Shark Familes Not So Nuclear

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Lemon sharks swim near the sandy bottom of ocean off the Bahamas. Lemon shark litters are oftered sired by more than one male.
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A single litter of shark pups can have anywhere from one to five dads, according to a new study that sheds light on the complex sex and family lives of many sharks.

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Multiple paternity appears to be very common among sharks and has been documented in at least six species so far: leopard sharks, small-spotted catsharks, bonnethead sharks, lemon sharks, nurse sharks and sandbar sharks.

The most widely accepted explanation for multiple paternity is what's known as "convenience polyandry."

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"Basically, the female doesn't have much say about who she mates with," lead author Andrew Nosal of Scripps Institution of Oceanography's Marine Biology Research Division told Discovery News. "If a male encounters her and wants to mate, he will."

"At this point, the female has two options," Nosal continued. "She can attempt to fight and escape, but may incur greater injury in the process. Or she can acquiesce to minimize physical damage to her body...As a matter of convenience, to minimize the chance of injury, the female may just go along with it, even though there appears to be no biological need to mate with more than one male per reproductive cycle."

Nosal and colleagues Eric Lewallen and Ronald Burton focused their study on leopard sharks living off the coast of La Jolla, Calif. The study has been accepted for publication in the Journal of Experimental Marine Biology and Ecology.

To determine the number of shark dads per litter, the researchers took DNA samples from 449 leopard shark pups from 22 litters. The average litter size for this particular shark species is about 20 pups.

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Over 36 percent of the litters were fathered by two males instead of just one. This would be like a human mother giving birth to quadruplets, with two of the kids having one dad and the other two having another dad.

For eight of the litters with two dads, half of those had an even paternal skew.

"In other words," Nosal explained, "the number of pups within a litter fathered by the first dad was the same as the number of pups fathered by the second dad."