In January 2018, a female crocodile living in captivity produced a clutch of eggs in her enclosure. Ordinarily, zookeepers wouldn’t have batted an eye at this, however, this case was different: Before she laid eggs, the creature hadn’t interacted with a male crocodile—or any other crocodile, period—for the last 16 years.
Now, researchers have finally unraveled what happened: The female had a virgin birth, also known as parthenogenesis, they reveal in a new paper published Wednesday in the journal Biology Letters.
Though scientists have previously documented parthenogenesis in other types of animals—including snakes, birds, fish and lizards—this is the first recorded instance among Crocodilia, the order of predatory, semi-aquatic reptiles that includes crocodiles, alligators, caimans and gharials. In these virgin births, the offspring are born with only DNA from the mother.
The new discovery hints at the possibility that dinosaurs and pterosaurs—the extinct relatives of both birds and crocodiles—may have also been capable of reproducing in this way. Researchers will “never be able to prove they could do it,” but the evidence suggests it’s “very likely,” as study co-author Warren Booth, an entomologist at Virginia Tech, tells the New York Times’ Veronique Greenwood.
Setting aside the possible evolutionary implications, the crocodile’s asexual birth is intriguing in its own right. The female American crocodile first arrived at Parque Reptilandia—a reptile park in southwest Costa Rica—in 2002 at the age of two. There, the female remained in “isolation from other crocodilians for its entire life,” the researchers write in the paper.
That’s why keepers were so startled to discover a clutch of 14 eggs inside her enclosure on January 17, 2018. Seven of the 18-year-old female’s eggs appeared to be fertile, so caretakers decided to artificially incubate them. After three months, the eggs still hadn’t hatched, so they opened them up to see what was inside. Six of the eggs contained unrecognizable contents, while one held a fully formed—but non-viable—crocodile fetus.
DNA analysis later revealed that the fetus and its mother were nearly genetically identical, with an important exception: the tips of the fetus’s chromosomes. This difference suggests that rather than joining with a sperm, the egg had fused with a “polar body,” or a small cellular sac that forms at the same time as the egg and contains chromosomes very similar to the mother’s. Usually, polar bodies simply die off, but scientists have noticed they fuse with eggs in other animals that have virgin births, too.
Today was spent packing to move to @VT_Entomology in Blacksburg, VA, and fielding interview with the @_newyorktimes_ , @ABCaustralia , @newscientist, @IFLScience , and more. Parthenogenesis is cool. Parthocroc!!!! Amazing. @JeffGoldblumNFT he got it right!!! pic.twitter.com/elNlUKROZZ— Warren Booth (@Warren_Booth) June 6, 2023
Researchers aren’t sure why some animals can reproduce this way, but they have a few theories. It might be an adaptation to give birth during long stretches without an available mate. Or, perhaps it’s a survival strategy for a species that’s approaching extinction, per Live Science’s Hannah Osborne. But it may also just be a random trait that persists without much purpose. After all, most species—including all mammals—do not reproduce this way, likely because the genetic diversity resulting from sexual reproduction typically makes offspring more resilient to disease, illness and other threats to their survival.
“You think, ‘Oh, well, every animal has sex,’ but the question is, why?” says Jenny Graves, a geneticist at La Trobe University in Australia who was not involved in the new study, to the Australian Broadcasting Corporation’s Bridget Judd. “Wouldn’t you do better cloning yourself? But it seems like recombining genes from two partners is a really good way of getting variation into your genome.”
Now, the widespread availability of DNA analysis tools makes it easier than ever for scientists to identify parthenogenesis—and rule out other possibilities, such as delayed conception. This and other possible virgin birth confirmations in the future will help grow “our understanding of a reproductive mechanism that until a decade ago was brushed off as an anomaly,” as Booth tells the London Times’ Rhys Blakely. “As such, we should be thinking more about the ecological and evolutionary significance of this trait.”