I have a species of intelligent human-like creatures. One of the things that makes them different from humans is that unprotected sex has a very high likelihood of resulting in the female becoming pregnant (about 80-90% chance, barring genetic or illness-related hindrances to fertility/virility). They also have a much shorter gestation period than humans, though birth is almost equally as painful.

The high rate of pregnancies is due to a quirk of their reproductive systems, wherein a female typically ovulates after experiencing orgasm instead of having a fertility cycle. Children grow at roughly the same rate as human children. They're also a K-selected species, meaning they put a lot of resources into raising a small number of offspring, rather than having a lot at once and leaving them to fend for themselves.

Because of the high-risk of pregnancy, their society starts giving sex-education to youth from a fairly young age in order to reduce teen pregnancies and the inevitable increase in death-from-childbirth that would cause (their tech is roughly around late medieval levels, just with better hygiene, so not a lot of options to reduce that overall). I am curious how such a difference in reproductive success would actually affect the species on a biological level, though.

Humans commonly have sex for pleasure because there is a (relatively) low chance of women getting pregnant from it.

Basically, with such a high chance of a female becoming pregnant after intercourse, would my species actually have a libido on the level of humans, or would it work differently? Would there be any desire on a biological level for casual sex (sex for pleasure or to deepen bonds rather than to have children), particularly in teens going through puberty?

Edit: Ignoring the potential use of contraceptives and abortifacients, since I'm asking on a pure biological level "how often would an intelligent K-strategist species with a high success rate in reproductive attempts experience the desire/urge for sexual intercourse?"

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    $\begingroup$ "I'm unaware of such behavior in other animals": Our kissing cousins the bonobos? $\endgroup$
    – AlexP
    Commented Aug 14, 2020 at 7:28
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    $\begingroup$ @AlexP don't forget the dolphins! And there are actually a score of animals doing it just for pleasure. $\endgroup$
    – Trioxidane
    Commented Aug 14, 2020 at 7:56
  • $\begingroup$ Yeah, I'm realizing now that it's more common than I was initially aware of. I'm editing that part of the paragraph out. $\endgroup$ Commented Aug 14, 2020 at 8:39

4 Answers 4


They Will Have a Mating Season

Humans are actually a bit strange for mammals in that we breed year round. Most mammals on the other hand choose a certain time of year to breed because the time of year they procreate will have a lot to do with the survival chances of the offspring, and these species tend to have much higher "success rates" for getting pregnant than humans since they work inside such narrow time tables.

This means your species would generally have little to no desire to have sex for most of the year, but then some seasonal/cyclic trigger sets them off into having an elevated libido. A good way to handle this with the female reproductive system working as you've described is for the males to undergo the cyclic libido bursts.

For a Monogamous Race:

Not everyone is on the same cycle. You might have several months where people stagger through cycles that only last a few days such that only a small portion of the population is on their cycles at a time. Libido bursts release pheromones that trigger nearby females into an elevated state as well. So, only females who spend enough time with him at this elevated state will also be triggered into wanting sex (as you might see with a female you live with). So husbands and wives will enter the cycle together, and typically come out the other side with a baby.

This may lead to a strong emotional tendency in men though to keep their wives away from other men. For example, If your wife works closely with other men and one of them cycles first, you might end up stuck raising someone else's baby which is selectively unfit; so, the evolutionary psychology and cultural factors of this race might lead to some very strong gender roles.

For a Polyamorous Race:

Everyone cycles at about the same time. In this system everyone more or less syncs up. Most of the cycle, literally nobody wants sex. Then all at once the whole planet turns into a giant college rage from which most women return pregnant.

This may lead to a stronger communal approach to child rearing since men will often have no clue whose kids are whose, and women will be having children at the same time as all their friends meaning there is a lot of room for sharing similar resources and skill sets.


Assuming that your species has a similar libido to humanity's, the most likely result of that sort of biology would be a much wider acceptance and practice of same-sex pairings and non-penetrative(or equivalent) intercourse. Penis-equivalent in vagina-equivalent results in a baby 90% of the time? Okay, fine. Have all the fun you want with the rest of one another's bodies, and just don't do that one thing.

Also, even in classical times there were early contraceptives and abortifacients. The effectiveness and safety of them left much to be desired, but with that much higher of a pregnancy risk then I'm sure there would be both a lot more R&D into them and a lot less stigma around their usage.

All-in-all, I'm sure they'll find plenty of ways to enjoy themselves without drowning in offspring. ^_~

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    $\begingroup$ Much appreciate the mention of alternate ways to "have fun". Somehow I'd completely forgotten about that =P. I'm less concerned about the potential use of contraceptives and the like since I'm mostly just curious how the thing would affect them on a purely biological level, ie would the species develop with an average human libido or a lower one? $\endgroup$ Commented Aug 14, 2020 at 8:24
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    $\begingroup$ Early on in the species development, they may not have the intelligence to properly correlate or communicate the relationship between sex and offspring which is a real problem for a highly fertile K-strategist species. Some more detail on how you would propose this species got past that evolutionary barrier would help round this out better. $\endgroup$
    – Nosajimiki
    Commented Aug 14, 2020 at 18:40
  • $\begingroup$ @Nosajimiki reproduction is such a basic biological urge that I'm sure that most species have the "sex causes baby" relationship figured out, even without understanding the biology behind it. Otherwise you wouldn't see such intense and elaborate courtship/reproductive strategies everywhere in the animal kingdom. $\endgroup$
    – Salda007
    Commented Aug 15, 2020 at 20:32
  • $\begingroup$ You don't have to understand an urge for it to guide you. If you catch your girlfriend cheating on you don't stop and think, "Shit! now I might get stuck raising some other guy's kid, I better hit someone to condition a monogamous partnership in the future." No, you don't need to think that because your jealousy response is automatic. You get angry, you hit someone, then latter try to make logical since of your actions. Likewise, most animals don't have sex because they want a baby, they do it because their emotions and instincts guide them to do it, and only latter might draw a conclusion $\endgroup$
    – Nosajimiki
    Commented Aug 15, 2020 at 21:20

Sex Isn't Fun All the Time

Sure an orgasm feels AMAZING but outside of certain conditions your species may just not feel the sex drive at all. I'm assuming that the quicker gestation but near-as-painful birth means that the embryo/fetus/baby develops to about the same level as a human baby in the womb, rather than some sort of marsupial-like birth where the baby is born very underdeveloped and small. So to me that indicates a more intensive energy demand on the mother to fuel the rapid growth. Growing an 8lb baby over 4 months is going to require more daily food intake than one which grows over 9 after all.

So we now have a species where sex=pregnant damn near every time, and a female has an extremely high energy requirement during pregnancy. You can still have year-round fertility, it just becomes tied to food. Think desert plants and animals that only breed after a rainfall. Maybe your species evolved from a region where they had to reproduce during infrequent and random periods where food/water was plentiful. In such conditions you'd absolutely want to become pregnant as quickly as possible once conditions were right, and you'd want to have the young reach viability rapidly before the good times ended.

In such a system males and females could evolve to simply NOT have a sex drive barring outside stimulus like a drop in air pressure or rainfall or the overwhelming smell of some flower/crop indicates that the conditions are right to breed. Because to get pregnant at a not-plentiful time is a waste of resources and potentially dangerous, as you don't have enough food/water to bring the pregnancy to term. But when the weather is good they could go from nothing to straight-up mindless orgy/more-or-less how humans act day to day as your plot demands.

You wouldn't really have to worry about random "teens" knocking each other up because outside those specific circumstances nobody feels like mating anyway. In your society when The Time Comes your aliens might be able to predict it well enough (better meteorologists than similar-tech humans?) that they can separate un-paired males and females from the opposite sex until the time passes. Wouldn't surprise me if by the level of advancement you want there would be a strong cultural/religious impetus to do so. After all, you're K-selective, losing a child SUCKS. For animals having a kid the instant things look good is a viable strategy, but for a civilized society who can go "hey harvest was only but so good, if ALL these young females reproduce somebody's gonna starve" there'd be incentive to keep "marginal" pregnancies to a minimum.

I'm reminded a lot of the aliens in "The Sparrow" by Mary Doria Russel. In it humans teach aliens how to grow their crops more efficiently, which pretty rapidly (less than a year iirc) leads to lots of mating and child-having brought on by the excess. The aliens still have what humans recognize as family units, they just don't reproduce unless their food supply is plentiful enough, but as the aliens are bronze-age it's something biological that triggers it rather than some sort of contraceptive/active planning on the part of the aliens.


They'll do what normal humans do if something keeps them from having vanilla sex...

If we examine the behaviours of those with medical reasons to not enjoy vanilla sex (The most common being Endometriosis, erectile dysfunction, and phimosis), those with legal or religious reason (eg Waiting for marriage for vaginal sex), or moral reason (eg Stop an STD spread) or those with a social reason (eg Open relationship where vaginal sex with outsiders is banned but other acts are ok, or embarrassed or uncomfortable with ones own genitals); we can actually learn a lot for this scenario. What do these people do instead of vanilla sex?

3 common things happen, so I guess society will split into 3 groups;

  • Those who abstain from sex and suppresses their urges.
  • Those who are loners who get themselves off.
  • Those who work the problem:

The first (abstainers), will be constantly failing, and the consequences will be bad every time. Think of any scandal rocking celibate groups (eg catholic priests).

The second will be hermits who've withdrawn from society. A lack of emotional maturity will makes it very hard to connect with others, making it a very addictive lifestyle that is damaging to social cohesion.

The third group will find the loopholes. (https://www.youtube.com/watch?v=na3lw1PzbCw) Any way the body parts can intermingle that doesn't make kids will happen regularly, oral, anal, and more will be common. Mutual masterbation, or mutual oral, will take the place of sex. Bondage and fetishes will develop. The quest to get off better without conception will lead to scientific advancement. They'll probably develop the vibrator and fleshlight before they have flushing toilets.

I suspect long term this society will become polarised towards abstainers and kinksters. I'm picturing mardi-gras vs armish.

The abstainers will be conservative and proper, believing if everyone followed their morals the world would be a better place. The kinksters would be progressive and open and accepting, believing anything between consenting adults is acceptable. You can basically map left/right wing politics onto this for more depth.

  • $\begingroup$ armish = Amish? $\endgroup$
    – StephenS
    Commented Aug 14, 2020 at 14:02
  • $\begingroup$ Much appreciated that you tried to give a solution, but your answer doesn't really answer the actual question asked. I wanted to know how the current specifications laid out would affect the species' libido on a biological level, outside of their intelligent choices. You've given me some options to think about if I do wind up having their sex-drives roughly equal to those of humans, just not the answer I needed. $\endgroup$ Commented Aug 15, 2020 at 19:18

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