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Why a ‘virgin’ crocodile pregnancy has ‘tantalising’ implications for dinosaur researchers

An American crocodile on the banks of the Tarcoles River near San Jose, Costa Rica.
An American crocodile on the banks of the Tarcoles River near San Jose, Costa Rica. Copyright EZEQUIEL BECERRA / AFP
Copyright EZEQUIEL BECERRA / AFP
By Lottie Limb
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What’s love or dinosaurs got to do with it? Researchers unpack the significance of the surprising discovery at a Costa Rican zoo.

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In the first known case of a ‘virgin birth’ among the species, a crocodile has been found to have made herself pregnant.

The 18-year-old American crocodile was kept away from other crocodiles in a zoo in Costa Rica. But that didn’t stop her producing a fully formed foetus inside one of her eggs in 2018.

Five years later, researchers have determined that the foetus was 99.9 per cent genetically identical to the mother, confirming it had no father.

This kind of asexual reproduction is known as ‘parthenogenesis’, and it’s been documented in a number of species including birds, lizards and snakes - but never before in crocodiles.

Experts at Virginia Tech credit the zookeepers at Parque Reptilandia for noticing that there were seven fertile-looking eggs within her clutch, which were then artificially incubated.

After failing to hatch three months later, the eggs were opened to examine their contents. Six contained ‘non discernable’ matter, but one had a fully formed non-viable foetus inside - a female, it was discovered upon dissection.

In their new findings published in the Royal Society journal Biology Letters, the researchers suggest that such ‘virgin births’ could be going undetected in crocodiles.

Why are ‘virgin births’ among reptiles on the rise?

There has been an “astounding growth” in recorded parthenogenesis over the last couple of decades, the researchers say.

The curious conception has been documented in birds, non-avian reptiles like snakes, lizards and ‘elasmobranch’ fishes - particularly in sharks, rays and sawfish.

An apparent rise is partly down to an increased awareness of the phenomenon, as well as leaps forward in molecular genetics. A finer science has enabled parthenogenesis to be "disentangled" from cases of ‘long-term female sperm storage’ - which can last over six years in some species and so is the main alternative theory for mysterious pregnancies.

Since the crocodylia group - also including alligators and gharials - lack sex chromosomes, this discovery is particularly interesting to researchers. The sex of crocodiles is controlled by temperature; as the Parque Reptilandia eggs were incubated at 29–30°C, the fully formed yet stillborn foetus was female.

Crocodile ‘virgin births’ could be missed

The discovery means that zookeepers may want to keep a closer eye on their crocodiles’ eggs.

“While it is not uncommon for captive reptiles to lay clutches of eggs,” the researchers write, “given the period of isolation from mates, these would normally be considered non-viable and discarded.

“These findings therefore suggest that eggs should be assessed for potential viability when males are absent.” And given that parthenogenesis can occur in the presence of potential mates too, instances of virgin births in crocodiles may be missed when females live with males.

Sometimes solo-produced snakes have been spotted because their colours or marking make clear a male wasn’t involved. But the phenomenon is harder to catch among camouflaged crocs without genomic testing.

‘Tantalising insights’ into dinosaur sex could follow

Parthenogenesis has now been documented in the two main branches of existing ‘archosaurs’ - birds and crocodilians.

The researchers say that offers “tantalising insights” into the possible reproductive capabilities of extinct relatives, particularly dinosaurs.

The most recent common ancestor of ‘squamates’ (snakes and lizards) and crocodiles diverged around 300 mega-annum ago, suggesting that parthenogenesis may be a primitive condition.

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If you’re wondering how it works exactly, the researchers have concluded that something called ‘terminal fusion automixis’ is the most likely explanation. This involves the egg nucleus fusing with its ‘sister nucleus’ - another small cell produced at the same time as the egg cell, but which generally can’t be fertilised.

Is parthenogenesis common in endangered species?

The American crocodile is a large apex predator which, thanks to its saltwater tolerance, can thrive from Florida to Peru. But it’s currently on the IUCN list of species at risk of extinction in the wild.

It’s been suggested that parthenogenesis may be more common in low-density populations, such as those on the verge of extinction, though further studies are needed.

“While it is disappointing that the crocodile parthenogen produced here failed to hatch, it is not uncommon to see non-viable foetuses and developmental abnormalities within litters or clutches of parthenogens, and long-term failure to thrive even for individuals born outwardly healthy,” the researchers add.

‘Parthenogens’ have reached adulthood in a variety of species, however, including turkeys, California condors, boa constrictors, and the whitespotted bamboo shark.

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So, they conclude, “the failure of this crocodile parthenogen to hatch should not be viewed as an indicator that all crocodile parthenogens will be non-viable.”

That could be valuable news for a vulnerable crocodile not wanting to go the same way as the dinosaurs.

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