This is the first known instance of a crocodile virgin birth, but virgin births (parthenogenesis) have been recorded in fish, birds, lizards, and snakes. Parthenogenesis is a natural form of asexual reproduction in some animals where viable offspring are produced from an otherwise unfertilized egg.
In a recent study published in the journal Biology Letters, a female crocodile living in isolation for 16 years at a Costa Rican zoo laid a clutch of eggs, a common practice among captive reptiles, even those without mates. After three months of incubation, one egg contained "a fully formed stillborn baby crocodile," a team of scientists found.
Mark Davis, a conservation biologist at the Illinois Natural History Survey (INHS) at the Prairie Research Institute (PRI) and co-author on the study helped examine the crocodile fetus' genomic makeup, ultimately discovering that the fetal genome resulted from reproduction without a male crocodile’s genetic contribution.
“This discovery speaks to the inferential strength of our modern genomics era,” said Davis, “We couldn’t perform this without the powerful genomic advancements made in the last decade, which allowed us to leverage a massive amount of genomic data, rendering these data irrefutable.”
Davis leads the Collaborative Ecological Genetics Lab at the University of Illinois Urbana-Champaign, which uses conservation genomics to assess the evolution of modern biodiversity, monitor biodiversity via environmental DNA (eDNA), and solve problems via community metabarcoding.
“It’s wild, but the data and the science confirm that this is what happened,” said Davis, “There animals are out there in our world – even in Illinois – that are parthenogens, or can produce offspring without a mate, but this is the very first time that anyone has confirmed this in crocodilians.”
Davis is referring to the asexual, all-female Silvery Salamander (Ambystoma platineum) which is found in Vermillion County and reproduces via kleptogenesis, a close kin of parthenogenesis.
Discovery delayed
The nest was constructed in a Costa Rican Zoo in January 2018, but this discovery was postponed for two years due to a lengthy permitting process required to deliver biological samples of the fetus from Costa Rica into the United States.
And discovery would be delayed once again with the COVID-19 pandemic, causing science labs worldwide to close their doors temporarily, and the scientific discoveries to come to a halt.
INHS alums assemble to perform analysis
Before the first DNA samples made it to American soil, Davis admits he was originally skeptical.
“As a scientist, you are trained to be skeptical, and there’s a strange thing with reptiles in that females can store sperm for a very long time, so I originally questioned if this was just a finding of record-long sperm storage,” said Davis.
But that skepticism slowly started to fade when his colleague and fellow co-author, Quetzal Dwyer, assured Davis that the female was born and raised in his zoo – and had never been in contact with a male crocodile.
“Their confidence helped us, but we knew we had to do our due diligence because nobody was expecting this and we didn’t want to contribute to the hype,” said Davis.
Warren Booth, an entomologist and Endowed Professor of Urban Entomology at Virginia Tech prepped the DNA from the samples, Brenna A. Levine, assistant professor of biology at Kean University (and former INHS visiting graduate research associate) performed the preliminary genomic analyses, while Davis, Joel Corush, a current postdoctoral researcher at INHS, performed rigorous testing in an independent confirmatory genomic analysis.
With data in hand, genomic analysis of fetal and maternal tissues revealed a 99.9% identical match, confirming a fatherless offspring, and a statistical impossibility that anything other than parthenogenesis had occurred.
“At this point, we were all thrilled, and it’s an awesome reminder of why we all do this job, but then to be able to collaborate with colleagues that I love and adore, and to publish such an important paper is just incredible,” said Davis.
Further implications
Parthenogenesis is a rare phenomenon that is thought to occur when a species faces challenging or unfavorable conditions, such as environmental stress or lack of mates.
Crocodiles' sex is not determined by their chromosomes, but instead by the temperature of the environment at which they are incubated, which adds complexity and nuance to this phenomenon, according to Davis.
“Not only is this the first time that this has happened in a crocodile, but it also has never been documented in a temperature-dependent sex determination (TSD) species,” said Davis.
It has been documented that global warming has disrupted sex ratios in TSD species, with warmer environments producing more female offspring, skewing sex ratios for turtles and crocodiles.
Davis notes that it’s too early to tell if there’s a link between parthenogenesis in TSD species and a warmer climate, but that this could be one way that reproduction shifts.
“As we continue to see sex ratios shift in TSD species, they could possibly do this to survive, but that level of certainty is not available right now,” said Davis.
Parthenogens inherit their asexual ability from a common ancestor, which introduces a possibility that this phenomenon could have happened in pterosaurs and dinosaurs. However, without the ability to retrieve dinosaurs’ and pterosaurs’ DNA analysis, questions remain. Nevertheless, the confirmation of parthenogenesis in crocodiles represents a seismic shift in our understanding of this phenomenon in vertebrates.