Endangered California condors can reproduce asexually, study finds
Conservation geneticists working to preserve endangered California condors have discovered two cases of chicks hatched from unfertilized eggs – the first known cases of so-called virgin births within the species.
This finding, included in a study published Thursday in The Journal of Heredity, is particularly noteworthy, as such cases are unusual in birds.
Parthenogenesis, the process by which females produce embryos that have not been fertilized by sperm, is more common in vertebrate species such as fish or lizards. Before the results were made public Thursday, other known cases of parthenogenesis in birds were limited to turkeys, finches and domestic pigeons, according to the San Diego Zoo Wildlife Alliance.
“Parthenogenesis is considered a rare occurrence in birds,” said Oliver Ryder, study co-author and director of conservation genetics at the San Diego Zoo Wildlife Alliance. “We found it in the California condors because we have such a detailed genealogical analysis of the entire population.”
California condors have long been an endangered species, with the global population falling to just 23 in 1982, according to the US Fish and Wildlife Service. At this point, the agency has removed all known California condors from the wild and bred them in captivity.
The species, which numbered 504 birds in 2020, has been closely watched and studied for decades, leading to findings like the one released Thursday, said Samantha Wisely, a conservation geneticist at the University of Florida who was not involved in the study.
The need to identify birds by sex in order to develop a successful breeding program led to the discovery of the two chicks.
Years ago, Dr. Ryder was asked to develop a system to identify the sex of California condors in captivity because males and females look alike. He also had to identify close relatives among the birds so that the parents were not matched. So he created a genetic database for all the condors in California.
In 2013, Dr. Ryder’s team noticed discrepancies in the database, which resulted in a reanalysis of all birds in captivity. Dr. Ryder’s team discovered two male chicks – one born in 2001, the other in 2009 – that did not match any of the males’ genetic profiles. This meant that none of the male condors had sired them.
“There was no paternal contribution,” said Dr. Ryder. “They only had genetic information from their mothers.”
The last clue that these chicks had developed from parthenogenesis was the fact that they were both males. Due to the genetic makeup of birds, single-breeding female condors can only give birth to male condors.
In the past, parthenogenesis was viewed as a somewhat hopeless form of reproduction, occurring when females were in small populations or in environments with few members of their own species, Dr Wisely said.
The captive condors, however, had been mated with males in an enclosure, but still reproduced by parthenogenesis.
According to Dr. Ryder, the discovery of “virgin births” in such a closely watched bird population has scientists wondering if more birds in the wild breed through parthenogenesis than previously believed.
“For other species, it seems like some kind of last resort effort to save themselves,” Dr Wisely said. “It will be really interesting to know the context in which this is happening in nature for birds.”
Another interesting aspect of parthenogenesis is that fatal genetic traits cannot be passed on from the mother.
Yet, said Dr Ryder, some less favorable traits can still appear in the offspring.
“Maybe they could, you know, not have the beautiful genes or something,” he said.