One of the hazards of breeding wild animals in captivity is that the forces of natural selection no longer act upon them. Specimens that thrive well enough of captivity are very often fundamentally quite different from those that would thrive in the wild. Those individuals who thrive well enough to reproduce may pass on traits that are entirely detrimental to living in the wild.
If the goal is to produce domesticated animals, as was the case in Belyaev’s fox farm experiment, one should rigorously select animals for tameness, docility, and general ease of handling. Of course, when one does that, the animal fundamentally changes.
We have been doing that for thousands of years. We have become experts at selectively breeding domestic animals for any number of traits. We do not care that our Yorkshire pigs are nothing like Eurasian wild boars. We do not care that our Herefords and Holsteins are nothing like the extinct wild Aurochsen. We are glad that our golden retrievers, Scottish deerhounds, and Pekingeses are not exactly like wild wolves.
These domesticated animals fit their place in our society. They have a utility that is fundamentally different from their wild ancestors, and most of that utility comes from our skills at selecting desirable traits.
However, when our goal is to produce captive populations of wild animals that can be reintroduced into the wild, we often have problems. Simply put, we cannot create the conditions in captivity that imitate natural selection.
Try as we might, we simply cannot make it so that prey species evolve under predation. We cannot allow predators to hunt prey in captivity. If we are dealing with endangered animals, those activities are simply too risky to even try. Further, they most likely would violate animal cruelty laws and could even violate conservation statutes. Endangered and threatened species are also too valuable for their genomes to ever experience possible risks of damaging or killing them. They must be treated as carefully as any museum artifact.
It is difficult to deny the reasoning behind such care. After all, every individual in a limited gene pool has importance, and we really shouldn’t be involved in what amount to canned hunts with endangered predators hunting endangered prey.
As noted earlier, once animals are removed from the pressures of natural selection, the animals themselves can evolve to fit captivity.
Take the case of one line of the endangered Mexican wolf.
Mexican wolves (Canis lupus baileyi) are the smallest subspecies of wolf in North America. If the red wolf is a different species, it is generally believed to be the oldest extant subspecies in North America. It has unique markers in its mtDNA sequence that make it stand out from wolves from other populations on this continent. It was native to the Southwestern US and northern Mexico. It was extinct in the wild until 1998, when 11 individuals were released into the Blue Range in Arizona. Currently, about 50 Mexican wolves live wild in Arizona and adjacent New Mexico.
However, as a subspecies, it does have issues with compromised genetic diversity. Mexican and US ranchers nearly wiped the subspecies out. The last of wild ones were all capture and then bred in captivity in hopes of preserving the subspecies.
One particular line of Mexican wolf was the Ghost Ranch colony. It was captured in New Mexico, and one of the founders of the line looked a bit like a wolf-dog. However, the rest all resembled wild Mexican wolves.
But as they bred over the generations, certain features began to pop up. Their heads got smaller. Their jaws got weaker. They were turning into dogs.
This line was bred at Carlsbad Caverns, and when it was decided to start breeding Mexican wolves with the possibility of releasing them into the wild, the status of the Ghost Ranch wolves fell into contention.
It is well-known that dogs and wolves will hybridize. In fact, it is probably not even correct to call wolf and dog crosses hybrids. They are now recognized to be part of the same species, Canis lupus. It was suggested that Ghost Ranch wolves were derived from hybrid stock.
If that were true, then it meant that releasing wolves derived from that line could be contentious. The Endangered Species Act and state conservation laws were designed to protect Mexican wolves, not Mexican wolf and dog crosses. It would mean that a wolf haters could kill them and then claim that they were shooting a wolf hybrid, which is not a protected species at all.
Of course, that is not a trivial concern.
However, it was also suggested that the Ghost Ranch wolves were actually the result of a kind Belyaev experiment. Generations of being bred in captivity and being fed a less rigorous diet had changed these wolves in the same way the foxes were.
Because of the potential problems, the Ghost Ranch wolves were euthanized. However, there are still a few individual wolves that are of the Ghost Ranch line. These animals are of great value.
You see, a later study of the DNA of Mexican wolves, including those of the Ghost Ranch line, found that there was no evidence of hybridization with dogs or coyotes. Not only were they free of that “contamination,” they were clearly genetically distinct from any wolf population in North America.
If only this sort of analysis had been performed before the majority of the Ghost Ranch wolves were euthanized, the Mexican wolf population would still have an important source of genetic diversity. Granted, they would probably would have those maladaptive doggish traits, but these possibly could have been bred out over time.
Also, there were some concerns that the McBride line, which is the main line that was used for reintroduction purposes, might have some issues with an inbreeding depression, and while there was no evidence of an inbreeding depression, it is known that it does have compromised genetic diversity. The Ghost Ranch and Aragon lines are also somewhat inbred, but it would make sense that allowing blood from these previously believed “unpure” lines would end some of these possible concerns.
However, there are very few Ghost Ranch wolves left. Twenty were found in the hands of a private owner, and these wolves were then added to the gene pool.
But because of the unintentional domestication of the Ghost Ranch wolves, it was decided to get rid of this part of the gene pool.
Of course, when one is already dealing with a very rare subspecies, breeding practices can be loosened. Texas cougars were released to augment the Florida panther’s genetic diversity. It is something that can be done, but it is generally frowned upon.
Releasing a wild animal that has possible domestic ancestry would always be a legal cheval-de-frise that wolf haters could use, so maybe euthanasia was a good idea at the time.
But all of this confusion could have been avoided if the wolves had been kept and bred to preserve as many wild characteristics as possible. This is not as easy as it sounds.
Although we are experts at selecting for tameness and docility in domestic animals, we are not so good at selecting for the traits that make animals survive well in the wild.
The best we can do is leave the animals alone in their enclosures and try not to put too many selective pressures for tameness on them.
However, the result of doing such a thing means that one is potentially producing dangerous animals that zoo staff cannot handle. In the clip about the foxes, the “wild” group that had been bred to retain those characteristics was far more aggressive. The “dragon” fox would probably have traits that would be good to have in the wild.
But those are just red foxes.
What if we are talking about Amur (Siberian) tigers?
Remember the San Francisco Zoo tiger that killed one person and maimed two others on Christmas Day in 2007?
That tiger had traits that would have made her survive well in the wild. Her aggressive behavior was strong enough that she could escape an enclosure that easily held other tigers.
Such an animal would be a liability in any facility, but her offspring might be well-adapted to eventually live in the wild. That is, if we ever figure out how to rewild captive tigers.
So in the end, we are left with a balancing act. The animals must be bred so they retain at least some of their instincts and physical traits. However, they cannot experience natural selection as their wild ancestors did. They have to be treated with care. They also have to be able to be contained in an enclosure, and they have to be tractable enough to be safe for staff to handle.
They also have to be able to breed in captivity, which is itself an important selective pressure. Not all individuals can reproduce in captivity. Male clouded leopards are often so aggressive that they kill their mates. To get clouded leopards to breed, males that are less aggressive toward their mates have to be used. That certainly will have an effect upon the behavior of their offspring. They may be less aggressive than typical wild male clouded leopards.
All of these things must be balanced in the light of maintaining a dynamic and diverse gene pool. Breeding animals that could eventually live in the wild is a much harder undertaking than breeding domesticated animals. We must balance lots of things with domestic animals, but with these wild animals, we also have to think as if we are imitating natural selection.
Not an easy task.
Natural History 