What do you get if you cross an octopus with a spider monkey? A loss of funding, a PETA protest, and a very stern letter from the university's genetics department.What do you get if you cross a giraffe with a zebra? An okapi, apparently. A cat with a bear? That gets you a binturong. What about a pig with an elephant? That's where tapirs come from.
None of this is remotely true, of course, but they are all things that I've heard from visitors before. Show people an unfamiliar animal, and they will instantly liken it to something familiar, saying that it looks like a cross between an A and a B. I think some guests think that there are only a few dozen species of animal in the world, and that the diversity of life that we see is the result of endlessly cross-breeding and mixing them. Of course, this belief goes back to ancient times - the earliest name of the giraffe, for instance, was Camelopard ("Camel-Leopard"), a fact which is immortalized in the species Latin name.
None of this, of course, is to say that hybrids don't happen - sometimes in the wild, far more commonly in captivity. And few things make zookeeping more complicated than hybrids.
A hybrid can occur on one of several levels. It can be the cross between two subspecies, such as an Amur tiger crossed with a Bengal tiger. It can be between two species, such as a lion with a tiger. Sometimes it can even be across genera, such as a puma crossed with leopard. Some hybrids are sterile, such as those between horses and donkeys. Others are fertile, such as those between brown and polar bears.
There are several reasons why people might produce a hybrid.
Sometimes they happen by accident. At one zoo where I worked, green tree pythons and carpet pythons shared an exhibit. The two closely-related species bred and produced a clutch of babies which were stunningly beautiful and horrifyingly irascible (we were forbidden to take pictures of them - the curator didn't want to publicize our mistake). Such roommate-pairings can occur in many mixed-species exhibits, with waterfowl and pheasants being notorious for hybridizing. Often, if there are suitable partners of the correct species, animals will still choose to match with their correct mate. While our babies were "whoopsies", some breeders breed them on purpose to create novel color patterns.
Sometimes it is done deliberately for a specific purpose Gyrfalcons, the largest of the falcons, are prized by falconers for their ability to capture larger prey. Being an arctic species, however, they tend to be susceptible to diseases when kept in temperate climates. Falconers have been known to cross them with other falcon species, such as peregrines and sakers, to produce a bird that is large but disease-resistant. Or you could look at the most famous hybrids in the world - mules and hinnies, the cross between donkeys and horses, used as work animals for millennia.
All too often, it's done just for the heck of it. No animal exemplifies that better than the liger or its counterpart the tigon. Crosses between lions and tigers are often larger than both and retain a slight mane (the legacy of one parent) and faint striping (from the other). Ligers are commonly produced in private collections, maintained for their "wow" factor. Some zoos then like to hybridize their hybrids - let's see how many of the big cats we can get in one! Zebras crossed with domestic equines - horses and donkeys - are another commonly encountered hybrid.
A small number of hybrids occur naturally - though often humans show their hand in the mix somehow, whether by modifying the environment or by introducing species. With the disappearance of wolves over much of North America, coyotes are filling the vacuum. Sometimes wolves, unable to find a mate of their own species, will pair up with coyotes. Genetic "swamping" with coyotes is one of the leading threats to the last remaining red wolves. Similarly, endangered Cuban crocodiles increasingly find themselves mating with the American crocodiles that are encroaching on their range; ditto for spotted owls and barred owls. Lately, polar bear/grizzly bear hybrids have been in the news, serving as another warning about global climate change.
From the perspective of a reputable zoo, it's hard to see any value in hybrids. Our goal should be to showcase and conserve animals as they are, not as we imagine they might be, and certainly not just tampering with gene pools for a giggle. This is especially true in the face of future reintroductions. A hybrid between two species is often less well-suited to life in the wild than either of the parent species.
Desperate times sometimes call for desperate measures, however. For example, in Tears of the Cheetah, Stephen O'Brien recounts how, in a last-ditch effort to save the rapidly disappearing Florida panther, conservationists decided to add fresh blood to the gene pool in the form of Texas cougars, another puma subspecies. Though controversial (some critics asked if the resultant animals could still count as Florida panthers), the decision is credited with saving the panther.
Of course it's not a hybrid's fault that it is a hybrid, and most keepers will maintain that the animal needs to be given care for the rest of its life, ideally being neutered or spayed first to keep it from contaminating a population (sorry, that sounded more eugenics-y than I intended). Doing so is admirable animal welfare, but comes at a cost. A hybrid animal takes up space, resources, and keeper time that could go to other animals that are far more useful for conservation purposes. In the case of long-lived, labor-intensive species, such as orangutans (lots of Bornean-Sumatran hybrids back in the day), the commitment can be extraordinary.
Once hybrid genes are in a population, they can stay there, hidden, unless a telltale trait emerges, or if someone thinks to do genetic screening. With hybrids, it really can be said than an ounce of prevention is worth a pound of cure... and by "a pound of cure", I mean "Twenty years of care, thousands of dollars in food and vet bills, and a perfectly good enclosure that's been held up for that animal's entire life."
One last note about hybrids. The two species involved have to be at least somewhat related to cross-breed, though it varies how closely based on the situation. You can cross a leopard with a jaguar, or a camel with a llama. You cannot cross an African elephant with a gorilla, or a walrus with a kangaroo. (Though I'm sure that out there, there's someone who'd love to try...)