The Ghosts of Yellowstone


Field assistant Jared Singer maps a partial elk carcass in Yellowstone National Park. Photo credit: Joshua Miller

Paleontologists often deal with time scales in the hundreds of millions of years, reading the messages of fossils to learn about life on Earth long before humans arrived on the scene. But bones aren’t limited to providing insight on prehistoric ecosystems. The skeletal fragments left behind by animals at their final resting place can be just as valuable as ecological data points when they’re 100 years old as when they are 100 million years old, a new study discovered. The “ghosts” of animals from decades past can give scientists the tools to study more recent ecological changes, due to climate change, invasive species and other threats to global biodiversity.

Yellowstone National Park, established in 1872, was the first of its kind not just in the United States, but in the world. For the past 139 years, the National Parks Service has both protected the western territory and studied it, regularly monitoring the wildlife that lives within its nearly 3,500 square miles of land. So for Joshua Miller, a graduate of the University of Chicago Committee on Evolutionary Biology, the Park offered a perfect testing ground for his study assessing the scientific worth of relatively young bones.

“Yellowstone is phenomenal; it’s one of our nation’s gems,” said Miller, now a postdoctoral researcher at Wright State University. “They have a huge amount of historical data on living populations, so it is a great place to look at the bone record and be able to match up what we see in bones to what we know has happened in the living community.”

Miller’s plan was elegant and simple: take a census of the bones he could find lying on the ground (no digging required) at different locations in Yellowstone, and see how well the story of the bones matches up with historical data on the local living species. Specifically, Miller would look at the bones of large mammals such as elk, bison, and mountain goats, estimating the abundance of each species over the last century by counting their skeletal remains. While a straightforward idea, and building off of work by researchers with similar interests, Miller said he initially received skepticism from Park officials who didn’t believe that a researcher could find enough bones to make estimates about historical animal populations. That assumption was proved untrue almost immediately, he said.

“It turns out everywhere you go, you run into bones,” Miller said. “We went up to the Arctic National Wildlife Refuge [on a later project], landed the plane and right next to the wheel was an antler. Bone accumulations are just rampant.”

In Yellowstone, Miller and his field assistants spent three consecutive summers laboriously analyzing forty different plots of land. The assistants walked slowly over a one kilometer area, planting flags wherever they came across animal remains. Miller, who had spent months at Chicago’s Field Museum looking at specimens and memorizing their bones, would then investigate their discoveries to identify the species. He also estimated how long the bones had been lying there, either based on how weathered the remains were, or by taking a small sample for radiocarbon dating.


Partial elk carcass in Yellowstone National Park. Photo credit: Joshua Miller

Slowly, a unique database of population changes over the history of Yellowstone was built by Miller. He then was able to compare his bone data to population surveys collected the old-fashioned way – by flying over the Park and counting the animals. Excitingly, there were few surprises, as the bone database matched known fluctuations in animal populations within the Park over many decades. For instance, the reintroduction of wolves in 1995 produced a decrease in elk populations, a shift reflected in Miller’s database by the high number of older elk bones contrasting with the lower number of more recent elk bones. The predictability was a good thing – it meant that the bone record was accurately depicting known ecological history in Yellowstone, and could therefore be used as a proxy for studying population changes in regions without historical survey data.

Other discoveries were totally unexpected and made little sense – at least initially.

In the very first region where Miller and his team conducted a search for bones, they came across a species that he did not prepare to see. After some study, Miller concluded the bones belonged to a horse, which presented a puzzle: horses do not currently live in Yellowstone.

“That was a total shock, I never expected it. It threw me for a loop,” Miller said. “It was both cool and astonishing; it was clear this would be a real adventure.”

The mystery was resolved by a conversation with a Yellowstone historian, who said that the United States Army cavalry was once in charge of protecting Yellowstone to prevent poaching and other disruptions of the natural land. By the early 20th century, cars gradually replaced horses as the main means for taking rangers and tourists around the Park, and the opportunity for horse bones to enter the record basically vanished. The fact that Miller found horse bones – which radiocarbon dating revealed to be from Yellowstone’s early history – was actually further confirmation that the bone record could accurately tell the ecological story of the park.

“That the bones could identify all of these really different population shifts over decades to nearly a century was a shock and awe moment for me – it said this really works,” Miller said.


Miller studying bone survey data sheets on Northern Range, Yellowstone National Park. Photo credit: Scott Rose

Now that the method has been confirmed in a well-studied cold weather ecosystem, Miller is taking his hunt for bones to areas with patchier historical records. Currently, he is working in the Arctic National Wildlife Refuge (ANWR), studying the migration and reproduction patterns of caribou populations that could be threatened by proposals to expand oil drilling in the Alaskan Refuge. Though the observational record for ANWR is nowhere near as thorough as Yellowstone’s (it’s nearly 10 times the size and has only been studied over the past few decades), Miller is finding that the bone record might be even more reliable in the far north, as cold temperatures slow the decay of skeletal remains found on the surface.

“Paleontologists have been interested in bones and what they tell us about extinct animal communities for generations. Up in Arctic, we can use the molasses pace of bone decay to our advantage and get historical ecological data useful for modern wildlife management,” Miller said. “It’s definitely using an old set of tools for a new set of tricks.”


Miller, J. (2011). Ghosts of Yellowstone: Multi-Decadal Histories of Wildlife Populations Captured by Bones on a Modern Landscape PLoS ONE, 6 (3) DOI: 10.1371/journal.pone.0018057

About Rob Mitchum (525 Articles)
Rob Mitchum is communications manager at the Computation Institute, a joint initiative between The University of Chicago and Argonne National Laboratory.
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