We even see some multi-male multi-female troops of mountain gorillas. And while chimps and bonobos have similar gender-mixed troops, the alpha female is the dominant one in bonobo society, not the alpha mail (as in chimps).
In general, animals that produce large amounts of unnurtured offspring tend to have larger females so as to lay as many eggs as possible, like arthropods, fish, amphibians, and many reptiles (but exceptions exist, like crocodiles and lizards).
There are many mammal examples, though it tends to come down to their unique history.
Baleen whales have larger females because fatter = bigger, healthier kids, more likely to survive in the cold ocean and to live longer so as to see them to maturity.
I’m not sure if there’s an agreed upon explanation for why hyena society is so aggressively matriarchal, or why female hyenas are so masculine. One idea I read is that it takes a long time for the bone crushing jaw strength to grow in, so mothers had to take care of them longer and they had to evolve to be more robust than usual.
I think the most popular explanation is resource partitioning so as to prevent overlap. Small males are better to go after quick, agile prey which are quite numerous. That way they can feed the family while the female takes care of the nest. When the female does hunt, it goes after more sluggish prey.
For other smaller male birds, an explanation is that sometimes the females prefer more agile mating displays, so large males are selected against.
One peculiar example are phalaropes.
Figure that one out.
Bringing it back to humans, this could be dangerous given the possibility of cultural assumptions but I think in our case it’s interesting that men, despite being larger, would be the drab ones, while women have the peacock tail. Certainly men show off in many ways, and collect gifts and ornaments to impress mates and make them stand out from the crowd, but they don’t have to be handsome to have high status. Meanwhile, men place high value on the appearance of women, often to the exclusion of most other traits, and in most cultures women spend a lot of time artificially enhancing their face and hair. And as far as I know women’s breast size being a result of male sexual selection is still the go to hypothesis (human breasts are ridiculously large compared to other primates).
Emphasis added. That second part is your cultural bias, and seems to be much more of a recent cultural phenomenon rather than something deeply rooted in our species’ make-up (pun intended).
[QUOTE=Der Trihs]
As I recall we’re middling. It seems to be related to how prone the species is towards a one-male-and-a-harem lifestyle; the more a species tends that way the bigger the males tend to be relative to the females, since they have to be big to fight off rival males.
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[QUOTE=Ahunter3]
That argument doesn’t parse. Somehow it reminds me of the person who said Jesus of Nazareth was taken to Golgotha to be executed, Golgotha being a word meaning “the place of the skull”, it having been given that name because Jesus was taken there to be executed.
[/quote]
I mean it doesn’t logically follow that the reason for the males collectively speaking being larger is that they have to be bigger to fight off rival males. They (collectively speaking) ARE their own rival males! If they all stayed non-dimorphically small they’d be just as adequately equipped to fight off each other as they would be if they (collectively) all got to be substantially larger than the females, since the “rival males” (each other) would, in either case, be of comparable size.
I can’t defend why it reminds me of the convoluted story of Golgotha, it just does 
ETA: in bio class I was introduced to the notion that female humans are smaller because lower bodily requirements means more likely survival and successful giving of birth etc. Human babies are large and hungry in the womb.
That would require either no evolutionary pressure, which is silly, because one-male-and-a-harem obviously is a situation with evolutionary pressure. Or it would require no potential genes causing dimorphism, which is also silly, since we know there are plenty of dimorphic species.
Sure, you could accuse Der Trihs of not communication clearly that “have to be big to fight off rival males” actually means, “will trend towards increasing size as those genes are favourable for fighting off rival males”, but in my opinion that is quite obvious in a discussion like this and you’re guilty of ignoring the basics of evolution to make a pedantic point.
I see your point, but I think the counterargument that would typically be made is that natural selection works on the level of the individual, not the collective. As soon as one male is bigger than the others, he becomes more likely than the others to pass on his genes.
Now, if there were some reason why a troop of non-dimorphic apes was more likely to survive than a troop of dimorphic apes, I could imagine natural selection operating at the level of the collective to favor non-dimorphism (although a real biologist might be able to knock that idea down).
Gorillas and [common] chimps live in troops. Groups of bonobo, appropriately enough, are often referred to as “parties”. 
Its dangerous to try to relate evolution to just so stories` but to some extent it can make sense…
In a harem setup, the 90-pound weakling never mates. Thus the biggest and strongest males have offspring. However, all the females would get to mate, so size only matters for males. Also, in a harem society, each individual woman is less important; the group can raise children, the group can gather while the male hunts, etc. Thus a (smaller) female who consumes less resources is a better survivor.
In a monogamous society where this is their only chance at reproduction for both, the size issue only relates in terms of competition with others for resources.
In both cases, presuming the male contributes to raising the offspring - which they have to do for humans, given the years of effort and resources necessary to raise one child.
Humans fall somewhere in between. Actually, humans have always been quasi-herd animals (villages, tribes) of allegedly monogamous couples generally, where maybe the alpha male might have more than one wife; or social structures dictate a way to ensure the eligible females are not stranded without support, while social concern for single males seems much less common.
Plus the tribe social structure removes much of the pressures of harem or solitary couples. Most of the harem models I think of involve herbivores, where everyone finds their own food; where hunting is involved, whether pack, couple, or solitary, the harem model breaks down. The hunters all have to be muscular and aggressive. If the females are involved, they have less incentive to settle for one male. Carnivores are better able to inflict dangerous wounds on each other; its harder to defend against one attacker after another if they are close to ripping each others throat out - whereas head-butting des not usually leave the winner seriously wounded for the next attacker.
The egregious displays of some males like birds are a simple situation - when the male does not contribute anything other than genetic material to raising a child, the most important thing is to display as conspicuously as possible that thy are healthy, implying good genes. This tends to result in colourful displays, rather than simple brawn or size, as the males are asking the female to choose, rather than fighting among themselves for the spoils.
One solution appears to be promising them 72 virgins in the afterlife…
We have only conjecture about what type of human sexual-social structures existed for 95% of the time that we have existed as a species. Even extrapolating backwards from the few H/Gs that have survived until modern times is problematic since most of them have been driven into marginal lands, which may significantly influence their social structures. We can’t even say if there was one particular social structure that was common, or if it varied by region/culture.
“Gorilla gorilla” is their species’ official name?? Wow, they got robbed! The most boring scientist in the world must have come up with that.
Yes, which is why there are small, medium, and large species that are sexually dimorphic. A small antelope is as capable of fighting other small antelopes as a gigantic buffalo is of fighting other gigantic buffalo. But even though the small antelope male doesn’t have to fight a giant buffalo for the right to make with the antelope females, he still has to fight with the other male antelopes. And that means if that male antelope has an advantage over the other males he’s more likely to breed, and more likely to pass that trait on to future generations. And those future generations of better fighters will have to fight with each other, so any with an advantage will be more represented, and so on and so on.
They aren’t larger because they need to be large to fight other large males, they’re larger because largeness gives them a combat advantage over smaller males, and therefore smaller males are selected against. You have to run as fast as possible just to stay in place. Except of course that there are other selective pressures on male animals, and larger males need more food and so on. So there is an equilibrium between the optimum size for your habitat, and the increased reproductive advantage of being larger. Same with other secondary sexual characteristics used in male-male competition, like antlers, manes, display feathers, noisemakers and so on.
But of course it’s not just human females that are smaller than human males, but lots of other mammal species. So it’s not something unique to the reproductive history of humans. I think the more logical answer is that in dimorphic species the females are optimally sized for the habitat requirements, and the males are suboptimal–they are larger than they should be to optimally take advantage of the resources around them, but sexual selection has forced them to be larger and saddled them with ridiculous crests, bright plumage, swollen skin patches, thick skulls, more aggressive behavior, and so on. But they can also drag the females along behind them, not all traits that improve male-male competition are isolated exclusively to the males, sometimes the females have them as well, even though they hardly ever use them fighting against members of their own species.
Orangutans get past this. The large, pouched, jolly looking ones practice “call and wait” mating. The smaller, less corpulent practice (I shit you not: this is an official term) “sneak and rape” strategies. A male can change form throughout his life. So a smaller male might have an advantage in some situations.
Cuttlefish do similar too, some males camouflage themselves as females and sneak in with the real females.
Yes, it certainly isn’t always true that larger males always have an advantage over smaller males. It depends on the particular circumstances and mating strategies of the species.
I read about that recently. In some species, some males will remain smaller and mimic females in order to sneak in and get some nookie while the big guys are duking it out.
We could try and change it to Magilla gorilla.
Actually, there are 2 extant species of gorilla, including* G. beringei*.
Some of you are so obsessed with this argument that you forget that bigger stronger individuals are better able to survive in the wild against predators. A big antelope will probably have a better chance to live long enough to mate than a small one. No?
Not necessarily, no. A larger-than-usual thompson’s gazelle may end up a little less agile than his smaller buddies and still not strong enough to fight off the much stronger cheetah hunting him. For him it might be a net disadvantage - bigger isn’t always better.
The were driven to extinction through competition with the much larger Grape Ape, and their numbers were further depleted through depredation from the Pink Panther and Snagglepuss. The Snagglepuss is now extinct as well, but modern scientists believe that it is not closely related to modern cats but most closely to the Crazy Nastyass Honey Badger.
Sow why wouldn’t the females get bigger, too, then? Sometimes bigger is better and sometimes smaller is better. We see this all the time-- larger species evolving into smaller ones and smaller ones evolving into larger ones. There are all sorts of factors that go into determining body size, but when comparing males and females, it’s probably sexual selection that is the main determinant in sexual dimorphism within a species.
Appropriate body size is a result of a number of factors.
Primarily, it is determined by food - available resources. Also, as mentioned, predator escape is a factor. A larger animal is more at risk of starving, and becoming weak, in times of less food. Similarly - an animal twice the size is, by the square-cube law, eight times as heavy. (1.5times the size, 3.4 times the weight). Not just the food issue, but heavier means more muscle and more energy required to outrun the wolf. An rabbit can outrun a human, and do a pretty good job outrunning a dog or wolf. Size is often less an advantage.
But in a fight-to-mate reproduction strategy, the “90 pound weakling” male of the group is unlikely to win, so there is a selection for the (relatively) larger males. this tendency is limited by the other factors above - food an agility. As I said earlier, in that situation it does not matter what the size of the female is, there is no size selection (unless the one with the biggest booty is more likely to get mated…)
Observations of basic human behaviour show a different set of tendencies. men do fight over women (think bar brawls) so to some degree size matters. Women also put on displays to attract men (clothes, makeup etc.), the opposite of your colourful male bird.
Men look for attractive women, because symmetry and free from flaws generally indicates good health, which can usually mean good genes and good child-bearing ability. Male preference for female shape has also been shown to appreciate good breeding capability.
Women, OTOH, are looking for good providers - who will stay around and help provide food for them and their children. (And Porsches, mink coats and jewelry, nowadays) Beyond basic appearance indicating good health, there is less selection for reproductive capability. Social status indicates “good genes”.
Humans allegedly evolved to be hunters running down the game on the savannahs. This is a cooperative venture, like wolf-pack hunting, so it’s a good assumption that the earlier humans also lived in groups and several men cooperated to hunt. The typical pattern observed in most hunter societies is monogamy; large polygamous marriages tended to be in societies where one man could “own” resources and did not have to work himself to provide for multiple wives and extended families (although two wives ore more occasionally was not unknown). Once humans had weapons, no one man in a tribe could safely defend himself against several others, so someone trying to monopolize too many women against the will of others would likely not wake up one morning with a spear or three in his back. The harem model does not translate well into a pack social arrangement. However, women selecting for “winners” likely helped select for somewhat larger males.
Along these lines, I recall a study that showed that male-female size was closest to equal in more monogamous societies, and significantly different in societies like the Middle East with a history of polygamy.