Evolution: Why are females so much weaker?

[Kazuaki Shimazaki]

I mean in the physical sense, of course.

Our recent thread on females in the military got me wondering.

Why are females weaker by such a great margin? Yes, one can understand some structural compromises like the different pelvic structure (which AFAIK is not so good for running) because of the need to carry a fetus-production facility inside (and the need of said fetus to get out of said facility).

But what about the others. They say the increased fat percentage is a reserve for pregnancy in starvation, but calories not a healthy body make. It is difficult to imagine a fetus coming out in any kind of decent shape from a mother that deep in starvation. Not to mention the chances of the mother dying trying to give the birth - and without the mother and her breastmilk the chances of a baby surviving is very low in the prehistoric times that made up most of our evolutionary history.

What about the smaller size - what good does that do? If anything a woman arguably needs to be bigger to comfortably accomodate everything. AFAIK a small woman even has trouble pushing her kid out from her womb.

So, are there any good evolutionary reasons for this? Discuss and Thank you.

[Sea Skimmer]

But what about the others. They say the increased fat percentage is a reserve for pregnancy in starvation, but calories not a healthy body make. It is difficult to imagine a fetus coming out in any kind of decent shape from a mother that deep in starvation. Not to mention the chances of the mother dying trying to give the birth - and without the mother and her breastmilk the chances of a baby surviving is very low in the prehistoric times that made up most of our evolutionary history.

Think of the fat as a capacitor rather then a battery and it makes perfect sense. It guards against temporary food shortages slowing down growth, not starvation.

[wolveraptor]

There have been two evolutionary trends regarding sexual dimorphism in sexually reproducing animals: one that sees females grow larger to defend their young (common in birds and reptiles), and another which sees them grow weaker (typical in mammals). I don't have a real reason for why females are so, but I'd just like to point out that the reason may have some taxonomic basis.

[Shrykull]

I mean in the physical sense, of course.

Our recent thread on females in the military got me wondering.

Why are females weaker by such a great margin? Yes, one can understand some structural compromises like the different pelvic structure (which AFAIK is not so good for running) because of the need to carry a fetus-production facility inside (and the need of said fetus to get out of said facility).

But what about the others. They say the increased fat percentage is a reserve for pregnancy in starvation, but calories not a healthy body make. It is difficult to imagine a fetus coming out in any kind of decent shape from a mother that deep in starvation. Not to mention the chances of the mother dying trying to give the birth - and without the mother and her breastmilk the chances of a baby surviving is very low in the prehistoric times that made up most of our evolutionary history.

What about the smaller size - what good does that do? If anything a woman arguably needs to be bigger to comfortably accomodate everything. AFAIK a small woman even has trouble pushing her kid out from her womb.

So, are there any good evolutionary reasons for this? Discuss and Thank you.

No real comment, but actually I saw in a show on disc science that a man could actually a carry a baby to term if it was a ecotopic pregnancy http://en.wikipedia.org/wiki/Ectopic_pregnancy that it could happen in the abdominal cavity instead of a uterus

[Zero]

It may have to do with the polygynous past. Monogomy is a reasonably new idea. In the past, males were expected to have several wifes. It's still so in many areas in the world. When most male animals want to take several women as their mates, they have to fight other males for the rights to the female, so males need to be bigger to win in a fight. If the males are larger in this situation, they're more likely to propogate, since they have several females with which to do so, so males in general in the species become larger and stronger, whereas women change slightly less.


If this went on for long enough, an uneven gender split would develope, because it takes more energy to create a male than a female, if the males are generally bigger. This is possibly also why the gender ratio isn't quite 1 to 1, and is slightly in favor of females. Of course, in humans, it hasn't been going on for very long, I'm sure, or the gender gap would be significantly different, with females being both weaker and dumber, since biologically their most important function is merely bearing children.

The monogomous notions in most western nations of the day probably actually have gone a long way in women's rights. In most polygynous societies, the role of women is significantly reduced compared to that of men.

Most of this is idle speculation, however. It's just something that makes sense to me. Sexual selection is usually responsible for sexual dimorphism in some way.

[Drooling Iguana]

My guess is that it's because males are far more expendable than females, and therefore better suited for the more dangerous tasks, which also tend to require more strength.

[Raw Shark]

I wouldn't say that lower muscle mass necessarily occurred as the result of evolutionary pressure because it provides an advantage, so much as a side effect of the hormonal differences that regulate the reproductive system. Additional testosterone in the form of steroids enables women to gain muscle mass easily, at the cost of throwing everything else out of whack.

[Broomstick]

Turn the question around - ask why men have to be stronger.

Because a female human is actually a pretty big, strong animal in her own right. In most human habitats, we're on the upper end of critter size. Yes, there are some bigger things out there - elephants, lions, etc. - but most species are smaller than us. Female humans are strong enough to forage for food, hunt game even if it isn't big game, haul their kids around, farm, and do all the physical labor required to exist. Among hunter-gatherers they typically supply the vast majority of the food consumed, so a man is more likely to starve without a woman than vice versa.

So why did evolution dictate that men expend more resources on bigger muscles?

Well, there's the fight-for-females aspect. Male humans do engage in dominance battles of several sorts, and certainly in non-western, non-modern societies the more dominant males typically had more females to mate with and tended to leave more offspring. Since in a "fair" fight the physically bigger, stronger male tends to win, evolution favors a size and strength increase in males.

Beyond that, though - vegetarian protests aside, humans are hunters. They may not derieve all of their food from hunting, but procurement of meat is a valuable source of protein and fat in non-modern societies. Lack of good protein might not kill you, but it can stunt the growth and health of developing children and make it harder for the body to maintain itself. In cold climates, or when the body is being called upon to do hard physical labor - typical situations, again, for non-moderns - fat is a valuable source of concentrated energy. Bigger, strong men tend to make better hunters, particularly better hunters of big game. Big game might be rarely caught, but procurement of such a resouce not only provides valuable nutrition but the sharing of meat resources also helps cement social bonds as well. This is seen in a primative way in chimps, who will exchange meat for sex and will cooperate to obtain meat in ways they don't bother to cooperate to obtain fruit. Pooling of resources and food-sharing is a prominent feature of all human societies, and non-moderns frequently have very specific formulas and rituals for dividing up the rewards of a succesful hunt.

Another factor is that, even if we are sizable animals ourselves, there ARE things out there that are bigger than us, and some of them will eat us if they can. Humans have a biological division of labor, with woman normally feeding and tending the young children and men defending their mates and offspring. A woman who is attacked will certainly try to defend herself, and/or her children, but in both modern and non-modern societies group defense has always fallen to the men under normal circumstances. A bigger, stronger defender is a better defender.

So, between the man-on-man dominance battles, the hunting, and the need for group defense evolution have favored stronger males, and stronger males tend to leave more offspring.

Now let's go back and look at the women. Women tend to gather plant life as their major food contribution. You don't need immense strength to pick fruit, harvest leaves and seeds, and dig for tubers. You need some stamina, but each individual effort is not that much. It's hard to say when we started making baskets and bags to allow people to haul more than their hands could hold, but in non-modern societies women frequently do carry substantial burderns over long distances. They tend not to move as fast as the men can, but then, since they aren't chasing games or fighting off predators as often they don't need "burst" strength. Thus, there is less evolutionary pressure to develop brute strength. There IS evolutionary pressure to develop the manual dexterity to make baskets, bags, and pots to carry things in, and in fact, on average women do tend to have a greater manual dexterity than men. This is also why women are more prone to joint injuries - men have traded off some of their flexibility for more stable joints, which are needed for their greater physical efforts. Women have more flexible joints, but they aren't as stable and more likely to fail under load. This is why so many elite women athletes have problems in this area - their muscles are making greater demands on their joints than was called for in the past.

Then we get to baby-making... Humans have huge babies for their size, our pregnancies are long (chimps and gorillas have, if I recall, 8 month gestations and their babies are smaller in comparison to mama's size than ours are), and then there's the lengthy breast-feeding and childhood to deal with. Ask any woman who's been pregnant about the fatigue involved in growing a baby. There's a lot of effort expended for every one human child.

Now, any organism has but a limited amount of resources to work with. After basic maintenance, there's only a small "allowance" left over for growth and development, not to mention the problem of how to divvying up resources for basic maintenance. For instance, human brains and vision consume an enormous amount of resources. Both require lots of oxygen and lots of high-quality fuel to run. That's why starvation in childhood can damage the intellect and also why, if you're deprieved of oxygen, your vision degrades very rapidly while other senses such as touch and hearing are maintained a bit longer. Humans have invested an enormous amount in their brains and vision. Those resources are unavailable for other parts of the body. Oxygen used by the brain can't be used by the muscles. If necessary for survival some oxygen can be diverted to the muscles but the results aren't always pretty - climbers on Mount Everest, where oxygen is at a premium and the body needs it to keep warm and keep moving, frequently get stupid because their brains don't get the resources they need to function at their best. Over the course of millions of years, as our ancestors came to rely more and more on their brains for survival, we might have traded the strength more typical of primates for increased brain power. We had only so much oxygen and energy to work with, and when the brain's needs were taken care there may not have been enough left over to build and maintain a chimp's level of muscle strength.

Anyhow, as we have seen, there is evolutionary pressure for men to "spend" their "allowance" on size and strength. Woman, however, have to build and maintain a significant reproductive apparatus. The species depends upon it. The resources and energy expended on a womb and functional mammaries can't be used for anything else. In fact, women with better reproductive systems, even if they are less physically fit otherwise, will tend to be favored if those better wombs allow them to leave more living kids behind them. The strongest muscle in a woman's body is her uterus, and given the importance of successful childbirth to the species, it's no surprise.

In order to support that long gestation and lactation (humans nurse their children an awful long time!), there is the problem of adequate food stores. Having just gone to all the trouble to evolve a kick-ass reproductive system and carry a baby for nine months, then squeezing it out, it would be a goddamned shame to have the milk dry up and the kid die because you were short of food for just a week. Hence, women have food reserves in the form of fat, which accounts for their more curvaceous shape. They convert more of their food to fat, rather than muscle, and because they have more natural insulation they don't have to burn as much fuel to maintain their body temperature, further conserving energy. In starvation situations, men die faster than women. In cold, men also freeze to death faster. This is somewhat offset by men tending to obtain and eat more calories from sources like hunting, and being more physically active, but that doesn't change the underlying situation. But, again, food converted to fat stores is food not converted to muscle or used for muscle maintenance.

Remember, we have that loooooong childhood. It's not just that a woman has to survive pregnancy and lactation - she has to survive years beyond that to give her child a real chance at survival. So these adaptions that are beneficial for pregnancy and the immediate aftermath also favor female survival over the long haul, by making her more resistant to starvation and cold. I believe, although I can't provide a cite, that women's bodies devote more to maintenance and upkeep than men's do, but it might well account for the tendency for women to outlive men when some of the pre-modern hazards are eliminated - it's like our machinery gets more frequent oil changes or something, allowing it to work longer before final breakdown. But that sort of maintenance requires energy and resources, which can't go to making a bigger skeleton or more muscles. It's more important that a woman be around for 18 years, even if the most she can lift and carry is, say, 80 lbs, than she be able to lift 200 lbs over her head or run a mile in 4 minutes. It's not that men can't care for children - they certainly do - but it's like saying there's no reason women can't hunt. They do hunt - but it's not as much a primary activity as it is for men. In pre-modern societies men take over childcare only when the woman is absent (i.e. dead). Likewise, women, particularly those with children, tend to leave hunting to the men although they will hunt if men are absent and the food is needed.

That ability to step out of the usual biological role, by the way, probably is also an adaption to our long childhoods and extremely helpless newborns. Men with the capacity (even if not the inclination) to change diapers, so to speak, and women willing to hunt if need be, are more likely to successfully raise offspring than those who adhere to ridgid roles that would allow a child to starve or die of neglect because they aren't willing to take on the duties normally falling to the other gender. Humans are also unique in that they will knowingly adopt unrelated children and raise them. Other animals can be tricked into thinking a baby is their own (farmers have long done this in attempt to save animals whose mothers die in birth) but only humans adopt in the sense of taking in strange children and raising them as their own, even where there are obvious physical differences and no biological relationship other than sharing a species. People willing to do this may pay a price in the form of resources that might have gone to their own offspring but don't, but in a group where this sort of adoption takes place it provides a sort of insurance that if they die someone else will successfully finish raising their children. Groups that do this probably had an edge over groups that didn't, and thus it's a common human trait.

In sum, there is a strength disparity between human males and females because, hypothetically, the resouces that women put into reproduction men use to increase size and strength. The humans that had this sexual dimorphism won the evolutionary race by leaving more children behind and eliminating the competition.

[Fire Fly]

My answer would be that males are expendable and females are not. Females require enormous energetic investments whereas males do no. All of that extra available energy, in males, is diverted to getting larger.

[Ariphaos]

Humans are actually fairly weak. An average bonobo female is far stronger than most human males. Except for upper body strength, female physical characteristics tend to come within 10% of males (or at least can). It should be noted that, for a woman's mass, her leg strength is actually often stronger, and her long-distance running speed is extremely competitive.

Humans have evolved for efficiency and endurance, to the point where, in long-distance races, humans can outrun horses. We evolved as travellers.

Male strength is a necessary inefficiency to provide additional protection, hunting ability, and, of course, sexual selection. The females evolved even more traits for endurance and child-rearing support instead.

[Master of Ossus]

It's consistent with other slightly polygamous animals to have males between 5 and 20% stronger than females, and also slightly larger.

[wolveraptor]

Strange that no such characteristics or behaviors arise in birds.

[petesampras]

Humans have evolved for efficiency and endurance, to the point where, in long-distance races, humans can outrun horses. We evolved as travellers.

Isn't this because two legged running is more efficient over long distances, rather than any superiority of human endurance?

As for the original question. Males in a large number of mammal species are bigger and stronger than females. The greater the degree of polygamy, the greater the difference as males have to compete harder to beat other males.

[Patrick Ogaard]

Strange that no such characteristics or behaviors arise in birds.

Among raptors, the characteristics are commonly reversed, with the females being big and buff compared to the males.

[wolveraptor]

That's what I said.

one that sees females grow larger to defend their young (common in birds and reptiles)

[Patrick Ogaard]

That's what I said.

one that sees females grow larger to defend their young (common in birds and reptiles)

That would be the one line I didn't catch. Pobody's nerfect.

Regardless, the ostrich would be one illustration of the avian version of the "big male" trend. Male ostriches are substantially taller than females, and a trifle heavier (by very roughly 15%). And, not so coincidentally, ostriches organize their reproductive behavior in a manner not that different from deer, or the majestic and tasty wild boar.

[Drooling Iguana]

Strange that no such characteristics or behaviors arise in birds.

Birds lay eggs. That changes everything.

[petesampras]

Strange that no such characteristics or behaviors arise in birds.

Birds lay eggs. That changes everything.

Absolutely. Bird eggs need to be kept constantly warm by a parent, which is why the majority of bird species are monogamus, in stark contrast to mammals. The root of superior strength and greater size in males follows mainly from polygamy.

[Ariphaos]

Isn't this because two legged running is more efficient over long distances, rather than any superiority of human endurance?

Yes, it is slightly more efficient, but so is sweating, the ability to have a variable gait, and in general lighter bone and muscle mass.

[xerex]

There have been two evolutionary trends regarding sexual dimorphism in sexually reproducing animals: one that sees females grow larger to defend their young (common in birds and reptiles), and another which sees them grow weaker (typical in mammals). I don't have a real reason for why females are so, but I'd just like to point out that the reason may have some taxonomic basis.

Birds and Reptiles lay eggs. mammals give live birth. I suspect that has something to do with the weakness or strength of the female