The average female grey nurse shark begins her pregnancy with about 40 embryos in her two uteruses. But it takes a year for the baby sharks to gestate, and by the time they’re ready to be born, there are only two left — one in each uterus. Why? Because by four months they’ve developed a full set of teeth, and the strongest begin cannibalizing the weaker ones.
To paraphrase the late Steve Irwin:
Crikey! That sounds very dangerous!