Crocodiles can reproduce without males – and perhaps dinosaurs can too

A fully formed crocodile fetus has been found in an egg laid by a female that has had no contact with males. This shows for the first time that “virgin birth” is possible in these animals.

The female fetus – which died at full term – is a near-genetic replica of its mother, a healthy adult who was kept in an isolated enclosure at a Costa Rican reptile park for 16 years.

Parthenogenesis, a form of asexual reproduction in which embryos develop from unfertilized eggs, occurs in some snakes, lizards and even turkeys. The discovery in a crocodile suggests it dates back to a shared ancestor of these reptiles and birds at least 267 million years ago — and suggests that dinosaurs and pterosaurs may have been able to reproduce without males, too.

Workers at the reptile park were surprised to discover their individually caged, 18-year-old female American crocodile (Crocodylus acutus) guarding a clutch of 14 eggs. Half cast shadows when held up to the light, as if they contained a fetus.

Because of its expertise in parthenogenesis, the park’s science team got in touch Warren Booth at Virginia Tech, who suggested incubating the eggs. When none of the eggs hatched, the team opened them and found six filled with unrecognizable contents, likely a mix of yolk and undeveloped cells, he says. One, however, contained a fully formed, but lifeless, female fetus.

Genomic Sequencing of tissues from the fetal heart and from the maternal slough skin revealed a 99.9 percent agreement — confirming that the offspring had no father and were the result of asexual reproduction.

Technically, though, the young crocodile wasn’t a clone, Booth says. Like all known cases of vertebrate parthenogenesis, the embryo was formed when the egg fused with one of its own byproducts called the second polar body, meaning it had two copies of the mother’s DNA.

However, the fact that the fetus was female had nothing to do with the parent’s chromosomes, as in crocodiles sex is determined purely by outside temperature – and the incubation temperature of 29.5°C (85°F) was just right for forming female fetuses.

The reason why some female reptiles, fish and birds sometimes reproduce asexually remains unclear. But it doesn’t seem to be related to a lack of males, as it occurs even when enough males are present, Booth says. “I think it’s controlled by a single gene, which could be activated by hormones or something like that,” he says, adding that more research is needed.

The early death of the fetus does not mean that offspring produced by asexual reproduction cannot live a full life, he adds. Many parthenogenetic offspring of other species mature and then reproduce sexually.

“You wouldn’t believe how often people send me Jeff Goldblum memes saying, ‘Life finds a way,'” says Booth.

Yet mammals can’t reproduce by parthenogenesis, he says. Unlike reptiles, fish and birds, their embryo formation requires genomic imprinting, meaning that specific genes from both the mother and father must be turned on and off at specific times for the embryo to form.