Ladies, leave your man at home.
It’s a tale as old as time — girl shark meets boy shark. Boy shark impregnates girl shark and girl shark gives birth to more than two dozen babies. Girl and boy shark go their separate ways and, years later, girl shark delivers three new pups — this time, without any genetic material from a male partner.
Sharks are capable of two modes of reproduction — sexual and asexual. And it’s not uncommon for species to switch between the two, especially if the animals were previously isolated in captivity. What is unusual is for the circumstances to reverse and for a species to become asexual after previously engaging in sexual reproduction.
But that’s exactly what happened in an aquarium in Townsville, Australia. In 2006, a female zebra shark named Leonie started mating with a male zebra shark named Leo. The two were breeding partners for six years, until the aquarium’s staff worried they were producing too many offspring. In 2013, the aquarium transferred Leo to another tank, leaving Leonie alone.
Leo’s departure should have signaled the end of Leonie’s childbearing years, but in 2014, she laid several eggs. None of them hatched. This is standard procedure; Christine Dudgeon, a biologist with the University of Queensland, says female sharks don’t need a male partner to lay eggs, telling CNN:
“It’s much like a chicken — they lay eggs whether they are fertilized or not, if the conditions are good.”
The next year, Leonie laid eggs again — this time, three of them hatched into pups named Cleo, CC and Gemini.
This is not the first instance of asexual breeding
‘Parthenogenesis’ is the process by which embryos grow and develop without fertilization. Virgin births occur mainly in plants and invertebrate animals, although there are recorded instances of vertebrates like sharks and lizards reproducing asexually.
In 2008, the Journal of Fish Biology released a study confirming that a female Atlantic blacktip shark at the Virginia Aquarium and Marine Science Center gave birth to a pup with no male genetic makeup. Unlike Leonie, this shark never reproduced sexually prior to its virgin birth.
When Leonie first became pregnant, scientists believed she may have stored sperm from her previous sexual encounters. This theory was disproven after scientists tested the pups’ DNA and discovered it only contained Leonie’s genetic material.
This is big news for science; the only other examples of captive female animals switching from sexual to asexual reproduction are an eagle ray and a boa constrictor.
Is this good or bad?
Zebra sharks are currently listed on the IUCN’s Red List of threatened species, so theoretically, the idea that female sharks have found a workaround for getting pregnant without men is a good thing. Leonie’s ability to switch from sexual to asexual is a clever reproductive strategy, evolutionarily designed to prolong her lineage until she can find a new man.
For females, shark sex sounds really rough. Male sharks indicate interest in breeding partners by aggressively biting them on their pectoral fins, often until the skin is raw. Female zebra sharks aren’t keen on this flirting method — they linger in shallow waters, making it more difficult for their male counterparts to access them. If wild zebra sharks are mating asexually — and scientists aren’t yet sure if they are — they might be doing it to avoid the painful process.
But as scientists are quick to point out, asexual reproduction has dangerous consequences. Leonie’s pups suffer from a lack of genetic diversity, as they only have their mom’s genes to rely on. This genetic homogeneity is a form of inbreeding and can hamper the species’ ability to adapt long term.
All scientists can do now is wait the seven years it will take for Leonie’s pups to achieve sexual maturity so they can test their breeding abilities. In the meantime, let’s hear it for the ladies doin’ it for themselves.