May 1, 2013
Unborn Sand Tiger Sharks Cannibalize One Another To Become Sole Survivor
Brett Smith for redOrbit.com - Your Universe Online
In some sexually reproducing species, the competition between males to pass on their genetic material ends with conception. However, for sand tiger sharks that competition extends into the female´s uteri where baby sharks cannibalize one another until only one remains.
According to a new study in Biology Letters, a female sand tiger shark typically mates with multiple males, but the results of in utero cannibalism could result in the production of offspring from only one.
“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,” lead author Demian Chapman, a marine biology professor at Stony Brook University, told The Washington Post. “This is demonstrating that embryonic cannibalism is actually whittling down the number of males producing offspring.”
After intercourse, any fertilized eggs settle in one of the female sharks two uteri and hatch when reaching a certain size. The first embryo to emerge in each uterus is usually the “winner” as it typically cannibalizes its unborn siblings.
Besides devouring its siblings´ still-developing bodies, the baby shark also eats any highly-nutritious unfertilized eggs that may be lying around. This protein-rich diet causes the hatchling to grow at a tremendous pace — eventually reaching more than three feet in length before being born.
In their four-year study, Chapman and several colleagues studied 15 recovered bodies of pregnant sand tiger sharks that died after getting caught in nets designed to prevent shark attacks. A DNA analysis of tissue samples from mothers and their embryos showed that females regularly mate with at least two males. When the marine biologists found hatchlings that died before devouring their siblings; the uneaten embryos were fathered by two or more different males.
The results of the study allowed researchers to conclude that some males will manage to mate with females but still not contribute to the next generation of sharks if their children are devoured in the womb.
Chapman noted that the entire process insured that a baby sand tiger shark is fully equipped to fend for itself after birth.
“That´s great for them, so when they´re born, they´re bigger than any other fish,” he said.
He added that female sand tigers abandon their young after giving birth, joking that they “probably run a mile” given their child´s already violent history.
The new mating study was performed on sharks from the open ocean, which could behave much differently than sharks in captivity. Male sand tiger sharks have been observed taking on a social hierarchy in captivity, with high ranking sharks fending off other males from their chosen mating partner. Future studies might be able to determine if the entire reproductive cycle were to maintain itself over several generations of sharks in captivity.
While Cannibalism in utero appear to be unique to sand tiger sharks, hatchlings of its closest relatives, including makos and great white sharks, have been observed eating unfertilized eggs in the womb.