Tribbles were "born pregnant"

Asexual reproduction was a mode of reproduction by which offspring arose from a single parent, and inherited the genes of that parent only.

Tribleustes ventricosus, unlike polygeminus grex, was a type of tribble that bred slowly. However, after Edward Larkin used his own DNA to genetically alter these tribbles, they were capable of breeding even faster than the polygeminus grex species. (ST: "The Trouble with Edward")

Doctor Leonard McCoy described tribbles as being "bisexual" [sic] (dual sexed) and were capable of reproducing at will. Born pregnant, and with sufficient food, a single tribble could breed in number exponentially, producing a litter of ten in twelve hours time. (TOS: "The Trouble with Tribbles")

Xindi-Insectoids also reproduced asexually. (ENT: "Hatchery")

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