Why some animals have ‘virgin births’

Some events defy the laws of nature.

In February 2024, a female stingray named Charlotte became pregnant in a small aquarium in the city of Hendersonville, North Carolina (USA).

The animal had not had contact with males of its species in more than eight years.

Charlotte left scientists at the Team Ecco Aquarium and Shark Laboratory disoriented.

How did she manage to conceive four cubs, floating in her tank without a mate?

One theory suspected two white-spotted bamboo sharks that occupied the same Charlotte tank.

The reason was suspicious bite marks found on the ray’s body, which could be signs of mating between sharks. But this would have yielded an unusual shark-ray hybrid.

Scientists believe the pregnancy may have been the result of a rare phenomenon called parthenogenesis.

The word comes from the Greek parthénos (“virgin”), while “genesis” means “creation”. In this process, an egg turns into an embryo without fertilization by sperm.

Charlotte is far from the first animal to conceive puppies on her own. Parthenogenesis is quite common in insects, such as mayflies, but is rarer among vertebrates.

Since a bonnethead shark gave birth in captivity in 2001, new cases have been recorded in sharks and reptiles. Charlotte is believed to be the first recorded case of parthenogenesis in stingrays.

The reason for parthenogenesis is a mystery. Some scientists suggest that it is a last attempt by females to transmit their genetics forward.

“The goal of evolution is to pass on your genes,” according to molecular biologist Kevin Feldheim. He uses genetics to study shark populations and the mating of these animals at the Chicago Field Museum in the United States.

“A female who is isolated from males, but who would normally give birth via sexual reproduction, simply doesn’t have that opportunity,” he explains.

Feldheim investigated another case of parthenogenesis among zebra sharks at Chicago’s Shedd Aquarium in 2008. He first needed to eliminate possible promiscuity among the aquarium’s residents.

“There was no direct evidence that males mated with females, but unfortunately the tank doesn’t have cameras running 24 hours a day,” he says.

Figuring out who the father of baby sharks is can be even trickier, as some female sharks can store sperm for months after mating, according to Feldheim.

The biologist then developed a paternity test to collect genetic markers, called microsatellites. “They are used in human paternity cases, such as on TV shows like CSI: Criminal Investigation and NCIS: Naval Investigation,” he explains.

The test results confirmed that the baby zebra sharks did not have the father’s DNA – only the female’s.

“The obvious question is how can this happen? And the answer is by parthenogenesis.”

In most cases of animal reproduction, eggs are produced in a process called meiosis. Cells divide and share genetic material and cellular machinery with each other. This process generates three cell branches called polar bodies.

Normally, these polar bodies are reabsorbed by the female. But in parthenogenesis, one of the polar bodies can fertilize the egg and form a viable embryo, mimicking sexual reproduction.

This process is different from cloning and has disadvantages, according to research scientist Kady Lyons, who studies sharks and rays at the Georgia Aquarium in the United States.

“The cells used are not a copy of the mother on carbon paper,” she explains. But, as the egg and polar body only contain parts of the mother’s genome, the offspring end up having less genetic diversity than her, as can be seen in “highly congenital individuals”, according to Lyons.

For some species, there are advantages to asexual reproduction. Some populations of whiptail lizards in Mexico and California (USA), for example, are made up only of females, who reproduce asexually.

The species has evolved an unusual way of maintaining its genetic diversity through parthenogenesis, doubling the number of chromosomes in the eggs of asexually reproducing females. This procedure has advantages, as it allows the species to colonize new areas and avoids certain pitfalls, such as sexually transmitted diseases.

But there is also a cost: Your DNA absorbs more harmful genetic mutations through parthenogenesis than through sexual reproduction in the absence of natural selection.

Unfortunately, not all species are as adapted to parthenogenesis as the whiptail lizard. Shark pups generated by parthenogenesis, for example, tend to be short-lived and rarely reach sexual maturity.

“There is a lack of genetic variation in puppies, which can generate the so-called expression of delusional recessive alleles”, explains Feldheim. In other words, despite their miraculous conception, vertebrates that are born through parthenogenesis can be short-lived.

Lyons participated in an artificial insemination trial on zebra sharks. Scientists studied the possibility of survival among offspring produced sexually and by parthenogenesis.

Kady Lyons says she wasn’t surprised to see evidence of parthenogenesis in stingrays. But Charlotte’s case did not solve all the mysteries surrounding this phenomenon.

“One thing we don’t know is whether there is a trigger for females to reproduce in this way,” according to her. “We just consider that when you have boys and girls together, they’re going to do what they need to do.”

But it is in unique situations that parthenogenesis occurs most frequently – such as among animals held by humans.

“Obviously, life finds a way”, concludes the scientist.