Female crocodile lived in isolation for some 16 years at Costa Rica zoo before producing egg with fully-formed fetus.

The female crocodile produced a clutch of 14 eggs within her enclosure in 2018 despite having no contact with males for about 16 years, researchers said.

While producing eggs has been documented among captive reptiles, the more puzzling fact came after three months of incubation when one of the eggs was found to contain a fully formed stillborn baby crocodile.

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

According to the study published in the journal Biology Letters on Wednesday, scientists tested the crocodile fetus’s genetic makeup. They found DNA sequences showing it was a result of facultative parthenogenesis (FP), or reproduction without the genetic contribution of males.

The research paper features an image of the mother and the fully-formed stillborn fetus, which tests later showed was female.

The phenomenon of FP, which some scientists have referred to by the shorthand of “virgin birth”, has also been documented in other species of fish, birds, lizards and snakes. The scientists said this is the first-known example in a crocodile.

In FP, a female’s egg cell can develop into a baby without being fertilised by a male’s sperm cell.

In making an egg cell, a precursor cell divides into four cells: one becomes the egg cell and retains key cellular structures and the gel-like cytoplasm, while the others hold extra genetic material.

Then, one of those cells essentially acts as a sperm cell and fuses with the egg to become “fertilised”.

“These findings therefore suggest that eggs should be assessed for potential viability when males are absent,” the study states.

According to one hypothesis, FP may be more common among species on the verge of extinction.

The American crocodile, Crocodylus acutus, is considered vulnerable and at risk of extinction in the wild.

The scientists who conducted the study said the “virgin birth” in Costa Rica could lead to new information about crocodile ancestors that walked the earth in the Triassic period some 250 million years ago.

“This discovery offers tantalizing insights into the possible reproductive capabilities of the extinct archosaurian relatives of crocodilians and birds, notably members of Pterosauria and Dinosauria,” the study said.