For the past few years, Georgia Tech palaeontologist Aaron Woodruff and his colleagues have spent their summers in a cave in Wyoming, searching for tiny fossils from the Ice Age. Their searches have uncovered thousands of remains of birds, lizards, snakes, small mammals and more, but they can't claim all of the credit for these discoveries: some of the fossils were gathered millennia ago by a much more seasoned band of bone collectors ... packrats!

Packrats' compulsive hoarding habits have found paleontological purpose. Image: Bryant Harper/Flickr

Packrats are notorious for gathering bits and pieces of their habitats, from seeds and leaves to twigs and bones. "Really, they'll collect anything that's interesting to them," Woodruff explained as he presented his research at the Southeastern Association of Vertebrate Palaeontology last month.

In environments full of fascinating odds and ends, the inquisitive rodents are often spoiled for choice. "They'll pick up things, and as they're travelling, if they see something else that they like, they'll drop what they were carrying previously and pick up the new thing and take it," Woodruff recounted.

Some of these scrappy acquisitions get eaten, and the rest go back home to the rat's nest (or midden), which is essentially its prized pile of trinkets. And then, just for good measure, the packrat pees on everything.

Over time, this urine crystallises and sort of cements everything together, resulting in a surprisingly resilient – if pungent – collection of the rat's favourite items. If undisturbed, these middens can stick around for thousands of years, like smelly little time capsules.

That's where palaeontologists come in.

Natural Trap Cave in northern Wyoming has been a famous fossil locality for decades; it's one of the world's best sites for Ice Age fossils, particularly for records of large extinct mammals such as mammoths, bears and camels. Recent excavations led by Julie Meachen have led to such exciting finds as the discovery of Beringian wolves.

The crevices along the walls of Natural Trap Cave are great spots for packrats to build their middens, full of seeds, bones, excrement and many clues to the past. Image: Justin Sipla. 

As the name suggests, many of the animals found in Natural Trap ended up there by accident. The cave's entrance is an easy-to-overlook hole in the ground above a 25-metre (80-foot) vertical drop. "If you're an animal that's running this direction, maybe from a predator, you most likely wouldn't be aware of [the cave] until you're about to leap over it," Woodruff explained. "And you'd basically be kinda screwed."

Meanwhile, animals like packrats were happily making their homes inside the cave: some of the middens here date back tens of thousands of years, and they're full of the remains of ancient plants and animals. The contents of each nest offer an excellent glimpse at the prehistoric habitat within about 100 metres (330 feet) of the cave.

For Woodruff, perhaps the most exciting thing about the packrats is their love for poop. The rodents really like collecting droppings and owl pellets, which often contain lots of small animal bones. This allows researchers to examine not only which little species were around in the not-too-distant past, but also which predators were eating them. That makes a packrat's midden a treasure trove of information about ancient ecosystems.

The microfossils that Woodruff has been studying from Wyoming's Natural Trap Cave include snake vertebrae, small mammal limb bones, rodent jaws and teeth, and more. Image: Aaron Woodruff

Jim Mead, Director of the Mammoth Site in South Dakota, has worked with prehistoric packrat caches for many years. "They're kind of our first curators," he said of the little mammals. "There is no animal in North America that consistently pulls in the variety, the diversity and the abundance that a packrat does."

Since packrat middens are full of organic stuff, they can be aged using radiocarbon dating. And since Natural Trap Cave has been home to packrats for so long, the middens within preserve a record of the local habitat through the ages. From the growing collection of microfossils, Woodruff hopes to learn how the ecosystem changed over time, particularly during the rash of extinctions that occurred at the end of the Ice Age.

In fact, the cave is such a hotspot for packrats that they're still there, building their nests along the craggy walls. "They like to kick bones down to us every now and then," Woodruff said. "One year, a packrat actually fell in. Somebody just put him into a bucket and they lifted him back up."

And working alongside packrats doesn't just mean scientists have to watch their heads – they have to watch their stuff, too! Recalling a research trip camping in the Grand Canyon, Mead said: "In the morning, our silverware would always be gone. The first time it was like, okay, let me go find the closest packrat midden. It was about 15 yards away, and there was our silverware."



Top header image: Alan Harper/Flickr