Sharks practice what is known as multiple paternity, where a single set of pups will have multiple fathers. However, tiger sharks (Galeocerdo cuvier) off the coast of Queensland do not share this trait, and this may be contributing to the endangerment of this apex predator.
Although cases have been recorded of human twins with different fathers, it's hardly common, but the situation is different for sharks and rays. Whether they lay eggs, have live young, or have eggs that develop inside the mother, sharks often have large clutches of young, and genetic analysis usually reveals members of the litter have different fathers, known as multiple paternity.
Dr Bonnie Holmes of the University of Queensland told IFLScience that how this occurs probably varies between species. In some cases, females may mate with multiple males in quick succession. However, sharks are also able to store sperm from the mating events distantly separated in time until the time is right for fertilization.
Now, Holmes reports in Royal Society Open Science, DNA tests of 112 unborn pups from four pregnant tiger sharks (killed in beach protection programs) found what she calls “genetic monogamy”. Only one pup even possibly had a different father from its siblings.
Genetic monogomy is unknown in any other shark species, although Holmes admitted to IFLScience many have not been tested. Tiger sharks are unique among the Carcharhinidae family in having their eggs hatch inside them (ovoviviparity), but Holmes told IFLScience her team checked paternity studies from ovoviviparous members of other families and found all used multiple paternity.
Instead of reflecting the method of gestation, Holmes thinks tiger sharks' genetic monogamy is probably the consequence of their ecology. As a largely solitary species that undertakes open ocean migrations, they may simply not encounter each other often enough for multiple paternity, particularly if they can't store sperm for the whole three-year gap between their litters.
Holmes acknowledges the phenomenon may also be localized. Tiger shark numbers have plummeted off Australia's east coast from a mixture of fishing pressure and culling programs. The low density may have reduced mating frequency, and Holmes said comparisons with the healthier Western Australian populations may prove revealing.
Either way, genetic monogamy isn't helping Queensland tiger shark numbers. Multiple paternity is a great way to ensure genetic diversity in a species; important for survival any time, but more so when a species is confronted with novel challenges, such as those humans are posing. Learn from the turtles, lady sharks.
Holmes would like to see further studies, since without them we are flying blind in trying to prevent local extinction. However, Holmes admits this is easier said than done, as tiger sharks aren't an easy animal to find, let alone study.
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