Bizarre Sexual Dimorphism
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Basic Information

Sexual Dimorphism is the technical term for the physical differences between males and females of a species.

Bizarre Sexual Dimorphism is a trope wherein a sci-fi novel or film takes this to an extreme level. The males and females might not even plausibly look like the same species.

Related Tropes

In Reality

Actually, there's plenty of species here on earth that display bizarre sexual dimorphism. Birds where the male is ridiculously colorful and has huge plumage, but the female is drably camouflaged. Insects where one sex is five times the size of the other. Queen bees. The male giraffe weevil has a neck three times as long as the female. In some species of deep-sea anglerfish the male is just a tiny parasite that lives inside the female. There's even a species of shrimp-like crustacean (in Order Tanaidacea) that has two different types of male - one with long sensory organs and one with huge claws.1

At the other end of the spectrum, there are species on earth that are naturally hermaphroditic, and yet other species that scientists can't determine the gender of short of dissecting them.

And that's just real animals, right here on earth. For aliens, or fantasy genre monsters, things could get a whole lot stranger.

Humans, with our external genitalia, breasts and various secondary sex traits are probably slightly closer to "bizarre" than we are to "indistinguishable". Generally easier to sex a human than a cat, for example.


1. Website: TV Tropes Wiki
2. Non-Fiction Book: The Science of Aliens by Clifford Pickover

Game and Story Use

  • Useful for those one gender races in the fantasy genre. Gorgon, Satyr, Lizard Man, Nymph, Dwarf, Succubus, etc.
    • Not sure how mermaids reproduce? If it really matters, just pick another species of piscean aquatic creature from your game system of choice, and make them the same species.
    • The same goes for any other "race" or "species" that is always depicted as being a single gender. Pick two, probably of opposing genders, and mix.
    • Alternatively, rule that they have at least two genders, but little or no dimorphism so that humans can't tell them apart without dissecting them.
      • This would go for dwarves - it certainly does for the dwarves of Terry Pratchett's Discworld - dwarves are a standard, two gendered mammalian species, but with so little dimorphism that they can struggle to tell which gender one another are, and non-dwarves have basically no chance at all. Add to that a culture which rejects any kind of gender differentiation and the result is - to humans at least - what appears to be an entirely male species. It's not that you never see a female dwarf, you just can't tell them from the males. (The appearance of a female gender identity following contact with humanity is a plot point - Pratchett missed the opportunity to have humans going the other way in search of gender equality).
    • You could also use the anglerfish method (as per the Pak'mara in Babylon 5) - one gender isn't obviously a creature.
    • A less extreme bust still bizarre dimorphism could have a species where one gender is non-sapient (like Kzinti2 females or Khepri3 males) or even sessile (such as the Xiang females from the 2300AD RPG).
  • If making your own species for a game, sexual dimorphism can make them memorable.
    • In cases of non-mammalian life, verisimilitude just about demands that the differences between the genders are nothing like what we have in humanity. Could be more or less, but certainly not the same. At least not if they don't give live birth and feed milk to their young.
    • When considering the differences between the genders, the GM should think about how the species' lives. Evolution will steer towards distinctions that matter and give increased chances of survival and success.
      • Male birds are colorful because they have to display to attract a mate; the females are camouflaged because they sit in the nest and incubate. If those roles are reversed, the coloration will be as well. If both genders raise the young equally, both should be equally camouflaged.
      • If one gender does most of the fighting or hunting, it may well be larger than the other, or have better natural weapons.
      • Alternatively, the larger gender may be larger simply because it needs more internal space for its reproductive system - the reason many female reptiles, fish and amphibians are larger than their males.
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