Shark 'Sibling Rivalry' Explained?

Sand tiger sharks were in the news the other day. This is the primary species of shark in The Maritime Aquarium’s “The Ocean Beyond the Sound” exhibit (a.k.a., what a lot of visitors refer to as our shark tank). Sand tigers also are the largest of the shark species native to Long Island Sound.

They were in the news not because a sand tiger shark attacked a person. It’s because of how they attack each other even before they’re born.

It’s been known for a while 1) that sand tiger females will develop eggs inseminated by multiple males; and 2) that sand tiger shark embryos will eat each other in utero – in other words, before being born.

What’s new is that scientists have concluded that there may be an evolutionary strategy behind how this happens:  the “winning” baby shark – the shark that actually will survive to be born – usually is the offspring of a male shark that is more aggressive than other adult males.

A news report quoted the lead author of the study – Demian Chapman, a marine biology professor at Stony Brook University – as saying, “This is demonstrating that embryonic cannibalism is actually whittling down the number of males producing offspring.”

For the father of the successful baby shark, he has outcompeted his rivals and it is his lineage that will live on.

Here’s how The Washington Post explained the study:

“Over the course of four years, Chapman and his six colleagues collected 15 pregnant sand tiger sharks that had died after being caught in nets set off Richards Bay, South Africa. By performing genetic tests on the embryos in different states of development, they were able to determine that while the majority of the females had mated with multiple males, in 60 percent of the cases they were carrying only babies from the same father, suggesting that all other male shark offspring had already been killed.”

The question still to be answered is this:  are these “winning” adult males able to mate first with females so that their offspring develop first, giving them an advantage over subsequent sharks fathered by other males? Or do the “winning” males simply produce offspring that develop, or gestate, faster?

Whichever the answer, the result is that all of that feasting on their brothers and sisters has paid off for the newborn sharks:  at birth, sand tiger sharks are nearly 3 feet long and thus much less vulnerable to predators.

(At the Aquarium, we’ve occasionally seen bite marks on the sand tigers that suggest evidence of mating behavior. No pups have been born, and that’s no real surprise. Sand tiger shark births in any aquarium are very very rare.)