The California condor is the largest flying bird in North America, with a 10ft wingspan that enables it to soar up to 15,000ft – nearly half the height of a commercial airplane. Now the birds can claim another superlative feat: scientists have discovered that females can reproduce without a male partner, in a rare phenomenon known as parthenogenesis.
Oliver Ryder, the director of conservation genetics for the San Diego Zoo Wildlife Alliance, has called the recent findings a “eureka moment”.
“Many scientists have experiences like this when they see the world in a way they haven’t seen before,” says Ryder, who co-wrote a paper detailing the discovery, released in late October. “I was really pleased to figure out something that was such a conundrum.”
Condors have been imperiled for a century. As Europeans settled the west, they often shot, poisoned and captured the condors, collected their eggs, and reduced their food supply of antelope, elk and other large wild animals. Their population declined steadily until only 22 remained in 1982, but a state-led recovery program has since successfully bred the animals back from the brink.
The journey that led to discovering the condor’s asexual reproduction also began in the 1980s, when the San Diego Zoo was tasked with developing a test to determine the sex of the few remaining wild condors, which were eventually all brought into the care of two captive breeding locations.
Ryder says the zoo continued to use DNA testing to determine the familial relationships between the condors, to avoid inbreeding closely related individuals. More recently, scientists decided to undertake a complete genetic analysis of the population, which had grown to more than 900 birds. They used condor samples collected over the course of the program’s 30-plus-year history, including blood, eggshell membrane, tissue from deceased birds and feathers.
When the scientists completed their investigation, there were two glaring cases that indicated something was amiss. Both involved chicks that came from an egg without genetic material from the male who shared the enclosure and was presumed to be the father. The results were confirmed by multiple samples, so scientists were confident it wasn’t a mistake.
Ryder remembers the conversation with his colleague about the strange cases. He got goosebumps as he realized that the condors had reproduced through parthenogenesis – a form of reproduction in which an egg can develop into an embryo without being fertilized by a sperm. Parthenogenesis is derived from the Greek words for “virgin birth” and several insect species including aphids, bees and ants are known to reproduce through the process, although the phenomenon is rare in birds.
Ryder says it’s not yet known how unusual this kind of reproduction is in the natural world, because detailed genetic analysis of large populations hasn’t been conducted. “Think about it: the cases where we do know about it, it’s overwhelmingly where animals are managed in a way where females don’t have access to males. It was only because we had these detailed genetic profiles of the condor which were developed in response to the need for managing generic diversity that we discovered this.”
The idea that these are “virgin births” is incorrect, he says: the females who reproduced this way had previously had many chicks through sexual reproduction. And birds don’t give birth; they lay eggs that hatch.
Ryder thinks this phenomenon is probably going on in more species than people know. There are reports of snakes in the wild producing offspring through parthenogenesis. He says it’s a “rare but possibly impactful part of reproduction”.
The condor is still considered a threatened species, and it remains unclear what solo reproduction will mean in terms of helping with conservation efforts. Condors can live into their 50s, but both of the male chicks born of asexual reproduction were relatively small and died before becoming sexually mature, at 1.9 and 7.9 years old.
Ryder hopes to use the same genetic tools to peer into past populations – California condor specimens from the 1800s and 1900s are commonplace in museums in the state and scientists could use them to assess how common parthenogenesis was in previous generations.
He adds that the condor is a remarkable species that has survived from the Pleistocene era. Right now, the condor’s population growth is limited by poisoning from lead bullets, when the scavengers eat animals shot by hunters. “It’s been critically endangered and there’s a lot of public support for recovery,” Ryder says.
Meanwhile, the bird,which has risen like the phoenix from near death, is rewriting scientific understanding. “This is really wondrous and amazing,” says Ryder: in the midst of an extinction crisis, scientists are still making big discoveries. “This is an unrecognized part of their natural history.”
