|This article is about the current version of DF.|
Breeding creatures are those which can produce offspring in fortress mode. Conversely, non-breeding creatures cannot reproduce in fortress mode, even though they presumably do so during worldgen. Breeding creatures are significantly more useful as stock for a meat industry than their non-breeding counterparts.
Currently the tokens necessary for breeding are not known. Toady stated that he "removed requirement that a creature have a baby/child state to breed completely" in version 0.40.06, however there are presumably other requirements (such as having a female gender).
When a male and a female of the same breeding species are present on the map, sooner or later the male will impregnate the female. Animal reproduction requires absolutely no contact between them, and in fact will occur regardless of distance, physical obstacles such as walls or locked doors, number of each gender, and even ownership. This has been jokingly referred to by players as spore-based breeding; even a male in a herd of wild animals outside the fortress walls can impregnate a female deep in the lowest level, and females can get pregnant again immediately after giving birth (much like dwarves). One way to prevent pregnancy is by caging all the females, but those that are already pregnant can give birth while caged (also much like dwarves). (Males who are caged are still able to breed.) Breeding also appears to be prevented while creatures are swimming.
Invaders normally are incapable of breeding, but they have been observed to arrive on the map pregnant. Additionally, invaders (whether soldiers or mounts) can become pregnant and breed while on the map if they possess the
[PET_EXOTIC] tokens (and a
[PET_VALUE]), but their parents will immediately attack their offspring unless they are separated from each other (such as via cage traps).
There is a per-creature-type population cap, observed to be around 50, past which breeding animals will not get pregnant; existing pregnancies will mature to term. Furthermore, animals will not reproduce if children make up more than 75% of their population - for creatures which take longer than a year to grow up (such as elephants, which take 10 years), this can slow breeding significantly. Once the population drops below these caps, the creatures will begin breeding again.