A muskox (Ovibos moschatus), photographed in Alaska. From Flickr user drurydrama.
Of all the mass extinctions that have occurred during earth’s history, among the most hotly debated is the one which wiped out mammoths, saber-toothed cats, giant ground sloths, and the other peculiar members of the Pleistocene megafauna around 12,000 years ago. It was not the most severe mass extinction, not by a long shot, but unlike the end-Cretaceous catastrophe 65 million years ago there is no single “smoking gun” that can account for the pattern of extinction. Instead the Pleistocene mass extinction remains a very mysterious event, but by looking at the natural history of one of the event’s survivors scientists have been able to get a better idea about how one of the suspected extinction triggers affected prehistoric mammals.
Today’s populations of muskox (Ovibos moschatus) are remnants of the Pleistocene herds which were once spread all around the Arctic Circle. The shaggy bovids are survivors of the events which wiped out so many other large mammals, but this does not mean that they were immune to ecological changes that may have played a pivotal role in the extinction. As illustrated by a new paper in the journal PNAS, the changing climate had a major influence on muskox populations, and by looking at what happened to them it may be possible to understand the fate of some of their extinct contemporaries.
Naturally much of what we know about Pleistocene mammals has been derived from fossils, but the bones do not just record the anatomy of the long-dead animals. Many of the Pleistocene fossils are recent enough that at least a few snippets of DNA can be obtained from them, and experts on the Pleistocene have been increasingly bringing together the more traditional aspects of paleontology with genetics to better understand the life of the past. In the case of the muskox, the international team of researchers behind the new PNAS study looked at 682 base pairs of mitochondrial DNA obtained from 149 prehistoric muskox specimens from North America and Eurasia dated between about 57,000 years ago to the present.
A diagram of fluctuations in muskox diversity (colored waves) over time (from right to left). Each color represents a different population: blue, Greenland; red, Northeast Siberia; orange, Taimyr; green, Urals; light blue, Canada. From Campos et al. 2010.
As might be expected given the status of current muskox populations, genetic diversity among living muskox is lower than in their Pleistocene counterparts. This is not the effect of a unidirectional, gradual dwindling trend, however. Between about 60,000 and 47,000 years ago there was an increasing trend in muskox genetic diversity, but that diversity began to decline before sharply increasing again around 20,000 years ago. After that genetic diversity began to fall once more before again rising in recent times (representing the expansion of muskox to Greenland). Roughly speaking, these increases and declines in diversity track changes in muskox populations over the past 60,000 years, but what could have caused these fluctuations?
Numerous hypotheses have been forwarded to explain the Pleistocene mass extinction, but for the past forty years or so the top contenders have been human hunting or climate change. Different authorities prefer different scenarios, but until now it has been difficult to determine how these ecological changes affected populations of large mammals. In the case of the muskox, the fluctuations in their genetic diversity do not appear to be tied to the arrival of human hunters in their habitats but to changes in climate.
As the authors of the new study point out, muskox live in cold, dry habitats in which the snow is just shallow enough for them get to the forage underneath it. These habitats can be greatly affected by shifts in climate, however, and naturalists have observed some populations decline by as much as 76% in a single year due to such changes. While the scientists behind the new study acknowledge that the muskox population dynamics and global climate models cannot yet be brought together to rigorously test the idea, from what is presently known it appears that the peaks in muskox genetic diversity match up with periods of global cooling. When things were cold and dry muskox genetic diversity went up, and when things got warmer (during interglacials such as the one we’re in) their genetic diversity went down. There is no indication that the activities of humans were responsible for these trends. As the authors conclude, “although humans may have played a significant role in the history of other large Beringian mammalian herbivores, to our knowledge this example is unique in showing there is no evidence that humans drove musk ox demographic fluctuations over the last 60,000 years.”
By itself this finding does little to resolve the ongoing (and sometimes acrimonious) debate over whether climate change or the depredations of humans were more important in driving Pleistocene mammals to extinction, but it does raise several interesting points. The most obvious is that we now know that not all large Pleistocene mammals were significantly affected by human hunting during the past 60,000 years, and, perhaps more importantly, by looking at preserved DNA from multiple populations scientists have found a way to document fluctuations in genetic diversity during prehistory. If this approach can be married to studies of paleoclimate and archaeology then we can better understand how populations of large mammals were affected by different ecological events. Combined with other new techniques, such as the ability to retrieve preserved DNA directly from soil, paleontologists will be more able to put hypotheses about what caused the Pleistocene mass extinction to the test.
Campos, P., Willerslev, E., Sher, A., Orlando, L., Axelsson, E., Tikhonov, A., Aaris-Sorensen, K., Greenwood, A., Kahlke, R., Kosintsev, P., Krakhmalnaya, T., Kuznetsova, T., Lemey, P., MacPhee, R., Norris, C., Shepherd, K., Suchard, M., Zazula, G., Shapiro, B., & Gilbert, M. (2010). Ancient DNA analyses exclude humans as the driving force behind late Pleistocene musk ox (Ovibos moschatus) population dynamics Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0907189107