That finding, included in a study published Thursday in The Journal of Heredity, is particularly remarkable, as such cases are unusual among birds.
Parthenogenesis, the process by which female animals produce embryos that have not been fertilised by sperm, is more common among vertebrate species like fish or lizards. Before the findings made public Thursday, the other known instances of parthenogenesis among birds were limited to turkeys, finches and domestic pigeons, according to the San Diego Zoo Wildlife Alliance.
“Parthenogenesis is considered to be a rare phenomenon in birds,” said Oliver Ryder, a co-author of the study and director of conservation genetics at the San Diego Zoo Wildlife Alliance. “We discovered it in California condors because we have such a detailed genealogical analysis of the entire population.”
California condors have long been an endangered species, with the world population falling to just 23 in 1982, according to the US Fish and Wildlife Service. At that point, the agency pulled all the known California condors out of the wild and bred them in captivity.
The species, which in 2020 numbered 504 birds, has been closely monitored and studied for decades, leading to discoveries like the one published Thursday, said Samantha Wisely, a conservation geneticist at the University of Florida who was not involved in the study.
The need to identify the birds by sex in order to develop a successful breeding program led to the discovery about the two chicks.
Years ago, Ryder was asked to develop a system for identifying the sex of the California condors in captivity because males and females look the same. He also had to identify close relatives among the birds so that relatives would not be paired. So he created a genetic database for all California condors.
In 2013, Ryder’s team noticed some discrepancies in the database, which prompted a new analysis of all the birds in captivity. Ryder’s team discovered two male chicks — one born in 2001, the other in 2009 — that didn’t match any of the males’ genetic profiles. That meant that none of the male condors had fathered them.
“There was no paternal contribution,” Ryder said. “They had only genetic information from their mothers.”
The final clue that these chicks had developed from parthenogenesis was the fact that both were male. Because of the birds’ genetic makeup, female condors reproducing on their own can give birth only to male condors.
In the past, parthenogenesis has been thought of as a somewhat desperate form of reproduction, occurring when females were in low-male populations or in environments with few members of their own species, Wisely said.
The condors in captivity, however, had been paired with males in an enclosure, yet still reproduced through parthenogenesis.
According to Ryder, the discovery of “virgin births” in such a closely monitored bird population is leading scientists to wonder whether more birds in the wild are reproducing through parthenogenesis than previously believed.
“For other species it seems to be sort of a last-ditch effort to save themselves,” Wisely said. “It will be really interesting to know the context in which it’s happening in the wild for birds.”
Another interesting aspect of parthenogenesis is that lethal genetic traits cannot be passed down from the mother. Still, Ryder said, some less favorable traits may still appear in the offspring.
“Maybe they could, you know, not carry the good-looking genes or something,” he said.
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