As everybody is aware, Kangaskhan poses many problems to Pokéscience. The number one thing that is of great concern is how Kangaskhan is born with a baby Kangaskhan already in her pouch. The second is how nobody knows whether baby Kangaskhan evolves into adult Kangaskhan. The third concern is how they are always female.
According to almost every Pokedex entry, anime episode and pretty much everyone in the world, Kangaskhan is baby Kangaskhan’s mother. It seems fairly well established, especially with the Parental Bond ability that mega Khan has. I have only one way to explain the anomaly of Kangaskhan hatching out with a baby already in her pouch and it relies on the theory that eggs are not eggs, but are actually ‘cacoons’ that house a baby Pokémon until it is ready to evolve. If this is the case, then the egg cradle actually holds twin Khans, but only one evolves upon ‘hatching’. Therefore the very first baby that Kangaskhan carries in her pouch is not actually her baby, but her sister. Since baby evolution is based on happiness and not experience, one could easily evolve before the other. After approximately three years or more when baby Kangaskhan is ready to leave the pouch because she is happy and well cared for, she evolves into an adult Kangaskhan and reaches sexual maturity. The original Kangaskhan can pop out another baby almost immediately because of her resemblance to the real life kangaroo. Kangaroos who are pregnant but also nursing can suspend the embryonic growth of the foetus until the older baby has been weaned. This process, called embryonic diapause, prevents the blastocyst (the very small embryo) from attaching to the wall of the uterus and developing. Mother Kangaskhan utilises the same method to give birth to a new baby as soon as the older one has left. Alternatively, she allows the development of the embryos, but bundles the babies up in a ‘egg’ if she is already keeping one in her pouch. Like many animals like Callitrichine monkeys of bears, Kangaskhan are very likely to have twins.
The second question, how nobody has seen a baby Khan evolves into an adult Khan, could be based on the protectiveness of the Mother while the baby evolves. But really, when it comes down to it we are taking the word of professors who can’t even tell whether we are a boy or a girl, so can we really rely on what they ‘see’ anyway?
An explanation of how she breeds without a ditto or male Kangaskhan is really quite simple. Pokémon breed within egg groups, which is essentially their genus or family, instead of breeding within species. Kangaskhan doesn’t need a male Kangaskhan to breed with but will suffice with anything in the monster egg group. It is much more difficult to explain how Tauros or Mr. Mine were bred before Ditto. However, all Kangaskhan are female, and this is of some concern.
In humans, sex is determined but the type of allosomes (a fancy word for sex chromosomes) one possesses. Females are determined by two X chromosomes and males are determined by an X and a Y chromosomes. The Y chromosome carries the gene SRY, which does some magical biochemistry to cause testis development. The X chromosome does not have the SRY gene so no male development occurs. Since this is the type of sex determination system everyone learns at school, it is easy to see why there is so much confusion about mono-gendered Pokemon. However, biology is never that simple.
I have two theories about why Kangaskhan are all female. The first is that the sex determination system of Khan is similar to that of the tuatara; namely, temperature sensitive sex determination. This system does not rely on the type of sex chromosomes one possess but rather the temperature at which the embryo was incubated. This occurs because enzymes function at different ideal temperatures, and since enzymes are the magic behind biochemistry, the enzymatic patterns determines the expression levels of the sex determining factors. This usually occurs in reptiles, but Kangaskhan is a mammal. However, Khan has some reptilian cousins (Tyranitar, Rampardos), which may mean they most likely both share a reptilian common ancestor for her to retain these genes from. However, now that they are mammals they retain a constant body temperature, and we all know that she keeps her embryos inside of her. If this is the form of sex determination that happens, all the embryos will be incubated at the same temperature and therefore be the same sex: female.
Alternatively, sex could be determined in the same manner as the disgusting little insect called the false spider mite. This mite has predominantly female eggs, but in a peculiar way. False spider mites determine sex in an XX XO system. Females are haploid, or have only one chromosome, and males are diploid, or have a chromosome pair like humans. The eggs of the mites are infested with a bacterium that somehow kills off sperm to prevent fertilization of the eggs, leaving only eggs that have one chromosome. This is likely due to the bacterium using the female egg as a host, which the males cannot produce. The bacterium preferentially kills off sperm so there are lots of females to lay eggs for them to live in. With haploid sex determination, no fertilization is needed to produce offspring. This also happens with bees, except that males are haploid and females are haploid. However, since there are so many female only Pokemon (Chansey, Mandibuzz, Jynx) and there are all from different eggs groups, it is easier to assume the latter theory than to former, because the former would suggest that were all closely related. Who knows, maybe it is a combination of the two?