How could evolution favour decreased sexual dimorphism in a humanoid species?

Question

What the title says, how can a biped human-like species evolve with decreased sexual dimorphism without compromising their survivability over the span of time?

Constraints:

• The species inhabit a planet with conditions, environment and biosphere similar to Earth.
• Technologically, they are on same level as humans.
• No magic, no aliens, no miracles.

Similarity (to humans):

• Evolutionary line (evolved from primates)
• Biped, similar internal anatomy.
• Dioecious (seperate sexes)

Differences (important):

• Same height, nearly equivalent weight, androgynic features

• Less pronounced voice box (adam's apple) in males

• Males with less broad shoulder (or opposite, with females having naturally broader shoulder) and less muscular overall.

• Males with slightly wider pelvic region (decresing pelvic girth can be fatal for female, though why would it increase in males, escapes me.)

• Secondary sexual characters less pronounced:

  • smaller sized breast being average,
  • more high pitch voice in males,
  • overall, little to non-existent facial and sparse body hair in males and females

Note: The species in not wholly without sexual dimorphism but they are as minimium as they can get. (as one can see from aforementioned soft limits).

Sexual dimorphism in humans is down to many factors and in many cases to cope with survival pressure.

One example, both sexes increased in stature, but females grew disproportionately larger (to an extent decreasing stature dimorphism) because of complex parturition. Tall height of females helped in increasing pelvic size without increasing valgus angle, giving them an opportunity to accommodate heavier babies which is advantageous because of their largely altrical (helpless) nature due to bigger brain size of newborn infant.

Another dissimilarity is in body mass: In a data sample from 129 nations men averaged 14.2 and 12% heavier than women. Women have considerably more fat and men have more lean (and muscle) mass . It is less pronounced in infants and children and increase dramatically with puberty (Lean body mass comprises organ, muscles, bones, etc)

For better understanding, visit this article on sexual dimorphism.

In account of all this, despite some complications, the anatomical and morphological body plan of humans is extremely efficient and had survived various selection pressures.

Taking note, is there any way a human-like species would evolve with decreased sexual dimorphism and more similarity in stature, weight, body structure, facial feature?

If so, how would it effect the behavioral, social aspect of this species?

Edit: I am asking, what would promote minimum/reduced sexual dimorphism in a hominid-like species (such that breast/ penis are only reliable external sex indicator of a person). Would it be fatal? How would it effect us?

(Don't know why the text in 'Edit Summary' section isn't showing up on main page.)

Answer 1

You want groups that live and work together, which maximizes female cooperation, making them more sexual will also help.

Confining this to a hominids makes it a lot easier, it removes a lot of factors. Two things correlate very strongly with sexual dimorphism in primates, the less social they are the more dimorphic and the more males can monopolize females the more dimorphic.

So first they need to be more social, that is they need to live in larger groups and not separate to do task during the day, the longer the whole group is together the less dimorphic they will be. Why because females gang up and prevent males form monopolizing a female. We see this bonobos any male that tries to monopolize females gets driven off by a group of females. Being aggressive and monopolizing a female works thus evolution encourages it, as long as it works, otherwise it is a waste of resources.

If you look at our two closest relatives Chimpanzee and Bonobo, Chimps are more dimorphic than us because the males and females feed separately far apart, so a male can monopolize a female this can be everything from outright rape to driving off competing males. In gorilla this is taken to the extreme with a huge male driving off all other males.

Bonobos have less dimorphism because they all feed close together and females can almost always protect each other. Male A tries to monopolize female A and gets driven off by female B, C, and D. Male A tris to drive off Male B and gets attacked by female A, B, C, and D and likely male C and D aswell. Aggression no longer helps male A reproduce so being larger than females offers no advantage. If being larger or mor muscular helps you get resources then that favors both sexes getting larger, not males more than females. This is why so much of dimorphism in humans are things that aid in aggression, size, musculature, a deeper voice which makes you sound larger, everything else is very minor. there is even a proposal that narrow hips in males may be to make your upper body LOOK bigger, since it does not appear to offer any other advantage.

The high sexual activity also helps since a female is going to have multiple partners no matter what the male does. There can be benefit if a male manages to monopolize a female but it is so difficult that there is not a strong evolutionary pressure to invest the resources to be dimorphic. The one thing you have to watch out for is male to male completion in bonobos violent completion is prevented by females but nonviolent means like penis fencing works, so you do have large penises proportionally. This may not be issue in an upright species though, since penis fence would be a LOT harder. Because a female will have many partners there is little benefit for a male to even try to monopolize. If a male wants to show off they do it through being more social not more aggressive.

Humans fall in between, because we are more social than chimps but we evolved in terrain that requires the group to split up to forage. Put humans in a place were they groups either has to stay together or food is concentrated in specific areas so they will naturally stay together and you should see less dimorphism. Communication helps too, humans can communicate past events easily which also helps punish aggression, but not as much as prevention. Without good communication humans would likely be more dimorphic than we currently are. Basically the more other females can tell when a female is being monopolized or when ingroup aggression is happening and the faster they can intervene the less dimorphic they should get. This can even help with things like facial hair which may have evolved to make it easier to tell male and female apart at a distance (lot of speculation here), but if individuals are always around each other display matters less since you know sex because you know who is who, and hominids are not starting with any easy to exploit display features. You can never get rid of dimorphism completely in an a species where mating involves a choice (do I have sex with A or B) but you can minimize it.

Body hair varies pretty widely in different human populations so you can basically handwave that. there are a lot of ideas about that but a lot of it comes down to climate, warm weather favors less body hair.

In short Females should prefer social males and they should always be able to help each other protect this choice and punish aggression. Also the more sex they have the less attempting monopolization helps.

https://www.eva.mpg.de/3chimps/files/apes.htm

https://carta.anthropogeny.org/moca/topics/sexual-body-size-dimorphism

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7337577/

Answer 2

You ask small differences

Compared to the animal kingdom, humans are already pretty similar. Height, weight, strength and all that are actually pretty similar. We notice the differences as we are taking extreme attention to such factors.

Many of these factors can be corrected over time by just slight adjustments in what is sexually desirable and what is expected. Women can look much stronger if they are expected, or are even desired to do strength tasks alongside the men. Doing such tasks and looking strong can be viewed as better for raising babies.

The same goes for much of your list. The in relative terms absolutely enormous breasts in western culture is something pretty recent, and exists due to simple sexual desires. This is also not present in every culture. If we tweak things further, men can have wider hips. Fat people used to be preferred when we had less food available, as it showed affluence. The extra fat on woman could thus be preferred in men, causing more similar fat. Even the beards is probably a sexual desire that made men with facial hair survive.

The voice is easiest. We often see in men that do not follow the cultural norm that they have a higher pitched voice. A large part of that voice is cultural and not biological. As people deviating from the norm tend to care less, they do not speak in lower 'manly' voices.

TL:DR

It seems like just minor tweaks to sexual desire can solve all your problems. Humans are already pretty similar, and your requests are evolutionary speaking minor. So earlier evolutionary selection in some useful and less useful traits is all that is required.

Answer 3

Sexual dimorphism in humans is down to many factors and in many cases to cope with survival pressure.

And there is 33% of your answer, 33% is in female sexual selection and the final 33% is in the rearing of babies.

Some of the factors IMO trace their lineage back (evolutionarily speaking) to the fact that Women have to Carry Babies, Birth them and then nurse them. For example the amount and location of Women's fat deposits (which tend to be all-over) vs Mens (which initially cover our visceral organs, providing a good barrier against slashing attacks).

Putting aside those factors (potentially have both sexes able to Breast Feed would be a good start) - next up you've got the combination of Survival pressure and female sexual selection.

I say Combination - because Women set the rules of the game and then Men play to win. Things like establishing dominance over other males to gain access to fertile females. Women are attracted to certain things and so tend to sexually select those things - often the things they select for are the most Masculine traits.

If we look at Dating app culture (I'm loathe to use that oxymoron) but there are things like '6 Pack, 6 ft tall and 6 figure income' - Women are signalling to Men what they want.

So for your world - you need it so that the characteristics that women select for aren't exclusively Masculine. I personally think this introduces some problems for species survival - namely if you aren't particularly attracted to the group that is required for making Babies, your species is likely to die out.

The next up is Survival pressure - evolutionarily speaking, Women are much more important than men.

All we have to do is run the maths - 2 tribes go to war, the outcome is that they are left with 100 people each. Tribe A has 10 men and 90 women, Tribe B has 90 Men and 10 women. Every 9 months or so Tribe A can produce 90 children, every 9 months Tribe B can produce 10. Tribe A will eventually grow larger and take over Tribe B (assuming Tribe B don't mount an all-out assault and capture the 90 Women, but that's another story).

So, you don't want to risk your most important people in the risky pursuits of big-game hunting or armed combat - unfortunately, both of those endeavors are ones where being bigger, faster, stronger and with greater endurance tend to be significant advantages. Which means that to ensure that the Men don't get bigger and stronger, you need to make sure there is no evolutionary advantage for them to be bigger and stronger.

To recap - have both sexes able to Breast Feed means that the initial rearing children is no longer sex-dependant (but you still have carrying and giving birth to babies) Have the Women not be sexually receptive and selective to 'masculine' traits (but as above - I think this dooms your species to dying out) Remove any scenario where being bigger, faster, stronger etc. has a competitive advantage (good luck on this one)

Answer 4

The evolutionary reasons for sexual dimorphism seem to be about ancient gender roles, and the expendability of males.

With no modern civilization or medicine, pregnancy in women was a severe liability to their health and nutrition. Although human women could fend for themselves for about 8 of the nine months, it became increasingly difficult to get enough calories and sustain enough mobility to hunt and gather, and then to do the same for another two years caring for and feeding an infant.

Step up males! Thus the evolution of tribes, and the evolution of love and the male role, of providing and protecting. Males have only about 5 minute, 100 calorie contribution to a successful pregnancy. But bonding and a lack of impairment lets a male contribute protection, and food production (hunting and/or gathering) to support his offspring for 3-7 years.

Our social structure evolves around that. Males fight the battles and do the dangerous hunting and compete with each other to mate, and they can be more sexually successful if they are larger and stronger and to an extent smarter.

Women do not have to compete to mate, and that is true even in modern times, for about 98% of women, if they want to mate they can find a male willing to mate with them, for free, within a day or two.

The same is just not true for men, many men go years on end, unable to find a women willing to mate with them for free.

If you want to evolve away sexual dimorphism, then take away the need for it. Make it a non-factor in survival of the mother or father, or make something else (like intelligence) much more important.

I believe the modern world is already doing some of that work, but evolution is a slow process. Physical size or strength in a man is no longer a good measure of their success or fitness as father, and I think we see sexual dimorphism decreasing in modern times. Women are more attracted to other qualities, such as fame, leadership or earning potential.

There is a reason that male band members can get thousands of sexually willing female fans, regardless of their physical "beauty", far more than most other professions, with the exception of some actors. The attractiveness seems to be fame and some quality of "leadership". And wealth can be the same.

Ultimately this is not entirely about gold-digging, it is about female evolved and natural instincts about choosing mates that have the resources and courage to protect and provide for their own offspring.

But in a non-physical society, that protection and providing doesn't have to be about physicality, it is about other qualities. It doesn't really matter if the band's lead singer is handsome, muscular or athletic, or tall, or dressed in a suit with trim hair.

(For many women these traits still matter; as I said, evolution is a slow process.)

It does matter that they appear to be famous, and followed, with the potential for wealth. They are standing on a stage in front of hundreds or thousands of fans, and that matters.

You evolve away traits by understanding why they exist, and then eliminating the conditions that make those traits preferable instead of detrimental. If possible, make the sexual dimorphism a detriment to survival and reproduction.

Psychology evolves too, but it will follow the same pressures in time. Sexual dimorphism is not present in all animal species. Study those that don't have it, and you will see why size and muscle are less important than other physical or psychological qualities.