The Genetic Jungle

A few years back, a pair of pheasants at our zoo had produced a clutch of eggs.  We ourselves had no capacity for more birds of that species, nor were they part of a managed Species Survival Plan.  Before pulling the eggs from the hen, I put out a question on a zoo list-serve asking if anyone was interested in offspring.  I received a fairly prompt reply from one zoo saying that would be interested in eggs, but only in eggs that would hatch out as females. 

Would I please do a quick in ovo egg sexing and incubate two female eggs for them.  My reply: "... we can do that?"  I felt like an idiot, but I was way too surprised to care.

It turns out, yes, yes you can.  You simply shave off a tiny piece of the eggshell, using either a Dremel or belt sander, without damaging the membrane inside.  Then, you delicately take a blood sample from the inside of the egg, send it out for testing (or run your own gel electrophoresis, if you are so inclined and so capable), and voila.  You can tell if the egg is going to be a male or female.  Then, take the eggs and - after carefully sealing up the tiny patch of shell you removed - place it back under the parent or in an incubator.

The science of egg-sexing has major implications in small zoo populations.  San Diego Zoo Global had been trying to establish a breeding program to save Storm's stork, a gorgeous dark blue stork from Southeast Asia.  The problem was that they kept producing surplus males - males that they would have to house and feed and spend time and resources caring for, but that were of no use to the population since there were no females around to pair with them.  A little in ovo sexing and soon only female eggs were being incubated.

It's a testament to any science that it goes quickly from being an unheard of possibility to a tried and true daily staple that we can't imagine doing without.  Few developments have rocked modern zookeeping more than the application of genetics to the field. 

I'd gone thus far in my career without ever coming across genetic testing of egg embryos, but I had used genetics in other areas.  For example, I'd often relied on blood tests to identify whether birds were male or female.  In some bird species, like pheasants, ducks (but not swans and geese), and many songbirds, the males and females look very different, often with the males having bright, beautiful plumage.  In others, such as raptors, males and females are different sizes - though with some overlap.  For others, however, the sexes look very similar.  Without DNA testing, a keeper could easily have two males languishing in an aviary together, wondering forever why they just won't breed.

DNA analysis can also be helpful in determining parentage.  This is most helpful when you have a lot of members of the same species sharing an enclosure.  Even in species which are reported to be monogamous, there can be some sneaky cheaters out there, and knowing proper parentage is needed for making future breeding recommendations.  Otherwise, you could easily end up pairing a female off with an animal who is really her father, having snuck by and visited her mother while her presumed father had his back turned.  DNA analysis at the Bronx Zoo, for example, revealed a whole lot of cheating going on in their scarlet ibis aviary. 

Birds aren't the only ones undergoing paternity tests.  When Ling Ling, the National Zoo's first female giant panda was paired with male Hsing Hsing, she was bred with him naturally AND artificially inseminated.  Her cub did not survive, but researchers were able to determine that Hsing Hsing was the father, not the AI donor. 

It is increasingly becoming common to do DNA testing to determine what species an animal is.  Two-toed sloths, we now realize, are represented by two species.  They look a lot alike superficially, but genetically are distinct.  A zoo that has a male Linne's two-toed and a female Hoffman's two-toed might not realize why their pair is failing to produce.  Similar problems plagued zoo populations of night monkeys, small nocturnal primates from the Neotropics, which we know realize are a hodgepodge of different species.  Or look at the recent revelations about the slender-snouted crocodile, recently carved into two species.

The future of genetics in zoos is full of possibilities.  We could look out for markers that would serve as warning signs of harmful traits, allowing us to filter those out of the population by not breeding those individuals.  Not to be too Frankenstein-ish, but at some point it may become necessary to genetically "tweak" some species in order to improve their survival.  For example, encouraging a species start breeding earlier in the year in response to global climate change, should that species ever be restored to the wild. 


The future is full of possibilities.... lots of questions, yes, but possibilities nevertheless.