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.
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."
"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.
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."
In the other four litters with more than one father, however, the shared paternity wasn't so even, with one dad clearly dominating the group.
Domination comes into play during mating as well. In leopard sharks, Nosal said, "The male will bite the female and wrap his body around hers to copulate."
The gestation cycle for this shark is then 10-11 months, longer than the human cycle of nine months. Females must quickly recover and then mate within one to two months to remain "on schedule" to give birth at around the same time the following year. The females therefore mate with multiple males within a short period of time, sometimes leading to the multi-dad litters.
Once the pups emerge, they are on their own.
"There is no parental care in leopard sharks or any species of shark," Nosal explained. "Leopard sharks are born live with all the instincts they need to find food and avoid predators. There is evidence of size clustering (that is, young leopard sharks hanging out together), which may function in predator avoidance or increased foraging efficiency."
He continued that one young shark might find something to eat, leading to a "commotion" that attracts others to the food source.
Andrew Griffiths of the Marine Biological Association of the UK and his colleagues performed a similar study on small-spotted catsharks and came to similar conclusions.
Griffiths and his team also believe that "the cost of avoiding mating attempts initiated by males may be high," so the females wind up mating a lot in a short period of time, resulting in litters with more than one dad.
Conversely, if no males are around, females of some shark species, such as white-spotted bamboo sharks, can give virgin birth. This is not ideal for genetic diversity, so hopefully male mates will eventually find their way to the virgin-birthed females.