I'm worldbuilding a human inspired species where females are larger than males. According to the explanations I've read about why that would happen with birds of prey, dimorphism comes from:

  1. Avoiding to compete with each other for the same type of prey - larger females hunt larger prey, males hunt smaller prey.

  2. Males are primary bread prey-winners, with smaller prey being more abundant, while females do more offspring nurturing.

  3. Larger females evolved due to larger females being better at protecting the nest home.

  4. (Questionable) A ZW sex determination system in which males are the homo-gametic sex ZZ, while females are the hetero-gametic sex ZW. So if a beneficial mutation appears on the W chromosome it can't spread as rapidly as in the XY system, Genghis Khan-way, because females are limited to how many children they could bear (the same as humans).

  5. (Questionable) Larger females could bear larger children that have a fitness advantage over smaller females.

Due to the above reasons, "ZW humans" developed a culture where smaller males were preferred and in which it was the females' job to protect the settlement and occasionally hunt some large prey.

Does my dimorphism explanation makes sense from a biological point of view?

My species have live birth like mammals - they don't lay eggs.


The Evolution of Reversed Sexual Dimorphism in Owls: An Empirical Analysis of Possible Selective Factors proposes 20 different reasons as to why females are bigger:

  1. Egg size.
    Large females can lay larger eggs than smaller individuals.
  2. Follicle protection.
    Larger females provide a better cushion for developing eggs than smaller ones.
  3. Breeding increment in weight.
    Females of many species increase in weight prior to egg laying.
  4. Incubation.
    Larger females incubate more efficiently than smaller individuals.
  5. Nest protection.
    Larger females are more effective in deterring predators than smaller individuals.
  6. Territorial defense.
    Small, agile males are better in the aerial defense of territory against conspecifics than larger, less agile individuals.
  7. Feeding efficiency.
    Larger females are more efficient at dismembering prey and feeding bits to small young.
  8. Foraging interference.
    It is more efficient for only one member of the pair to hunt alert and agile prey than to have two birds moving about alerting prey and thus interfering with each other.
  9. Energy conservation.
    Energy consumption is a function of body size, thus small males might forage and provide food for the young more efficiently than large females.
  10. Pyramid of numbers of prey sizes.
    Small prey are more abundant than large prey, and small males should do most of the hunting until the food demands of the young become sufficiently great, and the supply of small prey sufficiently depleted, at which time the female must join in hunting, capturing, and delivering larger prey.
  11. Sociability.
    Coloniality may inhibit the evolution of RSD.
  12. Anticannibalism.
    Large females prevent small males, who are presumed to have less of a "parental instinct," from eating their young.
  13. Pair-bonding.
    Formation and maintenance of pair-bonds is facilitated by female dominance, and large size facilitates dominance.
  14. Role-partitioning.
    Female dominance excludes the male from incubation, brooding, etc., and forces him into the role of provider of food.
  15. Female selection of males who are good provisioners.
    Male raptors begin to provision females well in advance of egg laying.
  • 2
    $\begingroup$ Are these proper primates, or just a human-inspired humanoid? On Earth, ZW isn't extant in mammals at all. $\endgroup$
    – John O
    Jan 14, 2022 at 13:55
  • $\begingroup$ @JohnO Proper primates as similar to humans as they could be, considering the earth like ecology and ZW system. $\endgroup$
    – ZW system
    Jan 14, 2022 at 13:56
  • 2
    $\begingroup$ It sounds like you've already done your research, and the answer is that it requires very little to justify dimorphism. I'm not sure why you feel the need to ask the question. Worldbuilding tends to tell you if something is plausible more than likely, and this is plausible. $\endgroup$
    – DWKraus
    Jan 14, 2022 at 14:05
  • 2
    $\begingroup$ (1) The Y chromosome carries very few genes. That is why women can do without it. It is extremely unlikely for a beneficial mutation to be located on chromosome Y. (2) Whether males are larger than females, or females are larger than males, or both are about the same size has no relationship whatsoever with the sex determination mechanism. (3) For example, in chickens, roosters are usually larger than hens, although they use the same ZW sex determination mechanism as hawks and owls. (4) I wouldn't put much faith in an article which uses "reversed dimorphism" as if it was something unusual. $\endgroup$
    – AlexP
    Jan 14, 2022 at 14:06
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    $\begingroup$ I'm in agreement with the other guys... other than actual ZW chromosomes, nothing about this is implausible at all. Switch to XY, and you don't even have to imagine these as a subspecies of human really. We see similar levels of group variability in extant populations. $\endgroup$
    – John O
    Jan 14, 2022 at 15:19

2 Answers 2


Well, my philosophy on talking about worldbuilding is to find reasons why it could happen as opposed to why it couldn't. There are a few extra reasons why a larger female, smaller male dimorphism could be beneficial.

Human Birth Sucks

Humans' births are actually incredibly dangerous for both the baby and the mother compared to many other mammals, due to the size of the baby's head, and the strange orientation of human's hips from our relatively new body plan. The first three months of a baby's life is even sometimes called the "Fourth trimester" due to human babies being pushed out so underdeveloped to give mother and child an easier birth and better chances. This entire problem could be done away with with larger women. The bigger she is, the easier it's going to be to get out a baby with an appropriately sized head (the head is really the limiting factor, as the brain doesn't grow during life nearly as much as other parts can). This makes for a strong selective pressure for larger females.

Don't Kill your Amazons

In our society, if you had a tribe of 10 people, and all but one of the males dies... the next generation will be the same size regardless (Ethics aside, you have five women bearing children). If on the other hand all but one of the females dies, your next generation will be severely hampered. This is likely why humans evolved for males to be disposable and have more dangerous roles like hunting and fighting. However, if you could mitigate the lethal risk to you female warriors by for example, making predators or dangerous prey likely to merely leave them wounded and not kill them outright, making it a societal taboo to kill others in conflicts, or giving them remarkable ability to recover, you make this risk considerably less important. Additionally, pregnancy is no big deal if you only have a small number of your hunters/warriors out occasionally. Unlike farming or actually caring for the children, your tribe/society won't grind to a halt because your amazon warrior needs to take a 9 month time-out.


Farming often requires year-round work, but the work (especially once domesticated animals come into play) doesn't require you to be large, and being large would be a downside in the much larger amount of energy required to keep you going. One thing you shouldn't be when farming however, is bearing the next generation! Small males working the fields and driving animals means maximum effeciency of creating food. Before agriculture, the same efficiencies apply to gathering food.

Evolution is not purposeful

The most important driving force here is of course that mutations are an accident, and as long as you don't have very strong selective pressures against something, if it appears it's likely to stick around. So, you don't even need particularly good reasons for an organism to be the way it is. Contrary to how we usually speak about it, evolution is more stumbling in the dark than a careful engineering process.


It is totally plausible. All you need is a reproductive system where males do not fight with each other.

Recycled from Is there a reason for a humanoid species to be 'biologically' matriarchal with females being slightly more dominant than the males?

Your females are much larger than the males.

They are twice the size of the males, females averaging 8 feet and 600-700 lbs. The physiologic reserves of these very large bodies make carrying a human-type pregnancy (or even twins) much less of a stress on the females of your humanoids. A tiny baby lashed across her body to her breast does not slow her down at all.

These large females do carry out the hunting of large prey because the large physical size of the females make them more formidable; also the huntress intends to eat most of what she kills, bringing back the remnants for the males at home. Female children put on a growth spurt early, before they are reproductively mature and are thus valuable allies and hunting partners for their mothers.

The considerably smaller males hunt small prey, forage, and do much of the child care for the weaned young. One female will have several males or "brother grooms" to spread around child care duties and improve the survival rate of offspring.

In a reproductive system where males fight each other, males are selected for size and strength and fighting prowess. Most mammals with this system have the males live separately from the females, possibly except for the alpha daddy but even he might stay away. Having a population of large males in the same society as the females and young puts extra food pressure on the female and young because the males eat so much - bad for the kids and hungry pregnant women. But that is our system, which we had to do because human males in groups are the original weapon of mass destruction, and if your opponents have that you must as well.

In a system where males do not fight each other, it makes sense for them to be smaller - this is the case for birds and fish and insects and snakes. They do not need to be as big because their bodies do not have to support the demands of producing offspring, just sperm. Having small, intelligent, creative, nonaggressive males to glue society together makes sense. The physically demanding work of carrying pregnancies, lactating and feeding babies, and overcoming large prey can be the job of the females

I humbly offer the system I proposed here: What would a matriarchal intelligent species look like?

Large plain females assemble at the lek every year. Males compete but not with fighting or butting heads - they dance, sing and look fantastic. Whoever puts on the best show gets to make most of the babies that year, after the show.

Of course there will sometimes be a female who is attracted to the funny looking crooner with the homemade beads in his beard who just sways back and forth as he sings. It takes all kinds. And as it turns out, those two are the parents of your protagonist.


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