In what evolutionary scenario can a hominid species evolve to have males and females of equal size and strength?

Question

Is there any possible way that a hominid species could evolve into a human-like race that features males and females of the same strength and build? I still would prefer there to be an aesthetic differentiation between the two sexes, but when it comes to physical performance and size, both sexes would be equal.

The only way these hypothetical hominids could be told apart was through other instances of sexual dimorphism. Differences I had in mind (if they're possible) would include-

1. Different hip and shoulder sizes between males and females, with males having broader shoulders and females having broader hips (Kinda like us).
2. Females have visible, pronounced breasts whilst males don't (also like us).
3. Facial features are slightly more rough and robust on males, such as the chin, cheeks, and brow, similar to the dichotomy of facial features we humans have between both sexes (again, like us).

Would it be possible for a hominid species like this to be capable of existing, and if so, what environmental pressures would it take to make them over the course of millions of years?

Answer 1

Same size requires no explanation.

humans are already close, there are plenty of species that do not show size differences, you can just say they are the same without explaining it and people will not be able to say anything unless you undermine it with behavior.

You have it backwards a size difference needs to be justified (although not difficult to do) the same size (monomorphism) is the default.

As long as the size difference offers no mating advantage they will be the same size. Essentially you need to minimize ingroup violence.

Large male size sexual dimorphism occurs when makes can dominate/control female mating, either by driving off other males or physically dominated the females. Bonobos have the least size difference in the great apes because females cooperate enough to minimize this advantage. Basically they usually gang up to drive off any male that tries. However males can drive off other males and occasionally catch females alone. Your hominids just need to be better at cooperative rape prevention, and have little male to male violence. Perhaps by feeding as a group instead of individuals or living in conditions that allow members of the group to stay in contact constantly so females and males can see any aggression happen.

This means mating has to be entirely voluntary, rape as reproduction needs to be impossible. You can help this by making estrus completely hidden, basically it need to be impossible to tell is a female is in a fertile portion of their cycle. Ideally physical conflict between the same sex can't lead to an mating advantage, so you probably want a a more cooperation based social structure like bonobos.

You may still end up with some minor differences due to the requirements for hominid pregnancy, any in something intelligent you will always have many competing mating strategies so some difference may exist, BUT you can make them small enough that any difference is swamped by normal variation.

Answer 2

The two main factors would be:

• sexual selection;
• reproductive success non-dependent on physical strength and body size.

Sexual selection is natural selection based on preferences for sexual partners. It is speculated that sexual selection is the main reason for higher-pitched voice and sub-optimal fat distribution in human females.

Your hypothetical hominids need to favour traits that you listed as desirable and select for them. As time goes by those traits will become dominant in the general population.

Reproductive success refers to an individual's ability to produce offsprings that become part of the breeding population once they mature. If greater strength and/or bigger body size result in reproductive success your species will eventually evolve to be bigger and stronger. Sexual dimorphism will occur if requirements for reproductive success differ between sexes.

A specific method for exclusion of strength and body size from traits important for reproductive success will depend on your species physiology, habitat, and social structure. Some examples of things that can affect reproductive success:

• hidden ovulation (like humans have) increases the necessity for the monopolisation (not sharing one's mate with others) of females by males. As seen in the comments this point is a bit tricky, so I will list some additional relevant points to consider:

  1. If males do not know when females are in heat, they need to avoid sharing female with other males if they want to make sure that the offsprings are theirs. Therefore, the necessity for monopolisation in terms of reproductive success. This does not mean, however, that hidden ovulation on its own will inevitably lead to sexual dimorphism.
  2. Monopolisation can be achieved through various means, including non-violent ones such as marriage. Violent means are most likely to lead to sexual dimorphism related to strength and size, while non-violent means may promote other traits (for example, intelligence).
  3. Hidden ovulation strengthens the position of females as choosers because it helps to conceal the fact that the regular mating partner is not the real father of offsprings. This may affect sexual dimorphism depending on the preferences of females.
  4. Hidden ovulation in isolation from other factors does not necessarily lead to sexual dimorphism. Its role should be examined in the context of all other factors.
  5. Hidden ovulation does not lead to monogamy or even stable mating partners. The majority of mammals that have concealed ovulation are promiscuous. One of the theories suggests that hidden ovulation evolved in order to increase paternal investment and decrease infanticide. This may contribute to greater sexual dimorphism if biological fathers and de facto caretakers are different males (for example, females may choose more aggressive males as fathers and less aggressive males as caretakers).

• a habitat full of dangerous predators and sex roles where males are the main protectors will favour strength (for fight) or agility and stamina (for flight).

• social structures with polygamous mating will most likely favour strength if violence is the main method of harem protection.

Another important aspect is male-male interactions. Species where male combat is common tend to have males stronger and bigger than females. If your male hominids do not use violence to win against other males differences in strength and body size will be less pronounced.

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Please do not see this answer as a blueprint for the evolution of your species. I just listed some examples of factors that can influence sexual dimorphism. Please also note that there is no one simple solution. There are always numerous factors affecting evolution. You should look at as many of them as possible and try to see how they interact. It is absolutely normal to have several contradicting forces shaping the evolution of a species. The outcome always depends on a combination of various factors and their interactions.

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I thought about it a little bit more and it seems that it might be hard to keep the same aesthetics as modern humans if you want to equalise strength.

One of the main reasons for the greater physical strength of males compared to females is body composition: Males have higher muscle-mass to body-mass ratio (men have more muscles than women when adjusted for body size and weight). If you want women to have body curves and attractive big breasts you will have to sacrifice muscles and exchange them for body fat.

Androgynous appearance might be more conducive to your ideal of equal physical strength and performance.

Answer 3

The species just needs to be strictly monogamous. There’s a rule of thumb that the less monogamous a primate species is, the bigger the difference in size and strength between males and females.

To quote Wikipedia:

A strong association between polygynous mating system and dimorphism in primates has been observed. Monogamous species tend to show lower degree of sexual dimorphism than polygynous species, since monogamous males have a lower differential reproductive success. Monogamous mating system seems to account for minimal dimorphism in hylobatids, in which females are codominant with males.

Answer 4

The sexual dimorphism you are trying to avoid (size) is based primarily on the advantage that greater physical strength gives a man, allowing him better access to women than he would otherwise be able to. The strictly aesthetic differences can be explained by simple sexual selection (men like bigger breasts and women like wider shoulders).

There are about 5 mating systems: monogamy, polygyny, polyandry, polygynandry and promiscuity. Polygyny and polygynandry tend to favor physically stronger and larger males that will monopolize mating access to females, whether individually (polygyny) or as group (polygynandry), you want to avoid those in the evolution of their hominids. Another aspect that correlates with these stronger males is sexual coercion, where the male will physically attack or intimidate one female in another, making her more reluctant to mate with other males, this is more associated with polygynandry or promiscuous mating, as it require the females to have access to multiple mates for the intimidation to result in a advantage.

Therefore, in order to minimize the gender disparity, you must focus on strict monogamy or promiscuity, with either an egalitarian or female-dominated social hierarchy to avoid sexual coercion. The strict monogamy is less likely, as it is usually not associated with mix gender groups and if extra-pair copulations occur it may favor higher status (usually the most dominant or strongest, but also those with more alliance/friends), which may lead to the dimorphism in size. Lifestyle also plays a role as the traits associated with physical strength and size may also make them better hunters, and individuals that can bring more food to the group may be attractive for the females, which can create a quasi-harem for some males in a promiscuous systems.

Overall, there is no recipe that leads to a specific result when it comes to evolution, but my suggestion would be promiscuity with a female-preference for less threatening males, not stronger or larger than them, but keeping the general preference for masculine traits in order to keep the dimorphism in other areas.

Answer 5

There's actually a Wikipedia article specifically on sexual dimorphism in non-human primates. According to the article, for gibbons, there is very little size disparity, and, for lemurs, females are larger. The article goes through the standard understanding of why the most common pattern among primates is for males to be larger, but it does not, for instance, trace what was special about the circumstances of gibbon evolution that led to less size dimorphism for them.

My understanding of the state of evolutionary science is that it is usually impossible to know these particulars on a fine-grained scale for a given species. Scientists are still researching how present-day social structures of these animals plays into sexual size differences, and apparently it's still rather mysterious. From this abstract about lemurs I see that some species of lemur are more monogamous, and others are polygynous, but even in the polygynous species, the males are not bigger, and researchers don't know why.

I would recommend researching gibbons and lemurs more deeply. If you can't get access to the whole articles like the one I just mentioned, you might ask the author if they will send you a copy. Researchers like it when people are interested in their work. I would just say to be polite and brief, tell them what you are working on, and also be aware of what you don't know.

My feeling as a reader is the model you as a creator come up with for why this human-like species is the way it is will be a part of what gives the story life and also a part of the social commentary that will inevitably be present.

Answer 6

As was already mentioned, size and strength difference in humans is relatively small. If you want to reduce it further, you need one or both of two things:

1. reduce mortality rate. Either make your species naturally healthy (most authors actually do this subconsciously - out of many characters who died in, say Game of Thrones, how many died due to disease, plague, NATURAL water/food contamination etc? ) or make them healthy through magical healing.
2. eliminate or massively reduce various pregnancy related problems either by making your species lay eggs (like reptiles) some other way.

Those two things would reduce the amount of time females need to be pregnant to grow the population, leaving them more time for other activities. Essentially you need to achieve what was achieved with modern medicine, but through natural means during your species evolution.

Why? The pregnancy historically made human females vulnerable and not able to perform hard labour or other physically demanding tasks for at least 4-5 months (first due to increased risk to the baby, then due to female body changes). Then the birth that is not easy today, but was almost traumatic experience throughout history. After that, females were still weak and needing protection for a further few weeks. What's more, pregnancy and birth was lethal - in XVIIIth century (and today in areas with very poor medicine) death ratio per pregnancy was about 1%. In war torn areas today it reaches 2%. This means that out of 100 females 5-10 died due to pregnancy related health issues. On the other hand massive overall mortality rate and especially children mortality resulted in the need for females to go through multiple pregnancies to sustain population rate. Remember that you need 2 kids survive till they have children per family on average to sustain population and more to actually grow. With Pre-modern child mortality this means 5-6 pregnancies on average per female - this means an average human female would be pregnant for 1/5th of the 20 years period between 15-35 years old. If you add the time to actually raise those children... Well you do not get much time for hunting or warfare.

This in turn result in all kinds of social rules and taboos that construct "traditional" roles for sexes, as the true role of the society in early stages of civilization is to raise as many children as possible... That's why farmers won against hunter-gatherers, simply by sustaining larger population from the same area.

So, if you want females of your species be more equal than human females in physical appearance to males you need to somehow reduce the impact of giving birth and raising children on the society. Egg laying lizard could be an answer, though those often (fe crocodiles) have females larger than males...

Answer 7

Caloric restriction and chronic disease.

Adult height, nutrition, and population health - a fine overview.

Synopsis: When humans starve as children, final body height and mass are more similar between men and women.

You could do this with the genotypes now in the human population including your own (assuming you are a human; no offense meant if not). When humans have caloric restriction and chronic disease in childhood, growth is slower for both sexes and so final heights of men and women are on average closer. That is true today and definitely historically. Menarche is delayed and so girls keep growing for longer. Overall body sizes at adulthood are more similar. Women still have secondary sexual characteristics which you want but men and women are more similar in muscle mass, fat content and especially height.

One can see this today in families who have emigrated from resource poor regions, or who reside in regions where disease prevalence / resource availability has changed in historic times. Children growing up under circumstances of good nutrition and less chronic disease are taller and more robust than their parents and grandparents, and male / female heights diverge more, I suspect in large part because females start menarch earlier and stop growing earlier.

Your people are the same people as we are, but their lives are harder. It does not take much imagination to see how that could be, because in some places it still is.

An interesting thing for speculative fiction would be a circumstance where calories were restricted but there was not an excess of chronic disease - a circumstance which is uncommon today but some people think may have existed for hunter gatherers living in low population densities in the remote past. Caloric restriction substantially increases the life expectance of rodents and nonhuman primates. Maybe your small people eat little but live long?

Answer 8

This all seems a bit serious, but the solution clearly lies in sociological change. Men's greater size is a direct result of more extreme physical effort, viz too much fighting, hunting, running, jumping, etc., throughout their evolutionary history.

If women took up some of the athletic slack, and the men sat around watching soap operas, hosing down the kids and engaging in meaningful conversation with like-minded househusbands, the difference would diminish over time. Maintaining their appearance would involve reducing calorific intake, increasing the effect.

Men would end their day by asking their dominant partners how the hunt or the battle went, assuming they returned from it. If not, they could be taken in hand by the victors. They would become accustomed to being judged by their appearance and having women fight for their favours, all while improving their domestic skills. Most men would happily go along with this. They just need to be asked.

Once women start taking bar fights and football more seriously, the tide will turn, mark my words.