I'm asking about the 4E Doppelganger/Changeling race, but I am also wondering about humanoid shapechangers in general in any edition of D&D/Planescape. (I.e. Can a Succubus assume male form? If so, can she impregnate someone? What if a Doppelganger got pregnant in female form, then became male? Would s/he be forced to change back in nine months when it was time to deliver?) Or are the changes only superficial? I think it'd be pretty cool to play a race that can be either gender as desired! I would think such a race would have to have a broader understanding of each other because the males would understand what it's like to be female and visa versa. To say nothing of the fact that they could become Elves, Dwarves, etc...
Can Doppelgangers/Changelings switch their gender, and if so are they, um, fully functional?
About the Succubus: It is my understanding that Succubi and Incubi are the same race/creature type, and that there's no "true" gender for either of them; merely the gender they prefer, or the gender they are assuming at the time. I could be mistaken.
I think that Succubus are exclusivly female as the Incubus are exclusivly male so they don change they gender, but as Anetra said they are actualy the same type of fiend (they have same stats, but also note that Incubus are less common than Succubus), fiend that have specialised in field of seducing mortals, one seduces males other seduces females...
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One-eyed, one-horned, flying, purple people eater says: "Monsters are nature's way for keeping XPs fresh."
According to Faces of Evil, all tanar'ri can change their sex (and be functional) with a few days worth of effort and will. It's related to their ability to evolve into new forms, but sex changes are comparatively easy for them.
According to "The Psychology of the Doppleganger" in Dragon #80, dopplegangers are sexless in their true forms, but reproduce by taking the form of male humanoids and mating with female humanoids. They cannot mate with their own species. They're born looking exactly like their mothers' species, but gain shapeshifting powers, telepathy, and knowledge of their true natures at puberty. Of course, if this is true, the half-doppleganger changelings of the Eberron setting would have to have another origin (since all dopplegangers are the result of dopplegangers mating with other humanoid races, true hybrids would probably have to be created through arcane or divine magic, or conceivably psionics).
Dragon #241 introduced a variant doppleganger known as the su-doppleganger. Created entirely from Suel humans during the Baklunish-Suloise Wars, su-dopplegangers are not truly related to other doppleganger types. They resemble true dopplegangers, but while standard dopplegangers have gray skin and eyes in their true forms, su-dopplegangers have pure white skin and blue eyes. Like standard dopplegangers, su-dopplegangers are sexless, but their reproduction is the opposite of that of the dopplegangers described in Dragon #80: they must take on female form, and can become pregnant with new su-dopplegangers after mating with a human male.
The other significant source (other than Races of Eberron) is The Complete Guide to Dopplegangers from Goodman Games. It's written by Keith Baker, so perhaps it reflects his assumptions about how dopplegangers work in Eberron.
In The Complete Guide to Dopplegangers, dopplegangers and mimics are different phases in the life of the same species. Dopplegangers eventually become sessile and evolve into mimics, and mimics evolve into dopplestadt, which are like mimics that take the form of entire buildings or neighborhoods. Dopplegangers don't reproduce sexually at all: rather, they're spawned asexually in spawning chambers within the body of a dopplestadt. The dopplestadt buds portions of its own flesh and allows them to separate; these form into new dopplegangers, with all the memories and personality of the dopplestadt that spawned them.
Steel dragons, according to "The Ecology of the Steel Dragon: Soulbond" in the Dragon Magazine Annual #3, can take the form of either male or female humanoids, and can reproduce in either form. Their offspring in these forms always belong solely to their humanoid parent's race (they're not half-dragons). They make new steel dragons by mating with other steel dragons in dragon form (presumedly as the sex they were born with).
Hm. That's an interesting life-cycle. Remember Nimicri, the Orb-Town on Gehenna? This gives me the notion of the doppelstadts worshiping Nimicri as a minor god and perhaps the First Spawner of their race.
Not knowing about this, I actually used doppelganger reproduction as a plot point in a recent arc: a PC was accused of various crimes involving children and an orphanage, and while it got complicated the upshot was that the dirty deed was partially done by a doppelganger in his form. Why? Because doppelgangers had kids (could still be spawned by doppelstadts, though with this I'd actually make mimicing and the doppelstadts a post-reproductive phase, or perhaps have doppelstadts spawn infants with input from motile doppelgangers) which had to be raised by other races; the infant would instinctively take on the race of its prospective parents by imprinting, and maintain that form until adulthood, as it learned about how to bluff its way through life as that species particularly well.
I seem to recall it being stated that Doppelgangers were created by sorcerers and were not a reproducing race in themselves. I have always had doppelgangers grown through vile magical rituals, like a kind of magically corrupted plant. Doppelganger pods hang from sprawling vines like ripening fruit until they are mature, at which point they hatch forth and go about their business ... unfortunately some of these hatcheries outlived their creators and continue growing new Doppelgangers as they spread out to commit their evils.
I'm fairly certain this is one of the things that was distinctively changed between 2e and 3e. I'm not familiar with 4e so I couldn't say if it has been changed further.
3e allowed a far largely degree of inter-racial reproduction than 2e, most obviously allowing all true dragons to produce half-dragon offspring, not just Gold, Silver, and Bronze as in 2e.
Regarding dopplegangers, Dragon #313 posited a half-doppleganger template that was stated to be the offspring of a doppleganger and another humanoid. This would imply that in 3e dopplegangers have to mate with each other in order to produce new dopplegangers. Perhaps they have to be in assumed form in order to do so? They are still genderless for certain, the doppleganger mindspy in Complete Warrior is referred to as 'it.'
I would imagine that for a doppleganger to carry a child to term it would have to remain in female form the entire time, otherwise a spontaneous abortion would result. The same would be true of demons and celestials that change genders, and likely even dragons that change shape (though a female dragon could return to a draconic female form as opposed to whatever form it mated in).
I would inclined to say that a creature that does possess a defined gender cannot produce offspring while in the form of another gender, if for no other reason than because if dragons and the like could, there'd be little to stop humanoids from using polymorph or shapechange to the same effect (one might rule that there's a sort of subconscoius regulation in effect, even if all the apparati in question function, a creature that defines itself as male, will not become pregnant as a female). Of course that does leave the extremely complicated question of taking on hermaphroditic forms (ie. Illithids), but perhaps such rare cases can just be avoided.
As far as I know, there's no details on this for any edition in 'general' D&D. However, as Changelings ("Lesser Doppelgangers", when all's said and done) are a part of Eberron, the old "Races of Eberron" sourcebook went into that sort of detail. To sum it up, Changelings (and presumably Doppelgangers, as they're one and the same now in 4e) begin life sexually neuter, neither male nor female. Early into childhood -roughly around 4 or 5, I think- they become either male or female: this is their "true" gender, the gender they will assume when they return to default form (the genderless form adopted by Reality Seekers is a deliberate, artificial form of their own creation). Once they attain puberty, they gain the power to assume the opposite sex.
As this is a physical transformation, Changelings are perfectly functional in either gender. So, yes, a female Changeling could get another woman pregnant by assuming a male gender, and a male Changeling in female form could become pregnant. Conceiving a child "locks" a Changeling, though I don't recall if it's only into that specific form or if it merely locks them into being female until they give birth. I don't know how far along the conception needs to be to lock them, either.
If you're interested, Changelings can't crossbreed; there's a flat 50/50 chance the child will either be a new Changeling or a member of the non-Changeling parent's race. There's a feat that actually lets the Changeling choose what race its children becomes, though I think it might only apply to children that the Changeling actually carries.