Spotlight on Zoo Science
June 17, 2009
Courting a Crane
National Zoo scientists and keepers come up with creative solutions when presented with the challenge of a genetically valuable crane that can't produce a chick.
Amanda is a beautiful 20-year-old. With a swan-white neck, a red mask, and striking black marking down her sides, she stands at a willowy four feet tall. She’s a white-naped crane, but she’d be shocked to hear you say so.
A member of a protected species, Amanda was hand-raised from the time she hatched. The first face she ever saw was human, and so that's what she thinks she is. She interacts with humans the way other cranes interact with cranes, calling to them and greeting known individuals. Her human-mindset means she’s very easy for her keepers to care for, but it presents a serious obstacle to natural breeding. For a long time, she did live with a male white-naped crane, but they never mated or laid fertile eggs.
The catch was that Amanda is a very valuable bird genetically. She has few living relatives and has never produced any fertile eggs or offspring, so her genes are very rare. And in a species that has only about 5,000 birds left in the wild and about 60 in North American zoos, every unique gene counts. Scientists work very hard to preserve all the genetic diversity in a captive population, which means they needed to figure out a way for Amanda to have healthy chicks, ideally chicks that know they’re cranes.
Amanda was already 20 years old when she arrived at the Zoo’s Conservation and Research Center (CRC) in Front Royal, Virginia. Cranes usually breed for the first time when they’re between three and five years old. She was sent to CRC because there was a very valuable male there, who also had never bred. He was the product of artificial insemination between two parents who had never bred before. The two cranes lived together amicably until breeding season came. Amanda’s mixed-up behavior confused her mate, and he ended up hurting her.
Since natural breeding was out, artificial insemination seemed to be the best answer. Normally, artificial insemination requires at least two people to restrain the bird and insert a semen sample into her cloaca. Catching and restraining the bird is stressful for the bird, and there is the very real possibility that either the bird or the humans will get hurt in the process. In addition, the scared bird tends to poop, which makes insemination more difficult and dirty, and less likely to produce a chick.
One keeper, Chris Crowe, thought that with Amanda there might be another solution. He decided to take advantage of her human proclivities: He’d play the role of the male bird, allowing her to be artificially inseminated “naturally;” without restraint or stress. In order to do this, he spent long hours in her enclosure, talking to her, feeding her, and generally getting her comfortable around him. As she grew more accustomed to his presence, he slowly began approaching her, until she was comfortable with him standing close to her and, eventually, would even allow him to touch her.
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Once breeding season came around, Chris would wait until Amanda "solicited" him by spreading her wings and turning her back to him, and used that opportunity to inseminate her with sperm taken from her erstwhile mate. Then everyone held their breath until one morning they discovered an egg in Amanda’s enclosure.
Just having an egg wasn’t enough, though. The scientists had to figure out whether the egg was fertile. Typically, bird keepers do this by "candling" the egg, or holding it up to a light source (a "candler") to peek inside the egg for veins and a growing embryo. This doesn’t work in white-naped crane eggs, though, because they are too dark. Instead, keepers float the egg in a pan of water or lay it on a smooth, flat surface and watch for movement. In a fertile egg, the embryo will twitch slightly in response to disturbances or scientists imitating an adult crane’s voice.
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| The white spots on the egg are the epoxy patches where keepers candled the egg and drew blood. |
Scientists strive to maximize the genetic diversity in the captive population of white-naped cranes. This means making sure that there are enough of each sex. Right now, the population needs more females, so keepers are doing their best to make sure as many eggs as possible each year hatch out female chicks. Determining a bird’s sex before hatching is hard but not impossible. When this sex-ratio imbalance became apparent several years ago, the call went out for keepers and scientists to figure out a way to determine an embryo’s sex while it’s still in the egg. Several institutions, including the National Zoo, figured out ways to do so. However, very few institutions have done this and only the National Zoo has used it regularly.
Usually, keepers would candle the egg to figure out where the veins are and draw blood out that way. But since they can’t see through crane eggs, they sand a very small hole in the eggshell, leaving the inner membrane intact. They use this tiny window to find the veins inside the egg, and then they draw a little blood out of a vein with a syringe. They patch the hole in the eggshell with epoxy, and let the epoxy dry in an incubator before they return the egg to the nest. The blood sample then goes off to a lab for analysis. Bird blood cells, unlike humans, have nucleuses, which contain the genetic code of the bird. Geneticists look at these cells for the signature that marks the individual as male or female. In birds, unlike in mammals, females are the heterogametic sex (ZW) while males are the homogametic sex (ZZ). In this case, the geneticists found a ZW signature: the embryo was female.
The nest the egg goes back to though, wasn’t that of the crane that had laid it. Because her lack of a mate and her preference for humans might endanger the egg or chick, a foster couple of
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| The five-day old crane chick sticks close to her grandparents. |
cranes incubated the egg. The foster couple, actually the potential offspring’s grandparents, are proven parents who weren’t recommended to have a chick of their own this year.
On May 23, after a month of lying ensconced in its cozy nest the egg hatched, revealing a healthy chick. She’s now in the care of her grandparents, where she’ll stay until she fledges in two or three months.
The breeding season is over for this year, but keepers hope to put Amanda in with a new male. She might be compatible with this male, and they might be able to produce chicks naturally. However, if they don’t, they’ll continue inseminating her artificially. There are fewer than 5,000 white-naped cranes left in the wild, and only 60 in captivity in North America, so every chick counts.
Related Information
White-naped Crane Hatches at Zoo’s Conservation and Research
Center
Note to Media: If you would like more information about this project, or any of the Zoo's conservation and science programs, please contact the Zoo's Office of Public Affairs.
