Ask Arthur Middleton to define the lifeblood of the Greater Yellowstone Ecosystem, and he may mention three major river systems. But he will quickly turn his focus to terrestrial wildlife migrations.

Middleton, an ecologist and research scientist with Yale University and the University of Wyoming, studies animal pathways. And Greater Yellowstone, the 22.6-million-acre region in Wyoming, Montana, and Idaho, contains the longest known pathways for mule deer and pronghorn ever documented: overland routes that rival in distance the movement of wildebeests on the Serengeti Plain and caribou in the Arctic.

“Rivers run in only one direction—downward, carried by gravity,” says Middleton. “But megafauna, using their ancient corridors, flow back and forth, up and down mountains, between winter and summer range. They pulse through the landscape like heartbeats, pumping and recycling caloric energy.”