Why do some animals have 'virgin births'?

It is a process that seems to defy the laws of nature. In February 2024, a female stingray named Charlotte became pregnant in a small aquarium in Henderson Valley, North Carolina, USA. Although she had not come into contact with a male stingray in more than eight years. This incident surprised the team of scientists from Eco Aquarium and Shark Lab.

How Charlotte gave birth to four stingrays mated to a male mate in her aquarium was a mystery.

One theory held two white-spotted bamboo sharks swimming in the aquarium with Charlotte as the possible culprit. This was due to some suspicious bite marks on Charlotte's body. Such markings may be indicative of mating behavior in sharks.

But it would lead to an unusual shark-stingray hybrid breed. Instead, scientists believe that pregnancy may be the result of a rare phenomenon called parthenogenesis. It comes from the Greek word parthenos, meaning 'virgin', and genesis, meaning 'creation'. This is where an egg develops into an embryo without being fertilized by sperm.

But Charlotte isn't the first animal to get pregnant 'alone or virgin'. The process of parthenogenesis is very common in insects, such as flies, although it is rare in vertebrates. More cases have been reported in sharks and reptiles since the virgin pregnancy of a bonnethead shark in 2001. Charlotte is believed to be the first recorded case of parthenogenesis in a stingray.

But when and why the process of parthenogenesis occurs is still a mystery. Some scientists believe that this is a 'last ditch effort' in female animals to pass on their genetics to their offspring.

'The purpose of evolution is to pass on your genes,' says Kevin Feldheim, a molecular biologist who uses genetics to study shark populations and mating behavior at the Field Museum in Chicago. A female that normally gives birth to children through sexual reproduction, but a female isolated from male does not have this opportunity.'

Feldheim in 2008 investigated another case of parthenogenesis in zebra sharks at the Shedd Aquarium in Chicago. First they had to rule out the possibility of sex between the fish in the aquarium.

"There was no direct evidence that the male fish had mated with the female, but unfortunately the tank does not have 24-hour surveillance cameras," he says.

He added that determining shark paternity can be more complicated because some female sharks can store sperm within themselves for months after mating.

Feldheim developed a paternity test called microsatellites for genetic identification of these fish to determine their parentage. 'These are used in cases of finding the genetic identity of parents in humans,' he says.

The results confirmed that the zebra shark offspring had no paternal DNA, but only genes from the female, he says.

Feldheim says the big question now is how did it all happen? So the simple answer is through parthenogenesis.

In most cases of animal reproduction, eggs are produced in a process called meiosis, where cells divide and share genetic material and other cellular machinery. This process produces three cellular branches or germ cells called polar bodies. Normally, these polar bodies reabsorb the material, but in parthenogenesis, one of these polar bodies can develop into an egg and form a viable embryo. This process mimics sexual reproduction.

It's a different process than cloning and has drawbacks, says Cady Lyons, a research scientist who studies sharks, skates and rays at the Georgia Aquarium.

'The cells being used are not carbon copies of the mother,' she says. However, since both eggs and polar bodies contain only parts of the mother's genome, the offspring are genetically less diverse than their mother, as can be seen in 'a monogamous organism'. .

Asexual reproduction is beneficial for some animals. For example, some populations of whiptail lizards in Mexico and California have become all-female and reproduce asexually. Animals have developed an unusual way of maintaining their genetic diversity through parthenogenesis by doubling the number of chromosomes in the eggs of asexually reproducing females.

This has some advantages, helping these species to settle in new areas and avoid harms such as sexually transmitted diseases.

But there is a price to be paid. Due to the absence of natural reproduction, the DNA of these animals carries more harmful genetic mutations through parthenogenesis than through sexual reproduction.

Unfortunately, not all animals can make their offspring as vigorous as the whiptail lizard through parthenogenesis. As in sharks, offspring produced by parthenogenesis are short-lived and rarely reach sexual maturity.

The offspring of these animals lack genetic variation, says Feldheim.

In other words, babies born through parthenogenesis in vertebrates can be miraculously young.

Scientist Cady Lines was part of an artificial insemination experiment in zebra sharks, where scientists studied the chance of survival between naturally born and parthenogenetic offspring. The study found that parthenotes (offspring born from parthenogenesis) lived an average of less than a year, had many characteristics that undermined their chances of survival, such as unbalanced swimming, wandering, Head diving and difficulty in eating.

She says she's not surprised to see parthenogenesis in stingrays, although Charlotte's case doesn't solve all the mysteries surrounding the phenomenon.

"One thing we don't know is whether something motivates females to breed in this way," she says. We just assume that when the male and female are together, they will do their thing.'

However, in exceptional circumstances such as when the animals are under human supervision, arthenogenesis occurs more frequently during this period.

'Obviously life finds a way,' she says.