Sand tiger sharks usually give birth to only two babies, but they can carry up to 12 embryos in the womb. About five months into a typical 12-month pregnancy, the largest embryos start to devour all but one of their cohorts, but scientists have been unable to figure out why.
It turns out that this bloody in-utero massacre was the mother’s fault, all because of the consequences of mating with multiple males.
In 2013, a team of Marine biologists led by demian chapman of stony brook university in the United States began their research. They decided to analyze the embryos of 15 sand tiger shark mothers at different stages of development. DNA tests have shown that in the latter stages of pregnancy, the few embryos that are more likely to remain appear to have only one father. This means that embryos from the strongest fathers are competing with other embryos and winning.