For sand tiger sharks, a deadly kind of sibling rivalry begins even before birth — and that's bad news for many would-be shark dads, a new study suggests.

While most sharks have multiple babies or "pups" at once, sand tiger sharks only give birth to two at a time — one from each of their two uteri. That's because the pup in each uterus has already eaten all the brothers and sisters that shared the womb with it — a phenomenon known as embryonic cannibalism.

"These guys have killed even before they're born," said Demian Chapman, who led a recent study on the sharks' mating system.

Sand tiger sharks are closely related to great white sharks and grow up to three metres in length.

Typically, a female sand tiger shark mates with multiple males and becomes pregnant with around six or seven embryos in each uterus, Chapman told CBC's Quirks & Quarks in an interview that airs Saturday.

But once the largest embryo reaches the size of 100 millimetres — about five months into the 12-month pregnancy — it breaks out of its egg capsule.

"And then it starts to go out and actively hunt and kill the other embryos," Chapman said

As soon as it has consumed all the siblings accessible to it, the pup spends the rest of the pregnancy eating unfertilized eggs that have accumulated in the uterus. When the two remaining pups are born, they are each a whopping one metre in length.

Paternity testing

Chapman and his colleagues were curious as to how embryonic cannibalism affects the paternity of the surviving pups, given that the mothers tended to mate with multiple males.

The researchers collected 15 pregnant female sharks that got tangled in nets intended to protect beach-goers from sharks in Richards Bay, South Africa. Five were in the early stages of pregnancy, when they had multiple pups. DNA testing showed that at that stage, the pups almost always had at least two fathers.

The 10 remaining pregnant sharks were in the later stages of pregnancy, when only one pup remained in each uterus. In this case, DNA testing showed that they were full siblings with the same father. That suggests other males may have successfully impregnated the female, but didn't get pups to show for it in the end.

In a paper published this week in the journal Biology Letters, the researchers suggested that embryonic cannibalism may increase the probability that the females give birth to the offspring of "competitively superior males" even if they were unable to determine which males those were prior to mating or to stop other males from mating with them.