The only commonly accepted facts are (though there are exceptions even from this):
- Yukkuri reproduction is extremely fast, but wide variations now exist.
- Yukkuri reproduction is sexual.
- All Yukkuri are hermaphrodites.
- The children are always a copy of one of the parents.
- The partners need to be different types of Yukkuri.
If the pregnancy is a reaction of the mother's body to injection with filling of another type of Yukkuri (as it is in some insemination methods), the requirement for the partners to be different types of Yukkuri becomes natural. However, the injection method varies between artists- in some comics and stories, injecting a Yukkuri with the filling of another will cause it to become a hybrid (or at least, give birth to hybrids).
The "traditional" combination is insemination by cheek rubbing, and then birth by stalk growing method, but since other combinations are possible, the next section will be split. This method has been overtaken in popularity by the Peni Peni method.
Note: Since the Yukkuri are hermaphrodites, henceforth "mother" will refer to the partner that gets pregnant and "father" to the one that does not.
There are several known insemination methods.
- The two Yukkuri rub against each other with their cheeks, and it is somehow decided that one of them is the father and the other is mother. See Rub Rub.
- The father inserts her Peni Peni into mother's Mamu Mamu and injects some of her filling.
- The father climbs on top and behind of mother, and rubs. The details are not explained, but the visuals bring "doggy style" position to mind. (Sometimes,the Yukkuri refer to this as 'anyaru')
- The father gets behind, beside or climbs on the mother, facing her, and begins rubbing, secreting a fluid to soften the mother's skin (dough). After a while,the father extends her Peni Peni and thrusts it,in- penetrating the softened dough and injecting some of father's filling. A similar way of reproduction is used by many sea and land snails.
- Insemination may also happen if the filling of another yukkuri is injected by any other method, for example,by human researchers, or the Yukkuri is splashed with it. It must be pointed out that only the filling of an aroused Yukkuri can induce pregnancy, in other cases it just works as food, a schizophrenia inducer, or modification of the DNA in the case of injection of all the filling.
After the insemination, a pregnancy and birth will follow. The selection of baby development way depends on the insemination method: Cheek rubbing results in stalk pregnancy and filling injection results in mammalian pregnancy.
An exception to the rules stated above is when the father is a Yukkuri Alice in Rape Mode: its seminal filling has a random pregnancy property, so it is possible to bear children in both ways.
The amount of pregnancy-inducing filling which a Yukkuri is exposed or injected determinate the number of stalks or children which will grow inside. In the case of Yukkuri of approximately the same size, the amount of filling emitted in one injection leads to one stalk or one inner children. This ratio varies with size differences: the bigger the Yukkuri, more offspring will spawn in a smaller Yukkuri with each filling injection or exposition.
- The mother grows a stalk from above its forehead
- A number of koyukkuri, usually about 4 (but in a normal range of 2-7), grow on the stalk like fruits (though a similar version exists.)
- When they reach a certain size (about that of a cherry tomato),the mother shakes the stalk and the koyukkuri detach from it. This action is called "birth" (even though they're conscious and able to speak as soon as they're born). The newborn yukkuri's first action is generally to announce to the world "Chake id eajy!"
- Once all the children are detached or born, the mother can shake the stalk until it falls off, at which point it can be eaten by the koyukkuri. It's said in some stories that the stalk contains essential nutrients and antibodies that strengthen the newborn Yukkuri. While in the wild,that poses a problem in delayed pregnancies (as the already born koyukkuri can't be fed until all of their siblings have fallen off of the stalk), pet koyukkuri can be fed steamed broccoli, or specially acquired stalks, coming from miscarriages at the Factory or in a Yukkuri Pet Shop
- The stalk is also described as bitter and hard to chew, so hard that if the mother Yukkuri doesn't soften it with its teeth, the koyukkuri may risk breaking their own, or being unable to bite through. Actually, eating the stalk first is a good thing, as a koyukkuri eating sweet things will be spoiled, and refuse to eat anything else.
- As a byproduct of the genetic memories every yukkuri has and the period of pre-birth consciousness, koyukkuri are born with full knowledge about parental and communal life, anxiously seeking to bond with their parents and sisters. If a koyukkuri falls later than the others from the stalk, the other siblings may cheer it actively
- Even if all the koyukkuri in a litter fall at a little interval between each other, the first one to fall will be considered by its entire family the "big shishter", and the last one will be the "widdle shister".
- This method is described as very fast, the whole process is completed within 5-7 hours, or one day at most. (Though, as usual, some sources vary, giving it duration in the range of several days to several weeks)
- Each intercourse creates another stalk, there is no limit for the number of stalks.
- Since each stalk drains some nutrients from the mother, there is a risk of terminal exhaustion. This is the typical outcome of Yukkuri Alice group raping an other Yukkuri.
- Stalk development generates a lot of stale filling which is absorbed and recycled in the mother's body. When stalk is cut in any place above the temple, stale filling starts to accumulate in the filling or the filling core, resulting in the death of the baby. Maybe that's the reason why a massively raped Yukkuri and all its stalks die: They are full of shit (Literally).
- Even a single stalk may risk killing the mother if she's been weakened by other causes. If an immature yukkuri gets pregnant, a stalk without koyukkuri will grow and sap out all the nutrients, eventually killing it. It may also happen to a Yukkuri Patchouli due to their generally low health.
- Yukkuris killed through such draining turn black and shriveled.
- Even if the mother dies, some children may be born successfully.
- Koyukkuri are extremely fragile,so the mother has to be careful as not to damage the children. Sometimes just moving (by jumping) may cause them to detach prematurely and die. And even a koyukkuri that has grown enough to detach may be damaged if it's not conducted properly.
- The fallen stalk may be used as food, but it often portrayed as too hard to be eaten by the weak children, in such case,the mother will chew it to soften it before feeding it to the children.
- If the koyukkuri, or stalk, detach prematurely, it is said that they can be kept alive and growing by feeding them sugar water. This can be done either by directly feeding them through a pipette (koyukkuri alone), or by immersing the root of the stalk in a bowl of sugar water (koyukkuri and stalk).
The order of development seems to be as follows: After the stalk grows, several buds form, which grow into the soon-to-be-born-koyukkuri. The body grows first, developing its eyes and mouth. The hair and accessories develop later in the process.
- Usually,a single child develops inside the mother's body. By the end of 2008 and beginnings of 2009, pictures of Yukkuri giving birth to several children through mammalian birth have become more common.
- When the child has grown enough, it's pushed outside through the Mamu Mamu opening.
- This method is usually described as slower than the stalk method, taking a couple of days to develop the child.
- This method is one of the most frequent, if not the only one, employed by Aquatic Marisas. By using the mammalian method of labor, the mother Marisas are able to make their children fall headfirst in the water, thus allowing them to balance on their tiny hats instead of drowing, as they would do with a stalk birth or a regular mammalian birth, falling with the bottom part in the water.
- The mother's body increases its size to accommodate the offspring, so it can't move around a lot.
- During development, the child or children feed on mother's innards to develop; so mother must have a constant supply of food to avoid death by consumption, which must be supplied by other Yukkuri.
- The birthing part may be painful and/or dangerous for a mother yukkuri.
- The mother will always announce its birth and complain about the pain by yelling a slurred "Gwibin bwird" while pushing its koyukkuri(s) out.
- If the mother is startled at the moment of birth, it will clench the Mamu Mamu muscles, which may result in crushing the child or propelling it out at high speed- even fast enough to kill it on impact.
- According to some sources, the ability to expel the child is a byproduct of the inner elasticity of a koyukkuri and the movement of Mamu Mamu muscles. As such,a miscarriage ends with the inability for the mother yukkuri to expel all the crushed filling and pieces of manju skin from its Mamu Mamu, heightening the risk of mold and infections.
- Sometimes,the child may be shown conscious and talking while being born (Much like stalk reproduction) especially if the labor was somehow delayed: in this case,the child may end happily begging its mother to "let cute yukkuchi being bown"
- Some varieties of the same Yukkuri type grow the children in the bottom of the Mamu Mamu, which forms a bladder to accommodate them. The feeding is done by a umbilical cord attached to the top of the head, like in the stalk pregnancy. Children cannot develop in mother's filling because they would be digested as food.
A variant of the stalk growing method, except instead some fruit grow on the stalk instead of koyukkuri. Several sub variants exist:
- Inside a large soft fruit,there's a hard shell (commonly known as Drupe Fruit) . The koyukkuri grows inside this shell protected from the outside. The fruit's soft tissue is a defense mechanism- a predator would eat the soft part, discarding the hard shell, thus the koyukkuri would survive the attack.
- A big fruit is formed and a koyukkuri begins growing inside. Then the fruit is detached from the mother and placed in a safe place (perhaps kept in a warm temperature). The child develops inside, gradually eating the fruit (unlike the Drupe Fruit method),and eventually breaks through the outer shell and is born. It's unclear what the advantage of this method is.
Both of these methods however, are used only among specific artists and are very, very rare compared to the extremely large body of works of normal development.
Frog Egg Sack
Suwako is the only known Yukkuri which lays frog egg sacks to reproduce.
One comic depicts an extra small sort of yukkuri known as House Yu or Cockroach Yu (the adults are the size of billard balls) adapted for living in human homes, usually under furniture. These yukkuris hatch from white eggs between one and two centimeters in diameter. The unhatched babies react to their own eggs being burned or crushed. Since the House Yu are a Yukkuris subspecies adapted to life as pests, the egg reproduction accounts for their constant need to reproduce and diminutive frames.
The yukkuris seemingly develop inside a bean pod of some sort, attached to the mother like a stalk (not to be confused for Stalk Reproduction). This is even rarer than the other two above, with only two pictures depicting it to date.
Some artists have drawn a certain type of koyukkuri, namely, the premature yukkuri. Premature yukkuri have a very distinctive appearance, having a very small patch of hair (if any) and a disproportionately small (though otherwise fully formed) accessory. Some premature yukkuris may not be able as well as their offspring, limiting their language skills to severely slurred cries of "eaji!" in a failed attempt to proper greeting their siblings. If able to properly feed and cared for, premature Yukkuris may grow up into regular koyukkuris, with their hair and accessories developing and increased social skills, equal to a "properly born" koyukkuri. However, such as with the runts in a mammalian litter, often parent Yukkuris and the other sibling may decide that the premature yukkuri is just "Uneasy" and stop caring for it, especially if there are low resources. In other case, the premature yukkuri will be simply cared of as another koyukkuri in the litter, with the other ko teaching the "widdle shish" how to grow up "Eajy".
A condition of developmental impairment in yukkuri sometimes described as a sort of yukkuri Down's syndrome known as Buriism. They seem to result when mothers are subjected to stress or malnourishment whereby they are unable to supply the nutrients necessary for the developing koyukkuri. When a yukkuri parent has a long stalk carrying multiple yukkuri, it sometimes has insufficient nutrient flow and koyukkuri hanging from the extremity (which end depends on the direction of nutrient priority chosen by the artist) have a greater chance of being buriistic. It's also possible that some yukkuri are more bean-paste-genetically inclined to produce the occasional special needs child. There seems to be no hard line for what is or is not a yukkuri with buriism and the condition runs from very mild (a few characteristics distinguishing a koyukkuri from its siblings) to profound. There is some overlap with Deibu (and their non-Reimu counterparts) types, which could be an aggressive, anti-social subtype of buriism. Normally however, a buri is more retarded/helpless/ugly than actively sociopathic. A buriistic yukkuri is generally distinguished by having (often profoundly) underdeveloped hair and accessories, distorted features (oversized cheeks, mouth very close to the eyeline, ridiculous expressions etc) and generally being much less cute than developmentally normal yukkuri. They may have poor bladder and/or bowel control, drool excessively,and display well below average yukkuri intelligence to the point of sometimes being unable to speak and/or say "take it easy" or recognize their own suffering. Despite these disadvantages, many buri yukkuri do survive to maturity, form buri-families and, when nurtured with proper social support, are able to lead full, easy-taking lives. In at least one case, buriism was portrayed as an infectious mutation.
The offspring of a Yukkuri Sanae are always Yukkuri Kanakos and/or Yukkuri Suwakos, no matter who the mate is. In order to get Yukkuri Sanae offspring, a Yukkuri Suwako must be impregnated/raped by a Yukkuri Kanako.
Wriggle Post-Mortem Reproduction
Yukkuri Wriggle only reproduces after death, releasing enormous amounts of miniature Wriggles from the interior of its corpse.
Peni Tengu Mushrooms (PeniTenguDake) is a mushroom used by Yukkuri Alices for getting very strong Peni Peni erections. However, this mushroom is teratogenic agent which alters the seminal filling and provokes some mutations if a Yukkuri is impregnated with it:
- Baby stalks become extremely hard, turning into trunks. Any babies growing on it are absorbed while it's hardening. These trunks are hard enough to pierce a Yukkuri and can break if enough strength is applied to them.
- Any babies which are able to born from these trunks have exaggerated sex drives.
Yukkuri Alices use them for refreshing without pregnancy, this is done by dry humping the mate's Mamu Mamu and ejeculating outside when they get to climax.
Alices have also been shown to have access to other medicines, all of which have a similar effect (peni peni erection, increased sex drive) though with varying side effects. (Whether this is the basis for the Factory's Viayugra, mentioned below, is not known.)
It is possible to castrate a Yukkuri to curb its sex drive and restrict the pregnancy options to stalk pregnancy only. Castration procedure is as follows:
- Yukkuri is aroused to expose the Peni Peni.
- Peni Peni is cut.
- The hole where Peni Peni was is patched with dough or Yukkuri skin.
This method has a few limitations:
- Yukkuri which don't form a pregnancy bladder risk to being eaten alive by the babies, unless babies die by despair or die by being accidentally eaten by their siblings or they are born by cesarean section or by going out through the anyaru
- Mutant yukkuri Alices may be capable of growing any amount of Peni Penis from any spot on their skins.
There is a castration set produced by a company called Yun-Ya. It is designed to castrate Reimus and Marisas in a painless fashion. The set contains:
- A lemonade flavored pill which contains an anesthetic.
- A small bag containing Viayugra, a Peni Peni stimulant. The medicine is administered as an enema.
- A castration clamp with four blades.
- A container with concentrated orange ointment for repairing the spot where the Peni Peni was cut off.
- A touch-up sheet, made from the skin of a two-week old koyukkuri.
- A spatula, which is used for reshaping the bean paste after amputating the Peni Peni.
The castration instructions for this set are:
- Give pill to Yukkuri, to make it sleep.
- Administer Viayugra via anal hole, to provoke a strong Peni Peni erection.
- Grasp Peni Peni with castration clamp.
- Turn castration clamp until Peni Peni is cut.
- Use spatula to put back the filling coming out of the hole.
- Apply ointment around the hole.
- Apply touch up sheet in hole.