Why would post-apocalyptic men be twice as large as women?

Question

I'm creating a post-apocalyptic world several centuries into the future. After the nuclear war whatever is left of humanity has regressed into the dark age.

Society:

The offspring of those few who survived the radiation and the following nuclear winter live in small tribes. They sometimes cooperate, and sometimes compete with other tribes. Humans are few in number.

Technology:

Technology regressed to the dark ages. The survivors hate technology and blame it for the fall. Anyone caught trying to learn or recover the before-the-fall knowledge is ostracized or put to death. Whatever is left from the destroyed civilization has been overtaken by mother nature like in Life After People. Rotting machines are mostly used for scrap metals by village blacksmiths.

Food:

Food production is limited to small scale horticulture and herding. Hunting and fishing are very important food sources since there are many animals and very few humans.

Radiation

Many places are highly radioactive and must be avoided. Some are mildly radioactive and people pass through them quickly if they are forced to. There are many dangerous mutated animals. Humans who show signs of mutations are killed.

Climate

The climate is much colder, with earth just recovering from an ice age. Humanity must rely on many food sources, since climate is very unpredictable.

In my story the average man is twice as large as average women. While the differences in modern world are 10-15kg, the average 25 year old woman in my setting weighs around 65kg, while the average 25 year old man is around 130kg. Obesity is unheard of. Men are bulky; not like modern body builders but like people who gained their muscles from a lifetime of hard work.

Is there a way to explain why dimorphism increased dramatically without resorting to genetic engineering or radioactive mutations?

Answer 1

You need some kind of environment where its :

1. Advantageous to be a bulky man
2. Disadvantageous to be a bulky woman.

For example let's assume that your tribe constantly fights with neighboring tribes, mutants, wild animals, giant spiders etc.

Big men are more successful in those battles and win more often, abduct more women and acquire more glory. That allows them to acquire more wives and have more extra-marital dalliances with the ladies. On the other hand thin men tend to die more often, kill less mutants, and hunt less boars. Parents are hesitant to give them their daughters, and few ladies find yoga instructors attractive. So bulky men tend to have more children on average then thinner men.

From an evolutionary point of view, it's advantageous to be a bulky man. Unfortunately, the above would also make women grow in size since bulky fathers tend to have bulky daughters.

So lets use culture for the second constraint.

Since there is so much fighting in your dog eat dogs world, man tend to die in droves and tribal warlord is always short on warriors. So he starts recruiting strong women. Bulky girls are pressed into tribal militia where they practice to use glaive & crossbow.

Unlike their sisters who must obey their fathers then their husbands when they get marry, the women warriors are granted same status as man. That means they have the freedom to go where they want, marry who they want, vote in the tribal council and even have a chance to become tribal chief themselves etc.

As a consequence of their more dangerous occupation bulky girls tend to die more often, they marry later (dalliances for serving women are strictly forbidden) and tend to have fewer children then civilian women.

Under above constraints its disadvantageous to be a bulky women, since you will be pressed into military service.

This cultural innovation makes the tribe stronger. Since our brave amazons are wo-manning the village walls and protecting livestock from two headed wolves, the warlord has more spare men to raid neighboring tribes. Which gives more loot and slaves. Tribe grows prosperous and conquers neighboring tribes. The remaining tribes quickly copy this practice. Now everywhere you go you hear the stories of glorious female warriors who protected their village from raiding parties & fire breathing mutants.

Keep these constraints for multiple generations and evolution will find a way to give you the dimorphism you want. Most likely your humans would be far more affected by the hormones, or smaller couples would have more girls or whatever.

Note:

All of the men must be warriors. Strong woman are recruited if they volunteer or if there is a shortage of men.

Excerpt from Dahomey Amazons

The Amazons were recruited in a number of ways. Sometimes they were volunteers – women who were sick of their bullshit day-to-day lives, poor women seeking battlefield glory, or even the occasional royal concubine who decided she was much more comfortable cutting people in half for the King than she was producing male heirs for him.

Answer 2

In my story average man is twice larger then average women. While the differences in modern world are 10-15kg, average women in my setting weighs around 65 kg, while man are around 130 kg.

That's B-I-G big.

Obesity is unheard of. Man are bulky, not like modern body builders, but like people who gained their muscles from lifetime of hard work.

That's not what people look like after years of manual labor, and when food is scarce from a "climate (which) is much colder then now".

Is there a way to explain why dimorphism increased dramatically without resorting to genetic engineering or radioactive mutations?

Possibly, if the only human survivors happened to be descended from American professional football players (specifically interior linemen).

Practically, no.

Answer 3

Is there a way to explain why dimorphism increased dramatically without resorting to genetic engineering or radioactive mutations?

First off, there is no such thing as radioactive mutations. A mutation may be caused by radiation, but it will be no different from a change in the same gene caused by chance or engineering. Just as if I commit a typo here that gets corrected, the text is no different if it is edited by you or me.

Now, the change may be part cultural, part selection.

Start with a template where, given a 20th century western lifestyle, healthy people would average 80-100 kilograms. Why? Because these people were selected by evolution after the apocalypse, for whatever reason. Say, if you have to wrestle against bears here and there, or if you have to haul what you've hunted large distances across uneven terrain, then more body mass helps you.

Next, all men are either into phisiculturalism by themselves, or forced into it. They've found a way to produce steroidal anabolizants with lower technology (I've seen anedoctal evidence for it, though I lack a source now, so handwave this away) and they eat and exercise a lot. Meanwhile, women are starved. Thus men acquire more mass than what would be a natural average, while women acquire less.

This food division is outright cruel, but is a thing in undeveloped countries. There is an educational cartoon from India which, in one episode, tries to teach people that giving girls smaller food portions is wrong so thay they drop this cruel custom. As far as I know, the custom has not died out yet.

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Alternatively, a random insertion mutation that adds growth-relates genes to the Y chromossome could do the trick. Such mutations may cause a lot more changes than just an increased production of growth hormones - the amount of possible birth defects and reasons for miscarriage could increase. Enlargement of males must be a really decisive factor for human survival for this tradeoff to be selected positively.

Answer 4

Just look at gorillas.

Such large differences in size already exist in an extremely close relative of ours. Gorillas have a large degree of sexual dimorphism and already weigh in at about your desired sizes. Adult male gorillas weigh between 136 to 195 kg and adult female gorillas between 68 to 113 kg. So, your post-apocalyptic males and females at 135 and 65 kg respectively are well within the realm of possibility both in size and in sexual dimorphism. In fact, males being roughly double the size of females in primates is not uncommon and can be found in Orangutans, Mandrills, Hamadryas baboons, and Proboscis monkeys.

Now, the question is how did such large sexual dimorphism arise? In primate biology, there is a well-known relationship between the degree to which males compete for access to females and the relative sizes of males and females. When a male’s reproductive success is strongly correlated to its ability to intimidate or fight other males there is a strong evolutionary pressure to increase in size in order to better compete. So, if you want to drive larger male size evolutionarily you want a societal structure in which the largest and strongest men are able to monopolize the reproductive capacity of many women.

Unfortunately, these sorts of evolutionary forces will be hard pressed to produce an appreciable change in such a short time span. I anticipate thousands of years at a minimum would be necessary to naturally grow humans to such sizes.

Answer 5

I suggest reducing both sizes. In a world where food isn't marketed, manufactured to be addictive, or even plentiful, it might not be unreasonable to have the average woman more like 45 kg. You could then more believably make the average man something like 80 kg.

In a world with very limited technology, there would be a multitude of advantages to men to be big and strong. If women were dependent on men for protection and food, they could well end up mostly petite. Remember that childhood malnourishment stunts growth, and being shorter is a huge head start in being lighter.

Answer 6

130kg is a really large man, and 65kg is not particularly small for a woman. Most 130kg men are very fat rather than very bulky. Even Arnold Schwarzenegger only weighed around 120kg as a bodybuilder.

So let's assume unusually small women and unusually bulky men: Women average 50kg and men average 100kg. This could be achieved without any genetic changes at all, if we assume that all men go through a strict training regime to become bulky, and all women are underfed and small. Girls would be fed as little as possible from birth, and boys would always be fed well and encouraged to fight and be physically competitive. Girls would stay small because they don't get enough nutrition to grow big (they would also be short). Boys who are genetically not able to grow large would often not survive the training regimen.

There would still be some small men and some large women, possibly because their families treated them differently from the norm. But the averages could make men twice as heavy as women.

Answer 7

Men and Women live apart

The most dimorphic (in terms of size) mammals are those that have males and females living apart. Sperm Whales have males in the order of 3 times greater mass than females, male polar bears are about twice the mass of females. In both cases, the males and the females live apart.

Living alone, being able to deal with the dangers in the wild, is easier if you have the bulk to fend off any of the nasty creatures the radiation has created. Killing one of those beasts should also provide food enough for a single male to live off for quite a while. Men would live in the more dangerous regions (for some reason -- perhaps to go to Valhalla or something) while women would live in more favourable regions, where there's no real danger.

Answer 8

It's not very hard to make males big and tall, like Richard Kiel. Just introduce some genetic anomaly which would make males generate much more Growth hormone and you'll have it.

But if you want your males to be very stocky without being fat, that would require a more significant change to human metabolism. To stay at 130 kgs weight, regularly tall (say, 185cm tall) human needs to eat a lot (and it has to be a very nutritious diet) and rest a lot too. For hunters gatherers, such height/weight combination would be detrimental. With more handwaving, another genetic modification may preserve muscle mass (today it quickly goes down with a lack of exercise or less nutritious diet), but as I said, this would be pretty bad for survival.

Answer 9

They may have Gigantism due to mutations caused by the radiation. There are some mutations that can cause Gigantism from early age, resulting in a much bigger and bulkier body in adulthood. If the anterior pituitary gland is damaged by a tumor, the body basically never stops growing.

But this mutation is not limited to men, so you would have at least some women with the mutation and some men without it. Or you need to come up with a clever explaination why women with this mutation and men without it don't survive, thereby driving evolution towards your goal.

There are several problems with this, though. Excess im Human Growth Hormone (which causes Gigantism) often leads to muscle weakness rather than strong muscles. It's also accompanied by Acromegaly, causing joint pain, high blood pressure, impaired vision, Diabetes and reduced sexual function.

One of the most popular people with Gigantism is "André the Giant", who was a professional wrestler.

Answer 10

My answer is cannibalism.

That solves part of the protein problem. It also explains why big women and weak guys don't make it.

Men fight and eat each other in semi-ritualised hand to hand combat to try to take over the harem of the other guy.

When famine strikes they keep up their strength by eating the women, starting with the biggest.

Ok, that may be a very different story from the one you want to write. But these conditions would over time lead to strong dimorphism.

Answer 11

Sexual dimorphism as you've described could easily occur within a few generations, by males having to fight for access to the females. Weaker, smaller males will quickly be killed off, and larger stronger males will be the only ones able to pass on their genes to the next generation.

Being larger would provide no reproductive advantage for females though, so they would stay pretty much the same size.

Answer 12

While the question is already closed I'd still like to give my version of an answer.

The early days after the apocalypse radiation was rampant. Many people died of radiation sickness. Many were born mutated or disabled. And as you say those born with mutations now are killed off. But in the early days after the apocalypse people gained mutations that they didn't even realize. A mutation that causes the person affected to grow to almost unnatural sizes. Bulky and muscular as you describe.

This mutation affects everyone though. Not just men. But why are women small then? This comes down to X and Y chromosomes. Men have a Y chromosome, which is just an X chromosome with a bunch of information chopped out. This is why color blindness is much more common in men than in women.

While a woman can have a recessive color blindness gene, they can also have a dominant gene that causes this gene to stay hidden and not actually give the person color blindness. But this dominant gene exists on that part of the chromosome that is missing in men. A man just needs this 1 recessive gene in order to be color blind.

Our mutation can work the same way. Women have this dominant gene that offsets the effect of this mutation the majority of the time. But since men don't even have this part of the chromosome the recessive gene affects all men that have it equally.

How would all people in this society get this gene? You only really need it to happen in a few people. And with the majority of humanity being wiped out there's a fairly small gene pool that this gene needs to float through. From here natural selection could favor the men with these genes. The larger men are better able to protect themselves and their families and localized societies. So this gene is preferred as the men with this gene do not die out. And they protect their small community which consists of people with this gene so they don't die out.

You can use this to create some legendary female warrior types from cultural legend and even some current characters because they have this same bulky build as the men in your society.

Answer 13

Malnutrition in childhood is detrimental to height. Perhaps women are thought of lesser importance than men in this society, and always eat last, whatever scraps are left at a table.

Man need to be strong, so they get best picks of food, and eat to the fullest, so they grow big and strong.

That causes the difference in height. Environmental variables are as important as genetics.

Answer 14

I think there are a number of links in this chain.

New food

The most obvious problem is that muscle requires protein. Lots of it, in a form which the body can take up easily. We don't currently have any way of doing this. Hunting has a good protein return, but hunts often don't return a win, and hunting anyway favours wiry hunters with either short-distance speed or long-distance endurance. Even in the animal kingdom, there are no big, bulky hunters. The closest you get are bears, and they're omnivorous scavengers.

So, we need your mutated environment to have come up with a new food crop. It has to have an insanely high protein content, grow prolifically enough that farmers can get high yields without having to work too hard (otherwise we'll be favouring endurance again), and keep well so that it'll tide them over winter.

New environment

Next, we need to encourage muscle-building. Farming is pretty good for this - there's a reason that armies tried to pressgang farmers rather than city kids.

There's a better option though. Back in the day, sailors were the ultimate tough guys. They weren't the scrawny ship rats from Pirates of the Caribbean - every last one was built like an Olympic gymnast, because spending all day climbing rigging will build that upper body strength.

So, let's suppose it's not too safe to live on the ground. Humans are now semi-arboreal - not as well adapted yet as an orangutan, but kind of starting on the way there. Even without major adaptation though, just spending most of your life climbing will give you the kind of muscles you need. Perhaps our new food source grows in the trees? Or perhaps we farm the trees and live in them?

We're only semi-arboreal though. We haven't yet lost our ability to stand upright, and we haven't yet got long enough arms to knuckle-walk, so our legs are still useful.

As an interesting related point, orangutans have a high sexual dimorphism.

Earlier birth

Humans have already evolved for birth to take place well before the child is a viable individual. To be as capable as the average lamb at birth, we'd need more like 3 years gestation. Earlier birth allows for smaller hips, and means the mother is not grossly encumbered by an oversized baby. Many other animals follow the same path, with differing development times. A radically smaller mother could not support a substantially bigger male child - but if the child is born at maybe 7 months then it wouldn't matter so much.

If (male) children get bigger, evolutionary pressures ensure this will just happen. If women don't give birth earlier, they and their son will die in childbirth. Women who give birth "prematurely" will become the new normal.

And with early childbirth, women can get away with being smaller than they currently are. Maybe they'd average 150cm and 50kgs - which only needs the men to be up to 100kgs, which is totally doable.

It's a man's world

Why would women become smaller though? Absent any health or dietary reasons, we're looking at sexual selection.

Suppose the birth rates become skewed, so we only get maybe 1 boy born for every 4-5 girls. This could result in a meerkat-like society where only one chosen female breeds but their relatives help parent their offspring. If men tend to choose the smallest, prettiest woman as their wife, then we'll have sexual selection in favour of small, pretty women.

Why might people follow that model, instead of the harem model? Maybe quality over quantity - raising a few children well and reliably is more successful than giving birth to many children who mostly die. Maybe a matriarchy develops to keep other women in line.