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The setting I'm building has a race of people, derived from modern humans, whose primitive survival as a species relies partly on very fast reproduction, and I think I want them to have a tendency for litters instead of being prone to one infant being born at a time.

I'm not satisfied with the similar threads On the Encouragement of Twins to be Born or Increasing the Rate of Multiple Births in Humanoids? or How can I Determine the Litter Size of a Species? because they're all about the evolutionary causes of races having litters. I'm just going to grant that there is an evolutionary problem, but I want to know if the solutions proposed would work.

I don't want to take the easy route and give this race more pairs of ovaries, so I have a couple suggestions and want to know how plausible the scenario is.

1: Simultaneous Ovulation

In this scenario, the females of this race would consistently release eggs from both ovaries at once, which would roughly double the chances of successful conception when in heat, and drastically increase the likelihood of twins. I'm pretty confident this would work, but I'd also like to hear your thoughts on point 2.

2: More Identical Siblings

In this scenario, something about the race's biochemistry causes post-fertilization cell division to be very common, meaning that any given fertilized egg is much more likely to develop into identical twins, triplets, etc. I think this would be very interesting to see but I'd like to know if it could happen. Is there any known phenomenon that causes identical siblings to be more common?

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    $\begingroup$ If you want rapid reproduction in a human-like species, you need to consider the consequences. Humans give birth to giant babies, and I don't think you can much reduce the average size without developmental problems/reduced average intelligence: there isn't really room in the body for more than twins without incurring huge health risks for the mother and the babies alike. In primitive times, pregnant women could generally remain mobile and fit until childbirth or very close to it; if twins and triplets are the common thing in your species, that will no longer hold true, a liability in nature. $\endgroup$
    – Palarran
    Commented Aug 20, 2019 at 1:09
  • $\begingroup$ @Palarran All good points, and I'm quite aware of them. $\endgroup$
    – Maddock Emerson
    Commented Aug 20, 2019 at 1:12
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    $\begingroup$ Humans who birth more than one infant at a time do not have litters. They have multiples. I don't know if English is your second language but in English litter is only used for animals and applying it a woman's offspring is considered very insulting. $\endgroup$
    – Cyn
    Commented Aug 20, 2019 at 1:39
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    $\begingroup$ @Cyn English is my first language, and I don't find the term litter necessarily insulting. It could be used in a deliberately insulting manner IRL, but this is a hypothetical about a fictitious race and the word is used in a detached and clinical tone. If you're still offended, I think that's a you problem. $\endgroup$
    – Maddock Emerson
    Commented Aug 20, 2019 at 2:08
  • $\begingroup$ Humans can't rely on fast reproduction as a survival trait, gestation is too long and the time it takes for human infants to become self-reliant is even longer. If anything, this just makes it worse by giving more resources to the young. $\endgroup$
    – Halfthawed
    Commented Aug 20, 2019 at 5:06

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The biochemistry is already in place in humans to satisfy Rule 1, all we really need to do is tweak it for your purposes.

As a rule, women older than around 35 are statistically more likely to give birth to twins or even more siblings due to their bodies naturally producing higher levels of Follicle Stimulating Hormone, or FSH for short. In older women, the naturally elevated levels of this is often described as a primary cause of multiple births as more eggs are released each cycle as a result of it.

In your environment, women could have higher levels of FSH from puberty, meaning more eggs are released per cycle, meaning that multiple births are far more prevalent. I don't have exact science to hand on the long term impacts of this on the female body but one can extrapolate that it may cause at least some women to enter into their menopause earlier than conventional human women do due to the accelerated release of their eggs. Also, multiple births can be an incredible strain on mothers to begin with so making them the norm could actually limit the number of times a woman can safely bring pregnancies to term as well. There is already some literature coming out in medical journals that show that women who have had 3 or more pregnancies have a higher risk of heart failure in later life than those who have had less, or even none. So it's important to factor in that the conventional human physiology may not be up to an extended series of pregnancies under these conditions.

Option 2 is also possible to some degree I believe, but I suspect it's more dangerous because the cell division causing identical twins in this manner is in essence riskier than the simultaneous fertilisation of multiple eggs so I'll leave others to explore this in more detail if they see fit.

As per the question, I'm not addressing the evolutionary pressures that would lead to this condition, only pointing out that the way FSM works on the female physiology seems to exactly meet your needs if it is elevated during the course of a woman's fertility through life.

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