Sunday 10 July 2011

How Black Bears Trekked to Ontario

Can you show me the way to Ontario...?
The Pleistocene is the geological epoch immediately before our own. It ended just ten thousand years ago, a time so recent on a geological timescale that the modern epoch isn't even visible on the chart of the Age of Mammals shown below - its effectively just the line at the very top of the column. It's hardly surprising therefore that, for the most part, the animals alive in the Pleistocene are the same ones that are alive now, barring those that have gone extinct over the course of human history.

The Pleistocene, however, represents the Ice Ages, when vast glaciers covered much of the landmass of the northern hemisphere (rather less so in the south, though, because Australia and Africa, in particular, don't get as close to the south pole as Europe, Asia, and North America do to the northern one). It ends with two events that occurred roughly at the same time: the warming of the world at the end of the last Ice Age, and the dawn of human agriculture. In the case of North America, humans only arrived at the very end of the Pleistocene, meaning that it represents the last time that the continent was untouched by human hands.

But what were wild animals doing at this time? During the last Ice Age, the climate was very different from today. That means that many animals can't have lived where they do now, because it simply wouldn't be suitable for whatever they eat, or for their ability to cope with, for example, heavy snow in winter. They must have retreated south, to warmer climes, hiding out in pockets called "refugia", and only heading to their present homes as the ice retreated.

We can see traces of this today, preserved in the genetics of living animals. By examining DNA, we can determine how different animals within a species are related to one another, and often we can discern patterns that show a clear division between animals in different parts of the continent. One might expect, for instance, that there would a gradual change in ancestry as you move across a continent, with animals being less and less related as they live further apart. But its often much more sudden than that - although there's usually some overlap in the middle, since they are, after all, still the same species, and capable of interbreeding.

In North America, for example, a number of animals show a sharp genetic divide between those living on the Pacific coast and those living elsewhere; during the Ice Ages, apparently, they retreated to two separate refugia, and they haven't been reunited for long enough since then for the lines between them to blur. We see this pattern, for example, in red foxes, pine martens, and chipmunks, among others. Its perhaps relevant that these are generally forest animals, suggesting that they were sheltering in woodlands that were, perhaps, separated by the Rockies.

The DNA in animals is found in two different locations within the cells. The vast majority of it is in the cell nucleus, where it is found in chromosomes, and includes genes inherited from both parents. A tiny fraction however, is found in structures called mitochondria, and this is only ever inherited from the mother. It is usually in this DNA that we can see the clearest signal of how past populations of mammals were divided between different refugia. That's because, for most mammals, females never travel far from where they are born. Males, by contrast, tend to wander widely, in search of mates, so the population mixing that does occur is largely due to them. An animal's female ancestors may have lived in roughly the same area for generations, perhaps thousands of years, and that enables us to reconstruct their past history.

The American black bear (Ursus americanus) is widespread across North America, being found almost wherever there are sizeable forests. Canada, in particular, has an awful lot of forests, allowing black bears to mingle and travel as far as they like (further south, deserts and the great plains create more of a barrier). The fact that there's no other reason for them not to interbreed across the country may make it easier to use their genetics to trace their distant ancestry, and, ultimately, their ancestor's movements.

The American black bear is one of two species of black bear, the other living in Asia. It is smaller, more herbivorous, and generally less aggresive than the brown bear, and seems to be happier in a wider range of habitats. They first evolved some time around 4 million years ago, as part of a burst of diversity that saw many new species of bear appear (not all of which survive). This was, itself, a time of significant climate change, and it was probably that that caused the ancestors of the black bear to separate from their nearest relatives. Crucially, though, it is well before even the earliest of the Ice Ages, during the preceding, Pliocene, epoch.

We have known for some time now that the black bears show the same split seen in many other forest dwelling North American mammals, that between animals on the Pacific coast (places such as California and British Columbia, and as far east as Alberta and Montana) and those living elsewhere. But, from the beginning, there was a hint of another split within the larger group - animals from Alaska, for instance, were genetically different to those in, say, Newfoundland. As well they might be, considering the distances involved. The problem was, nobody had ever looked at the places in between, to see whether this difference was a gradual one or not.

Until now; Agn├Ęs Pelletier and colleagues, of Trent University, recently published a study of the genetic diversity within the black bears of Ontario. This gives us a better look at what black bears were doing during the last Ice Age, and how they travelled from their southern refugia to Ontario, and the rest of Canada. We don't exactly know where those refugia were, although somewhere near Texas is likely, which at least makes it possible that they may have been separated by desert or similarly inhospitable terrain.

They collected black bear hair from a number of locations across Ontario, and examined its mitochondrial genetics. The crucial thing that you're usually looking for in this type of study is a "haplotype". These are combinations of genes, or of variations within the genes themselves, that are usually found together. So we're not looking for specific genes or mutations - which could be duplicated by sheer random chance - but complex patterns of the ways that genes and mutations are arranged that suggest animals must be closely related.

The study showed that there is, indeed, a more or less distinct line between two populations of black bears in Ontario. Those in the northwest (that is, roughly west of Lake Nipigon) had haplotypes found virtually nowhere else in the province, suggesting a clearly distinct ancestry from those elsewhere. Crucially, they were the same haplotypes as those found further west, indicating that the bears of Alaska and Newfoundland represent genuinely different populations, and that the line between the two runs through western Ontario.

The thing is, the difference still isn't very big - it's nothing like as big as the difference between the bears of Alaska and British Columbia, for instance, which you might reasonably expect to be fairly close. So the authors conclude that it probably isn't an echo of two separate refugia in the distant past, after all: the two populations seem to have separated more recently than that. So what does it mean? The conclusion they reach is that the difference doesn't reflect where the bears lived during the Ice Age, but how they got back to Ontario after it was over.

The bears trekked north across the eastern United States, as more forests opened up ahead of them and the ice retreated. These lands were empty, so that female black bears, which normally stay at home, had good reason to travel, finding new woodlands where there were no other bears to eat their favourite food. But they travelled by two different routes, either side of a tongue of inhospitable ice (there's a map here), one to the east of the Great Lakes, and one to the west. During those thousands of years, they were separated, even though they may have been together before that.

When the ice melted, the bears met up again, in what is now northern Ontario. At that point, they stopped travelling - no longer were the woods ahead of them empty, because the other group had got there first. They settled down, to live as bears normally do, with the females inhabiting their own patch of land, and effectively passing it on to distant generations of daughters. The males, of course, travelled about in search of mates, because that's what males do, and when you look at the DNA as a whole, it all blurs into one across the region. But, by looking at the traces in the mitochondria, we can still see the remnants of those long ago migrations, surviving for thousands of years in the genetic code.

[Picture from Wikimedia Commons. Cladogram adapted from Krause et al, 2008]

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