Natural History

Old Gray Guy is the large light gray wolf in the middle of this pack.

From NY Times:

In Ontario, in the winter of 1997, a particularly virile male wolf stepped onto the ice of Lake Superior and headed toward Isle Royale, an island about 15 miles offshore. There he radically changed the genetic makeup of an isolated group of wolves that had lived there since the late 1940s.

Researchers, who for many years have been observing the Isle Royale packs and the moose they feed on, did not realize at first that he was an immigrant, but soon his appearance and behavior became impossible to ignore.

He was larger than most of the Isle Royale wolves, and was so strongly territorial that he completely displaced one of the four packs, driving it to extinction within two years of his arrival. His own pack grew to 10 wolves, the largest seen on the island in almost 20 years. As he aged, his fur grew paler, almost white, a phenomenon known in other wolves but never before seen in the Isle Royale animals.

“We don’t know of any other instance — except when they first came — of wolves crossing the ice,” said John A. Vucetich, the lead author of a study of the wolves published online in The Proceedings of the Royal Society B last week. “The entire population is descended from a single female.”

The wolf population on Isle Royale is small under any circumstances — there were only 16 wolves there last winter, and the average is about 23. But by 2002, the new wolf, now designated wolf No. 93, and his offspring made up five of the six breeders. In his eight years of breeding, he produced 34 pups, and those pups produced an additional 45 progeny.

By analyzing DNA found in blood from some captured animals and in scat from many more, the researchers were able to determine that by 2009, 56 percent of the genes in Isle Royale wolves could be traced to wolf No. 93, or the Old Gray Guy, as he became known.

Such a change is clear evidence for a large difference in fitness between the Old Gray Guy and the other wolves, and scientists expected that the introduction of such a an animal into a small inbred group would produce a sharp increase in population — what scientists call a genetic rescue.

But in this case, it did not happen. A co-author of the study, Rolf O. Peterson, a research professor at Michigan Technological University, said that the population of Isle Royale hangs on by a thread, as it has for decades. The average reproduction after the Old Gray Guy arrived was no different from before. Yet this does not mean that he had no effect.

“The simple interpretation is that genetic rescue doesn’t work,” said Dr. Vucetich, an assistant professor of wildlife ecology at Michigan Technological University. “But what happened here is that when the immigrant came in 1997, in the decade that followed, the moose population declined radically. It’s plausible that we didn’t see an effect because the wolves were suffering from some other trouble that disguised the benefit.”

That, said Dr. Vucetich, is “an important lesson all over the world. When you do genetic rescues and it looks like it didn’t work, there may be a beneficial effect that you don’t see because of some other environmental event.”

What if wolf No. 93 had never arrived? Dr. Vucetich said that it is impossible to know for sure, but the Isle Royale wolves might have disappeared completely. It may be that the Old Gray Guy arrived just in time.

(Jess sent me an earlier article on Old Gray Guy from Science Daily, but I thought the Times had a better write-up on it.)

The authors of that study talk about the ecological factors that could have prevented the genetic rescue from happening.  Those factors would indeed mask any benefit from outcrossing.

But there are other factors.

This is a very small, very inbred population of wolves. It was suffering the effects of an inbreeding depression. An outbred wolf from Ontario shows up. When he mates with a local female, more of his puppies survive than the native inbred population. He winds up siring a large percentage of the a few generations of the wolves on the island, and because those are also quite outbred, they have better ability to swamp the gene pool. They are just better at surviving to pass on their genes than the pups born to the inbred population. This effect would be even more extreme if the prey population happened to be in decline. The outbred wolves may have had certain advantages from heterosis that kept them from succumbing to the stresses of famine.

He becomes like a popular sire within a closed registry breed– in fact, even more extreme.

Whatever benefits could have come from outcrossing are lost in this extreme natural version of the popular sire effect.

If more than one wolf had come across from Ontario and contributed genes to the Isle Royale wolf population, one might be able to make generalizable statements about genetic rescue. And maybe if several wolves had arrived at the same time as a moose population boom, things might have been different.

If Old Gray Guy hadn’t arrived, these wolves may have gone extinct already. His genes may have not have rescued them fully, but it is likely they ameliorated some of the worst effects of the inbreeding depression, preventing the population from collapsing entirely.

Isle Royale is an unusual place, but because of its size and isolation, it is a good place for scientists to design experiments about ecology and population genetics.

However, because it is so unusual, one cannot make such broad generalizations about the findings of many studies performed on the island.

We have seen genetic rescue save the Florida panther. Eight Texas cougar queens were released in South Florida in 1995.  Five of the queens had litters and then were sterilized.  The health and genetic diversity of Florida panther is now much better than it was before. The population has risen from about 24 to nearly 100 individuals. The population still remains isolated, and there can still be threats of an inbreeding depression in the future. No new genes are likely to come into Florida any time soon.

But genetic rescue works in certain situations.

But it won’t work with just one male individual entering the population when the population of the prey species is in decline.

Genetic rescue requires more individuals and relatively productive ecosystems to work.

That’s likely why it worked with the Florida panther and not with the wolves of Isle Royale.