Marine biology professor Demian Chapman of Stony Brook University in New York said when a female shark gets pregnant, it's usually with multiple offspring of several different male sharks, but as soon as the fetuses are old enough, they begin a cannibalistic battle for survival.
Scientists say they believe it's not a response to crowded conditions within the womb but represents an evolutionary strategy wherein the most aggressive male sharks father the ultimately successful baby and thereby out-compete rivals, The Washington Post reported Wednesday.
"For most species, we think of sexual selection as ending when males fertilize eggs, because once the male's fertilized eggs he's won, [and] there will be some genetic representation in the next generation," Chapman, lead author of the study published in Biology Letters, said. "This is demonstrating that embryonic cannibalism is actually whittling down the number of males producing offspring."
Female sand tiger sharks have two uteri and can mate with many male sand tiger sharks, but after a 12-month pregnancy they produce just two offspring, one from each uterus.
Like many other shark mothers, female sand tigers abandon their young after giving birth, and given the proven aggressive tendencies of their offspring the mothers "probably run a mile," Chapman said.
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