mother could, again, mate with various males but, instead of either using or discarding their genetic material, most of it could be stored and somehow (either in utero or after birth) be distributed among her female offspring, which would then use part of it to fertilize their own eggs and again pass on the remainder, et cetera.

That's about as far as my thinking has progressed. Please let me know what you think, and especially point out any amateurish mistakes which have slipped in. TIA! :)

 

[Darwin123]

The way I imagine this working is that a female mates with several males during The Rut, each of which fertilizes one or more eggs. The fertilized eggs are then subdivided into groups of a certain size. In each group, one begins to develop into an embryo, while the others remain inert and are ultimately encompassed by the developing one in such a manner that they end up lodged within its reproductive tract. Some time later, the offspring is born in the usual way, except that all females are "born pregnant", give or take. When they are well-grown, one of the eggs begins to develop in turn, again incorporating any remaining dormant eggs, and this results in their giving virgin birth. To a sibling, in genetic terms. How cool is that. The process can repeat for as many generations as there were eggs in the initial grouping, and ends with the generation which lives to see the next Rut.

Is this feasible, or did I overlook major problems in this chain of events, from either a reproductive or evolutionary point of view? If so, I also have a slightly less appealing alternative, somewhat similar to the way in which insect hive queens procreate, if I'm not mistaken - except that here it is spread out over several generations: The original mother could, again, mate with various males but, instead of either using or discarding their genetic material, most of it could be stored and somehow (either in utero or after birth) be distributed among her female offspring, which would then use part of it to fertilize their own eggs and again pass on the remainder, et cetera.

That's about as far as my thinking has progressed. Please let me know what you think, and especially point out any amateurish mistakes which have slipped in. TIA! :)

I don’t know how likely it is that a mammal-analog could be born pregnant. However, there is an insect on Earth that experiences alternating generations where one of the generations is “born pregnant”. This insect is the aphid.
The eggs that hatch in the beginning of spring have been dormant all winter. The generation that hatches from overwintered eggsnot only reproduces asexually, but is born pregnant.
Note that there is sex later in the year, when the plant may be dying. The overwintered eggs face an overabundance of food that only lasts a short time. The aphids may destroy the plant they are on later in the year. Therefore, the overwintered generation have to reproduce rapidly to take full advantage of the temporary resources. There is no direct competition between aphids early in the spring because of the overabundance of food. So reproducing fast is more important than being “fit”.
Later in the year, the edible plants may be in short supply. The aphids are competing directly with each other. It is then that the aphids practice sexual reproduction. Apparently, variety is more important when the resources are limited. Sex produces variety.
I don’t know how closely the example follows the scenario in your distant planet. However, the generation from the overwintered eggs is viviparous. Just like mammals! The later generations that have sex lay eggs. Just like reptiles or birds! So maybe your animals can change classes over the cycle of many generations.

Here are links to articles on aphids, followed by a relevant quote.
http://watoxics.org/files/aphids.pdf
“A generalized life cycle begins in the spring with the hatching of overwintered eggs
just as the new growth on plants is developing. These aphids are all wingless females,
each of which without fertilization bears succeeding generations of as many as 100 more
live wingless females. Incredibly, the aphids are actually born pregnant—even before
birth, the female’s own daughters are developing within her. This strategy of bearing live
young without fertilization accounts for the extreme rapidity with which aphid populations
can grow, making control difficult. Young plant shoots can quickly be completely
covered with a teaming colony of aphids.”

http://www.gpnmag.com/clone-wars-aphids
“There is much talk about how we as a society must stop
cloning at all costs. Just think about what would happen if humans were to
become like aphids. Sexual reproduction would no longer be necessary, and all
females would be born pregnant.
Well all right, that's going a little far, but that's what
happens to an aphid in the greenhouse. For example, the green peach aphid is
known to reproduce sexually outside in nature, but in the controlled
environment of the greenhouse, they reproduce asexually. They are all females,
and yes, they are all born pregnant. When aphids give birth, they give birth to
live young rather than eggs, and they are more or less born pregnant. They
cannot give birth until they become an adult, but the young inside of the
newborn aphid are already developing--their reproductive capacity is amazing.
It's exponential. Needless to say, pest populations can appear to develop
overnight. In fact, scientists of old used the sudden appearance of aphids on
plants as proof for spontaneous generation -- the theory that living organisms
can originate in nonliving matter.”

http://en.wikipedia.org/wiki/Aphid
"Only females are present in the population (although, a few species of aphids have been found to have both male and female sexes). The overwintering eggs that hatch in the spring result in females, called fundatrices. Reproduction is typically parthenogenetic and viviparous. Eggs are parthenogenetically produced without meioisis[37][38] and the offspring are clonal to their mother. The embryos develop within the mothers' ovarioles, which then give live birth to first instar female nymphs (viviparous). The offspring resemble their parent in every way except size, and are called virginoparae.
This process iterates throughout the summer, producing multiple generations that typically live 20 to 40 days. Thus one female hatched in spring may produce thousands of descendants. For example, some species of cabbage aphids (like Brevicoryne brassicae) can produce up to 41 generations of females.
In autumn, aphids undergo sexual, oviparous reproduction. A change in photoperiod and temperature, or perhaps a lower food quantity or quality, causes females to parthenogenetically produce sexual females and males. The males are genetically identical to their mothers except that they have one less sex chromosome.[37] These sexual aphids may lack wings or even mouthparts.[1] Sexual females and males mate, and females lay eggs that develop outside the mother. The eggs endure the winter and emerge as winged or wingless females the following spring. This is, for example, the life cycle of the rose aphid (Macrosiphum rosae, or less commonly Aphis rosae), which may be considered typical of the family. However in warm environments, such as in the tropics or in a greenhouse, aphids may go on reproducing asexually for many years."

 

[onomatomanic]

Thank you, that was extremely helpful. And here I'd almost given up on getting any replies at all. :)

I had not considered making any of the subsequent generations distinct from each other in a manner resembling that seen in aphids. The only way in which the generation which experiences The Rut would differ from the preceding and succeeding ones would be that there'd be no pregnant birth for that one, because there'd be no remaining fertilized eggs from the previous Rut for them to be pregnant with, and in order for them to be available for sexual reproducting when their Rut finally arrives.

However, having read up on the amazingly complex multi-generational lifecycle of aphids now, I'm liking that idea more and more. My humanoid protagonists' civilization hasn't evolved to the point at which they'd have means or motive to study the reproductive biology of the creatures in question to the degree which it'd take to figure out the intricacies I had in mind. So, they'd observe that these creatures give birth more than once between Ruts, and would find this curious as it differs from what they expect from mammals, but they'd have no idea of how they are able to do this. The only person "in the know" would be me, the author, and the only way to let the reader know would be to break the fourth wall.

If, on the other hand, I do make the generations more obviously distinct form each other, then there's more for the characters to observe, which is all to the good. Excellent! :)

Are there any mammalian examples of significant polyphenism (I think that's the right word), other than sexual dimorphism? Anything that mirrors, in whatever way, the sequential changes which occur over a full cycle of aphid generations? I suspect the answer is no, and that the reason is that there aren't any mammals which mature and die quickly enough for a permanent physiological adaptation to a specific seasonal climate to be of much use. Instead, some mammals have temporary adaptations, such as growing and shedding hair to adjust thermal insulation. Earth, unlike my planet, simply doesn't have any environmental cycles with a longer period and sufficient predictability for Evolution to be able to provide for them. Something like the 11-year sun-spot cycle would be the closest thing we have, and if that did cause major changes to environmental conditions, then it could conceivably lead to some such generational progression in the shortest-lived mammalian species. But it doesn't cause the former, so it can't lead to the latter. Does that make sense?

Regarding the viviparous/oviparous switch seen in aphids, that's something I should be able to adopt quite readily to my scenario, just as you suggest. After all, there really are egg-laying mammals, so it seems quite plausible to posit that the sexual generation would give birth using one method and the intervening generations via the other method. I haven't been able to think of a sensible scheme so far which would explain why the two methods are each preferrable at different points in the cycle, but that shouldn't be insurmountable.

The aspect of the aphid cycle I like best is that polyphenism in the sexual generation doesn't just allow the individuals to feed on the food source that is most abundant at that time of year, but that it also serves the purpose of preventing inbreeding (and this would be even "inner" than usual, since all of the male and female offspring of one original matriarch are as close to genetically identical as is possible): By enabling and/or forcing that generation to become migratory, one achieves population mixing at just the right time. I'm definitely going to try and incorporate that into my scheme, in some way. It just seems too elegant to miss out on... :)

 

[Darwin123]

“Are there any mammalian examples of significant polyphenism (http://en.wikipedia.org/wiki/Polyphenism).”
The naked mole rat (Heterocephalus glaber) is the only know mammal to be eusocial. This eusocial behavior is associated with significant polyphenism. The queen is significantly different from the workers both physically and behaviorly.
The life cycle of the naked mole resembles the life cycle of termites. Hymenoptera (ants, bees, wasps) also have eusocial lifestyles. However, the life cycle of hymenoptera is significantly different from naked mole rats. It is amazing how similarly termites and naked mole rats behave.

http://www.rosamondgiffordzoo.org/assets/uploads/animals/pdf/NakedMoleRat.pdf
“A naked mole-rat colony consists of 20-300 individuals (usually 70-80). The colony is led by the reigning queen and 1-3 males responsible for reproduction. The remaining colony members raise the young, defend the group from predators, dig and maintain tunnels, find tubers, roots and bulbs for food, and feed the queen and her babies.”

Here is a link to an article about termites. Note that the termite castes are not as rigid as hymenoptera castes. There is “promotion potential” in both termite and naked mole rate colonies.
http://www.fzi.uni-freiburg.de/InsectPestKey-long version/termit2.htm
“The simplified model of a termite life cycle indicates the three castes, the reproductives, the soldiers and the workers. … The so far poorly understood concept of caste determination does not seem to be definitive or too rigid.”

No known mammal practices asexual reproduction. I don’t know why that is so. There must be some developmental constraint. Aphids can reproduce asexually, but there is no exact analog in the mammal class of chordates. However, there are analogs to asexual reproduction that serve much the same purpose.
Inbreeding causes some of the same results as asexual reproduction. Inbreeding results in lines where the individuals are genetically identical. My conjecture is that under conditions where an insect would evolve asexual reproduction, a mammal would develop an inbreeding behavior. Some Earth mammals may have evolved a life cycle where some generations practice “royal incest”, while other generations practice “miscegenation”. This type of behavior could include a mammalian analog to the life cycle of aphids.
One mammal that goes through a stage of extreme inbreeding is the naked mole rate. Naked mole rat colonies end up being highly inbred, just like termite colonies. However, at certain times males leave the naked mole rate colony to find other colonies. I think this stage is somewhat similar to the “rutting” period in your story.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC53716/
“We found that DNA fingerprints of colony-mates were strikingly similar and that between colonies they were much more alike than fingerprints of non-kin in other free-living vertebrates. Extreme genetic similarity within colonies is due to close genetic relationship (mean relatedness estimate +/- SE, r = 0.81 +/- 0.10), which apparently results from consanguineous mating. The inbreeding coefficient (F = 0.45 +/- 0.18) is the highest yet recorded among wild mammals.”




One extreme form of polyphemism is called culture. Some behaviors and thought processes are taught by parents to families. The behavioral changes brought on by culture often end up causing large changes in phenotypic expression. Think about how different physically a weight lifter is from a couch potato. Yet they could have the same genes. Furthermore, the use of drugs and surgery is part of culture. This produces big differences in phenotypic expression.
Whales, chimpanzees and humans have all been shown to have culture. The most extreme form of culture is manifested by human beings, Homo sapiens.

“Something like the 11-year sun-spot cycle would be the closest thing we have, and if that did cause major changes to environmental conditions, then it could conceivably lead to some such generational progression in the shortest-lived mammalian species. But it doesn't cause the former, so it can't lead to the latter. Does that make sense?”
I suspect that there are insects with an 11 year population cycle. If so, it probably has very little to do with the sun spot cycle. I would most probably have to do with avoiding competition with other species of insects that have a 3, 5, 7, or 13 year cycle. In other words, long periodicity may be caused by historical contingency rather than driven by anything physical.
Maybe a closer analog would be those long cycles insects have that seem to occur with a period close to a prime number of years. For example, seven year locusts stay underground for six years and surface on the seventh year. Obviously, the astronomical events that provide the timing have merely a one year period. However, to avoid intraspecies competition the biological cycle ends up being much longer than a year.
Suppose that your planet has an annual cycle of any period. Say that the eclipse of their sun by a moon only happens every Earth year. I see no reason that a mammal-analog couldn’t have a life cycle corresponding to seven, or eleven, or nineteen years.
Rodents often have periodic population booms and then disperse in all directions. Their population cycle often occurs over the course of many years. The most famous example of such population cycles is of course the lemming.
http://en.wikipedia.org/wiki/Lemming
“The behavior of lemmings is much the same as that of many other rodents which have periodic population booms and then disperse in all directions, seeking the food and shelter their natural habitats cannot provide. The Norway lemming and brown lemming are two of the few vertebrates who reproduce so quickly, their population fluctuations are chaotic, rather than following linear growth to a carrying capacity or regular oscillations. It is unknown why lemming populations fluctuate with such variance roughly every four years, before plummeting to near extinction.”

That is may be the major error in your story. Suppose that the animals are impacted severely by this astronomical event. Suppose that most of them do develop a life cycle around this astronomical event. Mutations that enable an animal to avoid competition with other animals would have a positive fitness value. Therefore, at least species would evolve that would fluctuate with a period several times the period of this astronomical event.
If a hundred species evolve which rut on a one year period, then I anticipate that some species would evolve to rut only once every three years. This way, they get to eat everyone else during the two years they don't rut. Then another species develops a five year cycle.
This is what we see on Earth today. A large variety of periods that are integer multiples of the year. What suppresses the longer periods in your story?

 

[onomatomanic]

Excellent! The mole rat case is of course different from "my" kind of polyphenism in that the various morphs (?) coexist, but by combining its features with the aphid case, I should be able to flesh out my life-cycle idea nicely. Thanks again for being so helpful. :)

No known mammal practices asexual reproduction. I don’t know why that is so. There must be some developmental constraint.

That was touched on in several of the wikipedia articles on single-parent reproduction. There seem to be certain environmental condition which tend to trigger that reproductive mode in other vertebrates, but it appears as though mammals may not have that option. The favoured explanation for this is a phenomenon termed "genomic imprinting", which seems to be coupled to the XY-chromosome sex selection mechanism, which none of the parthenogenetic vertebrates employ. For example, this is from the article on parthenogenesis:

Induced parthenogenesis in mice and monkeys often results in abnormal development. This is because mammals have imprinted genetic regions, where either the maternal or the paternal chromosome is inactivated in the offspring in order for development to proceed normally. A mammal created by parthenogenesis would have double doses of maternally imprinted genes and lack paternally imprinted genes, leading to developmental abnormalities.

Naked mole rat colonies end up being highly inbred, just like termite colonies. However, at certain times males leave the naked mole rate colony to find other colonies. I think this stage is somewhat similar to the “rutting” period in your story.

Yes, those are definitely comparable scenarios. One aspect of my original idea of passing along pre-fertilized eggs from mother to daughter was that it avoided the problems one tends to associate with inbreeding and cloning. But, with all this new information, that doesn't seem all that relevant any longer. :)

What suppresses the longer periods in your story?

Oh, no, longer cycles are definitely allowed and employed among my mammal-analogs. What I'd like to disallow is merely mating (mating for reproductive purposes, specifically - social sex could still happen) between Ruts. On Earth, many species have an annual breeding season, which would mirror this, but short-lived species can and do breed more than once during that season, which would not.

The scheme I proposed was an attempt to reconcile the two factors, and that I unwittingly came up with something essentially similar to a real solution to the very same issue (the multi-generational cycle in aphids) boosted my confidence in the basic soundness of that approach a lot.

But that doesn't mean that my long-lived beasties have to reproduce every Rut. In the case of the dreadnoughts (mammoth-analogs), for example, the Rut would still cause the sexual frenzy in all females of reproductive age, but in those who have unweaned calves some internal mechanism (hormonal?) may override the externally induced fertility.

 

[Legios]

I guess there are many different ways you can use The Rut, what about a marsupial like gestational period, extremely short gestation and then use pouches to rear the young while inbetween ruts.

http://en.wikipedia.org/wiki/Marsupial#Reproductive_system

 

[Darwin123]

That was touched on in several of the wikipedia articles on single-parent reproduction. There seem to be certain environmental condition which tend to trigger that reproductive mode in other vertebrates, but it appears as though mammals may not have that option. The favoured explanation for this is a phenomenon termed "genomic imprinting", which seems to be coupled to the XY-chromosome sex selection mechanism, which none of the parthenogenetic vertebrates employ. For example, this is from the article on parthenogenesis:

This explanation is consistent with the fact that there are no parthenogenetic bird species. The birds have UV-sex chromosomes. For birds, the male is the homozygous gender (VV) and the female is the heterozygous gender (UV). Although the role of the genders is reversed, genomic imprinting determines the sex of the bird. Therefore, genomic imprinting of gender would prevent both birds and mammals from evolving parthenogenetic species.
The reptiles (squamata, crocodilians) do not determine sex by genomic imprinting. There are several species among squamata that have independently evolved parthenogenesis. Not to mention amphibians and fish.
Crocodilians may be a problem to this hypothesis, though. I do not know any parthenogenetic crocodilians. No one can tell right now for dinosaurs, which are extinct. It would be useful to know when genomic imprinting first evolved in the ancestors of birds. However, we may never know that.

 

[onomatomanic]

I guess there are many different ways you can use The Rut, what about a marsupial like gestational period, extremely short gestation and then use pouches to rear the young while inbetween ruts.

Do you mean that I could use my original scheme, but substitute larval newborns for fertilized eggs? It's an interesting idea, I'll grant you. The matriarch produces a vast litter, only a few of which go on to grow into dauthers right away, while the rest goes into some sort of developmental stasis. One could easily suppose that the newborns race to latch onto the available nipples - those who win grow, those to lose have to wait. When the daughters become independent, the mother distributes the remaining newborns between them, and so on down the generations until the original litter has been "fully processed".

Conceptually, I like it a lot.

Practically, though, it seems a lot more plausible to suspend the development of an egg into an embryo than to suspend the growth of a newborn, doesn't it?

Crocodilians may be a problem to this hypothesis, though. I do not know any parthenogenetic crocodilians.

A simple explanation would be that this is the statistically expected outcome whether or not crocs are fundamentally capable of parthenogenesis.

The phenomenon is very much the exception rather than the rule among species which seem to be capable of it, and since the number of extant crocodilian species is so small (~20, I think), it's unsurprising that none of them currently find themselves employing a reproductive technique which is only advantageous under specialized conditions.

That would be my guess - and a guess it is, nothing more. :)

 

[Legios]

Do you mean that I could use my original scheme, but substitute larval newborns for fertilized eggs? It's an interesting idea, I'll grant you. The matriarch produces a vast litter, only a few of which go on to grow into dauthers right away, while the rest goes into some sort of developmental stasis. One could easily suppose that the newborns race to latch onto the available nipples - those who win grow, those to lose have to wait. When the daughters become independent, the mother distributes the remaining newborns between them, and so on down the generations until the original litter has been "fully processed".

Conceptually, I like it a lot.

Practically, though, it seems a lot more plausible to suspend the development of an egg into an embryo than to suspend the growth of a newborn, doesn't it?

What if there are litters of newborns and they are impregnated soon after birth and the fetus and the new born develop at the same time under 1 mother and then after the youngling gets old enough also gives birth, sort of like a twofer? Haha

 

[onomatomanic]

What if there are litters of newborns and they are impregnated soon after birth and the fetus and the new born develop at the same time under 1 mother and then after the youngling gets old enough also gives birth, sort of like a twofer?

Well, the crucial factor of The Rut is that it would be the only time during which mammalian mating is allowed. So the newborns which were conceived during The Rut can't be impregnated after birth in the ordinary way. That was pretty much the whole point of instead having them be born pregnant in the first place. ;)

 

[Darwin123]

Do you mean that I could use my original scheme, but substitute larval newborns for fertilized eggs? It's an interesting idea, I'll grant you. The matriarch produces a vast litter, only a few of which go on to grow into dauthers right away, while the rest goes into some sort of developmental stasis. One could easily suppose that the newborns race to latch onto the available nipples - those who win grow, those to lose have to wait. When the daughters become independent, the mother distributes the remaining newborns between them, and so on down the generations until the original litter has been "fully processed".

Conceptually, I like it a lot.

Practically, though, it seems a lot more plausible to suspend the development of an egg into an embryo than to suspend the growth of a newborn, doesn't it?

That seems to be true in the case of seed plants. Seeds often have an indeterminate range of dormancy. However, I don't see much of that with animals.
The exception that I see is the wintered egg of an aphid. The wintered egg of an aphid hatches when it gets warm. The gestation period of the aphid egg varies with environmental conditions. Early in the spring, the aphids are viviparous meaning that the gestation period of the egg is zero!
So there does seem to be a grade in egg gestation for the case of the aphids. That may have provide an opportunity in gradual steps for the aphid to evolve its singular life style.
Some time in animals, there is a manipulable dormant stage of post ova development. Marsupials seem to be an exception. The marsupial mother can manipulate the dormancy in the embryo to a limited extent. That may be one reason that marsupials won out over eutharia in Australia. Maybe the most recent common ancestor (MRCA) of marsupials had an egg with a varying gestation period.
I haven't read of a varying gestation period for eggs in the case of monotremes. This doesn't mean there isn't any. However, I suspect that the marsupial MRCA wasn't very much like extant monotremes.
There is another thread in the Biology forum regarding the origins of viviparity in mammals. A lot of facts presented in this discussion would be relevant to your science fiction story.
You should be thinking about specific ways your "astronomical event" affects natural selection. Some of the posters in that discussion are coming to the conclusion that the viviparity in mammals may have originated as a response to cold climate. I just posted a response suggesting that viviparity may have originated in small mammals living in a "periodically disturbed environment."
I don't fully believe it myself. I favor the "cold climate hypothesis" of viviparity origins. However, the "disturbed environment theory" seems more consistent with the life cycle of aphids then the "cold climate" theory. Once the animals is forced to move by a disaster, they may find an overabundance of resources. Most everything is dead, so there is no competition. Reproducing fast would be useful at first. This would better explain why the animals are born pregnant.
One advantage of my "periodically disturbed environment" conjecture, in terms of your SF story, is that it would affect the evolution of both plants and animals. For instance, the mangrove is a viviparous plant. It lives in an environment that is impacted severely by erosion. It is harder to see how "cold climate" could lead to viviparity in a plant. Most plants don't have thermal homeostasis.
Note that I am not a true believer of my own theory. I doubt erosion was the main reason mammals developed viviparity. I have seen this speculation no where else. There at present is no evidence for the disturbance theory of viviparity, except maybe the mangrove tree. There is a little scientific evidence for the cold weather theory. However, you should not take full accuracy as a criteria for your story. Any scientific hypothesis that you write into your SF story just has to superficially sound plausible to an educated layman.
Maybe your astronomical event does both. It causes lots of erosion, so small mammals can't lay eggs without risk. They have to move around to avoid the effects of the erosion. Then, it creates cold weather for a long span of time. This would be the time when your animals are reproducing asexually, viviparously, and rapidly. Then, the land gets warmer and more stable for an even longer period of time. This is the "rutting." The animals reproduce sexually, oviparously and slowly. Then the astronomical event occurs and the cycle starts over.
Note the intraspecies competition maximizes during the rutting period. The rate of reproduction actually slows down during "the rutting." However, fighting for mates may become large. Food resources are limited. The small mammals will fight both for food and to protect their children.
Let's have a naming contest. You like "the rutting". Maybe this is what you want to say. However, let me suggest a pair of names taken from the language of locusts. The rapidly reproducing, stay at home stage of locusts is called the gregarious stage. The slowly reproducing, traveling stage of locusts is called the swarming stage.
Maybe for your mammal-analogs, you can use the following terms. The asexual stage of your mammal-analogs is the gregarious stage. Early in the gregarious stage, the mammal analogs are born pregnant, vivipariously. The sexual stage of your mammal-analogs is called the swarming stage. The mammal-analogs in the sexual stage lay eggs, oviparously.
The transition from gregarious to swarming can be a slow transition or very sudden, according to your wishes. However, the transition from swarming to gregarious has to be very fast since it is being driven by a short astronomical event.
I hope that you have some human drama to go with all this hard SF. It is easy to lose people with techno-babble, even quality techno-babble.
Why should the humans in your story even care how these mammal-analogs do it? Danger isn't enough. I suggest that the humans (or human analogs) in your story have some cultural taboo against some of the facts concerning your mammal-analogs. They don't see the problem not because they aren't smart enough to figure it out, but they are in denial.

 

[onomatomanic]

Okay, lots to reap up on and think about. Before I make a start on that, I'll quickly lay out the details of the seasonal changes I have in mind to spare you further guessing:

The planet is in an elliptical orbit about its suns, and the annual seasons are caused by the change in distance, as opposed to the axial tilt as is the case on Earth. The principal effect on temperature variations is that, instead of our relatively symmetric year, my planet has a short but steep heating period followed by a long but shallow cooling period. The "year" has six "months", and I've computed the average temperatures for each of those as the following (for temperate latitudes, starting at perihelion): 36 C, 27 C, 15 C, 4 C, -5 C, 4 C.

Every sixth year, the close passage of the planet triggers an instability in one of the suns which leads to a spike in brightness, which in turn further exacerbates the summer temperatures to the point that there are widespread wildfires during the day. Animals spend these days underground, anything else would be suicide - though there are one or two highly specialized predators which are able to withstand the heat and go around digging up a few of the others during this time. The nights between these days constitute the time known as The Rut: As the temperatures drop after sunset, it begins to rain. The animals come out of their holes and slake their thirst on the rainwater, which, by some pseudo-scientific process involving solar wind and aurorae, has been subtly altered. The physical response to this alteration is the sexual frenzy and fertility which is the whole point of the exercise.

So, in the wake of those fires, plants suddenly have lots of empty space to re-colonize, covered by lots of highly nutritious ash. By the time the first herbivores give birth, there should be plenty of fresh plants to eat, and by the time the first carnivores follow suit, the herbivore calves are nice and fattened up. As you say, it mainly has to sound plausible, and this does - to me, at least. :)

As to naming, there are certain rules, which result from the history of my protagonist culture. I won't go into the details unless you ask me to, but the upshot is that all natural phenomena have to have Germanic-based rather than Latin- or Greek-based names. So, "swarming" would work, "gregarious" would not.

I hope that you have some human drama to go with all this hard SF. It is easy to lose people with techno-babble, even quality techno-babble.

It's all human drama (well, humanoid, at least), with no techno-babble whatsoever. The opening paragraph describes the planet's suns, thus letting the readers immediately know that This isn't Earth, and that's all the exposition they get. After that, everything is seen through the eyes of the protagonists which are at home in this world, so nothing is being explained. What I'm going to try for is to give them just enough clues to puzzle out some of the underlying principles by themselves, if they are sufficiently attentive. The point of all this world-building is simply to get things straight in my own head, so that I don't have to make arbitrary decisions about the environment all the time but can figure things out from the established framework. In my experience, that gives the resulting fiction a noticeably more robust feel, even and especially if that framework is never actually being directly shown to the readers.

 

[Darwin123]

Okay, lots to reap up on and think about. Before I make a start on that, I'll quickly lay out the details of the seasonal changes I have in mind to spare you further guessing:

The planet is in an elliptical orbit about its suns, and the annual seasons are caused by the change in distance, as opposed to the axial tilt as is the case on Earth. The principal effect on temperature variations is that, instead of our relatively symmetric year, my planet has a short but steep heating period followed by a long but shallow cooling period. The "year" has six "months", and I've computed the average temperatures for each of those as the following (for temperate latitudes, starting at perihelion): 36 C, 27 C, 15 C, 4 C, -5 C, 4 C.

Every sixth year, the close passage of the planet triggers an instability in one of the suns which leads to a spike in brightness, which in turn further exacerbates the summer temperatures to the point that there are widespread wildfires during the day. Animals spend these days underground, anything else would be suicide - though there are one or two highly specialized predators which are able to withstand the heat and go around digging up a few of the others during this time. The nights between these days constitute the time known as The Rut: As the temperatures drop after sunset, it begins to rain. The animals come out of their holes and slake their thirst on the rainwater, which, by some pseudo-scientific process involving solar wind and aurorae, has been subtly altered. The physical response to this alteration is the sexual frenzy and fertility which is the whole point of the exercise.

So, in the wake of those fires, plants suddenly have lots of empty space to re-colonize, covered by lots of highly nutritious ash. By the time the first herbivores give birth, there should be plenty of fresh plants to eat, and by the time the first carnivores follow suit, the herbivore calves are nice and fattened up. As you say, it mainly has to sound plausible, and this does - to me, at least. :)

As to naming, there are certain rules, which result from the history of my protagonist culture. I won't go into the details unless you ask me to, but the upshot is that all natural phenomena have to have Germanic-based rather than Latin- or Greek-based names. So, "swarming" would work, "gregarious" would not.



It's all human drama (well, humanoid, at least), with no techno-babble whatsoever. The opening paragraph describes the planet's suns, thus letting the readers immediately know that This isn't Earth, and that's all the exposition they get. After that, everything is seen through the eyes of the protagonists which are at home in this world, so nothing is being explained. What I'm going to try for is to give them just enough clues to puzzle out some of the underlying principles by themselves, if they are sufficiently attentive. The point of all this world-building is simply to get things straight in my own head, so that I don't have to make arbitrary decisions about the environment all the time but can figure things out from the established framework. In my experience, that gives the resulting fiction a noticeably more robust feel, even and especially if that framework is never actually being directly shown to the readers.

Although I am not sure of the details, it sounds like you want your mammals to have both sexual and asexual reproduction. There are plenty of animals on Earth like this, but not mammals. So the rules on Earth for such animals with such alternating generations are fairly well known among scientists.
One problem is that sexual reproduction is generally slower than asexual reproduction. So a peak in sexual activity is not always associated with a peak in fertility.
When there is plenty of resources for the mammals-analogs (MAs), there should be a decrease in variation but a peak in birth rate. So this should be the stage for asexual, viviparous reproduction. When the MAs are faced with a shortage of resources, there should be an increase invariation but a decrease in birth rate. Therefore, the MAs at this point should have sexual, oviparous reproduction.
You are associating an increase in birth rate with an increase in variation. If there are such animals, then they are rare.
You want lots of frenzied sex. You want some ribald behavior for your MAs. FAir enough. However, why is it important that this be associated with immediate increase in fertility? Maybe the few offspring from sexual unions are more important than the many offspring from asexual unions.
In human terms, the sexual stage in the MAs should be a time of intrigue and menace. The frenzied sex is justified because that is one way to form important alliances. The asexual period of reproduction should be less frenzied, but care free.
It seems to me that the sexual behavior should take place just before the wild fires. The animals will need all the genomic variation that can get to deal with this rapidly changing and complex environment. Further, they will have to compete for holes. So they need to make alliances (families?) just before the fires. They will be raising families to prepare for this emergency.
The sex will be intense, and there will be reproduction. However, it will be relatively slow reproduction. Only so many animals can find holes. Better to raise a bad *** kid who will steal a hole then raise a family of wimps that will be burned with the first fires.
During the fires, there can not be any reproduction of any kind. Certainly not sex. However, frenzied sex would be part of the preparation for this crisis that is going to occur. These are animals that will die soon. They have to maximize the chance of surviving for their eggs. So yes, the sex could be frenzied. The animals will seek out the strongest, smartest, fastest and most aggressive mates. And they won't be shy about it.
The AMs may hide their eggs in holes. So some eggs overwinter in holes that protect them from the fires. If a mammal has too many eggs before the fires, then it won't be able to hide them all in holes. So the sexual mammals will limit the size of a clutch. Quality of offspring will trump quantity of offspring before the fires.
You claim that there will be an overabundance of food soon after the fires. I am not sure about that. If the plants have a lifestyle that parallels that of the AMs, then it is possible. The plant seeds also "hide" underground somehow. After the fires, the plants come on like gangbusters. So because the plants grow so fast, there is an over abundance of food.
The AMs reproduce asexually and fast after the fires. There is plenty of food for everybody. However, the offspring don't really need parental care. There is no competition for mates, because no one mates. The just pop out the offspring without any care. Since they don't have to hide eggs, they just give birth live. They are extremely unfrenzied. The offspring for several generations after the fires are born pregnant. The offspring pop out, one after another. These generations are relatively unaggressive, other than in reproduction. Quantity of offspring trumps quality of offspring.
How about spawning instead of gregarious? Spawn implies relatively undeveloped offspring. I don't suppose the offspring from the sexual stage will be highly developed.

 

[onomatomanic]

You're getting ahead of me now.

Most of the mogs (Mammal analOGS) reproduce only in the usual mammalian way, and they do this only once per lustre (a lustre is the six-"year"-period between two Ruts). This is a given, because it's important for plot purposes. My idea of how to explain it involves what I mentioned in the OP: By some undisclosed mechanism, the altered rainwater has a beneficial effect on mutation rates (more good ones, less bad ones). This is mirrored in the religion of my protagonists, who believe that this rain is the seed of the sun-god, who is thus the true father of every child. Human and animal males merely serve as an avatar or go-between.

This thread is only about a peculiar class of mammals which are small and short-lived and, for that reason, had to come up with a way to bridge the time between Ruts with multiple generations. Let's call these the rods (RODent analogS), perhaps. Since the non-mammalian animals can mate at any time of the year, it stands to reason that the rods are largely unchanged from the more primitive mammals of old, since they represent a sort of intermediate stage between non-mammalian and mammalian reproductive modes. Presumably, there is a common ancestor to all non-rodent mogs which split off from the proto-rods, had some advantage which allowed it to grow bigger and longer-lived. At some point, gestation times increased to the point at which multiple generations between Ruts became pointless, and the ability to produce them was lost. Nowadays, this lost ability represents a lower bound on non-rodent mammalian longevity. Something along those lines, anyway.

During the fires, there can not be any reproduction of any kind.

Typical day during the Rut:

- At noon, temperatures climb high enough to start fires, which burn during the afternoon and evening. Simultaneously, there is greatly increased evaporation and thus cloud-formation.
- At some point after sunset, the temperatures drop far enough for the moisture in the clouds to precipitate. The rain puts out the fires. The animals come out, drink, and go into orgy mode.
- At sunrise, they exhaustedly crawl back into their shelters. During the forenoon, the powerful sunlight thoroughly dries everything. Repeat.

As I said, this is plot-relevant, so the basic timeline here is nailed down. The details are not, of course - they just represent the best I could think of. :)

---

ETI: Oh, almost forgot - I'm thinking there'd be quite a bit of sexual cannibalism going on among the carnivorous mogs. They have to get through the Rut somehow, and there's no time for a normal hunt. If they're lucky, they'll come across a charred carcass or catch a herbivore "in the act" (easy kill), but if they're unlucky, a female may just decide that the male looks awfully tasty, once he's fulfilled his reproductive purpose.

I imagine this would tend to result in a sexual dimorphism opposite to that typically seen among our carnivorans, i.e. females larger and stronger than males?

The herbivores face the same problem, of course, but can tackle it simply by stocking their burrow beforehand.

---

You claim that there will be an overabundance of food soon after the fires. I am not sure about that. If the plants have a lifestyle that parallels that of the AMs, then it is possible. The plant seeds also "hide" underground somehow. After the fires, the plants come on like gangbusters. So because the plants grow so fast, there is an over abundance of food.

I was thinking the plants would tend to spread their seeds in autumn (before first frost), after which they go into a dormant phase. Just like the aphid eggs, come to think of it. That way, they're already in place when the brief spring arrives, allowing them to sprout straight away and thus be able to take the fullest possible advantage of the impending summer.

In the sixth year, that obviously doesn't work, as it would mean that the fires would consume all the fresh sprouts. So there'd have to be some mechanism which delays sprouting in this case, which shouldn't be a problem. When the fires have come and gone, what normally happens in spring happens belatedly, but with a vengeance, because - lots of room, lots of nutrients.

I'm pretty sure that's more or less what happens on Earth, in the wake of forest fires and volcanic eruptions, no? At first, everything looks really bad, but just a few weeks later, there's a riot of fresh green because ash makes such a great fertilizer.