Taking sibling rivalry to a new level

Very cool story in the Post today about how Sand Tiger sharks compete (and kill each other) in the womb:

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 ofthe 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.

Female sand tiger sharks have two uteri and carry hundreds of eggs. During their fertile periods, they can mate with many male sand tiger sharks. But each time they give birth after a 12-month pregnancy, they produce just two offspring — one from each uterus.

Wow.  I’ll leave final comments to Cosmo Kramer:

About Steve Greene
Professor of Political Science at NC State http://faculty.chass.ncsu.edu/shgreene

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