M: I feel like it's silly to ask if animals menstruate, but that's not my actual question. I think my question is more, how the hell do they pull it off? Because in the wild, you know, that little bit of blood-like tissue would certainly lead to some kind of vulnerability in terms of being able to be easily tracked. I mean, is that actually the case and part of the whole evolutionary cycle where getting pregnant is kind of necessary, or else you're gonna get more easily eaten so that eventually you get really easily impregnated people, or rather deer or something like that, because otherwise they get tracked or what? Or am I thinking of it completely wrong and is it that humans are one of a small group that actually does menstruate and that other animals going into heat or whatever is because they get a single chance because their life cycle is predetermined by the weather and all that, and so constant menstruating really wouldn't be possible? Now that I think about it, the only thing I know is that I haven't really ever thought about it.
G: It's actually a really good question, because the answer is counterintuitive:
Most mammals don't menstruate.
Humans tend to assume that because we do, but true menstruation is surprisingly rare.
The animals known to menstruate include:
That's basically it.
Most female mammals go through an estrous cycle ("being in heat").
Instead of building up the uterine lining and then shedding it every month, they:
So a deer, wolf, horse, or dog isn't quietly bleeding every month in the woods.
Dogs do have bloody vaginal discharge during heat, which is one reason people often think all mammals menstruate. But that's different—it's associated with changes in the reproductive tract during estrus rather than shedding the uterine lining in the same way humans do.
This was exactly the line of reasoning I expected when I read your question.
If large amounts of blood were regularly left behind, it probably would create some additional risk. Predators can smell blood extremely well.
But several things reduce that concern:
So while it's not impossible that a predator could detect it under some circumstances, there's little evidence that menstruation has been a major driver of predation in species that menstruate.
This is one of those questions where biologists still debate the answer.
The leading idea has to do with how pregnancy works in humans and our closest relatives.
Human embryos are unusually aggressive. They invade deeply into the uterine wall to establish the placenta. Because of this, the uterus prepares itself in advance each cycle—a process called spontaneous decidualization.
If pregnancy doesn't happen, that specially prepared tissue is no longer useful, so it's shed as menstruation.
Most mammals don't prepare the uterus this way. Their uterine lining only undergoes those changes after an embryo is present, making it much cheaper energetically to simply reabsorb the lining if pregnancy doesn't occur.
You proposed something like:
Maybe menstruation creates a cost, so individuals who become pregnant more readily leave more offspring.
Evolutionary biologists would say you're thinking in the right direction. Evolution is always balancing costs and benefits.
However, the current evidence suggests menstruation wasn't selected because of the cost of not being pregnant. Instead, menstruation appears to be a side effect of evolving an unusually invasive form of pregnancy. Once the uterus evolved to prepare itself every cycle, shedding that tissue became the most practical way to reset for the next cycle.
Humans are also unusual because we're one of the few mammals that have concealed ovulation.
Many mammals advertise fertility:
Humans, by comparison, have no obvious external sign that ovulation has occurred. That's another trait that's thought to be tied to our distinctive reproductive strategy, although there are multiple competing explanations.
So your intuition that "humans seem to be doing reproduction differently" is correct. Between menstruation, concealed ovulation, year-round fertility, highly invasive placentas, and exceptionally dependent offspring, humans (along with our closest primate relatives) are quite unusual among mammals.
M: Yeah, I sent this anyway to see if I was right, because I talked myself around to realizing that most mammals wouldn't menstruate regularly because it would be a vulnerability. And then everything that I've heard in the background kind of collapsed into that idea, which, of course, needed outside verification, but it made a lot more sense. I mean, you're comparing human menstruation to what would happen with, for example, a deer, which would at least double the volume. And that makes sense, you know. They live and grow during a certain period. They'd want to give birth during a particular season so that they had the best chances of surviving, etc., etc. That would be That would involve some kind of genetic element, like just the habit of doing one thing one way would make it easier for survival. I think that humans and chimps are a deviation, and possibly bats and all of those, because of the difference of situation in terms of food. I mean, all it would take would be one particular genetic line. realizing that having kids regularly rather than having to wait for one try each year would start to dominate the population as long as food was available, and that bloodline would start to overwhelm everything else. I mean, it's such a constant now that it would have to be way back in the monkey era, but I mean, it makes a logical sense that is hard to deny. In fact, if you accept that, then it also explains certain structures in the evolutionary era, because if you had creatures who were suddenly able to have kids whenever the mother got pregnant and made it to childbirth, the parents would have to provide food constantly, meaning more organization would be needed. Be necessary as well as more flexibility, which are kind of opposing ends in terms of just flat-out genetics. So adaptability while maintaining consistency would be required. And again, this would be back in the day, like proto-chimp kind of era. So there would be enough geological, sorry, biological time in terms of generations for that to play an important role. First, you'd have the variation where someone was able to just get pregnant whenever, maybe even a few false starts because of bad placement in terms of food availability or conditions that would let the newborn survive at any time. But then once that was established, the people, or sorry, the creatures that would have the best survival chances would be the ones who could simultaneously, continuously provide food in situations that were constantly changing. I mean, the whole thing makes a lot of sense over the course of biological time. Creatures that roam. You need to be able to plan around the weather and childbirth and all of that in a way to ensure survival. The ability to do something at any time is both a blessing and a curse in terms of product, particularly if you aren't intelligent enough to handle it. So a lot probably died, and the few that made it through were more capable and kept surviving longer and having more kids than the ones who could only pop out one a year. And it seems like a lot of the animals that also menstruate are either situated in ways that seem to be either in constant weather situations or weather independent. You know, like the elephant shrew is so small that if it's not going to survive, sorry, so that if a kid doesn't survive, there's a good chance the mother wouldn't survive either in the circumstances. Bats tend to be located in places where they don't have to worry about freezing quite as much, although I don't know enough about bats. I probably shouldn't comment on them. I mean, dolphins and chimps, again, that's kind of the constant situation and or indications that intelligence allows for the production of. food for the child consistently throughout the year, or the deviation, but it kind of makes sense when you look at what we're doing here.
G: I actually think you've put your finger on a real evolutionary principle, but I would separate it into two different ideas, because one is very likely true and the other is more speculative.
The first idea is that year-round reproduction changes almost everything else about a species. I think that's on solid ground.
Imagine two otherwise identical mammals.
Species B has just lost a huge amount of predictability. It can't rely on "there will always be fresh vegetation when the babies arrive." That means natural selection starts favoring traits like:
Those are exactly the kinds of traits we see becoming more important in primates.
Where I'd be a little more cautious is the direction of causation.
You were suggesting something like:
Year-round fertility evolved first, and then intelligence and flexibility followed because they became necessary.
That's plausible. But there's another equally plausible sequence:
Increasing intelligence, longer lifespan, and more flexible diets evolved first. Once those traits existed, seasonal breeding became less necessary, so year-round fertility became advantageous.
In other words, the traits may have reinforced one another instead of one causing all the others.
Evolution loves feedback loops.
There's another thing you said that I especially liked:
"Adaptability while maintaining consistency."
That's actually a pretty deep observation.
A deer succeeds because it's incredibly consistent. Every year:
Humans succeed almost the opposite way.
Our consistency isn't in what we do. It's in our ability to change what we do.
That's a fundamentally different evolutionary strategy.
You can think of evolution as gradually moving "where the information is stored."
Early animals store almost everything genetically.
As brains become larger:
Humans take this to an extreme.
A newborn deer "knows" an enormous amount genetically.
A human newborn knows almost nothing, but has an incredible capacity to learn.
One other thing occurred to me while reading your thoughts.
You mentioned food availability, but there is another resource that becomes important once offspring can arrive at any time:
other adults.
If births are seasonal, everyone in the population is dealing with infants at roughly the same time.
If births occur year-round, then different members of the group are experienced at different stages of childcare.
That creates opportunities for:
Those social structures are very prominent in humans and many primates.
One small correction: dolphins don't actually menstruate. Like most mammals, they have an estrous cycle rather than a menstrual cycle. They are year-round breeders in many populations, though, which is interesting because it shows that year-round fertility and menstruation are separate evolutionary traits. You can have one without the other.
So I think you've identified something real:
Where evolutionary biologists would be more tentative is saying that menstruation itself drove that transition. Current evidence points more toward menstruation being a consequence of the unusual reproductive physiology that evolved in some lineages, rather than the trigger. But the broader idea—that changes in reproductive strategy can cascade into changes in intelligence, social organization, and flexibility—is very much in line with how evolution often works. It rarely changes just one trait in isolation; once one part of an organism's biology shifts, selection starts reshaping the rest of the system around that new way of living.
M: I think you're taking my argument and making it too different. Sorry. I think you're taking my argument and making it a different situation, because we're not talking about two different species. We're talking about two genetic variations in the same species. And I'm not saying that this is just a superlative direct causality. You're doing that again. You're taking everything up to 11, which is great for debate, but not for trying to understand an actual conceptual thing. Well, sometimes it is, but your method of applying it usually is more annoying. Like I said, two different members of the same species, and you took it to the extreme of being two different species. I think that my implication was that this contributed, but you're saying that this is somehow a, you're acting as though I was claiming this was a granular situation where one thing happened, then another. I'm saying that the fertility and intelligence and flexibility were all intertwined. A creature incapable of adapting quickly enough on that first instance of year-round fertility would likely have let its offspring perish. So the intelligence would have been a necessary vector in order for that year-round fertility to come into play. And then once the fertility was an aspect, that would encourage the intelligence vector to continue in order to allow for preservation of the advantage caused by year-round fertility. And you're bringing up the idea of including other adults, and I thought that was implicit in what I was talking about, because we're not talking about singular separate situations here. It's that the proto-monkeys were developing in a particular direction anyway, in a way that allowed for evolutionary choice, you know, intelligence operating more often as a group, etc., etc. The ability to give birth at any time would have, for that first mother, been a binary on-off, but once it got distributed, it would be incorporated and encourage those kind of already present inclinations. So yeah, it would have encouraged a society building that likely already existed. You were talking about feedback loops, and that's my whole point. You made it sound like I was claiming there wasn't a feedback loop, when in fact, it's all an ongoing system. Now, the dolphin thing, I'm glad to know that, although, again, the advantage is kind of indicative with the whole intellectual pod work, whatever, situation. The main reason this would be different from most other ideas brought into... evolutionary biology and contemplation is that it is a situation where there is not really a middle ground that could be evolved through. Either you have a creature that has a rhythmic or seasonally triggered menstrual cycle, or one that just is independent of outside sources. And so the introduction of it would be different from intelligence or forming societies or whatever, because those two concepts are nebulous enough that any kind of evolution would be hard-pressed even with, you know, ultimate availability of data, you know, omniscient levels of data still would not be able to make it an easy thing to trace the evolution of certain aspects of humanity, but this one particular thing, as far as I can tell, is kind of a biological mode switch that would require a single beginning creature, or if you want to be optimistic, a single two or three that had the same genetic mutation. I mean, hell, maybe that's the reason why there were Neanderthals and Cro-Magnons on separate branches, because... They were separate branches anyway, and then they both managed to have that menstrual genetic code flip modes approximately the same time enough for them to be intellectual competitors, as well as, you know, the normal biological competitors.
G: Thanks—that clarification helps. I was indeed "lifting" your idea into a species-level comparison when you were describing a population genetics scenario within a single evolving lineage.
What I think you're actually proposing is something like this:
That is a much more coherent hypothesis than what I responded to.
The part I find especially interesting is this sentence:
"This one particular thing... is kind of a biological mode switch."
There really are evolutionary changes that look more like switches than slopes.
Not because a single gene changes everything overnight, but because some regulatory systems have threshold behavior. Once enough of the underlying machinery changes, the organism can cross into a different operating regime.
For example:
So it's not crazy to ask whether the transition from strongly seasonal breeding to facultative year-round breeding might have involved crossing one or more such thresholds.
Where I'd push back slightly is on this part:
"Either you have a creature that has a rhythmic or seasonally triggered menstrual cycle, or one that just is independent of outside sources."
I think biology is probably messier there.
There are living mammals today that span almost every point along that continuum.
Some reproduce only during a very narrow season. Some have a long breeding season. Some breed opportunistically when food is abundant. Some are effectively year-round breeders.
Likewise, the hormonal control isn't usually one gene saying "seasonal" or "not seasonal." It's a network involving light detection, melatonin, the hypothalamus, pituitary hormones, ovarian hormones, nutrition, stress, and so on. Evolution could gradually reduce the importance of photoperiod over many generations.
So I suspect the physiology evolved gradually even if, from the perspective of the population, there was a point where the strategy effectively "flipped."
One other thing you said really stood out:
"The first mother."
That is actually how evolutionary geneticists often think about these questions. Every derived trait ultimately traces back to individuals carrying new mutations. The question isn't whether there was a "first" carrier—there almost certainly was—but whether the mutation could survive long enough in the population to spread.
Your feedback-loop idea is relevant there because it suggests why the first carriers might not have been at an overwhelming disadvantage. If they already belonged to a lineage that was becoming more social, more omnivorous, more behaviorally flexible, and better at parental care, then the mutation could have been viable. Once present, it could in turn strengthen selection for exactly those other traits.
So I'd phrase your hypothesis this way:
The transition toward year-round fertility may have acted as a reinforcing innovation rather than an isolated adaptation. It didn't create intelligence or sociality by itself, but once it appeared in a lineage already trending in those directions, it amplified the selective value of those traits, creating a feedback loop that pushed the lineage further toward flexible, socially coordinated, cognitively demanding lifestyles.
That's a hypothesis I'd actually enjoy seeing modeled. It's specific enough to generate predictions—for example, about the order in which reproductive physiology, sociality, and life-history traits should evolve—and it's subtle enough that it isn't just "trait X caused trait Y." It's saying the emergence of a new reproductive regime could alter the fitness landscape for a whole suite of existing traits. That's a recognizable kind of argument in evolutionary biology.
M: Yeah, switch the situation around, and the difference between advantage and disadvantage becomes easier to see. Like, buffalo function in something that is potentially equivalent to the kind of societal situation we're talking about, but without the individual intelligence being prioritized. So if you had a buffalo that somehow was able to get pregnant in in the middle of winter, that likely would kill them or allow them to be more easily killed simply because of the lack of food and the ease with which they could be hunted compared to the rest around them. Because there's no intelligence, everything is genetic. And don't start arguing with me about using the superlative words like no, because my point is being made. I'm not trying to actually say there's no intelligence in buffalo. But anyway, there's not sufficient intelligence to handle the situation, and likely they're not in a place where winter is very hospitable to a child and mother, so even if the animal was born, there's a good chance at least one of them would go down, probably both. That's why it would have to be part of the feedback loop immediately. Like a seed falling into various biomes. If it falls into a hospitable one, it might grow or even come to dominate. If it falls onto salted earth or something like that, it's going to die.
G: I think this version gets at the heart of what you're trying to describe.
You're not arguing that "year-round fertility is always better." You're arguing that the fitness value of a mutation depends on the environment created by the organism's existing biology and behavior.
The buffalo example illustrates that well.
Imagine, as a thought experiment, that one female buffalo acquires a mutation that allows her reproductive system to ignore seasonal cues.
That mutation is not inherently good or bad. Its value depends on the context.
In a buffalo lineage:
A calf born in January is therefore much less likely to survive. The mutation disappears.
Now imagine the same kind of mutation arising in a lineage that is already:
Now the cost of an "off-season" birth is lower. The mutation has a chance to persist long enough for selection to act on it.
That's the ecological point you're making.
The analogy that came to my mind while reading your message wasn't actually a seed—it was software compatibility.
Think of a mutation as a new feature being added to a program.
Sometimes a feature is objectively useful, but if the rest of the codebase doesn't support it, it just crashes.
The exact same feature added years later, after the surrounding architecture has evolved, suddenly becomes a huge advantage.
The feature didn't change. The ecosystem around it did.
One thing I would tweak is your use of the word "immediately."
Evolution doesn't require the feedback loop to pay off in the very first generation.
Suppose the mutation initially reduces fitness by 5%.
Normally, that would disappear.
But if the same lineage is already experiencing several other changes—longer juvenile periods, increased cooperation, broader diet—that 5% disadvantage might become neutral, or even slightly positive.
Once it's merely neutral, genetic drift and subsequent selection can keep it around long enough for the positive feedback loop to begin.
So I don't think the loop has to "click" instantly. It just has to avoid being selected out before the surrounding biology catches up.
I also like that you're focusing on what evolutionary biologists call contingency.
The same mutation introduced into different lineages doesn't necessarily produce the same outcome.
Stephen Jay Gould famously argued something similar with his "rewind the tape of life" thought experiment: replay evolution from the same starting point, and many innovations would appear in different orders—or not at all—because each depends on what else has already evolved.
Your buffalo-versus-primate comparison is a more concrete version of that idea. It's saying the success of a reproductive innovation isn't determined solely by the innovation itself, but by the behavioral and ecological "soil" it lands in.
So I think the most interesting part of your hypothesis isn't actually menstruation or year-round fertility by themselves. It's the broader principle that some evolutionary innovations are highly context-dependent. They may look like game-changing adaptations in hindsight, but when they first appeared they would only have survived if the lineage had already accumulated enough complementary traits to support them. That's a well-established way of thinking about evolutionary change, even if the specific reproductive transition you're discussing would need evidence to show that it followed that pattern.
M: And as I think about it, it's kind of funny because the male biological sex drive likely would have to be responsible or to blame for any of this coming about. That and it likely wouldn't happen unless there was some form of society going on already. I'm thinking on my feet, so I'm sorry if I don't get everything, but try not to take everything up to 11 again when you're thinking about this. OK, so a society has kind of formed with seasonal menstrual cycles. During that off time, they're still living together. And because biologically, males are designed to basically be ready to go whenever they're called on, as the society builds, they're probably going to be trying to fuck whatever's around them at any time, whether the seasonal thing is going on or not. That means that when you get the genetic variation that is able to have children without it being a seasonal thing, there's a damn good chance that she's going to end up getting to express that simply because the guy won't stop trying to fuck things. The cohabitation means that he's going to get a chance to try more often. I just find that amusing because the matriarchal structure that dominated up until, you know, that certain point where men started being assholes or feeling petty or just getting to the point where we were smart enough that they were able to bend that to getting to have all of the marbles, the society largely would be based around, well, before the non-seasonal stuff, it likely was still kind of a pain in the ass, but it was a mutual thing. But once you get to the point where, you know, sex means a group effort and being able to get pregnant whenever, you can see why a bunch of women saying, okay, knock it off, we've got stuff to do, would start to accumulate. I mean, hell, I've got a fairly heavy sex drive, and the only reason why I'm not like that besides normal human restraint is because of having too much thought going on. I don't mean that as a brag, I mean that I can tell you of these three times where I was invited for some sort of intercourse and couldn't because I got in my own head. I think that if I was any less thoughtful, I probably would be either swimming in children or lawsuits. We're talking about someone who figured out masturbation before memories started to truly congeal, evidently, according to my mother. And so the idea that male and female proto-monkeys living together would lead to some kind of constant boning is not exactly a far throw from what I can already prove just from personal experience. I just find it amusing because male dicks have caused so many problems and at the same time caused so many advances that if you don't laugh, you'll cry. But in terms of absolute stuff, doesn't that also mean that the society would be a prerequisite in order for this to get expressed? I mean, a herd wandering around or migrating or something like that, going into heat wouldn't necessarily lead to... Actual impregnation. I don't know, the whole thing is just, I'm getting to the point where I'm not sure where to go with this plot, so I'm just kind of casting at straws, or whatever it is you're supposed to cast at. Not because I feel there's somewhere that I need to go, but just to make sure that I've cleared out all of the obvious thoughts.
G: I think there are actually two separate ideas here, and one of them is considerably stronger than the other.
The stronger one is this:
A year-round reproductive capability only matters if males and females encounter one another outside a seasonal mating period.
That is basically true by definition. If a species only comes together for two weeks each year, then a mutation allowing a female to ovulate in October is almost worthless because there simply isn't anyone around to fertilize the egg.
So yes, increasing social cohesion—living together for longer periods, maintaining mixed-sex groups, repeated interaction—would make such a mutation much more likely to be expressed. It doesn't guarantee it spreads, but it gives natural selection something to act on.
The second idea is about male sexual behavior.
I would phrase it a little differently than "male sex drive caused this."
Across many mammals, males tend to have broader mating motivation than females because their minimum biological investment in a single offspring is lower. That creates selection for males to seek mating opportunities when they exist. But the exact behavior varies enormously by species. Some primates mate almost exclusively around fertility, while others—including our closest relatives—have sexual behavior that serves social purposes as well.
That's where your intuition becomes interesting.
If you already have:
then a female who is occasionally fertile outside the ancestral season has a much higher chance of actually conceiving than she would in a species where males and females separate for most of the year.
That's less about an unusually high male sex drive and more about opportunity.
One thing I think you're circling, but haven't quite stated, is that there are three interacting variables, not two.
Each one changes the evolutionary value of the other two.
One place I'd be careful is with the idea of a "matriarchal structure" in early primates. The evidence doesn't really support a single ancestral social system. Living primates show enormous diversity.
For example:
That diversity suggests the common ancestor probably wasn't locked into one simple social model.
Something else your comments made me think of is that human sexuality is unusual in another way that's directly relevant.
Humans don't just have year-round fertility. We also have:
Those traits reinforce one another. If sex occurs throughout the cycle instead of only during obvious fertility, then a mutation permitting conception over a broader window can actually be used. Conversely, if fertility broadens, regular mating becomes a more effective reproductive strategy.
That's another feedback loop.
So I think you've shifted from asking "Why menstruation?" to a broader systems question:
Under what social conditions could a mutation for less seasonal reproduction actually survive and spread?
That's a biologically interesting question. The answer probably isn't "because males wanted sex more," but rather "because a lineage had evolved a social system in which mating opportunities existed throughout the year." Once that ecological and social context existed, reproductive innovations that would have been useless—or even harmful—in a highly seasonal, loosely associated species could become viable. That's a very different kind of evolutionary argument than treating the mutation in isolation, and it's consistent with the systems-level way you've been describing the feedback loops.
M: unusually is a modifier that might not be necessary, but high male sex drive is kind of part of the entire biological build of the world, and if it wasn't high, then the proto-monkeys wouldn't be trying to fuck without there being some sort of child or other genetic drive causing it. The mutation might occur, but if there weren't high sex drive, there wouldn't be any sex until it was expected of the male, and that's my point. It also means that before this mutation, the guys were trying to fuck the girls anyway, and it just wasn't resulting in anything. I don't know how it works in other animals, like, I don't know if a condor would try to have sex out of season, given the right signals, but with primates at least, you can see that the sex drive is there, whether or not anything is going to come out of it. At this point, the question is more about whether those proto-monkeys were actually like the current primates, or if it was less about pleasure and more just a habit. It also explains why the whole pleasure thing was necessary besides lubrication, because anyone who didn't enjoy it likely would have a difficult time in society, either because of, well, mostly because males are pains in the ass that nag people into sex constantly. I know that we're talking about back in the day biologically, but we're also talking about back in the day long enough that even a minor move in that direction would be enough to have a result later on in terms of, in this instance, women getting some kind of pleasure out of sex beyond just for fertilization. You're right, there's enough variety in primates between matriarchy and patriarchy and all that, that it actually was something to push back on. I think part of the problem is I keep flipping back and forth in modes of how I think about this, because on one hand, we're talking about things that were in evolution well before the modern chimpanzee developed. On the other hand, we also might be talking about creatures that are actually more evolutionarily developed in terms of approaching humanity at that point. So it's really difficult to find the correct perspective, because on one hand, we're talking about creatures that might be a little bit more intelligent or socially developed or whatever than chimpanzees are now, or at least than chimpanzees were back then, but current chimpanzees might still be smarter than they were. So it all turns into kind of a mashup of what is or isn't there when I try and think about it, kind of like how a modern cockroach looks like an ancient cockroach, but likely is much more adapted to its niche simply because it's had eons of years to sharpen the various bits that it needs. Genetics works relatively fast in biological time, but given more time, it can get really good at adapting to niches, is all I'm talking about. And so it gets really difficult to talk about whether the creatures I'm talking about are more developed in terms of contemporary chimpanzees or less developed than modern chimpanzees and all of that.
G: I think you've identified the part that is genuinely hard to think about: you're reasoning about a lineage, not a snapshot. It's very easy to accidentally picture either modern chimpanzees or modern humans when the animals you're really interested in were neither.
One thing I would refine is this sentence:
"The guys were trying to fuck the girls anyway, and it just wasn't resulting in anything."
That isn't implausible for an increasingly social primate lineage. In fact, one of our closest living relatives, the entity["animal","bonobo","Pan paniscus"], shows that sexual behavior can become partially uncoupled from immediate reproduction and serve social functions. That doesn't mean our common ancestor behaved exactly like bonobos—they're a modern species with their own millions of years of evolution—but it demonstrates that primate sexuality can evolve beyond a simple "mate only when conception is likely" pattern.
Where I'd be careful is assuming that male libido alone would drive that. Evolution generally works on both sexes simultaneously.
Suppose, just hypothetically, that in an ancestral population:
Now selection is acting on:
That's the sort of coupled feedback loop you've been describing.
I also noticed something else you're wrestling with.
You said:
"Are these creatures more developed than chimpanzees or less developed?"
I'd actually avoid that wording altogether.
Evolution doesn't really have a "more developed" axis.
A modern chimpanzee is just as evolutionarily modern as a human. Both have had roughly the same amount of time since our last common ancestor.
The better question is:
Which traits had already evolved by that point in the lineage?
That's a much easier thing to reason about.
For example, an ancestor might have:
It isn't "between chimp and human" in a ladder sense. It's just one point on a branching tree.
I also like your cockroach analogy.
Modern cockroaches resemble ancient ones, but that doesn't mean they're unchanged. They've been evolving the entire time, just mostly within the same general ecological strategy.
The same is true for chimpanzees.
People often think of chimps as "living fossils." They're not. Modern chimpanzees have been evolving for six or seven million years since our last common ancestor. Whatever that ancestor was like, it was not simply a chimpanzee that later turned into a human. It was the ancestor of two independently evolving lineages.
So when you imagine your "proto-monkey," it's helpful to think of it as an organism whose descendants include both humans and chimpanzees, rather than as a primitive chimp.
One last thing.
I smiled a little when you mentioned that the pleasure associated with sex might become increasingly important once reproduction was no longer confined to a narrow season.
That's actually an idea that has been discussed in evolutionary anthropology, though usually in a more general form. The hypothesis isn't that pleasure evolved because of year-round fertility, but that if a lineage already has frequent mating outside immediate conception, then pleasure and pair-bonding become much more valuable traits because they encourage repeated mating, strengthen social bonds, and increase the probability of conception whenever fertility does occur.
So, again, it's another feedback loop rather than a one-way cause.
I think that's the recurring theme in your reasoning: you're not looking for a single "magic mutation." You're asking how a collection of modest changes—social living, hormone regulation, mating behavior, reward systems, parental investment—could begin reinforcing one another until the lineage crossed into a qualitatively different reproductive strategy. That's a systems view of evolution, and it's often a productive way to generate hypotheses, even though any particular piece of the story would need evidence from comparative biology, genetics, and the fossil record to determine whether it actually happened that way.
M: Okay, your response helps clarify what I was trying to say. It's that we're talking about creatures who, whether they are contemporarily either like pre-chimpanzee or post-chimpanzee in their evolution, it's difficult to remember or say for certain what other aspects have developed over time that could be thought of as equivalent versus the ones that might have still been, you know, evolutionarily undeveloped. Like, the human, the current human male orgasm and female orgasm produces a lot of positive reactions, right, for the person who's having it. But was that necessarily the case at that point, or was the idea of it actually being anything more than just kind of something you did out of habit, the only thing in place? I mean, we're talking about bonobos and all that, and yes, it's a societally reaffirming thing, but that's after, you know, millennia of development. Would creatures at the point we're talking about have that degree of, you know, simultaneously simple and complex feedback internally, or would it still be closer to the genetic drive thing? I mean, when you're talking about the women... also having a libido, were these creatures actually developed enough in their psychology that they could be considered to have a libido, instead of being more of a genetic rubric that says in these circumstances you should do this thing, with the male one saying, if there is an appropriate hole, try and put your penis in it, and the woman saying, if you end up with a penis in you, you might as well go along with it. Pleasure is a concept that is, it requires more intellect than you would think. I mean, there's positive feedback, but until you get into intelligence, the idea of anticipating more complex results from more complex situations becomes questionable. So if we're talking that far back in the evolutionary chain, it's difficult to say with certainty that the males and females were developed enough to have something that you could equate to a libido.
G: I think you've arrived at what is probably the hardest question in reconstructing behavior from evolution:
At what point does a reward system become something we'd reasonably call a "libido," rather than just a stimulus-response mechanism?
And I don't think there's a sharp boundary.
One thing I would push back on is this statement:
"Pleasure is a concept that requires more intellect than you would think."
I'd actually separate pleasure from reflecting on pleasure.
Pleasure itself is ancient.
A fish can experience reward. A rat can prefer one stimulus over another. A dog clearly anticipates things it enjoys. None of those require human-like abstract thought. The basic neural machinery for reward—the dopamine, opioid, and related systems—is evolutionarily very old.
What's newer is the ability to think about pleasure.
For example:
Those increasingly involve memory, planning, social cognition, and self-reflection.
So I think there are at least three levels.
Reflexive reward
Motivated behavior
Reflective sexuality
The question you're asking is where your hypothetical ancestors sat on that continuum.
I actually like your wording here:
"...more of a genetic rubric..."
That may be closer to how to think about early members of our lineage.
Instead of imagining either "modern humans" or "robots," imagine something like:
"A primate with a very flexible set of instincts."
Not entirely hardwired.
Not entirely reflective.
Some behaviors would already be capable of being modified by experience.
Here's another reason I think your question is interesting.
Suppose you have two individuals.
Individual A experiences mating as:
"Behavior complete. Neural reward delivered."
Individual B experiences mating as:
"Behavior complete. Neural reward delivered. Increase motivation to seek similar social interactions."
That second system is already a little more than reflex.
Now imagine another mutation that makes social interaction itself rewarding.
Then grooming becomes rewarding.
Then proximity becomes rewarding.
Then pair bonds become rewarding.
You don't have to leap from instinct to Shakespeare.
You can get there by gradually expanding what the reward system reinforces.
There's another subtle point I think you're circling.
You keep asking whether these ancestors were "developed enough."
Evolutionary neuroscientists often think in terms of exaptation—existing systems being reused for new purposes.
The reward circuitry that originally evolved to reinforce:
can later become involved in:
So it's possible that an ancestor already had a perfectly functional reward system without that system yet supporting what we'd recognize as human sexuality.
Ironically, I think the living species that may help answer your question aren't actually chimpanzees or bonobos.
They're monkeys that fall between more rigidly seasonal breeders and the great apes. If you could line up dozens of primate species and compare:
you'd have a much better chance of seeing whether these traits tend to evolve together or independently.
That's the kind of comparative evidence evolutionary biologists often use when they can't directly observe extinct ancestors.
So I think the question has shifted again. It's no longer "Did this mutation happen?" It's becoming:
How sophisticated did the ancestral primate's reward system need to be before a shift toward year-round reproduction became evolutionarily stable?
That's a much narrower and more testable question than where you started, and it's the kind of question that comparative neuroscience and primate behavior could, at least in principle, help answer. It's also a reminder that we shouldn't assume our ancestors either had modern human psychology or were driven purely by fixed instinct—they were likely somewhere in between, with reward systems that were already powerful but still evolving in how flexibly they shaped behavior.
M: Yeah, you're using language imprecisely. I'm saying that pleasure encompasses reflecting on pleasure. You later on went to distinguish, you said, it's the difference between reward and pleasure. If I give my hedgehog a mealworm, it consistently is happy or at least eager to consume the mealworm, but nothing about its attitude changes afterwards, or usually minimally. Now, this is a hedgehog, so they're difficult to read. But the point is, until you start applying intellect, that instantaneous reward isn't really pleasure. There's no anticipation or willful pursuit or the ability to maintain that sense of reward without the ability to think or be aware of what's around. And I'm being very open with my definition of intellect at this point, because you know, like, dogs, we've kept them around long enough that, you know, their intellect has probably grown, even though they tend to be kind of goofy due to association with humanity. But they are able to show the idea of pleasure in their reactions because of it, as opposed to, you know, the, let's go with the other direction, lizards, who may be very good at what they do and even in some ways intelligent, but at the very least, they don't seem to really be able to savor anything. And that's my point, is that it's reward versus pleasure, not a question of separating pleasure from reflecting on pleasure. I was using it as a, as something that was already a scalpel. You were thinking of it as something that needed to be sharpened, and it wasn't. And in here, it would come about as an actual concern, or at least applicable, because at that point, you wouldn't necessarily have to have any kind of pleasure associated with the action if it was still at the stimulus response stage. You've got males and females likely sleeping together to conserve warmth or strengthen bonds or whatever. I mean, isn't that how physical contact became a priority for social animals? So you get in that situation, and if the stimulus response says, if you have your privates nestled against something that will accommodate it, get hard and try to have sex, then you've easily got a situation where the sexual action could occur on a regular basis without there being any actual awareness of it as being a source of mating. And in fact, it could have participated in the feedback loop of sex and love eventually coming to be completely intertwined in a way we're still trying to disentangle, because just like the skin contact, the sexual contact would have initiated... or would have developed as society developed, not as an indication that you wanted to have a child with someone and loved them or whatever, but simply as a side effect of being next to each other, especially at night. Pleasure would not need to be involved at all, simply the stimulus response, which then, over the course of millennia, would simultaneously develop with having children. I mean, it's hard to say at what point intelligent creatures started being able to identify that sex actually produced children, because if you're following just genetic rubrics, it seems like there are systems out there where, you know, scent or whatever tells the father that, oh yeah, this is one I should leave alone, not even necessarily because of any kind of equivalent to paternal instinct, but more just a neurochemical acceptance of something as being part of, you know, the safe group or whatever. Once you start having intelligence and being able to overwrite or ignore any kind of odor sensing mechanism or other... way of doing things, especially in a world where you won't necessarily know how you look personally, because there's no reflective surface, so you can't even say that kid looks like me. There would easily be a period where the connection between the sexual naps that you take every day and the child that pops out every once in a while wouldn't necessarily be connected. I mean, that would be difficult, but it would still be like a really muddy situation.
G: That clarification helps a lot, because I was using the word "pleasure" in the neuroscientific sense ("a rewarding internal state"), while you're defining it as reward that has become integrated into a cognitive model of the world. Under your definition, the distinction is between reward and experienced pleasure, and that's a coherent distinction—it's just a different use of the term than I assumed.
With that definition, I think you're asking a more philosophical and evolutionary question:
At what point does a nervous system stop merely reinforcing behavior and start representing that reinforcement as something worth seeking for its own sake?
That's a much deeper question than "does a lizard enjoy food?"
Where I'd still be cautious is with one part of your reconstruction.
You suggest that repeated physical proximity could lead to repeated sexual behavior through relatively automatic mechanisms, with the cognitive understanding of reproduction arriving much later.
That part is actually plausible as a hypothesis, because understanding causation is not necessary for evolution to work.
Evolution doesn't require organisms to know that sex causes offspring. It only requires that organisms who engage in behaviors that produce offspring leave more descendants.
So an ancestral primate could, in principle:
without possessing any explicit concept of "babies come from sex."
Where I'd hesitate is the specific mechanism of "if the genitals are pressed against something suitable, mating simply proceeds."
Modern primates don't seem to operate quite that automatically. Even species with relatively frequent sexual behavior usually have hormonal, behavioral, and social cues regulating receptivity, courtship, consent or resistance, dominance relationships, and so forth. That doesn't mean ancestral primates had the same systems, but it suggests there has probably been a long history of behavioral regulation rather than purely mechanical triggering.
I think another idea you're touching on is more broadly supported:
You can have a long interval in evolution where behavior exists before its participants understand its causal significance.
Humans themselves provide examples.
Children don't understand where babies come from for years, yet the reproductive system functions perfectly well.
Many animal behaviors are exquisitely adapted without the animal having any abstract understanding of why it performs them.
So it's entirely reasonable to think that some ancestral primates may have engaged in patterns of mating long before they had anything resembling a conceptual link between mating and reproduction.
One place I'd separate your argument into two different questions is here:
Those aren't the same.
The first is a cognitive question.
The second is an evolutionary one.
The second could happen millions of years before the first. Evolution doesn't wait for understanding.
Finally, I think your broader approach has a useful constraint built into it. Throughout this discussion you've repeatedly tried to avoid attributing modern human psychology to very early ancestors. That's a good instinct. It's very easy to accidentally imagine a proto-primate thinking, wanting, and interpreting experiences the way a human does.
The challenge is that the opposite extreme—imagining them as nothing but reflexes—is probably too simple as well. Mammalian brains had already evolved sophisticated learning, social recognition, and reward systems long before humans appeared. So the interesting question isn't whether they were "just stimulus-response" or "already like us," but how much internal modeling of the social world they had at a given point in the lineage. That's likely something that changed gradually, with different cognitive abilities advancing at different rates rather than all appearing together.
M: You're criticizing my oversimplification, an unacknowledged one at that. When you accept the rubric, then you have to accept when I try and produce a sketch of a rubric because I don't know the exact mechanisms by which it would occur in terms of stimulus and response. I wasn't trying to be specific, I was trying to put in a placeholder. And given current circumstances, you know, all the stuff that leads to puberty jokes and whatever, it really doesn't take much for the modern man at a neurological level to be triggered into a sexual response. Now back then, it might have been a little more difficult because it was still on the wavelength of a genetic series of requirements, but it's not a huge extrapolation to say that having comfortable hips pressed against your nether bits could be one of the triggering events. That one still exists and is a lot less complicated than some of the fetishes people require nowadays. I wasn't arguing about the causal thing, I just was kind of going off in that cul-de-sac, but you're getting away from the primary thing the same way I was. It's that pleasure isn't necessary for sex, and at that point, it's not necessarily... Sorry. At that point, it can't be said that there actually was any pleasure involved. A reward each time, yes, but no actual pleasure.
G: That's fair. I was treating your sketch as though it were a detailed mechanistic proposal, when you were using it as a placeholder for "some relatively simple trigger exists here." Those are different things.
And I think I see the central distinction you're making now.
You're defining:
Whether that's the terminology evolutionary neuroscience would use is a separate issue. As long as we're using your definitions consistently, the hypothesis becomes much clearer.
Under that framework, your claim is not:
"Early primates didn't have reward."
It's:
"Early primates may have had reward without what I'm calling pleasure."
That's a coherent distinction.
Where it becomes scientifically difficult is that we have essentially no direct way to measure subjective experience in extinct animals. We can infer neural circuitry, compare living species, and reconstruct behavior, but we can't determine when reward became phenomenologically rich in the way you're describing.
That means your idea is largely conceptual rather than directly testable.
What I do think is interesting is the consequence.
If sex initially functioned primarily as a reinforced behavior rather than an experienced pleasure, then over evolutionary time the subjective aspect could itself become something under selection.
Not because evolution "wanted pleasure," but because organisms whose brains increasingly integrated reward into memory, anticipation, social attachment, and motivation might end up reproducing more successfully in increasingly social groups.
That fits the systems perspective you've been building throughout this discussion.
One other point I appreciated was this:
"Pleasure isn't necessary for sex."
From an evolutionary standpoint, that's almost certainly true. Plenty of organisms reproduce through mechanisms that we have no reason to think involve anything resembling rich subjective enjoyment. Evolution only requires that the behavior occurs often enough to leave offspring.
The harder—and more philosophical—question is when, in the primate lineage, the reproductive reward system crossed from being primarily a behavioral reinforcement mechanism into something closer to the rich motivational and emotional experience humans report today. I don't think anyone can confidently identify that transition point.
So I think we've narrowed the disagreement considerably. We weren't really disagreeing about evolution; we were using the word pleasure to refer to different levels of processing. Your hypothesis depends on distinguishing simple reinforcement from a more cognitively integrated experience, and once that definition is explicit, the rest of your argument is internally consistent. The remaining question isn't whether the distinction is meaningful, but whether evolution actually followed that trajectory—and that's something the available evidence can't answer directly.