For sand tiger sharks, a deadly, cannibalistic battle inside the womb is part of evolution


It’s a tough world from the moment of conception for a sand tiger shark. When a female gets pregnant, it’s usually with multiple offspring of several different male sharks. As soon as the fetuses are old enough, they begin a cannibalistic battle for primacy in utero, with only one surviving.

Now scientists have concluded that this is not just a response to crowded conditions but represents an evolutionary strategy that allows the most aggressive male sharks to father the successful baby and thereby outcompete sexual rivals.

“For most species, we think of sexual selection as ending when males fertilize eggs, because once the male’s fertilized eggs he’s won, there will be some genetic representation in the next generation,” said Stony Brook University marine biology professor Demian Chapman, lead author of the study published online Wednesday in Biology Letters. “This is demonstrating that embryonic cannibalism is actually whittling down the number of males producing offspring.”

It’s still unclear whether the evolutionary strategy works because the most-aggressive fathers get their offspring growing in utero before others, thus giving them a developmental advantage in the cannibalistic battles to come, or if they produce offspring that gestate more quickly.

Written By: Juliet Eilperin
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  1. Yes isn’t nature cruel ? Survival is all that matters, plus making the next generation of course. I don’t suppose sand sharks have much concept of that. They do what they do, or die.

  2. It’s amazing what a truly nasty place it is and how that nastiness has to a very large extent helped shape us.

  3. The very stuff of life! Don’t like it? Tough.

    Want to make something up and pretend, go ahead, but don’t lay it on children.

    And don’t even begin to imagine it’ll make a jot or tittle of difference to the truth, which is infinitely more interesting and exciting than anything anyone has ever made up or pretended about anything.

    There it was right under our noses for all those millions of years until just over a century ago when two very clever chaps clocked it, to our mutual benefit, but there are still folks who want something more or different; there’s just no pleasing some people.

  4. Yes, survival. Still quite disturbing though. So… only the strongest, most aggressive male survives the ordeal but isn’t that kind of wasteful and unnecessarily cruel? Could this be an evolutionary “work-in-progress” towards the female of the species breeding only one foetus in the majority of pregnancies (like humans or elephants)?

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