I remember them saying the dinos were all female to prevent reproduction, but that they reproduced anyhow because of frog DNA used to make them. I thought at first this meant some turned into males to breed with the remaining females, but somehow or another I realized that meant they may have reproduced asexually instead.
Which is the case? And are there males now or is parthenogenesis still going on?
Doctor Grant, in the source novel surmises that some of the dinosaurs have turned male, then impregnated the remaining females. At no point does he seem to change his opinion;
“It's not frog DNA,” Grant said. “It's amphibian DNA. But the phenomenon happens to be particularly well documented in frogs. Especially West African frogs, if I remember.”
“What phenomenon is that?”
“Gender transition,” Grant said. “Actually, it's just plain changing sex.” Grant explained that a number of plants and animals were known to have the ability to change their sex during life-orchids, some fish and shrimp, and now frogs. Frogs that had been observed to lay eggs were able to change, over a period of months, into complete males. They first adopted the fighting stance of males, they developed the mating whistle of males, they stimulated the hormones and grew the gonads of males, and eventually they successfully mated with females.
“You're kidding,” Gennaro said. “And what makes it happen?”
“Apparently the change is stimulated by an environment in which all the animals are of the same sex. In that situation, some of the amphibians will spontaneously begin to change sex from female to male.”
“And you think that's what happened to the dinosaurs?”
“Until we have a better explanation, yes,” Grant said. “I think that's what happened. Now, shall we find this nest?”
@Valorum has answered for the book. In the film Grant says, on finding some hatched eggs in the wild:
Well, on the tour, the film said they used frog DNA to fill in the gene sequence gaps. They mutated the dinosaurs' genetic code and blended it with that of frogs. Some species of West African frog are known to spontaneously change sex from male to female in a single-sex environment. [chuckles a bit] Malcolm was right. Look.
I suppose it’s only a theory and it isn’t actually confirmed, but he seems to think it plausible enough that some of the dinos changed their sex.
I know there's already an accepted answer on this BUT parthenogenesis is also taking place.
It's explicitly mentioned in Jurassic World: Dominion. Owen says that since the raptor Blue was created in part with "Monitor DNA" she can have offspring via parthenogenesis, which is how they explain Blue having a baby raptor despite Blue being the only raptor around.
The female-to-male transitioning is also happening based not only on the statements from the first book/movie but also in later films the audience is told about male dinosaurs (the "buck" T-Rex comes to mind). So theoretically whether a certain dinosaur species can change male/female or females reproduce without males depends on what sort of "extra" DNA was added when the dinosaur species was first created.
