As explained elsewhere, we primates have certain visual cues we rely on to tell a woman from a man; men are generally taller and more muscular. Women are shorter, more curvaceous and have breasts. This is because we are Sexually Dimorphic animals.
This is also the case in several other animal species, particularly those who also rely mostly on vision, and communicate largely with visual displays. The most common kind of dimorphism is when males and females of the same species sport different colors and markings.
Then there are a few species (relatively speaking) in which males and females look wildly different. One sex may even have an entire physical feature that the other lacks (other than, you know, the obvious ones) — if the other sex doesn't sport his/her own trademark physical feature. In a very few species, males and females may be different enough to look like completely different animals.
And science fiction writers, when inventing aliens and fantasy races, love to run with this. There are several alien or fantastical creatures in which the sexes are wildly different. This tends to run towards the extreme end of the Earth scale and beyond: they could be greatly unequal in strength or intelligence; one sex could even be reduced to an inert breeding machine or parasite like certain species of insects and fish. The most common example is for males to be bestial or ugly by human standards, while the females are Cute Monster Girls. Whilst we want to say "oh, it's all biological and totally rational", a substantial amount of this derives from Most Writers Are Male (as well as most viewers also being male) and will opt to make their female members of their fantasy race as close to human as possible regardless of what the males look like for easy opportunities for Fanservice and Male Gaze exploitation to boost the number of eyeballs watching.
Tends to go hand-in-hand with Bizarre Alien Biology. See also Cute Monster Girl, One-Gender Race, Humanoid Female Animal, Mr. Seahorse, The Ugly Guy's Hot Daughter and Bee People. For a (relatively speaking) more subtle sex difference, see Tertiary Sexual Characteristics.
Taken to the logical extreme, this leads to Hot Skitty-on-Wailord Action.
May occur in conjunction with Bizarre Alien Sexes, and in some such cases, "Bizarre Sexual Polymorphism" may be a more technically accurate term. Indeed, that name redirects to this trope.
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Anime & Manga
This appeared to be the case for the plants in Trigun, with the girls in bulbs with the big strange eyes and all, and the only two guys walking around looking like blond men. But later manga material shows that Vash and Knives really are different because they're mutants (probably throwbacks; the odds that the people who engineered the plants didn't use human DNA are vanishingly small), because some female independents come from space; the twins are just even weirder.
Grand Princess Fire Dragon from Maoyuu Maou Yuusha looks like a beautiful human girl with little horns and a flaming tail. Her father is a bona fide Fire Dragon.
Even more confusingly, there are also female Guardians who look like Guardians. They originally came to be when Kyle Rayner (possessing at the time the godlike powers of Ion) recreated the Guardians years after Hal Jordan (at the time possessed by evil entity Paralax) wiped them all out. Of course in the current series all this is ignored and the Guardians are shown in flashbacks to have been always both sexes.
There are also the Controllers, who were also the same species as the Guardians/Zamarons, but appear to be tall, bald humanoid men with orange skin. It's safe to say that the whole species is just weird.
In Phil Foglio's XXXenophile, Martian females are all gorgeous, buxom, four-armed women, and the males are tiny, squat, furry, mostly shapeless (and, incidentally, non-sapient) hexapods with disproportionately large sex organs.
This is then given an extra twist at the end of the story. After making "First Contact" with a human explorer, a giant battlesuit bursts into the room- and the Martians assume that the giant robot must be a human female.
The Badoon, a Marvel Comics alien race, have scaly reptilian males and fur-covered females. The females are also stronger and tougher, but less violent and cunning than the males, and the sexes don't interact except during their once-in-a-lifetime mating frenzy. The females also have Non-Mammal Mammaries, despite laying eggs, though it's possible that they do suckle their young.
Rom Spaceknight's enemies the Dire Wraiths come in two flavors: the technology-using males, who look like ugly, fanged, gangly-limbed Pillsbury Doughboys; and the sorcery-using females, who are pure Starfish Aliens.
The Dire Wraiths are eventually revealed to be offshoots of the Skrulls. For a race of shapeshifters, appearance isn't that important.
In the pre-Crisis version of the Vega system, one world's male inhabitants look very much like humans. The females look like giants snails. To quote the gamebook, "This seems to work for them."
In Legion of Super-Heroes, male Dryads look like large masculine-seeming humanoids made of stone. In some continuities, female Dryads look like large masculine-seeming humanoids made of crystal. (In others they look like the male dryads with narrower waists and Non-Mammal Mammaries.)
Male Warlocks from Nemesis the Warlock walks on a pair of goat-esque hooves, while female have centaur-like bodies.
According to Word of God on Hivefled's interpretation of Homestuck troll biology, the trolls we actually see walking around are all chromosomally male. The enormous egg-laying Mother Grub is the only female. The difference between male-appearing and female-appearing trolls is more like that between worker and drone bees; the "females" have a large internal pouch in the place a uterus would be (crudely known as a "gutbucket") for carrying genetic material to the Mother Grub, and venom sacs for defending themselves during the long pilgrimages this entailed.
Films — Live-Action
The Devaronians of Star Wars (See picture here◊). The males look like devils. Hairless, with pointed teeth and large triangular horns. Also kind of small. The females are tall, hornless, covered in thick fur, and look like humanoid lemurs.
Some artists, especially the great Jan Duursema, draw female Devaronians that look a little like the lemurs shown above, and more like hornless versions of the males (or red skinned humans with black spots), as seen here.
Female Devaronians vary a lot, from the fairly hairless Sian Jeisel to Aven'sai'Ulrahk. Darth Maladi◊ looks like she was fairly hairy at one point (the picture does not show it clearly, but she sports impressive side burns) but the fact she sports Sith Tattoos indicate she might have undergone some epilation.
Another Star Wars Expanded Universe example are the Cathar. Men look very much like saber-tooth lions who happen to be bipedal and have opposable thumbs. Women tend towards the Cat Girl end of the scale.
Though, rather humorously, female Cathar looked just as animalistic as the males until they decided Cat Girl Juhani in Knights of the Old Republic was a Cathar. Perhaps explainable due to the two being subspecies of one race.
Not to mention the Twi'leks, where the men range from "mildly ugly" to "monstrous" while the women are almost all attractive. The men also have human-style ears, while the women have little closed-off domes instead.
Maybe a case of Beauty Equals Goodness, seeing as nearly every male Twi'lek seen is a corrupt evil bastard, while all females are good people or at least innocent slaves.
Probably the case, as looking at some "good" male Twi'leks, like Shado Vao, shows them to be good looking.
Unless of course it's Darth Talon, because Evil Is Sexy. Only for females, though. Which brings us full circle back to the dimorphism trope.
The novelization of Star Trek IV: The Voyage Home would have you believe that this◊ is a male Deltan◊. No other source corroborates this, and it can be assumed to be an error. The "male Deltan" is later established as an Efrosian, the only other seen member of the species being the Federation President in the Star Trek VI, due to reuse of makeup. In the Star Trek: Titan novels, the Efrosians come from "Efros Delta", so you could call them Deltans, but they aren't those Deltans.
Downplayed with the Gorn. Rather than Non-Mammal Mammaries, female Gorn are distinguished by simply being bigger than male Gorn. Unfortunately Gorn also never stop growing until they die, so an old Gorn male could potentially be mistaken for a young Gorn female or vice versa.
Evil Alien Conquerors; the male aliens look just like human males, but the females look... just plain weird.
In the movie version of Land of the Lost, Shaka, a Pakuni, is a hairy ape-like creature. The "hideously ugly" females of his species look like human women.
In Zone Troopers, a group of American soldiers in WWII discover crashed aliens in Axis territory. The males are tall, pretty, blue-skinned Aryans. The females, as it's later discovered, are hairy bug-eyed mole people. Needless to say, the soldiers had a bit of a shock with that revelation.
In 1953's Mysterious Mesa of Lost Women Dr. Aranya injected members of the Theraphosidae family with human pituitary hormones. His subjects grew to the size of human beings and developed reasoning powers. He found he could telepathically command them. He transplanted their brains back into humans, granting them the capacities and instincts of the giant spider. His females proved near-indestructible, and might live hundreds of years. The males dwindled to dwarfish proportions.
In Howard the Duck, at the beginning of the movie we can see another planet or parallel world inhabited by anthropomorphic ducks. The female ducks have breast –even when ducks are not mammals- as can be seen clearly because the titular character also checks a porn magazine.
Weird exception: With all the Starfish Aliens in Wayne D. Barlowe'sExpedition, it's interesting to note that this trope is hardly used at all. Indeed, it's often hard to tell if the terms "male" and "female" even apply; many species are functionally hermaphroditic, and mating impregnates both individuals. The closest thing would be the female Sacback, who lives her adult life buried underground as her male counterpart roams around finding food for her. Other creatures make up for it by having bizarre life-cycles (a giant ocean-going beast starts out as a tiny flyer, for example).
The Termagant Trogs in The Edge Chronicles. The dominant females are massive hulking bald Amazons. Their husbands are skinny, shrimpy and rather pallid creatures usually kept locked up.
As if that wasn't weird enough...when the females come of age, they undergo an induced Metamorphosis from sweet, ethereal little redheaded girls into the aforementioned bald (and bad-tempered) Amazons. If they miss the associated ceremony, they're stuck in the juvenile form.
In David Gerrold's The War Against the Chtorr series, the bunnydogs/bunnymen and libbits were eventually discovered to be the male and female of the same species.
It's heavily implied that the entire Chtorran "ecology" may not have the concept of "species" at all.
Myth Adventures by Robert Asprin includes a dimension called Trollia, where the men are Trolls (enormous, warty, frightening creatures right out of a fairy tale) and the women are Trollops (gorgeous and human-looking except for green-tinged skin and hair). Despite their smaller size, the women are as strong or stronger than the males.
Theodore Sturgeon's "The World Well Lost" centers on a pair of fugitive "loverbirds" from the planet Dirbanu, which has shunned contact with Earth. The loverbirds are initially assumed to be a male and female, but they manage to explain, via some illustrations, that male and female Dirbanu are vastly different in appearance. In fact, the main reason why the Dirbanu dislike humans is due to homophobia, because they perceive all human relationships as being homosexual.
The Cygnans in the novel The Jupiter Theft had human-sized females and insect-like parasitic males that were permanently attached to the females.
The Kzinti are catlike people, and while the males are of humanlike intelligence, the females are not. However, ancient Kzinti females were entirely sapient—their current state is the result of intentional breeding for unintelligent women, with the help of genetic engineering technology. That's what happens when you uplift a bronze age species. Strangely enough, Kzinti (in Man-Kzin Wars) consider the human sexes to be separate alien species, based on behavioral differences. A human female (Manrrett) is considered to have apparently faster reflexes, higher pain tolerance, and greater intellectual insight, than a human male (Man).
The Puppeteers, who are already fairly strange looking, are a subversion. What they refer to as "females" are actually a different, non-sentient species with its own males and females; true Puppeteers are parasitoids, with both sexes sporting similar-looking genitalia used to deposit gametes in the flesh of a host "female".
Grogs. The adult females are large furry cones with a mouth, which cannot move from the rock they attach themselves to. Young females are something like alien bulldogs, and young males are akin to chihuahua, neither of which are sapient. Adults telepathically control the young into breeding, and use the same telepathy to force prey animals to leap into their mouths, since giant immobile cones aren't good at hunting.
In Flatland, men are polygons (whose number of sides and symmetry indicates their social class) and all women are single lines— hysterical, dangerously sharp, and not too bright. (The blatant sexism and classism is a satire of the attitudes that people actually had when it was written, as explained in a foreword added by Abbot when people missed this.) Other details of their physiology, including how they reproduce, are never explained. In Flatterland (a 2001 sequel by Ian Stewart), it is implied that the females utilize the males in detaching a segment after sometimes folding the segment into a new shape with roughly the same number of sides as the male. The females are also confirmed to be multi-sided beings that are rotated into a third dimension, which just brings up even more questions, including already-answered questions, the answers to which were suddenly made invalid.
In the sequel, The Scar, there is a race of mosquito people, the Anophelii, whose women are vampiric winged creatures that have fang-like protuberances that extend from their mouths to penetrate their victims. The extremely passive males have mouths that are described as being like anuses. Yeah. The females are incredibly strong, can fly and have a ravenous hunger that overwhelms any higher brain functions they might have, whilst the males are earthbound, vegetarian and quite intelligent.
In Larry Niven's Draco Tavern stories, all the Chirpsithra were female. There are males, but the Chirpsithra won't talk about them.
In one of the stories, the Chirp males are revealed. They're the "red demons", essentially mindless beasts.
At least, they are at the current day. Chirps are an old species. One story features one that's visited Earth before (and looks quite different to the modern ones, and it isn't just her extensive medical modifications, as she's a different height). Her last visit was in the Precambrian.
The Piggies and Buggers in Speaker for the Dead. Especially the piggies. The buggers are about as sexually dimorphic as real bugs. Both species call humans weird for having sexes so similar that both can perform the same societal function.
The piggies' life cycle and reproduction is strange enough that it really deserves to be explained in full detail (though put in spoilers). Both sexes start out small and grublike, but the dimorphism starts quickly. The males develop into the pig-like form that the humans in the story first assume were the only form, but will develop into enormous trees upon death and retain their sentience upon being ritually vivisected.
The females, on the other hand, almost all stay small and grublike, and after a certain time are carried by the pig-like males to the trees, and are impregnated by said trees via pollen. Then, when the young they're carrying are ready to be born, they eat their way out of their mother. In addition, the occasional female will develop into a pig-like form like the males to serve as a matriarch, and eventually also be vivisected to become a "mothertree", which holds all the young and nourishes them with its sap. "Bizarre sexual dimophism" doesn't even begin to cover it.
There's also the apparently female-only deer-like creatures that are impregnated by the grass-like male half of the species. The xenobiologists spend half the book puzzled over evidence of genetic exchange (which implies the species don't reproduce by parthenogenesis like some earth species do), yet without any apparent presence of males.
This is all a relatively recent development from an evolutionary standpoint, caused by the Descolada, a highly-adaptable virus that literally unravels DNA strands. The only things that can survive it are those who have adapted to use it.
The insect-like society depicted in Frank Herbert's Hellstrom's Hive had breeding-machine females not conceptually unlike the piggies.
In the Uplift Storm trilogy, male urs are much smaller than females, and generally less intelligent (though sapient; a male urs is a secondary character in the series). Females have pouches to carry their mate around.
In Dennis L. McKiernan's Mithgar series, dragons are the male of a species. What are the females? Of all things, krakens!
Dwarves are interesting. Only the males are seen by outsiders. Anyone that visits a Dwarvenholt that is considered friendly might catch a glimpse of a veiled, graceful figure that swiftly departs, said to be a "Chakkia" (the dwarves call themselves Chak). One character caught a glimpse of a Chakkia without the veils: she was beautiful. Another character, able to see the "true form" or light of living things, stated that he did not believe the Chakkia to be dwarves at all, their light was too different.
In another book, a seer also commented (to herself) that they were clearly not the same species as the male dwarves, then recognised them as... something else that she wouldn't tell us about, wondered if their state was some sort of divinely ordered punishment, and then claimed it was none of her business and turned away. If he keeps writing enough books he MAY explain all this.
Bob Shaw's "Warren Peace" has the Squelchers, an alien race with no less then six different sexes, each one with its own unique appearance, and with a reproductive cycle where each sex fertilizes the others in turn. The forms look so different that, to the vast majority of the universe, the species only consists of the fourth sex, which resembles an orange haired saggy sasquatch (kind of like a blown up balloon that's developed a slow leak) with multiple eyes in a ring around its head (usually covered by its fur), oversized feet that let it wade on water, and two giant red nipple-like gamete sacs positioned one above the other on its torso. The fifth sex, the only other one mentioned, is described as being indistinguishible from a tree, except for the presence of a pair of two dual-pronged ovipositors (they look almost identical to staples) sprouting from its trunk.
Inverted in Terry Pratchett's Discworld, where male and female dwarfs look pretty much identical. Both males and females have beards and dress identically in lots of layers of thick clothes and chainmail. In fact dressing noticeably "feminine" is considered outrageously shocking. A large part of Dwarven courtship involves tactfully finding out what sex the other dwarf is...
Later novels have Dwarf society slightly imitating humans, with female dwarfs adopting Tertiary Sexual Characteristics for style and comfort. A discreet ribbon in the beard, steel-heeled boots, chain mail that doesn't chafe...
Played straight with the pictsies, tiny Bee People whose female breeders ("keldas") start out the same size as males, but grow twice as tall and spherically-fat after a lifetime of birthing hundreds of offspring. A kelda's rule over her clan is absolute, as she's mother to most and wiser than any.
Which isn't saying much as male Pictsies are impulsive and slow to think, even for a Proud Warrior Race. They'd probably all be dead without a kelda to keep them in line.
Played straight with the Igor clan, who are human (just barely) and do it intentionally. Both male and female Igors are brilliant surgeons who incorporate body parts from corpses into themselves, and can look however they want. Males choose to look like the traditional scarred hunchback, and speak with a heavy lisp. Females (or Igorinas) however follow a different trope, and are quite lovely (though they often keep a nice looking scar or stitch-mark to signify their heritage).
Similar to Discworld, Lord of the Rings inverts the trope with the dwarf race, with it being stated that female dwarves are often mistaken for men due to being similar to the males in voice and appearance. As every male dwarf we encounter in the book has a prominent beard, some readers have interpreted this as meaning that the female dwarves even have beards.
In the films, this became a rather comedic scene where Gimli humorously explains this phenomenon to a human woman. The human woman (Éowyn) then turns around and looks at Aragorn to see if Gimli is pulling her leg, and he mouths, "It's the beards," and gestures at his chin.
Confirmed in The Silmarillion, which says "For the Naugrim have beards from the beginning of their lives, male and female alike."
Subverted in Bruce Coville's My Teacher Is an Alien with Kreeblim, a female alien who looks very different to the previously introduced male one, Broxholm. The human narrator initially assumes this trope is in play, but it turns out that they are of completely different species.
In Frank Herbert's Dune series, Bene Tleilax females have been genetically altered, and serve as their axlotl tanks, basically giant wombs on life support.
In The Dirdir by Jack Vance, the titular Dirdir race has a complex sexuality. A male will be born with one of twelve different sex organs, females one of fourteen. Each type matches one or more of the others. Mating is complicated by the great secrecy surrounding sex: no-one wants to be "outed" as a particular sex since there are a host of restrictive sexual stereotypes waiting to be applied.
In Everworld, the Hetwan males are vaguely humanoid Insectoid Aliens, while the females are described as transparent bags of organs with wings. How they reproduce: Hetwan males tear the females apart and eat them, and new Hetwan form at the males' waist during the process. Ugh.
In Edgar Rice Burroughs's Tarzan series, the people of the lost city of Opar consist of a tribe of stunted, hairy, almost apelike men ruled over by a beautiful, entirely human-looking woman. It's implied that the inhabitants degenerated by mating with great apes, but somehow the degeneration didn't affect females the way it did males.
In Man After Man, one species of post-Homo sapiens hominid developed the ability to hibernate. As males slept for most of the year, whereas females remained awake to nurse their young except in the depths of winter, the sedentary males wound up having a lifespan several times as long as that of the nomadic females.
John Varley's Gaea Trilogy features a dimorphic intelligent species in which the gas-inflated males are living blimps and the deep-diving females are organic submarines. They begin life as sexless, snakelike animals, then choose which adult sex to metamorphose into when their consciousness and race-memory emerges. Mating takes place at the ocean's surface, aside from which the two sexes never interact.
In Sergey Lukyanenko'sSpectrum, the main character meets several members of a race of Reptilians (according to the book cover, they look like Narns). Later, his Love Interest explains that the non-sapient animals they travel with are actually their females, as she saw one of the aliens mating with one of the animals. That or bestiality. The cause for this appears to be an ancient cataclysm that affected all known races, causing many of the bizarre biological and psychological features of the aliens.
In Today, Mom there are genetically engineered sphinxes: lion-sized sentient beings created to survive on Venus to terraform it to human-habitable conditions. The males were working on Venus, the females were kept hidden on Earth. One day males revolted, declared independence and smuggled females to Venus. They were very shocked to learn that the females are ordinary, non-sentient domestic cats. At one point a sphinx shows his family portrait with his mom and two sisters.
In Negotiators it's humans, who are bizarre. Aliens cannot believe there can be such huge psychological differences between genders and disregard the information as savage chauvinism. But it's the habit to come to terms with people you don't fully understand, what makes humans the best negotiators.
In the Steerswoman series, female demons are tall, spray acid from under their arms, and can make sculptures (out of a wax-like substance that their bodies produce). Male demons are much shorter and cannot make these sculptures. Since these sculptures are how demons communicate, this is a major social barrier.
Retief: In Retief and the Pan-Galactic Pageant of Pulchritude, an alien comments to Retief on the 'remarkable sexual dimorphism' of Terrans, after Retief slips a ringer into the titular beauty pageant: a female Bengal tiger. The alien is apparently used to sexual dimorphorphisms extreme enough to make this plausible, and doesn't notice the difference between human males and females, except to suggest that the human with the large protrusions on his (actually 'her') upper thorax might want to see a doctor about it.
Medea: Harlan's World was a collaboration between a number of major SF writers to create a single setting, and a series of short stories set on the titular moon. In Flare Time by Larry Niven, the reproductive cycle of the native sentients known as fuxes is described. They start off as six-legged females. At around seven they have their first litter, during which their rear body segment tears off, leaving them as four-legged females. The hindquarters contain the eggs, and function as a nest which the newborns eat their way out of. Around seventeen they have a second litter, leaving them as two-legged males, with the male organs exposed through the loss of the second body segment. The male guards the nest until the young are born, then goes into heat for about three years, and eventually ends up a 'post-male'.
The Agletsch in Ian Douglas's Star Carrier novels are peaceful spider-like creatures whose hat seems to be information exchange (they don't even have colonies, just trading outposts). In the second book, it's revealed (to the readers, not the characters) that the "spider-like" Agletsch are, in fact, their females. The males are the tiny non-sentient slug-like parasites that all females carry attached to their bodies. The males feed off the females, but fertilize them for reproduction. This is first brought up when a racist maître d' of an expensive restaurant tries to kick out two Agletsch who come in, calling them "gentlemen". A main character corrects him, and points out they're "ladies".
The Pak'ma'ra from Babylon 5 have sexual dimorphism similar to some types of deep sea fish: one sex is a complete creature, while the other is much smaller and permanently attaches to its partner like a parasite (where most of its organs and nervous system atrophy, with only the reproductive organ remaining). But unlike in such fish, the Pak'ma'ra seen in the show are the males, while the hump on their back (only ever seen covered by clothing) is the female Pak'ma'ra.
The Mers (essentially carnivorous seals from the future) from Primeval. The Mer Queen is about ten times the mass of a regular Mer (which can easily overpower a human). Fortunately, Mers, being big, not terribly fast, and lacking in any sort of natural armor, are particularly vulnerable to bullets.
This trope is played with in the first season of Farscape, where a Yenen named Staanz (played by a male actor) turns out to be an attractive female of the species.
According to some traditions, the male equivalent of a mermaid is a "merrow." Merrows are hideous but friendly to humans, as long as said humans don't try to steal their wives. Of course, mermaids are hot, so humans try to steal their wives a lot, actually, thus incurring the merrows' wrath.
In fact, lots of stories feature fugly (or at least Hollywood Homely) mermen and drop-dead gorgeous mermaids, probably because Most Writers Are Male. There are always exceptions, such as mermen of Scandinavian folklore (who tend to be Bishounen as a rule), and mermen featured in more modern works (especially those penned by and for women).
In the Dark Crystal film, the two Gelflings look quite similar, but possess a significant hidden difference, leading to this interesting exchange:
Jen: Wings? I don't have wings! Kira: Of course not. You're a boy.
The Skaven of Warhammer fantasy take this to extreme of the females being non-sapient and even more rat-like than the males—"breeders", as they are called, basically look like Ogre-sized or larger female non-anthropomorphic rats, made even bigger by the fact they are swollen so huge with litters that they can't walk.
The current Codex seems to show that there are normal size sapient females.
Likewise, in a non-canon third-party Dungeons & Dragons sourcebook, The Slayer's Guide to Troglodytes from Mongoose Publishing, the titular lizard-people are made up of sapient humanoid males and non-sapient females that look like huge bloated lizards.
In Castle Falkenstein there are no female Dwarfs. Dwarfs always mate with Fairy women-if the child's male, it'll be a Dwarf like dad, otherwise, it'll be whatever sort of Fairy mom is. (There are male Fairies.) See Gender Equals Breed.
Let's talk about sphinxes shall we? There are technically four sexes: Androsphinxes (male human head) Gynopshinxes (female human head) Criosphinxes (ram's head) and hieracosphinxes (hawk's head) of respectively Lawful Good, True Neutral, Chaotic Neutral and Chaotic Evil alignment. Gynosphinxes areAlways Female, the others are Always Male. A gynosphinx mating with a hieraco- or criosphinx always produces hieraco- or criosphinxes as offspring, while if mating with an androsphinx they produce twins, one of which is a gynosphinx and the other an androsphinx. This means that, for obvious reasons, gynosphinxes don't like mating with crio- or hieracosphinxes much, which means that the other two reproduce pretty much entirely through rape. Androsphinxes on the other hand don't like mating at all much, and it is noted that gynosphinxes will pay adventurers handsomely for the location of an androsphinx. In other words, Dungeons & Dragons monsters are screwed up.
And very likely going extinct, if gynosphinxes aren't having enough daughters to replace themselves each generation.
There are also medusae —complete with snake hair and petrifying gaze, with 10% being poison-blooded Snake People for added measure—and maedars—which look more-or-less like bald human men—which are the male and female of the same species. Since 4th Edition this dichotomy in dimorphism has lessened; while male medusas are still bald (and have a gaze that poisons anyone that it falls upon rather than a petrifying gaze) they are quite obviously recognizable as members of the same species as the females.
Second Edition stated that maedars were extremely rare (with a medusa able to breed with human men, according to some sources, basically with a bag over her head until she was done, when she would take it off and petrify her paramour), and they had two other really useful abilities: They were strong enough to smash rock with their bare fists with ease, and they could do Stone to Flesh by touch at will, unlimited applications per day. They used these two abilities to smash petrified creatures to bits (killing them) then convert the bits back to flesh for food. Needless to say, medusae very much wanted one for a mate.
The Talislanta game does this with not one, but two species:
In the first example, the stooped, wrinkled females are Gnorls and the gangly, monkey-like males are Weirdlings. The former live in elaborate cavern homes, while the latter live in simple burrows; they inhabit different regions, but come together every 50 years or so to mate. Both sexes were named separately by outsiders, who'd thought they were different races at the time, and no Gnorl or Weirdling will admit what (if anything) they call their mutual species.
In the second example, female Batreans are Green Skinned Space Babes, complete with hyper-seductive pheromones, while their male counterparts are grotesque, hulking ogre-ish creatures. The females are intelligent and live in Polynesian-style villages, while the moronic males shamble around in the jungle, looking for edible things to hit.
In one of the many Star Trek role-playing games, Orions are presented as having green-skinned females and gold-skinned males. No canon source bears this out. On the live-action shows, no males are seen until Enterprise and they're all green. Many of the Expanded Universe sources present Orions as having a range of skin colors, but pretty much universally present males and females as having the same range of colors, which generally doesn't include gold.
Pathfinder has the lashunta. The women look like elves with antennae; the men are squat and hairy. Their in-game stats actually reflect this.
Though, in a slight subversion, the brutish men are just as prenaturally intelligent as the females, though more prone to rash decisions.
The abovementioned sphinxes also appear as well, with some minor tweaks.
Mortasheen has the Q-Lex, which has a small parasitic male hanging onto the large, predatory female. This is based on actual mosquito sexual dimorphism, which is appropriate, given that this creature is a human/mosquito hybrid.
The Grekim from the RTSAchron have three sexes (octo, pharo, and sepi). Octos crawl along the ground with six limbs out to the side like an insect or crab, with the remaining 2 held out front like arms. Sepis float slightly above the ground with their tentacles dangling beneath them. Pharos walk around with six limbs directly beneath them, and the other two out front. The changes in body types get even weirder with the 'higher' classes. Did we mention that they are all cyborgs and they use Time Travel as their weapon of choice? Starfish Aliens indeed.
In Pokémon, there are a number of species that are gender exclusive (Jynx, for example, is only female). But some of these Pokémon are considered the opposite sex of a different species. The Nidoran pairs come to mind, familiar to most people, but Tauros found his female counterpart in Miltank, and later generations continued the trend. This might be Gender Equals Breed, though.
It's debated whether or not Miltank and Tauros are opposite genders of the same species—unlike Volbeat/Illumise and the Nidoran family, Miltank eggs can never hatch into Tauros.
Burmy, based on a bagworm, evolves into a Mothim if male or a Wormadam if female. Their appearances are VERY different, as Mothim is a moth and Wormadam is still a pupa. (Like real bagworms.) Wormadam actually has three appearances, but that's unrelated. Other examples: In the anime, a Latias produced a Latios egg like the Volbeat/Illumise thing. Some species also have sexual trimorphism: Only male Kirlia can evolve into Gallade, but both genders can evolve into Gardevoir. Only female Snorunt can evolve into Froslass, but both can evolve into Glalie. Combee can be both genders, but only females evolve into Vespiquen, males do not evolve. Also Rufflet/Braviary and Vullaby/Mandibuzz are considered counterparts, but they don't produce each others' eggs much like Tauros/Miltank.
Also some species have ACTUAL sexual dimorphism, as in visual differences in the same species based on gender, starting in the fourth generation games. Most of them are tiny differences (such as female Rattata having shorter whiskers) but a few species are very different. Hippopotas/Hippowdon are quite different in color. Male Unfezant have more colorful plumage than the brown-and-gray females, and a trailing red crest. Male Frillish are blue, females are pink...and when they evolve the male Jellicent gets a huge Pringles mustache while the female gets a fluffy collar. And then there's Meowstic, which takes it up another notch- the offense-oriented females and support-oriented males not only have different enough fur patterns to be two separate species, they also have different hidden abilities and learn different move sets.
In Darkstalkers, Succubi are gorgeous women and nymphettes. 60% of Incubi, on the other hand, are gigantic mutant insectoid dragons.
Several races in World of Warcraft. Note that Trolls and Tauren were much less dimorphic in the original alpha of the game, but the females were changed to be more humanoid following player feedback that they weren't good-looking. Blood Elf males were given more visible muscles because they were said to be too skinny, despite having the closest body shape to an real human.)
It should be noted, a lot of the dimorphism in playable races comes mostly from the playerbase complaining, Trolls, Tauren, and Blood elves were extremely close to one another in body shape in their early alpha models, granted, female trolls were terrifying, Tauren women were...meh. and Blood elves were actually very good looking (. We didn't know if this would have applied to female worgen, as at the time they weren't even playable in alpha, and the player responses were already negative.
The Draenei males are huge and bulky, with large, ridged tails, forehead plates, and catfish whiskers. Their kinswomen are lithe and willowy (they're almost the same height as the boys, but would appear to weigh perhaps half as much, if that) with short, thin tails and prominent horns. They also have catfish-like tendrils, but theirs sprout from behind the ears instead of on the face.
Trolls, especially the Forest and Ice Trolls where the males have thickly muscled bodies. The playable Darkspear are still glaringly dimorphic; females have tiny tusks, pointy little noses, and can be Cute Monster Girls, if given the prettier facial feature; males have huge tusks, beak-like noses, and their elongated bodies would be at least a foot taller if it weren't for a permanent slouch. (When they stretch to full height in their idle animation, you can see they are the tallest playable race.)
With Orcs and Tauren, the males are bulging with muscles and their posture is strongly hunched. (Tauren seem to have their head come out of the chest.) The females are definitely the burliest among the playable races, but are noticeably more humanoid in their overall proportions. Orc have prominent fangs which are much smaller in the females, and Tauren bulls have teeth showing through their lips which are absent in the cows.
Night elves have a milder case. Males are at least seven feet tall, with wide shoulders, tiny waists, and lanky, muscular limbs. The females have more human proportions. (Ironically, in lore and among NPCs their warriors are mostly females.)
The Naga as well, which is often pointed out by Richard A. Knaak and other Warcraft authors. The females are lithe and still very elven from the waist up, despite their four arms and serpentine tails, but the males are decidedly draconic from the waist up, while still only having two arms. It is also implied that the brains of the males have degenerated quite a bit, which explains why only the females are ever spellcasters. Except for some males.
Possibly justified as the Naga were created when Azshara, the elven queen, made a pact with the Old Gods to save her people.
And the Worgen: the men are big, bestial wolf-men; women look like characters out of a furry webcomic.
Averted with the Gnomes, Goblins and Blood Elves, where the dimorphism is comparably normal. Gnomes share almost the same height and proportions. In Goblins the only major difference is that the women have lighter and smoother skin. Blood Elves have some differences, with men being more muscled and women having a mild form of Hartman Hips and a "thinner" face.
The Pandaren from the Mists of Pandaria exspansion. While both sexes are approximately orc height and fairly heavily built, you've got rounded, curvy females with the option of a long tail like a red panda (an animal more closely related to weasels and raccoons), and rather blockish, bulky males with stubby bear tails.
Every species in Sword of the Stars, except the Humans and the Liir (who are hermaphroditic), have some kind of this or another. The Hivers are Bee People so this is to be expected (not to mention that workers and warriors are sexless to begin with); Zuul females are approximately three times the size of the males and animalistic while the males are weedy psionicmasterminds with a specialty in Mind Rape; Tarka males that become fertile (which only about one in a thousand do, and which can only be accomplished by eating unfertilized Tarka eggs) approximately double in size; and Morrigi females are basically dragons while the males are more akin to birds.
This also receives a kind of reverse-lampshading in the fluff, where it's noted that due to the death of sexism in Human society and the lack of dimorphism between men and women, most other races actually have a hard time telling the difference, which can be quite comical. The Hivers are the exception - they can detect airborne estrogen, so they can instantly pick out the females. Unfortunately for female Human spacers, Hiver battle strategy targets the females first.
Comes into play in Grandia when you meet Milda who doesn't look any different from any of the other long-eared beastfolk of the game. Then you get your first glimpse of the males of her tribe when you meet her husband Darlin, who is...a large, robed, bipedal cow.
Those huge, blind, nigh-invulnerable Berserkers in Gears of War? That's a female Locust Drone. The male drones could pass for humans in poor lighting conditions. It's also stated that Berserkers are tied down during sex so they don't beat the Drones to pulp during the Act. Oddly though, the Hive Queen is decidedly more of a Gorgeous Gorgon.
It's that Myrrah is human.
On the MUD Dark & Shattered Lands, there's the Leonine race, which may just take the cake. The males are Wemics, basically centaurs with lions instead of horses. The females are Felars, pretty much just catgirls, which can have fur on as much as ninety or as little as ten percent of their bodies. Yes, the males are quadrupedal and the females are bipedal. Cue a huge number of off-color jokes...
The female Covenent Elites from Halo, as seen in the Halo Legends animation anthology do not resemble their male counterparts much. Although they have the same coloring, they look considerably more humanoid.
However that was deemed Noncanon; later female Elites just look like the males.
Male Sadida from Dofus and Wakfu have long green hair all over their face, whereas the women appear more normal. Much more blatant with male Sram from Dofus, who look like skeletons, while the females look completely normal.
The Orfa, an alien race from the 4X game Ascendancy, have a staggering 17 distinct sexes. The game theorizes that the only reason that they even evolved to sapience was the extreme sexual competition that arose from their ridiculously complicated mating system.
Subverted in the Monkey Island series with the mermaids and mermen. Mermaids are fairly exotic and beautiful as a sailor might expect, but there's a problem— telling them apart from the mermen can be quite difficult for outsiders. All merfolk look pretty much the same and there's no external dimorphism between sexes, so if a Human man mistakes a Merman for a Mermaid, you can expect a squicky situation.
Orcs in the Russian MMO Allods Online are dimorphic enough to give World of Warcraft's draenei and trolls a run for their money. The males are hulking, hunched brutes with huge tusks; the females are athletic-looking gray-skinned women with Cute Little Fangs.
Lineage 2 has male dwarves looking like old bearded men, albeit quite short — they are dwarves after all, while females are quintessential loli girls.
Though less extreme than some other examples, the insectoid Kephera of Lusternia straddle this trope. The males are about four to five feet tall, dexterous, hardy, and generally warriors; the females are six to seven feet tall, considerably more intelligent and charismatic, and much slower/bulkier due to their thoraxes. Their society is matriarchal, with one Queen having many mates.
The Rap Men and Rap Women in Rhythm Tengoku count, as do the Love Lizards from Rhythm Heaven.
In Breath of Fire, males of the Blacksmith Clan are hulking minotaurlike oxmen. Females look like human women with horns.
In the MMORPG Dream Of Mirror Online, the females of the Shura race are basically foxgirls, while the males are massive walls of muscle with short, thick, hairless tails, and one small horn in the center of their foreheads.
Mostly averted in Guild Wars 2, but played straight with the Norn. Males are hulking brutes with shoulders that would put World of Warcraft to shame, while females look like tall humans.
In League of Legends Yordle female look like tiny, pointy-eared, blue-skinned women with white hair- quite squat and with rather wide faces (depending on the artist) but basically humanoid. Yordle males on the other hand vary from looking like short gnome-like fellows with massive facial hair (but not blue skin) to resembling bipedal hamsters- either way they look nothing like the females. Compare Tristana◊ with Teemo◊. This largely stems from them originally being 2 seperate races (Yordles and Meglings) who were retconned into 1.
Done uniquely in Startopia. Every single alien race you see has the same model for males and females (notable only in each individual's info, if at all). The Dahanese Sirens (referred to in everything but their intro as just "Sirens") are the only distinctly male and female species, simply looking like humans with wings. However, what we see as humanoid male and humanoid female are, actually, just the reverse according to the employee management screen.
In Dota 2 there are two representatives of the Slithereen race, living underwater. The females look noticeably more humanoid, while the males have much more prevalent fish traits. Compare Slardar◊ and Naga Siren◊. However, Slark, a bipedal fish-creature is also described as Slithereen- it's thought by some that Slithereen is an alliance rather than a race.
Mother Deathclaws in Fallout: New Vegas look noticeably different from their male counterparts, having blue skin, a broader neck, swept back horns, and a more erect stature. However, in Fallout 1 and 2, they were identical to the males.
The Donkey Kong games tend to make female apes look more anthropmorphic than the male characters, typically having himan-like proportions, walking upright and wearing clothing (Dixie Kong is the exception, since she essentially looks like Diddy Kong with a wig).
At The Heart Of It All, written by the creator of Concession featured a cute female alien who was discussing another alien race with Immy, the protagonist: "Their females look much like us, but their males are giant room-sized beasts with twelve tentacles who only think with the most primal urges of eating and reproduction..." Cut to Immy zooming off in a rocket.
Thuxians from Buck Godot Zapgun For Hire exhibit a fairly mild form of this trope, assuming that the one male and one female seen in the comics are typical examples of their sex. Thuxians look vaquely like Xenomorphs in the Alien movies (they have a similarly shaped head, no visible eyes and somewhat similar tail). Al (male) is green, the back of his head is rounded and his face is shaped so that he looks to have a large nose (he doesn't seem to have nostrils, though). Tal (female) is blue and her head looks more like that of the Xenomorphs, extending further back and not having a nose like Al has.
In particular, firstborn children are considered to "belong to the goddess" — like actual hyenas, the first pregnancy is almost always fatal to the child. A firstborn who survives is seen as an omen and a lucky charm, a blessed person who was given back by their deity.
The author, Ursula Vernon, based a huge amount of the work on the real-life characteristics of the anthropomorphized animals; while the mythology is taken from real world myths and legends. Definitely someone who did the research, and did it well.
It helps to know that Ursula was an anthropology major in college and a post-grad student of Weird Knowledge in real life.
In Lore Sjoberg's Book of Ratings entry for "Hostess Products", it is suggested that the cake and frosting portions of a Hostess cupcake are an example of this.
This picture of a massive whale-like alien creature followed by tiny specks. A zoom in reveals that these specks are insectoid creatures and the males of the species (The description states that they are the size of a seagull).
Truly extreme Monster Buster Club example: Grampa Smith, in his true form, looks like a sort of blue-skinned fish-fly thing with four limbs. Cathy's true form, on the other hand, has six tentacle-like limbs, white skin with pink spots, three crests on her head, and a squirrel-like tail. This is assuming that Grampa Smith and Cathy belong to the same species; Cathy may well be adopted, seeing that we never hear anything about her birth parents. Or Grandpa Smith is just what happens when his species gets old.
Storm Hawks—Subverted in the case of the Wallop species. As Finn says "they all look like dudes from behind"
Superman: The Animated Series: Behold Mr. Mxyzptlk and his girlfriend, Gsptlsnz◊. Note that they're both 5th-dimensional imps with godlike powers, so it's possible that they both just choose to look that way.
From Wakfu, all male Sadida have their entire head, including the face, covered by green hair—and this is true for the juveniles too. The females also have green hair, but their faces are devoid of pilosity.
A throwaway gag on South Park has a male Gelgamek, a stocky green individual marginally larger than a male human of similar age and social standing, mention that the Gelgamek vagina is three feet wide, and filled with razor-sharp teeth. One can only imagine what the rest of the Gelgamek female looks like.
In Star Wars: The Clone Wars, Female Dathomirians (Dathomirians are a hybrid between Zabrak and humans), have chalk-white skin and shaved bald heads, resembling Rattataki. Male Dathomirians, however, resemble their Zabrak forefathers (like Darth Maul) with skin tattooed red or gold, and horns. Basically, while the males hardly look any different from regular Zabrak, females lack horns and thus look no different than humans.
In Adventure Time, in the episode "Her Parents" the parents of Lady Rainicorn visit Jake, and we discover that the male Rainicorns are far shorter than the female ones. Also that rainicorns eat human flesh.
This turned out to not actually be true, since the longest of Jake and Lady's corndog pups was a male while the other four, three female and one other male, were quite short compared to their mother.
In Treasure Planet the dog-man alien scientist Doppler winds up a couple with the cat-woman captain Amelia. Fair enough, just one of those soft sci-fi things. However, their children turn up in the epilogue—a litter of kitten-girls and puppy-boys. This could, of course, be Gender Equals Breed, but this trope is also a possible explanation.
The Martian Queen in Duck Dodgers is obviously sexier and more humanoid than any other Martian. However there was a tall, burly male Martian who attempted a coup. Marvin is apparently just the runt of the species...
In some species of anglerfish, after the male attaches himself to the female, his body degenerates until only his testes remain. He ceases to exist as an independent organism, and the female has sperm on tap for the rest of her life. No surprise why the angler fish won the titles of The Most Extreme in the episodes "Oddities" and "Appendages". There's a song about it. And a small comic on it, too.
Whalefish and bignoses are membersnote female and male, respectively of the same species of abyssal fish so wildly different from one another that for the longest time, scientists thought that they were entirely different One-Gender Race species. Not only that, but they have a juvenile stage which is completely distinct from both of them and was itself mistaken for a third species.
In several fish species, males come in two varieties: conspicuous, muscular macho-males who use aggression to monopolize access to females, and small, inconspicuous ones that resemble females so they can sneak into harems and fertilize eggs without the macho-males noticing them.
A great many birds have brightly colored males and plain, often larger females. In fact, the word for a male hawk or falcon is "tiercel", which comes from the French word for "one third", because male raptors are a third smaller than females. This makes a certain evolutionary sense — the female has to carry eggs, so a larger body is needed. Males can get away with being smaller (and thus needing less food) because birds generally don't physically fight except in extreme circumstances — it's too easy to hurt a wing and be crippled. Similarly, males normally are bright and females drab so that a nesting female is hard to spot but a displaying male easy to find. (There is also a theory that some species, like peacocks, are essentially advertising their fitness as a mate by showing off a major handicap — a big tail says "Hey, I can walk around with this strapped to my butt and still avoid predators! I'm an awesome provider!" Zoologists are rather stumped by the peacock - despite what the obvious human assumption would imply about displaying it, actual observance of female peacock's mating preferences don't seem to be consistent with anything about the tail - one tail seems, as far as zoologists can tell, as good as another.) The purpose of the size difference in male and female birds of prey is also to discourage competition for food between a mating pair. The larger female can catch large prey (and have plenty to feed her chicks) without worrying about the small male getting in her way.
Peafowl are good examples of extreme difference between the sexes, as are most Galliforms (chickens, pheasants, turkeys, etc.).
For an inversion of the usual trend, see the Eclectus Parrot. Males are a well-camouflaged green, females are a gaudy red, blue, and purple (Eclectuses are one of the only parrots to practice polygamy, and the females' bright colors make it easier for the male to find his ladies on their respective nests). They were actually classified as two different species until (it is rumored) someone caught them mating.
Another odd inversion are the Phalaropes. For some reason, this group of little Arctic shorebirds have reversed the usual avian gender roles. Females are brightly colored and fight over males, who are drabber and stay with the nest. They have been described elsewhere as "an entire Genus of Wholesome Crossdressers."
The huia, a strange bird native to New Zealand which was driven extinct by overhunting, had an entirely unique form of sexual dimorphism. The male and female had the same feather markings, but their beaks were shaped very differently. They had separate niches and ate in different ways: claims that they had to work as a team to eat are based on a misunderstanding.
Snakes are very rarely dimorphic, but some species of constrictors have a similar size discrepancy to birds of prey, for similar reasons.
In some members of the boa taxon, males have residual stubs of toes on either side of the cloaca, which are used to stimulate females during mating. Far from conspicuous, but for a snake to have any trace of limbs is bizarre in itself.
Spotted Hyenas are a case of Bizarre Sexual Monomorphism: Females have identical-looking genitals to males, with a "pseudopenis" and "pseudo-scrotum".
Very common among arthropods:
Spiders are infamous for examples of extreme dimorphism between the sexes. There is a tendency of the males to evolve into wanderers, spending their lives looking for more sedentary, web-bound females, resulting in them evolving to very different lifestyle requirements. For instance, in this picture◊ of a Golden Silk Spider, the two spiders are the same species, with the larger being the female.
Male Salticids (jumping spiders) are often brightly colored and have very large "boxing glove" pedipalps in front, while females are brown or gray with small palps (P. audax, a common lab spider, is an exception to the color rule). Probably because Salticid females are very aggressive hunters, some species' males do elaborate (and hilarious) dances to increase the probability that females will recognize them as a mate rather than a snack.
In some species, the male will, after starting the process of sperm transfer (which is typically done with a pair of LEGS, spiders are weird), rotate their abdomen up toward the female's head and try to get her to EAT HIM. This is advantageous for the male, because it will provide her with valuable nutrition when she's finishing up the eggs he's fertilizing and putting them in their egg case. Given the longevity of these male spiders, and the travel time to the next female, he's extremely unlikely to ever encounter a second female anyway.
Scale insects are even more extreme than the spiders in terms of body-dimorphism. As juveniles, males and females look much alike, but when the females mature, they sink their mouths into a plant and become something that looks less like an animal than a wart. Mature males develop wings and fly around looking for females (for a few days, before they die).
Even more so than the scale insects, barnacles of the genus Sacculina are... unusual. Males are free-living, wandering animals that don't resemble typical barnacles at all. Females resemble neither barnacles nor the males. As adults, the females parasitize female crabs and look like a generic mass of cancerous tissue rather than a separate animal at all. The adult males also discard their crustacean exoskeleton and implant themselves into the females, becoming basically a packet of sperm. here's a site to read more about ''Sacculina''...
Fig wasps. Males are tiny compared to females and wingless.
Bagworm moths. The females are wingless, eyeless, near-legless breeding machines. The male moth mates with the female while she is still in the cocoon, and in some species, particularly the asexually-reproducing ones, the young hatch out of the female Alien style.
Eusocial insects have sexual trimorphism: the reproducing females look different than the Worker females, which look different from the males.
The adult females of some species of firefly are virtually indistinguishable from the larvae (i.e., grubs), while only the males are the elongated beetles we all know and love.
Siafu/Driver Ants. The females are not small for ant standards (being about an inch long). The male is about the size of a large sausage, hence the term "sausage fly" for him. The "sausage" part is a bloated abdomen that makes them look like very obese dragonflies. This abdomen? Contains sperm. When they breed the females release a pheromone that attracts the male, who usually have nothing to do with the females (for good reason!). The females chew off all his limbs. Then they rip open his belly and impregnate themselves with the contents while the male lies dying.
Inverted by various functional hermaphroditic species, in which "male" and "female" exist in the same individual at once. Played with by a few species that change sex according to age; which "sex" is young and which is old usually depends on which would be more advantaged by large size.
Humans' sexual dimorphism isn't as great as that of our near relatives. Gorilla and orangutan males are enormous compared to the females, while male and female gibbons sport different colors of fur, plus an inflatable throat pouch for males of some species. It's even more extreme in the larger monkeys, such as baboons, mandrills, or proboscis monkeys.
Boobs! Er, to elaborate, human females are about the only creatures with naturally permanently swollen breasts, and they don't even need to become pregnant for them to get like that (pregnancy just swells them more). Other mammalian females have rather flat teats until they get pregnant. (Though some human women don't grow much in this area, either. Art also shows that what was average before the Industrial Revolution would be pretty small today.)
One theory is that this is due to the relatively flat faces humans have compared to other primates - our lips don't protrude nearly as much, so without those inflated breasts to suckle on, babies would mash their noses against tougher tissue, making it harder to breathe. The "aquatic ape" theory has things to say about buoyancy during breastfeeding instead. The actual reason remains a mystery.
The word manatee comes from a Caribbean word for breast. The extinct giant manatee is reputed to have had breasts.
Also, human male penises are gigantic compared to other primates (yes, even your Teeny Weenie is massive compared to a gorilla's; "hung like a gorilla" is not a compliment), leading some to believe that before the advent of clothing they, like human breasts now, served as a sexual display in addition to their other functions.
Another thing is that humans don't have a Baculum (penis bone), as one of the only two species of primate (including other apes; they are sporadically present among mammalian species) without. Erection is supported only by blood pressure so, in conjunction with the already over-average size, it means that sexual performance is directly connected to good health (both physical and mental).
Which just makes "boner" an ironic name.
We and gorillas both have a gestation period of about 8.5-9 months and we are both Hominids. A healthy human newborn will weigh roughly twice what a healthy newborn gorilla will and a human will be helpless for far longer. Because of this, human reproductive organs (both male and female) have to be larger.
The recently discovered fossil of an adult male Australopithecus afarensis revealed him to have been five to five-and-a-half feet tall. Compared to his female conspecific Lucy, at three-and-a-half feet, this is quite a substantial size difference between sexes, even if it's assumed he was tall and Lucy was short for their species.
Because humans practice body modification - at least to a far greater extent than any other species - this is most commonly used to vastly exaggerate these factors (though it depends a great deal on the relevant culture for, say, Long Hair Is Feminine). In some cases, this has led to a cultural expectation for this trope to apply when it actually doesn't. For example, in Western culture in the 21st century hair in the armpit is regarded as something men should have and women shouldn't, despite the fact that most women naturally do. Sure, men are usually hairier, but a woman who's healthy and fertile is unlikely to have none.
Some of these body modifications can also result in some forms of artificial sexual dimorphism, based largely on those same cultural expectations. In a few examples, the Kayan people of Burma have women who wear stacked brass coils that appear to greatly elongate their necks. In China, the notorious practice of foot binding meant that a good proportion of Chinese women had feet shaped differently then the males. This can even include things such as earrings and tattoos. Traditionally, earrings were seen as something only women wore and a woman with tattoos was seen as unseemly. While these may no longer apply in modern Western cultural standards, there are still some exceptions (for example, men don't normally wear earrings in both ears or large earrings like the giant hoop variety, and a woman with a fully tattooed arm is generally seen as unusual or noteworthy).
Elephant seals, whose males are about 3-4 times bigger than the females, and have a proboscis-like snout (hence the name).
Male rotifers (a type of freshwater zooplankton) are barely a quarter the size of females, and are so single-mindedly dedicated to mating that they don't even have mouths. The males starve to death after only a day or two, whereas the females (which eat algae) last a week or more after hatching.
Like spiders, some species of octopus have much larger females. An extreme example is the argonaut, where the female is about 5 times larger than the male, and secretes a calcareous egg case that looks like a nautilus shell, using a pair of highly-modified tentacles.
Female blanket octopi are about 2 meters long, while males are a few centimeters long.
Well known pterosaur Pteranodon is known to have differed so much between sexes that, like the creatures above, caused the males and females to be mistaken for different species initially. The male Pteranodon was much larger (an 18-foot-long wingspan was the average and some findings suggesting that even larger wingspans were possible as well) and had an enormous head crest on top of his head. The female was smaller (12-foot-long wingspan) and had a tiny or even non-existent crest.