Debra Abercrombie

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ScienceShot: Survival of the Fittest, Inside a Shark's Womb

And you thought human sibling rivalry was intense. Sand tiger sharks (Carcharias taurus) eat all of their brothers and sisters —before anyone has even been born. A new study shows that the shark pups' "embryonic cannibalism" serves a Darwinian purpose, ensuring survival of the fittest. Researchers studied 15 pregnant sand sharks carrying up to nine eggs in twin uteri. The eggs sported enough genetic diversity to indicate that the litter had multiple fathers. But not all of those dads got to pass on their dashing good looks to Junior. When sand shark eggs hatch, the first hatchling in each uterus (shown, at right, with embryo at left) eats all the rest, leaving only two pups per litter. In about 60% of the sharks studied, the two surviving hatchlings had the same fit father, the team reports in the latest issue of Biology Letters. Embryonic cannibalism may favor faster growth rates or "the precocious development of eyes and teeth," the team writes, "which enhances their ability to locate and consume their siblings." Fighting over the top bunk seems trivial in comparison.

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