Line drawing of several Sirenians smoking together. There is a large phocid wearing an elaborate hat with a fringe of reeds holding a pipe, a shorter selkie with a reed cloak and sandals, and a tiny shortwing harpy perched on the phocid's tail. The people of Siren in today's Era of Enlightenment do not closely resemble their own ancestors nor the precursor Humans on which their genome was based. Although there are families of distinct morphologies or 'types' of people, three of which are pictured above, it is important to note that the people of Siren largely live in mixed groups and that their cultures are geographically linked; it is more common for a Sirenian to self-identify as being from x region than as a phocid or a harpy or the like. I myself would sooner describe myself as a scholar from the Spire than a shortwing.

 

1. Size Chart (open in new tab for full size)

A line-up of the different types of people on Siren showing their sizes relative to one another. From left to right and in order of decreasing size there is a phocid, a zeta, a selkie, a longwing harpy, a landstrider harpy, and a shortwing harpy. In silhouette there is a figure of an unmodified human standing by the phocid's tail.

Extant Sirenians can be broken into two major groups: Aquatics and Harpies. Follow the links to find anatomical and evolutionary charts of these groups.

Finally a moment must be spent discussing the matter of Precursors...

1.1 Unaltered Humans

We have no evidence beyond fossil record of unaltered humans (known to us as 'precursors') and, until recently, could not speculate on their appearance and physical traits. For centuries, the were known only from skeletal remains. Their appearance may be odd, but we can see ourselves in their skull and hand morphology, and the fact that they - and we, by extension - do not appear to bear any relation to other animals on Siren. Therefore, it was a natural conclusion to reach that we were the living descendants of humans. And by 'we' I mean shortwing harpies; it was not accepted in earlier eras that aquatics or longwing harpies were related to precursors. These theories have now been proven false.

2. Reproduction

Two phocids cuddling

2.1. Sex

All people of Siren are the same sex and can perform the reproductive roles of insemination and pregnancy (barring injury, congenital variation, hormonal issues etc that might prevent someone from fulfilling these roles). This means that every GMO human was designed to have the same anatomical setup; internal testes (one for harpies, two for everyone else), ovaries (one for harpies, two for everyone else), uterus, cervix, vaginal canal, and penis which can be everted for use (i.e it is usually stored inside the body; in this case within the vulva proximal to the vaginal opening). Because they are all mammals, they all have mammary glands and two nipples. Selkies and zeta also have a two-chambered pouch with a muscular opening which can be entirely shut for diving and swimming. Everybody menstruates, however, this is a very quick (1-2 days max) stage and produces very little blood.

Sex between people of different body types is entirely normalised and happens literally everywhere but conception can only occur with two aquatic Sirenians of the same type (two phocids, two selkies, two zeta) and between all harpies. This is because harpies all arise from the same original population while the aquatics come from entirely separate populations and are far more genetically distinct.

When penetrative sex occurs between two parties, usually it's one penetrating the other. But because of the genital layout, it is possible for mutual penetration to take place if one of them sort of goes reverse cowgirl about it and there are no tails in the way. This is called 'doubling' and a lot of very vulgar terms besides. People being people, any other imaginable sex act is possible within the limits of biology.

2.2 Pregnancy and gestation

Gestation periods and pregnancies differ dramatically between body types. Phocids have the longest gestation period and can be pregnant for up to 11 months, giving birth to a single relatively precocious child who must be large and developed enough to be able to swim unassisted from birth. 

Selkies and zeta, as marsupials, are only pregnant for three months before the very undeveloped foetus is born and transferred by hand to the pouch. It's relatively common for the marsupials to have twins but triplets present an issue in there being only two chambers of the pouch. A surrogate nursemaid has to be located otherwise the third foetus is unlikely to survive. The baby is able to emerge for short periods at around 9 months but won't remain outside fulltime until almost two years of age. While nursing a child, an aquatic marsupial cannot be submerged for long periods of time. Baby needs to breathe too. 

Harpies all have the same gestation period, a cool four months. The foetal growth is inconsistent; it seems to grow slowly the first three months before entering a developmental spurt for the last few weeks. This minimises the amount of time a flighted harpy has to sit around unable to fly. Longwings have relatively good heavy-lifting abilties in flight and tend to handle it well, while shortwings are grounded for the last two or three weeks. During this time they need to find a safe space and, if they live in a small village or if they're solitary, they need to plan to be tied to that safe space for about a year, as the baby is born very premature compared to a regular human baby and requires constant care. 

In many villages with high shortwing populations, a communal nursing bower becomes a necessity. The one in the Spire is particularly large and infamous; every shortwing citizen of the Spire is obliged to spend a couple of months working in the nursing bower every few years. It's similar to jury duty, we get a summoning letter and have to drop our work and report for duty. Birthing parents will leave pretty much asap, and likely have no further contact with their child after that. It's the nursery workers' job to raise the children in similar-age groups, feeding them and providing their first lessons. Shortwing children grow and mature very rapidly after birth and are considered teens at around 7 [Earth] years old. They are named by an official from the university who assigns each kid a unique prefix which will be used as the first part of their adult name.