Scientists at a zoo in Costa Rica have discovered an unprecedented case of a crocodile who made herself pregnant. This resulted in the development of a fetus that shared a 99.9% genetic similarity with its mother. The phenomenon has been observed in birds, fish, and other reptiles but this is the first time it has been documented in crocodiles.

In 2018, a solitary female American crocodile (Crocodylus acutus) that had been living in captivity for 16 years surprised researchers by laying a clutch of eggs, one containing a fetus, a female like her mother. Genetic analysis now showed the crocodile produced the eggs without any input from a male, a process commonly known as virgin birth.

While the eggs didn’t hatch, the finding suggests that crocodiles’ ancient relatives, the dinosaurs, could have shared these reproductive abilities.

Crocodiles and birds belong to a group of reptiles called archosaurs, which forms a distinct clade. If we trace back the branches of this clade, we find that it also included dinosaurs and flying reptiles.

“[T]his discovery offers tantalizing insights into the possible reproductive capabilities of the extinct archosaurian relatives of crocodilians and birds, notably members of Pterosauria and Dinosauria,” writes the team of researchers in their paper.

How did this happen?

A crocodile egg cell undergoes multiple divisions, resulting in a final product carrying only half of the required genetic material. Simultaneously, three smaller cellular sacs are formed as byproducts, typically destined to deteriorate. However, one of these sacs occasionally merges with the egg, forming a cell with a complete set of chromosomes.

This is likely what happened with the crocodile in Costa Rica, the researchers said. When the team at the zoo noticed it had laid eggs, they contacted Warren Booth, a researcher at Virginia Tech, who suggested incubating them. When none hatched, the team opened the eggs and found one of them contained a fully formed, but lifeless, female fetus.

By conducting genomic sequencing on tissues extracted from both the fetus’s heart and the mother’s shed skin, the researchers established a striking 99.9% similarity between the two. This provided unequivocal evidence that the offspring was generated through asexual reproduction, with no contribution from a male parent.

Virgin births such as this one have been observed in over 80 vertebrate species, including lizards, snakes, sharks, and rays. However, most of these happened within captive animal populations. While scientists initially believed this was something rare, they gradually realized that virgin births, or parthenogenesis, were more prevalent than previously thought.

However, mammals remain the main exception. Unlike reptiles, fish, and birds, mammalian embryo formation requires a process known as genomic imprinting. This mechanism involves specific genes from both the mother and the father being activated and deactivated at precise timings, facilitating the development of the embryo.

The researchers propose in their paper that virgin births could be more prevalent among crocodiles, yet have remained unnoticed due to the lack of focus on such occurrences.

“Given that (virgin births) can occur in the presence of potential mates, instances of this may be missed when reproduction occurs in females co-habited with males,” they wrote.

The study was published in the journal Biology Letters.