The Sirkin brain is better suited for processing relatively large quantities of precise data. In short, they have Intuitive Mathematician and Absolute Timing, and are sometimes compared to biocomputers in terms of capabilities. They're also much more likely to possess Photographic or Eidetic Memory than other races. Scientists also notice a very minimal set of parental instincts, and a slight aversion to power struggles, but these traits are rather subtle in a typical Sirkin's life.
Anatomy and Metabolism
Sirkin facial structure is characterised by higher level of symmetry than other races on average (they have Appearance: Attractive or better and Honest Face). Nariyása hybrids also possess eye-protecting membranes, just like their non-Sirkin parent. Anatomically, hybrid Sirkin resemble their non-Sirkin parents. The artificial Pure Sirkin are characterised by near-albino hair and eyes, and a very pale skin.
An anatomical peculiarity that is inconspicuous 99% of the time, but is glaring once known about, is that a female Sirkin does not have an uterus . . .
. . . And this where the fun begins.
Sirkin reproduce strictly by hybridogenesis, and a stealthy one at that. They rely on another Continental parent to gestate the fœtus, which is never a pure Sirkin. But let's not get ahead of ourselves.
Sirkin do not produce the same types of gametes as other species. Instead, their gametes are essentially symbiotic colonies that are capable of forming a fœtus under the right circumstances, or exchanging genetic data horizontally otherwise. After an intercourse with a Sirkin, they try to infest the partner's reproductive system. If successful (which it isn't always), they will stay there for years, perhaps decades. They will gather genetic data from the host's old or dead cells, and even from the host's gametes. They also don't like competition, so they'll fight off infections (granting +3 to HT against reproductive illnesses).
For a variable number of years (3-20 is often the quoted range), the colony stays mostly passive. After that, it starts taking measures to ensure progeny.
In male hosts, there's a simple tactic: part of the colony goes 'adventuring' along with the non-Sirkin gametes. These can either directly fuse with the female gamete, or, at a later stage, 'hijack' the zygote (i.e. eject some of its genome and replace it with their own). There's some chance of either occurring - or none.
In female hosts, the colony waits until it detects the presence of both gamete types (i.e. of the host's gametes and a non-Sirkin male's gametes), at which points it acts as above (again, with some chance of either or none occurring). However, if the opportunity doesn't present itself for many years (and very, very many generations for the colony!), the Sirkin gametes will just start trying fertilize the female gametes. Either way, once a fœtus forms, the colony either joins the fœtus, or dies off.
Note that this mechanism means that twins, only one of which is Sirkin, are quite possible. Also, while this results in hybrid somatic DNA, a Sirkin's reproductive system initially contains only Sirkin DNA (but see below).
Aside from hybridisation, there's horizontal genetic exchange among Sirkin. This changes the genotype of the gametes, but has no effect on the rest of the Sirkin's genetic code.
A method was developed in the Justicariate that allows artificially triggering the Sirkin gametes to form a fœtus without the presence of non-Sirkin gametes. However, this still requires gestating it somewhere. Thus, exo-wombs are used.
A Sirkin body grown by such method 'reverts' to the default pigmentation of the race, i.e. nearly none. While it is not always easy to identify such an individual as unerringly Sirkin, their near-albinism sure makes them stand out in the crowd.