Sweeping comparisons between wild animals and domesticated pets are dangerous given the unique and often mutually exclusive conditions in which those two groups often find themselves. Wild animals must hunt for their food, compete for resources, suffer predation, lack medical intervention, self-select their mates, and exist in numbers based upon a complex interaction between their own merits and the conditions in their environment. Pet dogs are fed from a bowl daily, compete only for our affection and attention, are largely free of predation, have ready access to modern veterinary techniques and treatments, have their mates chosen for them–sometimes from dogs long dead or on other continents, and exist in numbers based upon human concerns and rarely on their own merits or the environmental carrying capacity. Their selection is very much unnatural. But there are very few scientific studies of domesticated pets versus numerous investigations into wild populations, so dog lovers would be remiss in not learning lessons from our pets’ wild cousins. One particularly interesting ongoing scientific inquiry is the study of the wolf and moose populations on Isle Royale Michigan.
Wolves colonized Isle Royale, a wilderness island in Lake Superior, North America, in 1949 or 1950. The population is isolated from mainland wolves by a channel of frigid water, 24 km wide. In many, but not all years, this channel freezes for several days or weeks. Although an occasional ice bridge makes immigration possible, the analysis of mitochondrial DNA and the Y chromosome suggests that the population was originally founded by only one female and two males.
This makes Isle Royale analogous to a small dog breed based on a few founders or even a single kennel that rarely brings in any new blood. Nova Scotia Duck Tolling Retrievers, Leonbergers, Cavalier King Charles Spaniels, Vallhunds, and many other breeds were founded by only a few sires and dams and have had very little influx of blood since and there are quite a few old time breeders who would only occasionally bring in a sire from outside their own moderately sized kennels. As with the above breeds, the mostly-closed gene pool on Isle Royale became steadily inbred over time.
By the late 1990s, the population’s estimated inbreeding coefficient had risen to 81%. Fifty-eight per cent of Isle Royale wolves showed congenital bone deformities compared with only 1 per cent in two outbred wolf populations. Some of these deformities could reduce individual fitness, particularly components of fitness associated with predation and reproduction.
So the first lesson to be learned from the Isle Royale Wolves is that isolation leads to inbreeding and inbreeding is detrimental. In dogs, closed registries and kennel blindness are a form of isolation and in both cases we see rising inbreeding followed by increased expression of otherwise rare diseases.
Some of the most studied wolf packs in the world are in serious jeopardy. Researchers report that the occurrence of debilitating bone deformities in wolves marooned on Isle Royale, an isolated island in Lake Superior north of Michigan, has risen sharply over the past five decades due to inbreeding.
A genetic defect now common in the Isle’s wolves causes bones in the spine, the vertebrae, to grow gnarled and crooked. Also found in domestic dogs – close wolf relatives – the bone malformations can pinch nerves in the spinal cord, causing pain that makes it tough to walk and can lead to paralysis of the back legs and tail in severe cases, according to research published in February’s issue of Biological Conservation. Back in the 1960s, about a quarter of Isle Royale’s wolves appeared to have the anatomical abnormality, but now the percentage of afflicted wolves has risen to nearly 60 percent of the population. “In normal, healthy wolf populations without inbreeding, you are only supposed to see this kind of defect in about one out of a hundred animals,” says paper coauthor John Vucetich, an assistant professor of wildlife biology at Michigan Technological University (MTU) in Houghton. The deformity, discovered during autopsies of recovered, dead wolves, has grown so rampant, Vucetich says, “we haven’t found a normal wolf in the past decade.”
The second lesson to learn from the Isle Royale Wolves is that hybrid vigor is real and powerful and that inbreeding depression is real and powerful.
We used molecular techniques to document the consequences of a male wolf (Canis lupus) that immigrated, on its own, across Lake Superior ice to the small, inbred wolf population in Isle Royale National Park. The immigrant’s fitness so exceeded that of native wolves that within 2.5 generations, he was related to every individual in the population and his ancestry constituted 56 per cent of the population, resulting in a selective sweep of the total genome. In other words, all the male ancestry (50% of the total ancestry) descended from this immigrant, plus 6 per cent owing to the success of some of his inbred offspring. The immigration event occurred in an environment where space was limiting (i.e. packs occupied all available territories) and during a time when environmental conditions had deteriorated (i.e. wolves’ prey declined).
This event is an excellent example of what one outcross might do for a small breed and especially a single kennel. The immigrant wolf’s offspring were true hybrids, a mix of the formerly isolated pool and fresh blood; versus having an outside pack supplant the locals in toto, as one might expect in a contiguous geography that was not isolated like the Isle.
We have here an analogous situation that fits existing human patterns of behavior regarding dog breeds and strains that have virtual barriers instead of physical ones.
The astounding aspect of this immigrant on Isle Royale is just how potent his genetics were to effect change on the island. None of the existing males could even compete with him and he became the sole sire. His initial success is likely caused by the inbreeding depression in the inbred Isle wolves.
The high fitness of this immigrant wolf was also associated with distinctive behaviour and physical appearance. First, he was physically larger than most Isle Royale wolves. As alpha male of the Middle Pack, his high fitness was also reflected by his dominance over other ISRO packs. Specifically, he exhibited strong territorial behaviour that completely displaced West Pack, driving that pack to extinction by 1999.
This immigrant wasn’t necessarily a super-wolf, he was in all probability a young male that was driven out of his birth pack’s territory for being non-competitive with that pack’s alpha male. His success on Isle Royale shows just how compromised the inbred population had become.
It’s an open question if and how much the influx of new genes has changed the bone deformities which had come to define Isle wolves. Surprisingly, before the results of the bone study were published, Isle Royale was used as an example of a wild population that was thriving and unharmed by inbreeding and isolation. This is why I’m cautious of anyone who argues from ignorance regarding their ability to inbreed and avoid disease. This is the third lesson: don’t assume that inbreeding can exist in high levels without detriment and don’t cite wild populations if no one has ever done a detailed health study to document the true health of the population.
The new results offer the first evidence of the wolves’ closed population leading to a decline in natural fitness. This is important, Vucetich says, because for years some policy makers and conservationists have pointed to the apparent health of the Isle Royale wolf packs as an indication that small animal populations can maintain proper genetic diversity. “Isle Royale is not this robust place that some people thought it was,” says Vucetich.
Now, not all the lessons are positive ones. Given the isolation of the island, the complete genetic sweep of the immigrant male, and the small population size the rates of inbreeding swiftly ticked back up. The smaller the population size and the greater degree of inbreeding done following new blood, the shorter time you’re going to reap the benefits of that new genetic material. Outcrossing can forgive a myriad of sins, but it needs to be used in measure to the problem. On Isle Royale, this new wolf didn’t simply add to the sires on the island, he supplanted them and then bred with his children, creating more inbreeding instead of extending the benefits of outcrossing.
This isn’t an advisable strategy for refreshing a dog breed, essentially going from one popular sire to another. Diversity in breeding males should be maintained over each generation and proceeding to breed father to daughter–and similar–after a single outcross will swiftly return the gene pool to inbred status. The other issue that researchers have is that to complete a “genetic rescue” one must be able to document the benefit on a population level, an this is often done with the most crude methods, namely demography (head counting). An increase in population size was not observed here, although there was a precipitous drop in the wolves’ main food source, Moose, during this time and the wolf numbers did not suffer either.
Genetic rescue, in which the introduction of one or more unrelated individuals into an inbred population results in the reduction of detrimental genetic effects and an increase in one or more vital rates, is a potentially important management tool for mitigating adverse effects of inbreeding. … The immigration event occurred in an environment where space was limiting (i.e. packs occupied all available territories) and during a time when environmental conditions had deteriorated (i.e. wolves’ prey declined). These conditions probably explain why the immigration event did not obviously improve the population’s demography (e.g. increased population numbers or growth rate). Our results show that the beneficial effects of gene flow may be substantial and quickly manifest, short-lived under some circumstances, and how the demographic benefits of genetic rescue might be masked by environmental conditions.
This is not a problem with outcrossing at all, rather it’s a problem of the limitations of the crude science of demography. Head counting might speak to quantity, but it is wholly lacking in useful information about quality.
Demography is likewise a poor tool to employ when analyzing domesticated dog health. The numbers of dogs within breeds has little to do with their vitality and much more to do with fashion and the whims of a handful of breeders. No one would claim that a Pug is popular due to competitive natural gifts of robust health and fitness, rather they are much like the inbred wolves on Isle Royale, they are artificially supported by beneficial conditions, easy access to food, and a blunting of natural pressures against their survival; in the wolves’ case it’s the benefits of living on an island, in the case of the Pug it’s being coddled by owners and breeders willing to spend a pretty penny on their upkeep.
In open and competitive environments, population numbers can serve to estimate vitality when better data has not been taken, but it seems that on Isle Royale, the wolves have a sheltered niche as the apex predators with ample food supply. The Moose can’t migrate away and there is little in the way of competition for the wolves. In such conditions, even sickly inbred wolves can reach a carrying capacity at about the same numbers as more robust wolves can. It’s possible that more wolves could have thrived had the main food source not plummeted, or it could be that even at the lowest levels the moose populations were not small enough to be a significant factor in the head count of the wolves.
Without a marked improvement in population size on the island and without documentation of the rates of disease and bone deformities improving with the influx of the immigrant wolf, the technical definition of a “genetic rescue” has not been met with the current state of knowledge about the Isle Royale wolves.
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