So I have a park on a server my friend hosts and I made a mammoth enclosure with 1 male and 3 females. Every time I'd come back to the enclosure there would be multiple adult mammoths in there, so they were giving birth to adults. So I decided to isolate the male. After that it seemed that it was the male mammoth that was giving birth to the babies.

You can read about the mammoth here. "Adult mammoths can breed every five minutes as if there is a male and female present. The breeding probability is handled by how many individuals are nearby", taken from the wiki. However most F/A related mobs only require a male and a female in present for breeding freely.