Sibling-eating shark embryos put new spin on natural selection

For a sand tiger shark embryo, the uterine experience is not so much “safe and nurturing” as it is “Hunger Games” arena.

In each of a pregnant sand tiger shark’s two uteri, several egg sacs are fertilized, but only one baby emerges -- a 3-foot-long cannibalistic victor who has killed and devoured its siblings.

What the embryos experience in the womb is really less like a battle and more of a race. The first embryo to break free of its egg sack and start killing and eating its siblings will be the one who eventually gets to swim out of mom.

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This intrauterine behavior has fascinated scientists for more than 30 years, but a new study, published in Biology Letters, sheds more light on how it affects the reproductive success of males in the species.

Led by marine biologist Demian Chapman of Stony Brook University, the study looked at 15 pregnant sand tiger sharks that had been caught in nets off the coast of South Africa. The embryos’ DNA revealed that most of the time, the eggs in a sand tiger shark’s womb have been fertilized by several different males. And yet, in most cases, the two siblings who ultimately made it out alive had the same dad.

“In most systems, as soon as a sperm fertilizes an egg, the male is a success,” said Chapman. “But in this system, the competition and the selection is happening even after that.”

Chapman proposed two scenarios that might determine which male’s genetic material will survive to the next generation. In one scenario, the first male sand tiger shark to impregnate a female would likely be victorious. The embryos he fertilized would have more time to develop and therefore be more likely to win the life or death intrauterine race.

In another scenario, some males may carry genetic material that promotes super fast embryonic development, leading their embryos to win the race and eat the slow pokes.

Evidence supports the first option. In captivity, an alpha male sand tiger shark starts to guard a female when she shows signs that she will soon start ovulating, Chapman said in an interview with the Los Angeles Times.

“He does that until he mates with her, and then he doesn’t care anymore,” he said.

But he said, it’s unclear whether sand tiger sharks exhibit the same behavior in the wild.