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I want to have a species based on humans, where everyone is born as female, but switches gender to male later in their life. Assuming that transformation takes 4 years (*) hermaphrodite is born as woman, at 27 starts transformation and at 31 becomes man.

Assuming that anatomy of sequential hermaphrodites is equal as possible to humans, would humans outcompete hermaphrodites assuming same technological level in the stone age era.

Advantages:

  • Nearly double population growth (humans have longer fertility window but it starts to drop after 27)

Disadvantages:

  • Infertile during the transformation period
  • Smaller number of physically strong soldiers
  • Might be less willing to take risks, since there's no young men to do that

(*) I plan to ask is this realistic time period in separate question

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  • $\begingroup$ Welcome. Sounds like an interesting world you live in. I also tried grouping all my related questions in my initial post. I think you will be better off if you separate your sub questions into separate posts allowing answers to focus on one at a time. As it is I think this may be closed as too broad. $\endgroup$ – EveryBitHelps Oct 7 '16 at 20:40
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    $\begingroup$ Yes, this is definitely too broad as it stands. I count a minimum of four separate questions here just in what you've written; depending on how you split this up, it might be as many as six. The line about "sequential hermaphroditism versus two genders" is a major idea that demands a question all to itself, for instance, without the clutter you're adding to it here. $\endgroup$ – Palarran Oct 7 '16 at 22:02
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    $\begingroup$ I narrowed down to single question, is this acceptable format? $\endgroup$ – Yuvato Oct 7 '16 at 23:10
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    $\begingroup$ I was gonna reply that switching costs are the main disadvantage but then I found this news.yale.edu/2009/02/02/… $\endgroup$ – Platypus Oct 7 '16 at 23:50
  • $\begingroup$ Read "The Left Hand of Darkness" $\endgroup$ – Thucydides Oct 10 '16 at 2:51
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Well, if this species is anatomically similar to humans, you have to consider one of our species' biggest design flaws: maternal mortality. In pre-modern times, it's estimated that up to one in twenty births resulted in the mother's death. If this is the same for your species, it's easy to imagine that for much of their history they will experience quite an unequal sex distribution among the population, as many women won't live long enough to make it into manhood.

But the other factor that skewed gender distribution in human history was warfare. In our species' history a big war would reduce the proportion of young men. In your species, there won't be any young men, so there's a question over who fights this species' wars instead. If it's the older men, then this would be a further factor that would reduce the proportion of men in the population. But my suspicion is that it would be young women and girls who'd primarily be press-ganged into military service. This might make your societies more apprehensive about fighting wars, as it's much harder to rebuild your population after losing a critical mass of your mating-age females than a similar number of mating-age males.

Once your species develops medical technology on a similar level to ours, the gender disparity is going to completely flip, as more women survive childbirth and more men live older lives. Your societies will face an unprecedented shift where the gender split goes from 60% female to 40% male, to 40-30% female to 60-70% male. For a civilisation completely unused to this, it would be fascinating to see what this sudden surplus of testosterone would do...

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    $\begingroup$ Why would girls/young women fight war, when they could surrender and still reproduce? If human history is any guide, child bearing women were quite often the spoils of war. As soon as the men of one side are dead I doubt that war will last much longer at least in pre mass destruction era. $\endgroup$ – Chonin Oct 8 '16 at 16:15
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I'm going to go out on a limb here and say

Not if the hermaphrodites do it right

TL:DR: Don't assume that just because women can breed that they will try to.

Many other answers here are applying modern (or at the least, human) sociological values to an otherwise alien society. If this sexually hermaphroditic society develops a different approach to gender roles (or, more likely, the entire concept of gender) and adopts a more arbitrary caste based system, then you will quickly see similar roles to men/women developing but split along caste lines.

Strapping young girl? Military caste. Go forth, try your best to kill or be killed. Take risks. Don't breed, as death in childbirth is the worst shame a warrior can know. If you grow old enough to become a man then you have an awful lot of experience and can be used to train the raw recruits, not to mention stud the next generation of warriors into existence.

Likely to get killed in the first skirmish? Don't worry: There's always a need for the working caste. Feel free to breed if you want to, but don't expect any special treatment from the other castes. You're here to primarily support the other castes, including taking care of other people's children if required. If you grow old enough to become a man then bully for you.

Wide hips? Good pedigree? Mother has a history of twins? Breeder caste. You get food, protection and your pick of the men. You're likely going to die giving birth, but hey, at least you perpetuated the species. If you don't die and manage to live long enough to be a man then clearly you're good at this whole 'giving birth' thing, and the next generation could use some of your genetic material. Go forth and multiply.

This system requires more social interaction than the human system, and as such tribes can be expected to be more closely knit. If the death rates are similar to those of the human's you're trying to outcompete then you should come out ahead of the game (or at least drawing) by having specialised roles that are not gender specific, and as you'll likely develop specialties pretty quickly (your 'alphas' will likely be female, your wise men will be just that, you won't have any arbitrary traits defining social structure from the get go) your social structure may well pip humanity to the post and make up for any breeding disparity.

Plus it would make for a social structure that is simultaneously incredibly PC and hugely discriminatory.

Disclaimer: I mostly wrote this answer to be contrary. I have no idea if this would occur and, in fact, no way to simulate the social evolution of a pre-historic population, but it's a possible outcome!

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The key human advantage is risk-taking

In human societies, men are generally less important to the reproductive process than women. Therefore men can be risked without incurring much risk to the population as a whole.

This is a powerful advantage for developing technology and for expanding through violence. Women, in general, had to reproduce to keep the species going; thus was true for all of history until the 1800 when improved nutrition and sanitation allowed many more children to survive. But men were never that necessary; one man can do the job of 10, so to speak, when it comes to making children. Therefore, an effective society will utilize its young men in high-risk, high-reward occupations to attempt to gain competitive advantage for that society. High-risk, high-reward things might include hunting dangerous things (like mammoth for food, or lions to keep them from eating your kids), long distance trading for valuable rocks from far-away places, or fighting your neighbors for prime land to steal their womens.

Risk taking humans can out-compete the more fecund sequential hermaphrodites in direct competition

So applying this principle to your sequential hermaphrodies, they are likely to be at a significant disadvantage in direct competition with humans. Sure they can reproduce faster due to having more women around, but if they fight battles, they have a relatively higher penalty for losing men in combat because there are proportionally less men. Each man lost in combat has relatively more negative effects on the ability of the society to produce children, compared to humans.

They also don't have the available expendable people to send out to do scouting or trading, unless the young women do this...in which case they are forfeiting their advantage in reproductive rate. Finally, it is worth pointing out, if combat is more or less concentrated in the males, the males of the sequential hermaphrodite would have significantly less experience with combat when they reach their 30s; and experience is the primary advantage of older men in combat and as combat leaders.

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    $\begingroup$ Women are quite capable of engaging "risk taking" behaviors like hunting and exploration and even combat - the rampant sexism displayed in these answers is disappointing. $\endgroup$ – pluckedkiwi Dec 20 '16 at 18:39
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    $\begingroup$ @pluckedkiwi They can...but then they might not have kids. The point is not that they can't take risks, but that they shouldn't to optimize reproduction. You didn't take the time to understand that my post was about risk, not about the abilities of the sexes. Your blind anti-sexism is dissapointing. $\endgroup$ – kingledion Dec 20 '16 at 18:43
  • $\begingroup$ is your position that a woman must either spend their entire lives at home doing nothing or be rendered infertile? Women can serve in the military, even having engaged in combat, and still have children. Women can go rock climbing or skydiving or any number of other "risky" activities, and still have children. While this isn't recommended while heavily pregnant, that is a relatively tiny portion of a woman's life. There is no loss of fertility if a woman goes hunting (maybe you think the angry spirit of the killed animal curses her womb?). $\endgroup$ – pluckedkiwi Dec 21 '16 at 17:57
  • $\begingroup$ @pluckedkiwi Without child care services and with predators around, someone has to look after the kid 24/7 for a couple of years. Without formula, someone also has to breastfeed the kid for at least a year. If you take 2 years of dedicated care and multiply that by the 4 kids (50% child mortality), then the mother is occupied for 8 years of her life in order to create the next generation; even longer to increase population size and account for high adult mortality as well. Risking the woman at any point before that is a bad investment on propagating the species. $\endgroup$ – kingledion Dec 21 '16 at 18:09
  • $\begingroup$ you are presuming that only the birth mother can nurse a particular child and this somehow takes all day, and that all women have kids all the time. If presuming these people are more similar to humans, any lactating woman could feed a child (of which there will be ample even though they would not be constantly breeding anyway else population growth would be truly explosive), and anyone can look after several children at a time (no need for a one on one babysitting 24/7 for 2 years). And by the way, replacement value here is having just one child, not two. $\endgroup$ – pluckedkiwi Dec 21 '16 at 18:51
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Can they breed with humans? Many scientists believe Neanderthals didn't go extinct but were absorbed through interbreeding with traditional humans. Given that your population has fewer men than women it's possible that the females of your species might interbreed with human males especially if your mails stay behind and the females go to war leaving their husbands behind perhaps for years.

As a side note I don't think this species population what really grow much faster than humans. Most human women continue breeding up till their thirties and some even up to their mid-forties men can continue breeding from puberty to their sixties. Whereas in your species the females only viable breeders until they reach 27 long before a human female would be still capable of breeding at this time. As for your men they are capable of breeding till 31. That's twenty-nine to 30 years of Lost potential when it comes to greeting. So in terms of population I actually think human population would be much higher

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  • $\begingroup$ I think you've fundamentally misunderstood the premise. It isn't that women are only fertile to 27 and men only fertile to 31, it is that everyone is a woman until 27 and becomes a man at 31. Since fertility is primarily at younger ages, especially in more primitive circumstances, losing the ability to give birth after 27 is not a significant detriment compared to having doubled the number of potential mothers at younger ages. As men are not as functionally limited in breeding, the reduction of potential there is irrelevant. $\endgroup$ – pluckedkiwi Dec 21 '16 at 19:06
  • $\begingroup$ @pluckedkiwi I understand the question fine you misunderstand my answer. This species starts life out as female and is fertile as a woman from the time of puberty up to the age of 27. During this time it is infertile as male. Then during the transition it is infertilie both male and female . At age 31 transition is complete. It is now fertile as a male but infertile as a woman. I do not think this method of reproduction is as efficient as our. $\endgroup$ – Bryan McClure Dec 21 '16 at 20:06
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If life expectancy tables are correct, and the average life expectancy hovered between 20 and 30 for most of history, most of your females would not have an opportunity to live long enough to become a father. Since males would be rare, they would be much too valuable to risk in war. If we assume the average age of death is 30, and the people don't become fertile men until age 31, more than half of your population would not survive to reproduce as males. This would almost certainly remove the possibility of pair-bonding. A human couple can reproduce at age 14 or so, while among hermaphrodites, 14 is the age of the female, while 31 is the minimum age of the male.

If we are looking at human physiology as an analog, then most of the society would be females and thus on average be both physically weaker and less aggressive their human counterparts.

Humans would reproduce faster, be more aggressive, have more aggressive males to hunt, fight, and compete than the hermaphroditic society. They could tolerate more men being killed. Furthermore, human females could tend to the young while the men fight and hunt, while there could be no such division of labor in the hermaphroditic society.

Edit While infant mortality does skew the life expectancy statistic, it is undeniable that life expectancies were much lower long ago. Consider maternal death at childbirth. In countries where moms do not have access to quality medical care at birth, about 1 in 6 die in childbirth (Unicef note the rate is per birth, not over her lifetime.) This article does a good job at refuting the Live Science piece quoted in the comment. In it, the author cites age of skeletons in the catacombs.

if human lifespan had really not changed in 2000 years, then 35-year-olds shouldn’t have left their skeletons very often in the Roman catacombs. Unfortunately (for them), we find those 35-year-old bodies. A rough estimate (gleaned from tomb inscriptions that give ages) is that half of Romans who lived to age 15 – and therefore escaped juvenile mortality – were dead before age 45.

In a world where there is no antibiotics, no surgery, no dental care, no false teeth, no refrigeration, no stable food sources, and danger at every turn, death came quickly and often. A scratch could lead to sepsis. Food could turn in the hot sun and lead to food poisoning. Starvation, war, plague, and violence were constant companions.

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  • $\begingroup$ Life expectancy is significantly skewed by child mortality - if people survived to reach puberty, they were likely to survive into middle age. The idea that most people died in their 20s is a thorough misunderstanding of what life expectancy represents (says nothing about the mode). $\endgroup$ – pluckedkiwi Dec 20 '16 at 18:32
  • $\begingroup$ @pluckedkiwi is correct it's only the infant mortality that skews the life expectancy livescience.com/10569-human-lifespans-constant-2-000-years.html $\endgroup$ – slobodan.blazeski Dec 21 '16 at 0:31

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