Yinrih reproduce much like salmon. They are semelparous (only reproducing once in their lives) and exovoviviparous, which is a reproductive strategy not found on Earth.

Both female and male yinrih have a cloaca and lay eggs. Male and female eggs are externally similar, such that most languages do not differentiate between them in common speech, but both genders must contribute to a clutch in order to have a viable litter. Fertilization, as it were, begins when the male and female eggs are placed in a clutch together. The eggs meld together to form a complex structure called a womb nest. The womb nest possesses a simple heart and circulatory system, with a highly vascularized dermal layer for gas exchange.

Embryos form and grow within individual amnions that line the walls of the womb nest. Each amnion is attached to a central artery running down the center in a bus configuration. This arrangement is what gives interstellar womb ships their name.

Gestation takes about 144 Earth days. The process of emerging from the womb nest is called yeaning. Offspring are called kits from the time of conception until they are weaned, at which point they are called pups or puppies. Age is reckoned from the time of conception, and there is no concept of the firstborn. However, in very large litters, the kit closest to the heart gets more nutrients than the kits further away. The furthest kit tends to become a runt, and the closest may be quite big.

The number of parents can vary between two to twelve, as long as there are an equal number of mothers (called dams) and fathers (called sires). Each kit is a genetic combination of the contributing parents. Culturally, more is almost universally seen as better, to the point that accusing someone of having only one sire and dam is akin to calling someone inbred. LItter-size is proportional to the number of parents, with a surplus of 1.5 kits per parent being the usual statistic.

A group of yinrih parents is called a childermoot. Because they are semelparous and because reproduction does not involve physical contact, yinrih completely lack a libido, and regard their fellow parents more like fellow teachers in a school. The drive to reproduce instead manifests as broodiness, an overt desire to have and raise children.

There is no concept of marriage. A childermoot is only expected to remain together until the children are raised, which takes about 53 Earth years. One is expected to remain in the childermoot during this time, however, which gives rise to the phrase “You put your egg in this nest” meaning to make a long-term commitment.

Paternal care is much stronger in yinrih than it is in humans. Sires instinctively guard the womb nest as the kits gestate. In the wild, the womb nest was a target for ovoraptors as well as conspecifics from other groups seeking to keep the number of enemies down.

Kits come out of the womb nest blind and immobile, their eyes opening after a few days. Kits will nurse for a few years, licking milk sweated from their dams’ paws (sires do not lactate). This gives rise to the mild oath “By the palms that nursed me!” or simply “Palms!” to mean something like “oh wow!” or “holy crap!”

Yinrih do not recognize blood relationships beyond those between parent and child and between litter mates. Those relationships are extremely strong, however. Grandparents do not typically interact directly with their grand-pups, only doing so indirectly through interactions with their own children who are now parents. Childless adults, especially women, may play a special role in the lives of their siblings’ pups, usually as the ‘cool aunt’ who sugars them up and sends them home with a can of silly string and a noise maker.

Child rearing styles vary throughout place and time, but three strategies are common at the time of First Contact. All parents and the entire litter may share a single home, Parents may live singly, with each parent taking care of a few pups at a time in rotation, or lone parents may concentrate on the same few pups throughout their puppyhood, with the other parents and siblings being regarded as very involved aunts/uncles and cousins.