Science

How Two Female Birds Had Baby Birds With No Help From Male Birds

“It was a bucket of water in the face.”

A large bird perched on a rock spreads its wings
San Diego Zoo Safari Park

Two baby birds were born in California without any help from males—that is, they were each produced by a female bird alone. The San Diego Zoo Wildlife Alliance released a report Thursday detailing the discovery, which reveals that condors, a critically endangered species of bird, are able to asexually reproduce.

A few years ago, the Wildlife Alliance researchers began tracing the parentage of the California condors they keep in captivity. (The goal was to better track the birds, after an incident in which a couple of birds accidentally had their wing tags switched.) But when researchers analyzed the genetic profile of two of the male baby chicks, they noticed something strange: The genetic markers used for parentage determination indicated they couldn’t have had a male parent. Each bird had just one parent.

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Oliver Ryder, the director of conservation genetics at the Wildlife Alliance and one of the co-authors of the report, filled Slate in on what it was like being a part of the discovery and what it means for the future of the critically endangered species, as well as updated us on the fate of those baby birds. The conversation has been condensed and lightly edited for clarity.

Meredith Moran: Was it a huge shock that these birds could reproduce without help from a male?

Oliver Ryder: It was a huge surprise. I mean, it’s not what we were looking for. It was a bucket of water in the face. It was quite a moment when all the data came together with the realization that that was the only reasonable explanation—that we discovered that there were two chicks that had been hatched that had no male contribution.

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So female birds were around the males, but chose not to reproduce with them. Why?

They were with males that they had reproduced with sexually, but I don’t know whether they chose to not do it this time. I don’t know if they were aware or not, and I’m not really sure how to test that. But we have the facts that they were capable of reproducing sexually, and did, and then for reasons that we can only speculate about, they didn’t. And then one of them at least started again.

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One of them has started reproducing sexually again?

Yes, sexually again. I don’t have the latest records, but possibly both of them.

So we know that these endangered birds can reproduce without males. If all the male condors disappeared, would the population be OK?

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We don’t know. I mean, I would assume—but we don’t have any evidence that condor females would continue laying eggs in that situation. We really don’t have a good idea yet of the frequency of asexual reproduction in condors. We know it’s happened twice, and that’s remarkable.

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But, you know, depending on the frequency, or what it would look to trigger it environmentally or in the background of an individual bird, we don’t know about that, so it would really just be speculation.

Why can birds occasionally reproduce asexually, but not humans?

It’s complicated, but humans have a higher level of gene regulation that marks the maternal and paternal genomes. The male has to provide genetic information to a mammal embryo to produce a placenta. There are contributions that have to come from males in order to have normal development, whether it’s a male or a female, so parthenogenetic development [which is what these condors did] only results in tumors in people.

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The paper notes that both of the chicks that were birthed like this are “now deceased.” One of them was kept in captivity and the other was released into the wild. What happened?

One died of an infection, and the other—we’re not exactly sure why it died, but it was malnourished and didn’t successfully compete with other birds for food.

How is it decided which condors remain in captivity and which get released into the wild?

It’s not a process that I participate in closely, but my understanding is that they want to produce animals with a wide genetic diversity, so that when they reproduce in the wild, they’ll have as large of a gene pool as possible. So there’s a genetic consideration and then there’s a sex consideration—you want to have a basically equal sex ratio.

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All condors before release go through a kind of training for reintroduction into the wild. They’re trained to avoid landing on power lines. Their flight skills are assessed, and to some extent, their behavioral skills are assessed. All of this is done by the experts in the big production team and they make those decisions.

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Do female condors reproduce asexually in the wild?

That’s something we’re waiting to find out.

How much do you think this finding will change the process of the captive breeding program going forward?

I think it’s very important to monitor this to understand the species as well as possible, but I don’t think that the day-to-day management is going to change much. I do think that if we get a sample from a chick to identify a set and it turns out it’s a male and then, when we do the parentage analysis, it’s all homozygous, you know, we will have found another, and we’re going to pay very close attention to how that bird develops and its behavior.

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