The Na'vi are monogamous creatures who mate (Na'vi name: muntxa si meaning "mate with, marry") for life. The mechanics of their reproduction are similar to that of humans and other Terran mammals. However, their unique physiology creates an experience fundamentally different from that of humans.
Choosing a mate
Traditionally, once a young Na'vi has passed their clan's rites of passage and are accepted as an adult, they are allowed to choose a mate. Though it is typical for male Na'vi to choose a female mate, and vice versa, same-sex pairings are not uncommon and do not appear to be stigmatized in Na'vi culture. The process of selecting a mate can take many years, though arranged marriages also occur in Na'vi culture, such as between the future Tsahìk and Olo'eyktan of a clan.
After an appropriate mate has been selected, the Na'vi couple will mate before Eywa, often at a sacred site such as the Tree of Voices. During the mating, the couple will connect queues in an act called tsaheylu. This creates a state of unified body consciousness, in which both parties access the physical sensations of the other. A single instance of this act will create a powerful emotional bond that lasts a lifetime. The intertwining of queues is both highly erotic and profoundly spiritual, but is not the actual act of sexual reproduction. The tsaheylu bond between Na'vi during mating should also not be confused with the non-erotic tsaheylu bonds Na'vi make with animals and plants, like the "mental reining" that Na'vi use to control animals or the use of the queue to access the Pandoran Neural Network.
The couple will remain bonded until the next day, when they return to their clan as lifelong mates. If a Na'vi's mate leaves them or passes away, it is unknown whether their partner can or will choose another mate.
Sexual activity outside of the bonded pair is not uncommon, but is almost always associated with fertility rituals and other rites, and has never been reported to lead to a disruption of the pair bond.
A Na’vi female will typically produce from one to ten offspring, often spaced widely apart in time. Na’vi females have two breasts, and nurse their infants for up to four months. Directly after birth, newborn Na'vi perform their first instance of tsaheylu with their mother to form a strong emotional bond and with her, then they are brought to the Tree of Voices to connect to the Pandoran collective consciousness for the first time. Some Na'vi elect to abstain from reproduction, despite being sexually active; it is not known how contraception is accomplished in this case, although the Na’vi appear to be quite sophisticated in this regard.
Though they nurse their young and possess belly buttons where they were once connected to an umbilical cord, the Na'vi are non-placental, thus the details of their internal biology and gestation period are unknown.
Despite being hybrid creatures, avatars are able to reproduce with Na'vi. In these cases, there is a chance for the children of such unions to inherit human features such as eyebrows, five fingers, and human-like noses, but it is also possible for them to be born near-indistinguishable from a native Na'vi.
- James Cameron's Avatar: An Activist Survival Guide pg 30
- Avatar: Tsu'tey's Path issue 1
- Lo'ak, Jake and Neytiri's biological second-born, has eyebrows and five fingers, but their other two children, Neteyam and Tuktirey, appear mostly native.