Take my enemy, please

Relocated kangaroo rats can thrive when territorial rivals move with them

Even solitary creatures do better when a new place has the same old jerks next door.

SETTLED IN Stephens’ kangaroo rats relocated to this new habitat were more likely to survive and have more pups when moved with their dear familiar enemies. Ken Bohn, San Diego Zoo Global

Stephens’ kangaroo rats, on the U.S. list of endangered species since 1988, live by themselves most of the time in plots of California grassland that they defend from nearby members of their species. When conservationists moved animals to safer homes away from development, familiar rivals relocated together fared better than kangaroo rats grouped with strangers, says conservation biologist Debra Shier of the San Diego Zoo Institute for Conservation Research.

This boost may come from what’s been called a “dear enemy effect,” Shier and institute colleague Ronald Swaisgood say online October 6 in Conservation Biology. Animals tend not to scrap as aggressively with familiar holders of neighboring territories as with complete strangers. Among the kangaroo rats, the researchers noted that those relocated along with their dear enemies indeed spent less time fighting and more time foraging than kangaroo rats surrounded by unfamiliar neighbors.

Talk is increasing about relocating imperiled animals or plants — either from shrinking wild habitats or captive breeding centers — as a solution to conservation challenges including climate change, says Mark Stanley Price, chair of the International Union for Conservation of Nature’s Task Force on Moving Plants and Animals for Conservation Purposes. Yet it’s a complex process. “While the last 30 years have seen some spectacular species returns, there are many, often undocumented, failures,” he says. The work by Shier and Swaisgood “should prove to be an exemplary milestone in translocation biology.”

Previous relocation attempts with kangaroo rats haven’t produced any documented new, lasting populations, Shier says. Almost 600 Stephens’ kangaroo rats relocated in 1992, for example, disappeared within only 11 months.

This species of kangaroo rat, one of 19 in North America, had a small range of only a few California counties to begin with, and — as Shier puts it — the same tastes in real estate as a lot of California developers. Farms and sprawl claimed more and more of the flat grasslands the kangaroo rats favor, and invasive grasses threaten to choke the remainder with a carpet so dense that the animals can’t find little patches of open ground for fur maintenance by sand-bathing or communication by foot-drumming.

In their relocation experiment, Shier and Swaisgood took animals from three failing habitats. The researchers identified which rats burrowed where in the neighborhoods, and then moved 54 of them to new, protected ground in 2008. In 2009, they moved 45 more animals. In both batches, about half the kangaroo rats had their usual neighbors nearby and half didn’t.

Monitoring the animals revealed better survival in the group moved with their familiar foes. This group also had 24 times as many pups as the group moved with strangers, in part because only three out of 20 females in the stranger group survived their first six months.

Three years after the project began, the original population of 99 has now grown to about 400.

Keeping familiar rivals nearby could be reducing the stress of the move, says physiologist Molly Dickens of the University of Liège in Belgium. In her studies of animal relocation, the process inevitably meant stressful unpredictability, novelty and lack of control. Reducing the stressors after the move, she says, can give animals a chance to recover and avoid developing physiological vulnerabilities from chronic stress. 

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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