I'm world-building for a SF-story and would like some feedback on an exobiological 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. One of them 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? 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: 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!