For decades we had thought the wolves of Isle Royale to be isolated and highly inbred, but had also somehow managed to avoid inbreeding depression, the negative consequences of inbreeding. In particular, Isle Royale wolves had rates of survival and recruitment that were similar to other healthy wolf populations. This perspective was important because Isle Royale seemed to some an important exception to the idea that small populations experience an elevated risk of extinction, in part, because of their vulnerability to inbreeding depression.
It was not until 2009 when we discovered the wolf population had long suffered a high incidence of malformed vertebrae (backbone). One third of the skeletons we inspected had a particular kind of malformity known as lumbrosacral transitional vertebrae (LSTV). By contrast, only 1 in 100 wolves suffer from LSTV in healthy populations. Moreover, the incidence of malformities has been steadily increasing over time as the population steadily became more inbred. The incidence of malformity has increased such that we have not detected a normal skeleton in the past fifteen years.
Last year, we discovered another surprise. The origins of this discovery traced to the late 1990s, when we began to systematically and intensively collect wolf scats. They are a source of DNA, and allow us to learn about the population’s genetic history. We stock-piled the samples until we found enough funding to analyze them.
As we analyzed the samples, what caught our attention was wolf #93, who was first detected through his scats in 1997. He carried several alleles that had not previously been observed in the Isle Royale population. These alleles and other genetic patterns indicated that wolf #93 was an immigrant from Ontario, Canada, very likely the first immigrant to Isle Royale since the population was first founded in the late 1940s. Patterns of genetic relatedness also indicated that #93 began reproducing in Middle Pack in 1998.
From field observations made more than a decade ago, we knew that Middle Pack was taken over by a new alpha male sometime between February 1997 and February 1998. We also know that 1997 was one of only two years in the past 15 when an ice bridge connected Isle Royale to the mainland for several weeks. In 1999, during a research flight, we observed the alpha male of Middle Pack defecate on a frozen lake. When the pack left, we landed the plane and collected the scat. The DNA in that scat matched that of wolf #93.
Wolf #93 seemed to be an extraordinary wolf. He was physically larger than other Isle Royale wolves. He exhibited strong territorial behavior that completely displaced West Pack, driving that pack to extinction in 1999. Under his leadership, Middle Pack grew to 10 wolves by 1999, the largest pack size observed on Isle Royale in almost 20 years. We reported all these observations before knowing that the alpha male of Middle Pack was an immigrant.
In addition, and prior to knowing that wolf #93 was an immigrant, we observed the alpha male of Middle Pack turn very light in color, almost white, as he aged. At that time, we dubbed him “The Old Gray Guy.” While turning light colored with age is not uncommon among wolves in general, this had never been observed before on Isle Royale. Before knowing that wolf #93 was an immigrant, we reported two other whitish-colored alpha wolves, and in 2010 we observed a fourth light-colored alpha. We later learned that these wolves were descendants of wolf #93.
With the immigrant’s arrival and the new genetic material he brought with him, the population’s inbreeding coefficient dropped dramatically from 0.81 to 0.09 in just four years (one wolf generation).
This immigrant event on Isle Royale represents an important opportunity to better understand genetic rescue, which is a potentially important conservation tool that involves introducing one or more unrelated individuals into an inbred population as a means of mitigating inbreeding depression. However, the effectiveness of genetic rescue is not well understood because the opportunities to closely monitor an isolated population before and after a known immigration event are limited. For this reason, the Isle Royale immigration event represents a special opportunity.
The hallmark of genetic rescue is an increase in a population’s vital rates after immigration. However, the evidence for increased vital rates in the Isle Royale population is equivocal. There was no statistically detectable difference in survival or recruitment after his arrival. However, even important differences can be difficult to detect. Moreover, coincident with the immigrant’s arrival, moose on Isle Royale declined dramatically in response to food shortage, severe winter, and tick outbreaks. A clear response to the immigration event may well have been disguised by lack of food for the wolves. If so, it may be important to recognize that deteriorating ecological conditions can mask the beneficial effects infusing new genetic material. But the story does not end here.
The Old Gray Guy’s genetic constitution was so superior to that of native Isle Royale wolves that he soon chose to mate with a wolf that shared half of his genes. That is, he sired 21 offspring with his own daughter (wolf #58; see also event [a] in the pedigree below). Two of the offspring from this parent-offspring mating began breeding with each other (wolves #135 and #147) when they established Paduka Pack in 2007 (event [b]). The inbreeding did not stop there. In 2003, the breeders of East Pack were full sibs (wolves #62 and #102) who had been born to the immigrant and an unrelated, Isle Royale wolf (event [c]). In other words, by 2002, five of the population’s six breeders were either the immigrant or an offspring of the immigrant. In the end, wolf #93 was an alpha wolf for 8 years (1998-2006), gave birth to 34 offspring, has 22 grand offspring (and counting).
The Old Gray Guy was successful, but perhaps too successful for the benefit of the population. The dramatic success of the immigrant and his offspring led to quickly rising rates of inbreeding by 2003.
On Isle Royale, the last wolf unrelated to male #93 died in 2007. By 2009, 56% of all the genes in the Isle Royale wolf population trace back to The Old Gray Guy. He initiated a genomic sweep of the Isle Royale population.
We once thought Isle Royale wolves had avoided inbreeding depression despite being isolated and highly inbred. The discovery of bone malformities in 2009 suggest they hadn’t avoided inbreeding depression. And discovery of the Old Gray Guy’s identity indicate that the wolves haven’t been quite so isolated. We could not hardly have had a less accurate impression. One of the great rewards of long-term research is an opportunity to validate an ancient wisdom, that says: The more we know, the less we understand.