Why do shark embryos eat each other in the womb?

Sand tiger sharks (Carcharias taurus) usually only give birth to two babies, but they can carry as many as 12 embryos in their womb. It was known for decades that about five months into the typical 12-month pregnancy, the largest embryo would begin to devour all but one of its littermates, but scientists couldn’t figure out why.

It turns out that this bloody, in-womb carnage is the mother’s fault, because by mating with multiple males, she’s set in motion an all-out paternity war.

In 2013, a team of marine biologists led by Demian Chapman from Stony Brook University in the US decided to investigate, analysing the embryos of 15 sand tiger shark mothers at various stages of development. The sharks had accidentally gotten themselves caught up in safety nets and died. DNA tests revealed that in the later stages of pregnancy, it was more likely that whichever few embryos were remaining had just one father. This means that the father who sired the biggest, strongest embryo competes with other males at passing on his genes and wins.

“In some species, the struggle for paternity continues beyond the point where the female [mates with] the male,” said Chapman to Tia Ghose at LiveScience. These embryos are so vicious, it’s rumoured that one bit a researcher’s hand when he was dissecting its mother’s uterus, he added.

Of the 15 mother sharks the researchers analysed, says Ghose, 10 were carrying just two embryos, and the remaining mothers, which were in earlier stages of pregnancy, had five to seven embryos. These earlier embryos came from two different fathers. “Basically, that loser father ultimately provided food for a rival male,” Chapman told LiveScience.

While it’s not clear why one shark father is more likely to produce a larger, hungrier shark baby over another, it’s always advantageous to the mother, because the young she births will be the strongest and most likely to survive and carry on her genes. It also means she doesn’t have to bother about being too selective with her mate choice, and risk injury from resisting undesirable males. The researchers said this was an example of how sexual selection is often like an evolutionary arms race between males and females, just as it is with ducks and their bizarre genitals.