When you’re witnessing a disaster like the current extinction crisis unfold, doing something feels better than doing nothing. That’s one reason captive breeding programs are so common—it’s difficult to stand by and watch a species careen toward oblivion. But a new study says our interventionist urges often lack scientific support and, in some cases, do more harm than good.
This claim may be hard to swallow, since you’ve probably heard stories about captive breeding bringing species back from the brink. For example, in 1986 only 18 black-footed ferrets remained in the world. Since then, a successful captive breeding program has brought the numbers for North America’s only native ferret to 500 in the wild, with 300 more breeding in zoos and other sanctuaries. The Arabian oryx went extinct in the wild in 1972, but captive breeding has brought this gorgeous horned ungulate back to a population of approximately 1,000 in the deserts of the Middle East.
Fondness for captive breeding might have to do with media bias: Reporters tend to write more about the programs that are both dramatic and successful. You are less likely to hear about captive breeding failures, even though there are plenty—raising and breeding a wild animal in captivity can be extremely challenging.
The puzzle for conservation biologists is separating the ferrets from the songbirds—which species would more likely prosper from time in captivity and which should be left to fight for survival in the wild?
Some species simply refuse to mate outside their normal habitat. Take Lonesome George, the Pinta Island giant tortoise who lived in captivity in the Galápagos for 41 years, as biologists tried to coax him into copulating with a female of a closely related species. His caretakers tried just about everything—they even considered showing him videos of tortoise pornography (though it’s not clear if that ever happened). But the 100-plus-year-old George just wasn’t in the mood. He died in 2012, taking his species with him.
Even if George had left a brood behind, his lone set of chromosomes was all that remained of his species. Such low genetic diversity doesn’t bode well for a population, and inbreeding is a huge problem for captive breeders. The Association of Zoos and Aquariums tries to manage captive gene pools by swapping breeding animals between zoos across the globe, but genetic bottlenecks still can occur. You also don’t want to take too many animals (and their genes) out of circulation in the wild.
Zookeepers are currently trying to breed the Asiatic Lion, which is down to about 175 wild individuals in a single state in India. The program has not gone smoothly. In 2002, an effort to crossbreed the species with African lions was abandoned after 30 of the animals died from a mysterious disease, and dozens more appeared to be suffering from the same affliction. Since the disease likely resulted from genetic weaknesses in the Asiatic breeding stock, all the zookeepers could do was isolate the sick lions and keep them comfortable until they died. Captive breeding of Asiatic lions continues, but a black-footed ferret-style comeback seems a distant hope.
Releasing animals born in captivity into their natural habitat can also be tricky. Songbirds, for example, are very difficult to re-introduce to their natural habitat, because their peers teach them the local “language” in the wild. They can’t learn the songbook in captivity. Re-introduction can also place the wild population at risk, as it did in 1991, when a once-captive toad brought the deadly chytrid fungus Batrachochytrium dendrobatidis to the island of Mallorca.
Making sure species have suitable habitat to return to is also necessary for successful outcomes. Creating a robust captive population doesn’t do much good if the animal’s natural home is bulldozed over during its stint in the zoo. Another challenge is artificially replicating very specific habitats, such as that of the Kihansi spray toad, a tiny amphibian whose survival requires the light mistings of a single waterfall in Tanzania. It can be done, but figuring out when to invest in such strategies is key.
The puzzle for conservation biologists is separating the ferrets from the songbirds—which species would more likely prosper from time in captivity and which should be left to fight for survival in the wild? The authors of the new paper, published in the Journal of Applied Ecology, examined the case of the great Indian bustard. Thanks to hunting and habitat destruction, this bird species consists of approximately 250 individuals surviving on the dry plains of India and Pakistan.
The authors’ crystal ball (actually a computer model) had bad news for any bustard breeding enthusiasts out there. Bustards are not good candidates for a zoo-based program. At about 40 pounds, these heavy birds can, remarkably, still fly—but they are clumsy. According to the researchers, bustards crash into things and break bones in artificial enclosures. Captivity also shortens their lives more than it does for similar species, suggesting that bustards may find zoo life stressful.
The computer predicted that a captive program would need at least 100 birds to prevent a serious genetic bottleneck, which would almost halve the already small wild population. Comparisons to similar birds released from zoos also suggest that captive-bred bustards would fare poorly upon re-introduction, dying young and failing to reproduce. (For reasons not entirely understood, captive breeding seems to select for genetic traits that are harmful in the wild, like low sperm volume and egg-laying rates.) While allowing the existing bustards to take their chances in the wild is no guarantee of survival, the computer indicated that would be the better course of action.
You might ask why we’d need a computer model to tell us that a species that breaks bones, gets stressed out, dies young in captivity, and fails to reproduce upon re-introduction is a bad candidate for captive breeding. That would be the wrong question, though.
The researchers’ point was to create a computer model with the right dials—one that could work for all types of species, with adjustments that can be made for reproduction and survival rates in captivity, along with the number of animals that must be extracted from the wild. Until now, there have been few tools to help conservation biologists decide whether to initiate a captive breeding program. These models could also potentially help conservationists argue to national and international authorities that, in some cases, the only option is to save as much of the species’ habitat as possible, or to stop poaching, or to curb pollution—because our halfway houses don’t always cut it.