In January 2018, a female crocodile at a Costa Rican zoo laid a clutch of eggs. She was strange: she had been living alone for 16 years.
While crocodiles can lay sterile eggs that don’t develop, some of these clutches looked pretty normal. And one of them – in a twist familiar to anyone who has seen « Jurassic Park » – continued to mature in an incubator. In this case, life has not found a wayas the egg eventually produced a perfectly formed but stillborn baby crocodile.
In an article that appeared Wednesday in the journal Biology Letters, a team of researchers reports that the baby crocodile was a parthenogen, the product of a virgin birth, containing only its mother’s genetic material. While parthenogenesis has been identified in different creatures such as king cobra, sawfish AND California condors, this is the first time it has been found in crocodiles. And because of where crocodiles fall on the tree of life, it implies that pterosaurs and dinosaurs may have been capable of such reproductive feats as well.
Here’s what a virgin birth looks like: As an egg cell matures in its mother’s body, it repeatedly divides to generate a final product with exactly half the genes an individual needs. Three smaller cell sacs containing chromosomes, known as polar bodies, are formed as byproducts. Polar bodies usually wither. But in vertebrates that can perform parthenogenesis, a polar body sometimes fuses with the egg, creating a cell with the necessary complement of chromosomes to form an individual.
That’s what appears to have happened in the crocodile case, said Warren Booth, an associate professor at Virginia Tech who has studied the eggs. Dr. Booth is an entomologist whose main focus is on bedbugs, but that’s how he is an extensive sideline in the identification of parthenogenesis. Genome sequencing of the parthenogenetic crocodile suggests that its chromosomes differ from those of its mother at the tip, where there has been little shuffling of its DNA, a telltale sign of polar body fusion.
This is exactly what happens in the parthenogenesis of birds, lizards and snakes, Dr. Booth said, suggesting that this group of animals inherited the ability from a common ancestor. But crocodiles evolved long before many other modern parthenogenetic animals, which suggests intriguing possibilities about the creatures that got in the way.
« What this tells us is that it’s very likely that this also happened in pterosaurs and dinosaurs, » said Dr. Booth.
Why do animals produce parthenogens? While some parthenogens can survive into adulthood and mate, they’re not always the healthiest creatures, Dr. Booth said. But the increasing ease of DNA analysis, which makes animals born this way easier to identify, has shown that they aren’t all that rare.
« It’s much more widespread than you think, » he said.
It is possible that parthenogenesis gives a species the ability to survive long periods when no mate is available. A new individual, carrying largely the same genes as its parent, might live long enough for a mate to arrive, thus enabling sexual reproduction, which tends to produce more robust offspring.
But it’s also possible that parthenogenesis is simply a trait that doesn’t have enough disadvantages for evolution to eliminate it, Dr. Booth said. It’s not necessarily a stress response or even a lack of mates. In 2020, scientists discovered it lizards can mate and then lay clutches of eggs where some are normal offspring and some are parthenogenous. That’s Dr. Booth’s insight: It’s an ability that can be turned on or off, and is perhaps controlled by a single gene.
So, did dinosaurs do it, as the discovery of parthenogenesis in crocodilians suggests? Parthenogenesis is best confirmed with DNA analysis, a process that has allowed scientists to distinguish it from delayed conception. where a female stores sperm for up to six years before using it to fertilize eggs. Without the possibility of recovering the DNA of dinosaurs and pterosaurs, which does not persist in fossils, there is no certainty.
« We’ll never be able to show that they could, » said Dr. Booth. « But it suggests they had the capability. »