Tiger sharks' monogamous habits are increasing their risk of extinction, according to a new study published in the Royal Society Open Science journal.
Sharks practice what is known as "multiple paternity" – a set of shark pups born at the same time will have several fathers.
This happens when female sharks have sex with different male sharks one after the other in a short period of time. Female sharks also have the ability to collect sperm from a sexual event and store it until the time is right.
But tiger sharks don't really reproduce in the same way, which increase their chances of getting extinct sooner than other sharks, found Dr Bonnie Holmes, from the University of Queensland.
Dr Holmes looked at 112 unborn tiger shark pups from four different mothers killed in beach protection programs and identified what she calls "genetic monogamy". Only one pup had a different father than the others.
Tiger sharks are already an exception among sea predators, as their eggs hatch inside them, but they are the only sharks known to be quasi-monogamous.
Holmes and her team tested eggs of several subspecies of sharks, but found the pups all were from multiple fathers – except for the tiger sharks.
You're the shark that I want
But we don't know for sure that tiger sharks actually want to be monogamous. According to Holmes, they might be forced into monogamy by their environment.
They are known as a solitary species that has to undertake open ocean migration, so it's likely they don't get to meet a lot of other tiger sharks. Also, multiple paternity would require them to store sperm for more than three years, the time gap they leave between litters.
Holmes also suggests that the tiger sharks of Queensland might be the only ones of their species to be monogamous. Culling programs and fishing off the Australian coasts haven't been too kind to tiger sharks, which in turn could result in less mating on their part. To confirm this possibility, Western tiger sharks will have to be studied as well.
The study explains that monogamy isn't good for tiger sharks, as it keeps them from evolving to face the new challenges humans impose on them. Multiple fathers ensure a certain level of diversity in the litters.
A tiger shark's gestation period lasts between 14 to 16 months, and they can give birth to as many as 80 pups at the same time.