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Is there any way to explain how did humans in my planet end up with extreme dimorphism without resorting to polygyny nor "magic"?

I want men to be around 6ft (183cm) - 230 lb (105kg), while women should be around 5ft 10in (178cm) - 112 lbs (52kg). The planet is similar to earth and was colonized several generations ago. In short I want both of them t be quite tall, but men to be built for strength, while women should be very agile.

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    $\begingroup$ Skinny does not equal agile. $\endgroup$ – SPavel Jan 8 '17 at 16:15
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    $\begingroup$ What's extreme about this? You're describing, with some tolerance, me and my lady ;) I'm just bit taller and heavier. $\endgroup$ – Mołot Jan 8 '17 at 16:15
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    $\begingroup$ Cultural/societal reasons? Tall people mean that they are well-fed; so if bulked-up men and skinny women are considered attractive then that's going to be what they aim for. In fact, basically Western society as it is now - just with far less obesity (probably because people have much less sedentary lifestyles) and with a very high stock placed in physical attractiveness. $\endgroup$ – Matt Bowyer Jan 8 '17 at 16:19
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    $\begingroup$ Seconding Mołot's comment: What's so extreme? Your "extreme" men are just a little taller than ordinary men; while your "extreme" women are considerably taller than ordinary women, they are still only 4 cm taller than the average Turkish woman. The difference in weight is less than two times greater than usual--an average European man weighs somewhere around 80 kg and an average European woman about 65. What you are describing looks very much like a country of American men married to Brazilian women. $\endgroup$ – AlexP Jan 8 '17 at 16:31
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    $\begingroup$ @TGar: The point is that the heights and weights in the question are within normal human range. As Cort Ammon said, they represent mild dimorphism, only a little more marked than the mild dimorphism already exhibited by humans. In a novel or story this can well just be declared to be so, no special explanation needed. $\endgroup$ – AlexP Jan 8 '17 at 16:41
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Assuming man's weight comes from muscles, since you want strength this looks like a match between Arnold Schwarzenegger 6'1" ft./ 235 lbs or Marcus Fenix from Gears of War and lingerie model Candice Swanepoel 5′ 10″ / 120 lb. Arnold Though this dimorphism is far from extreme like in angler fish it is far from normal. If your planet is a colony I suggest to use founder effect where women colonists were lingerie models while male colonists were linebackers & bodybuilders

NFL players

Then you only have to use some kind of selection mechanism where the slender boys & bulky girls fitness is very low. Eliminate those for few generations and evolution would do the rest. I think.

Chart source

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    $\begingroup$ If you could only swap a guy in your example into someone real... $\endgroup$ – Mołot Jan 8 '17 at 18:33
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    $\begingroup$ @Mołot suggested real persons are welcome assuming they are 6ft (183cm) - 230 lb (105kg) and muscular $\endgroup$ – slobodan.blazeski Jan 8 '17 at 18:53
  • $\begingroup$ So Lu Chen Hui is too heavy and too muscular? $\endgroup$ – Mołot Jan 8 '17 at 18:57
  • $\begingroup$ @Mołot Too tall, Less height same bulk Height: 6'2" (188cm) Off-season weight: 298 pounds (135kg) asiamusclenetwork.blogspot.de/2015/03/lu-chen-hui.html $\endgroup$ – slobodan.blazeski Jan 8 '17 at 19:01
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    $\begingroup$ Unless the women sit around all day painting their nails and the men do EVERYTHING, the women will also need a lot of strength for a stone age and/or subsistence agriculture lifestyle. Hauling around firewood, lugging water from the river, hoeing crops, threshing the grain, grinding the flour, pounding the laundry clean on rocks and other traditional 'women's work' is labour intensive and creates physically strong women. They might not have muscles like a blacksmith or Arnie (who works out to maintain that shape), but they certainly won't be 'office girl' wimps like your lingerie model. $\endgroup$ – DrBob Jan 11 '17 at 15:44
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Hmm. "Without polygyny" is the tricky bit, since that's the primary reason male mammals & birds are bigger than females of their species. Humans dumped the biological version of polygyny in favour of pair bonding (and corresponding reduction in infanticide). But we kept the size difference, probably as we evolved 'job specialisation' (say, for instance, groups of men hunt big game in endurance chases, individual women hunt small game by short chases).

Birds of prey often have different sized sexes (female bigger). It's not exactly clear why, but each sex hunting a different size of prey is one suggestion why.

So maybe combine the human 'job description' and the bird of prey 'niche separation'? Have a situation where the two sexes have always hunted very different prey. Women climbed trees and chased down colubus monkeys through the canopy the way modern chimps hunt - they'd need to be small and agile. Meanwhile men ran down antelopes and boar on the forest floor - they'd need to be big and burly to rugby tackle the animal to the ground and beat it to death.

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  • $\begingroup$ Founder effect? $\endgroup$ – slobodan.blazeski Jan 8 '17 at 17:39
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    $\begingroup$ Height is a problem with the Founder Effect, because height is a multifactorial feature - your height is the sum of the size of your ankle bones, leg length, spine length, skull height and so on. And it is not sex linked. So a tiny woman and a giant man may produce children of small, medium and large height. The small ones will not necessarily all be daughters and the large ones not all sons. My 4'8" grandmother has a 5'11" daughter, for instance. $\endgroup$ – DrBob Jan 11 '17 at 15:28
  • $\begingroup$ Height difference is quite small 6ft (183cm) vs 5ft 10in (178cm). It's the weight (muscles) size that's way too large 230 lb (105kg) vs 112 lbs (52kg). Selection mechanism for this sizes is left as an exercise for the reader. $\endgroup$ – slobodan.blazeski Jan 11 '17 at 20:20
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If your planet has genetic engineering (which is not a big stretch because Earth is right on the cusp of designer babies) then all you need is a cultural predisposition for what the ideal male and female forms are. If parents want their male offspring to be big and strong, and they want their female offspring to be thin, athletic, and agile, then that's what they're going to choose when designing their babies. Within a couple generations, that's what they're going to look like.

All you need is a culture that presents these forms of masculinity and femininity as "ideals" in popular media. In other words, you need Hollywood and popular magazines.

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  • $\begingroup$ Larry niven messed with the idea of a species using genetic engineering to push their sexes to the most "desirable" body plans, huge hulking males small demure women,becasue it was a heavily sexist society they even ended up making their females idiots, border line animal intelligence. You could do something similar. Pushing your sexes to the most desirable body plans in not even that far-fetched once genetic engineering is introduced. $\endgroup$ – John Jan 9 '17 at 1:31
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I would say that is not so problematic, the main issue is to find jobs/roles for men and women, that were strongly separated between them in the history of your planet and that had an important role in their lives (so it could affect their look).
The height is not so different so the thing you should focus on it the weight. There are plenty of possibilities that could determine their weight and with weight strongly connected strength.

Especially the strength is ability traditionally useful for men (as women were at home taking care of children), so you just need the reason for men to fight and for women to not doing such a muscle activity.


(for example very safe caves all around the planet and less of vegetation so the women could hide, but they wouldn't be so helpful outside, and the men can be always outside trying to get something to eat.... but here I am making stuff up, there is a lot of other possibilities for you, like riding specific animals, defend some specific kind of homes or building them etc.)

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    $\begingroup$ Childcare is only "not such a muscle activity" in the modern western world, where cars, supermarkets, prams, vacuum cleaners, washing machines, mains water, central heating and other inventions make it easy to feed, clean and carry your child. Women had to be much physically tougher before all these things were invented. $\endgroup$ – DrBob Jan 11 '17 at 15:52

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