California condors can reproduce asexually, San Diego Zoo discovers

SAN DIEGO — He’s got his mother’s eyes, and his father’s … nothing.

A scientific team led by the San Diego Zoo Wildlife Alliance announced earlier this month that California condors can reproduce without having sex. Researchers made the finding after genetic tests showed that two condors born in captivity didn’t have fathers.

Yes, fathers. And that’s not a misspelling of “feathers.”

This phenomenon, known as parthenogenesis, has been seen in certain insects, fish and reptiles. There have been a few cases among birds, too. But no one knew until now that female California condors could have offspring without males, a head-scratching finding that raises questions around how often this occurs and whether it matters in the wild.

Another unanswered question: Why did this happen? The mothers of both condors had each mated successfully before and were housed with a male at the time they reproduced asexually.

“I thought it was pretty remarkable,” said Kevin Burns, an ornithologist at San Diego State University, who was not involved in the study. “That raises the issue that we should be looking for this more, I think.”

It’s no accident the discovery was made by the San Diego Zoo Wildlife Alliance, which runs the zoo and Safari Park. The non-profit organization has played a key role in helping California condors claw their way back from the brink of extinction.

By 1982, there were only 22 of the iconic birds left. By the end of 2019, that count rose to 525, with 306 condors flying freely across California, Arizona, Utah and Baja California. A captive breeding program run out of the Safari Park has accounted for much of that rebound, and the zoo and Safari Park have hatched more than 160 condors over the years.

Throughout this ongoing effort, researchers have collected a vast repository of condor blood, feathers and tissue.

Researchers didn’t expect any major surprises when they began testing samples from more than 900 condors around 2013. That all changed when Leona Chemnick, who was then a researcher at the zoo’s institute for conservation research, told director of genetics Dr. Oliver Ryder she needed to talk with him about some puzzling findings.

Ryder, who was leaving the office, asked Chemnick to walk and talk as he headed to his car. As they did, she explained that two condors born in captivity didn’t seem to have genetic material from their fathers at any of the 21 different DNA regions Chemnick checked (and double-checked).

What was also odd, she added, was that while each condor had two DNA copies at each site, those copies were all identical.

That stopped Ryder dead in his tracks.

Did the birds only inherit maternal copies, he asked?

Yes.

Were they males?

Yes.

“I said, ‘You’ve just shown that there’s parthenogenetic development in California condors,’” Ryder recalls. “I still get a chill and goosebumps when I tell that story.”

Ryder and colleagues reported their results in the Journal of Heredity.

, the official publication of the century-old American Genetic Association.

Most animals (us included) inherit two sets of genes, one from mom and one from dad. The fact that researchers couldn’t find evidence the birds had DNA from a father was a telling sign that they didn’t have one.

The sex of the hatched condors fit, too. That’s because while it may seem bizarre for a female bird to have male chicks on her own, sex in birds and people isn’t determined the same way.

In humans, females have two X chromosomes, while males have an X and a Y. There are exceptions, such as people with Klinefelter syndrome, who are XXY males, but that’s the general rule. In birds, it’s the opposite. Males have two Z chromosomes while females have a Z and a W, meaning that an unfertilized egg already has the genetic material needed to form a male chick.

It’s happened before, says Reshma Ramachandran, a researcher with Mississippi State University’s department of poultry science, noting that reports of parthenogenesis in pigeons, quails, chickens and turkeys date back to the 1960s. In these cases, researchers believe there’s a hitch in the normal steps of cell division and development that cause eggs to only carry half the DNA they need to form a living creature, resulting in an egg with a full set of DNA from just one parent, though scientists haven’t fully worked out all the details.

“It was very interesting. I was not totally surprised,” she said of the recent study. “Based on my research experience, I’m expecting more incidents like this in the wild and in domestic flocks.”

But such cases are likely to be few and far between, adds Ramachandran, who has been part of teams that have looked for parthenogenesis in turkeys and quails.

Despite 15 years of trying, scientists never got a single quail to hatch by parthenogenesis, though they saw clear signs of early embryo development in unfertilized eggs. They did, however, have one male turkey grow to adulthood, something other researchers have also seen in turkeys and chickens. But the bird was small and failed to fertilize hens even after researchers tried artificial insemination.

Those issues echo the fates of both condors. One of the birds, hatched at the Safari Park in 2001, was released into the wild but died at Big Sur’s Ventana Condor Sanctuary in 2003. The other, hatched at the Los Angeles Zoo in 2009, died at the Oregon Zoo in 2017. The latter bird was underweight and suffered from a curved spine, and neither condor reproduced before it died.

That raises the question of whether parthenogenesis in birds is important in the wild. Right now, scientists aren’t sure. Some speculate that rare and spread-out species such as condors could benefit from reproducing asexually if they can’t find a mate. But that’s only true if the offspring survive and pass down their genes to the next generation.

In other species, however, it’s clear that parthenogenesis is a successful strategy. Some female-only lizard species only reproduce this way. In other cases, sharks, snakes and scorpions can alternate between sexual and asexual reproduction, using parthenogenesis when males are hard to come by.

There’ve been a number of reports from zoos where female Komodo dragons, pythons and other critters have had offspring without ever seeing a male. But scientists wouldn’t have spotted the condor cases without their detailed genetic analysis. After all, Ryder says, if you see two birds caring for a clutch of chicks, it’s natural to assume the male’s the father. This new finding calls that assumption into question.

“One of the meanings of this is that we might think we understand life, but we shouldn’t take it for granted,” he said. “There may be more going on out there than we know about.”