Reminder that they can conceal their catgirl peripherals at will - Yuki's just bad at keeping that up while Kozue presumably doesn't feel any need to. Nevermind now the effectively magical full-body transformation into normal-sized cat because what is conservation of mass. ʅ (ツ) ʃ
The implication is that the Catulus phenomenom is entirely unnoticeable (except perhaps by in-depth genetic scan but that's not exactly something routinely done to children) until the physical symptoms appear at around 12 years of age, after which the afflicted are effectively shapeshifters with three forms - the baseline catgirl, regular human appearance with the Catulus traits concealed, and the more or less uncontrollable full feline state. Riko's offhand remark would also seem to imply some individuals sport two sets of ears - God only knows how the already-dubious cranial anatomy is supposed to work in those cases...
I always wondered about how their anatomy changes in regard to this condition. Aside: it somewhat irks me that it was referred to as an illness in the manga. Maybe a poor choice of words on someone's part? I will refrain from being too hung up on it.
Back to the point; I always rationalized it as either: (1) having typical external ears that are vestigial and cannibalized by the body upon reaching the requisite age at which symptoms (if that's the right word to employ here) manifest themselves. Or, (2) not being born with typical external human ears. Perhaps one of, if not the only early sign(s) that the girl may be a carrier of the mutation(s) that define the syndrome. Both of my rationalizations (correct or not remains to be seen) imply that the girls' actual auditory organs (tympanic membranes and such) are at the top of their heads from birth.
Or even crazier theory: they are born with "normal" ears that as they age toward 12 years old are slowly cannibalized while replacements develop at the top of the brain or skull (take your pick). Personally, I don't like this one.
Much as I like the cuteness of it all, those last two chapters were so super-tropish and cheesy, I don't know. It's just always the same in these type of manga, isn't it?
Since when are characters using physical exercise to help with their depression a common trope actually shown in media? (As opposed to presenting characters taking long runs as running from their problems.) I would love to see far more stories use this cheesy trope.