“The translocated animals were literally let out of a livestock trailer and started looking around at their new environment,” says Matthew Kauffman from the University of Wyoming. “And they almost entirely failed to migrate.”
Kauffman knows this because the translocated sheep were often fitted with radio collars, allowing him and his colleagues to compare their movements to those of bighorns that lived in the same place for centuries. Within those longstanding herds, between 65 and 100 percent of the sheep migrated. But in the translocated herds, fewer than 9 percent migrated—only the sheep that had been moved into established populations that already knew the land.
The team also used satellite images to measure how closely the sheep were tracking the waves of emerging greenery. Then, they compared the animals’ performance to two kinds of simulated sheep—naive ones that moved around at random, and omniscient ones that had perfect knowledge of the local plants. “Some of the recently translocated herds tracked the green wave no better than the ones that wandered randomly,” says Brett Jesmer, who led the work. The older herds did far better—“not as well as the omniscient ones, but closer,” he says.
“This changes how we think about wildlife habitats,” Kauffman adds. “Wildlife researchers have always focused on the physical landscape. How much grass is there? How many conifers? Then you can ask how good that habitat is for a sage grouse or a grizzly bear. But our work suggests that the true measure of habitat quality for mobile animals is both the physical attributes of the landscape and the knowledge that animals have of how to make a living there. Put naive animals into awesome habitats and they may perform really poorly, while animals that know how to exploit landscapes that have been degraded could do really well.”
Scientists have long wondered how migrating animals know where to go. In some cases, that knowledge is innate. Sea-turtle hatchlings read the Earth’s magnetic field to head off in specific directions, while hybrid songbirds will travel along routes that are halfway between those of their parents. In other cases, learning clearly matters. Whooping cranes get better at migration with age, and groups that include at least one elder are much better at staying on course.
A once-captive dolphin has introduced her friends to a silly trend.
Ecologists have long speculated that ungulates—hooved animals such as deer, bison, and sheep—also learn to migrate, since many species seem to adopt the movement patterns of their mothers and peers. By studying the translocated bighorns, using data gleaned from their collars, Kauffman’s team has finally confirmed this long-standing assumption.
To an extent, ungulates can find emerging greenery through local smells and sights. “But they also possess excellent spatial memory,” Jesmer says. “They can remember when a path greened up and time their movements to go to that area the next spring.” Their mental maps are the foundation of migration. They’re the difference between an animal that’s just going after nearby shoots, and one that’s moving long distances across the terrain in anticipation of greenery that it knows will arrive.