American crocodile floating in a river in Costa Rica
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Stories of virgin births, where young are produced without fertilization, have been told throughout history. Mars the ancient Roman god, Horus the ancient Egyptian god, and Qi from ancient Chinese mythology were all born of virgins. But virgin births actually happen in the natural world.

The first evidence of a virgin birth in crocodiles was reported in a captive American crocodile, A pointed crocodile, who was housed alone for 16 years in a zoo in Costa Rica. She laid a clutch of 14 eggs, seven of which appeared viable and were artificially incubated. The eggs did not hatch and the contents of six of them were indistinguishable. But one contained a fully formed fetus, genetically identical to its mother, showing no evidence of input from any male.

This is not the first case of virgin birth in the animal kingdom. Baby lizards, snakes, sharks and birds, including the California condor, have all been documented hatching from unfertilized eggs.

How to explain virgin births?

Species can reproduce either sexually, by combining genetic material from two parents, or asexually. Our ancient ancestors were asexual and essentially made clones of themselves. Plants reproduce in similar ways, including splitting, budding, and fragmenting.

However, this produces many genetically identical organisms, and a lack of genetic variation means individuals cannot adapt to changing conditions. If the environment is bad for one member of a species, it is bad for all and can lead to extinction.

Sexual reproduction in species such as humans requires sperm to fertilize eggs and create an embryo. In evolutionary terms, sexually reproducing species are considered to be more advanced, as their offspring are genetically diverse, with unique gene combinations from their parents.

This diversity can be important if a species is to adapt. It also reduces unwanted genetic mutations, which are often associated with inbreeding (when close relatives mate).

American crocodile floating in a river in Costa Rica
The virgin mother was an American crocodile like this.
Uwe Bergwitz/Shutterstock

Virgin births are a form of asexual reproduction because they do not require genetic information from sperm. But, unlike other forms of asexual reproduction, they require an egg. Unfertilized eggs are often produced by females – you may have had unfertilized eggs for breakfast this morning from a domestic hen – and if unfertilized eggs are not eaten, they eventually perish. But there is an exception. Virgin births, known as parthenogenesis, occur when an unfertilized egg develops into an embryo.

However, it will not necessarily be genetically identical to the mother – it depends on how the egg develops. Parthenogenetic young may be full or half clones of the mother. Half-clones are produced when embryonic cells divide in two before multiplying. Full clones are created when an embryo multiplies whole cells.

Thus, half-clones have even less genetic diversity than full clones. Not only do they lack the genetic diversity of organisms created during sexual reproduction, but they only inherit half of the genetic diversity from their mother.

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Some species, called facultative parthenogens, alternate between sexual and asexual reproduction. They mainly depend on sexual reproduction, but can use asexual reproduction if necessary.

Virgin birth, which usually results in female offspring, is believed to be triggered in several situations. For example, when there are not many men around. This is often reported in captive animals, including the bonnethead shark, where the animals are kept in single-sex enclosures.

pass on genes

Even when there are males around, females can still use parthenogenesis. For example, last year a female zebra shark hatched several offspring whose DNA did not match any of the males in the Chicago aquarium where she lived, baffling researchers. Maybe the female just didn’t like the males she lived with.

If the environmental conditions are bad, asexual reproduction requires less effort than sexual reproduction, because the female does not need to waste time and energy to find a mate. For example, many cases of parthenogenesis have been found in geckos, snakes, and lizards that live in harsh, dry climates such as high altitudes.

Female animals can also reproduce asexually to take advantage of a favorable change in conditions. The prickly cheek crayfish is native to the United States but was introduced to Europe where the climate is more temperate. It has invaded many European rivers by reproducing asexually. Although many invasive species are larger and stronger than local species, parthenogenesis is another factor that may contribute to their success.

Genetic testing technology that can more easily identify parthenogenesis is helping researchers discover that more and more species are capable of virgin births. The revelation of parthenogenesis in the American crocodile suggests that there is a common ancestral connection between archosaurs, or dominant reptiles, which include dinosaurs, pterosaurs (flying reptiles), birds, and crocodiles. As parthenogenesis occurs in birds and crocodiles, it is possible that dinosaurs also had virgin births.

The Virgin Crocodile Mother is eerily reminiscent of a scene from Jurassic Park where scientists claim there is nothing to fear, that they can control the population of the park by ensuring that all dinosaurs are born female, so that there will be no young products naturally.

But in the words of the film’s chaos theory expert, Dr. Ian Malcolm (played by Jeff Goldblum): “life finds a way.”

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