After 30 Years of Breeding Condors, a Secret Comes Out

‘Virgin birth’ might be more common in animals than we thought.

close-up photo of california condor
Justin Hofman / Alamy

When you get to be as endangered as the California condor, your sex life becomes a highly public affair. Since 1983, when the number of California condors in existence was a mere 22, biologists have been carefully breeding the birds in captivity. They kept track of who mated with whom, how many offspring they had, and when those offspring were released into the wild. All of this is logged in the official California-condor “studbook.”

So it was quite a shock when, a few years ago, scientists conducting DNA tests as part of routine research found two condors with unexpected paternity. These two birds—known by their studbook numbers as SB260 and SB517—were not related to the fathers recorded in the studbook. Actually, they had no fathers at all. A full 100 percent of their DNA had come from their respective mothers. “We were confronted with this inexplicable data set,” says Oliver Ryder, a conservation geneticist at the San Diego Zoo Wildlife Alliance.

The only possible explanation was a strange one: The eggs that produced these two condors must have essentially fertilized themselves without any sperm. The phenomenon is known as parthenogenesis or, colloquially, “virgin birth.” (The two mothers in this case weren’t technically virgins; they had previously produced normal chicks with the male they were housed with. As I said, not much sexual privacy when you’re a California condor.) Parthenogenesis has been studied in other birds, like turkeys and chickens. It’s also been documented in snakes, lizards, sharks, rays, and bony fish—both in captivity and more recently in the wild. Many of these discoveries were accidental, and all of these accidents have scientists wondering if parthenogenesis is not as rare as once thought.

In the case of the condors, Ryder and his colleagues had used DNA markers to help manage the breeding program for years. It helped them minimize inbreeding and develop a test for chondrodystrophy, an inherited bone disorder common in condors. After captive-bred birds were released into the wild, the team even rappelled down cliff faces to study the parentage of their chicks. The biologists ultimately accumulated samples of blood, eggshell membrane, feathers, and tissue from more than 900 condors over the course of the condor-management program. A few years ago, they decided to analyze the DNA from all of them. That’s when the oddity in SB260’s and SB517’s paternity showed up.

Unfortunately, by the time scientists realized the birds were genetically unique, both of the condors had died, so they weren’t able to study how SB260’s and SB517’s unusual parentage might have affected them. When the birds were alive, they weren’t so remarkable that the zookeepers thought to do a special postmortem exam. “To the people taking care of them, they were another condor,” Ryder says.

But both of the condors did have some documented health issues. SB260, a male hatched at the San Diego Zoo Safari Park in 2001, died two years later after being released into the wild—he was always small and did not integrate well with the wild birds. SB517, a male hatched at Los Angeles Zoo in 2009, had a curved spine and trouble walking. He was never released into the wild and died in captivity at about age eight. (California condors usually live for decades.) “They certainly weren’t, shall we say, shining specimens of the condor,” says Demian Chapman, a biologist at the Mote Marine Laboratory and Aquarium, who has studied parthenogenesis. That’s not uncommon for parthenogenetic animals, also known as parthenotes.

No one can say definitively that the issues in these two male condors were caused by parthenogenesis, but scientists have observed similar patterns in other species that produce virgin births. In turkeys, for example, male parthenotes tend to be small in size with poor semen quality, says Reshma Ramachandran, a poultry scientist at Mississippi State University. And Warren Booth, a biologist at the University of Tulsa, told me he’s seen skeletal changes in snakes similar to SB517’s. “Just a couple of days ago, I was dissecting tissue from parthenogenetic pit vipers,” he said, and those vipers too had short, stunted spines with malformed skulls. (Booth was not involved with the condor study, but he did handle it as an editor at the Journal of Heredity.)

What’s cool about the case of the condors, Booth said, is “they hatched live parthenotes and those grew to some level of maturity.” The discovery of parthenogenesis across more and more vertebrates has some scientists thinking that parthenogenesis is not always a dead end—it might even be adaptive in certain circumstances. In boas and pythons, Booth has been able to get female parthenotes to breed with males and have viable offspring. In the wild, parthenogenesis could help these reptiles recover from severe population loss. ​​(Although parthenogenesis has now been found in many vertebrates, mammals seem incapable of it because some of our genes are selectively turned on, depending on whether they’re inherited from the mother or father, so we need both.)

In species where parthenogenesis has been extensively studied, the process begins not long after the egg itself is created. When a cell divides in two to make an egg cell, the other half becomes a polar body, which contains a near-identical copy of DNA. Normally, the polar body disintegrates. But studies of other birds have revealed that on occasion, the polar body somehow merges again with the egg, acting like sperm fertilizing it. Because of birds’ chromosome system—ZZ makes males and ZW makes females—all avian parthenotes are males. If an egg with a W chromosome merges with its polar body, the resulting WW embryo will not be viable. Only the ZZ parthenotes ever hatch.

But that doesn’t explain why some females go through parthenogenesis but not others. The poultry industry—which, given its interest in bird breeding, has extensively studied parthenogenesis—has found that a number of factors influence it in turkeys and chickens. One is genetics, says Ramachandran. Different poultry breeds have significantly different rates of parthenogenesis, ranging from 0.16 percent in Barred Plymouth Rock chickens, to 3 percent in commercial turkeys, to 16.9 percent in Beltsville small white turkeys. Poultry scientists have also succeeded in selecting for parthenogenesis, increasing the incidence in Beltsville small white turkeys more than threefold, to 41.5 percent in five generations. Environmental factors—like high temperatures or a viral infection—also seem to trigger poultry parthenogenesis.

In condors, biologists are trying to understand more about what happened in the two parthenotes SB260 and SB517. They are sequencing the pair’s full genomes—and they plan to sequence those of hundreds of other condors too. (The DNA analysis in this paper relied on 21 DNA markers, not the full genomes.) Ryder says he hopes to use this more complete genetic information to understand mutations and guide the condor breeding program. Every condor today, after all, is descended from a tiny genetic pool. The original population of 22 has now grown to just over 500, but the species remains critically endangered. California condors may be capable of remarkable reproductive feats, but they still need all the help they can get.