Published online 9 August 2008 | Nature | doi:10.1038/news.2008.1021

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Cougar's movements betrayed by claw analysis

Stable isotope analysis helps to retrace animal's steps.

The last months of a cougar’s life have been reconstructed by analysing the chemical isotopes in its claw. The technique could be a boon for conservation biologists and land managers who need to track the movements of animals, say the scientists behind the study.

cougarThe mighty cougar carries a journey log in its claws.PUNCHSTOCK

Cougars, like all animals, carry an invisible history of their movements in non-living tissue such as claws and hair. These contain characteristic ratios of stable isotopes, unique to a particular location and determined by the food and water they consume.

For example, water that contains deuterium, the heavy isotope of hydrogen, tends to rain from clouds more readily than regular ‘light’ water. So as clouds move across the country, they leave a distinctive gradient of heavy to light hydrogen in soil and groundwater — and in the creatures that live there.

Stable isotope tracking has seen “an explosion of applications” since pioneering work on monarch butterflies in the mid 1990s, according to Keith Hobson, an isotope ecologist at Environment Canada in Saskatoon, Saskatchewan.

Along with colleagues Viviane Hénaux and Larkin Powell of the University of Nebraska, Lincoln, Hobson applied the technique to the claw of a cougar that met its end on a highway near Greta, Nebraska in November 2005. Hénaux presented their analysis at the Ecological Society of America meeting in Milwaukee, Wisconsin, on 5 August.

Detective work

Hobson used stable isotope mass spectrometry on a powdered portion of the claw, to measure the ratio of hydrogen to deuterium; and of carbon-12 to carbon-13. As the claw grew, it recorded the ratios that were unique to each part of the state.

The team then created reference maps showing variation in the isotope ratios across Nebraska. The maps were based on the isotope ratios found in the lymph nodes of deer shot in different parts of the state, which had been collected as part of a separate study on chronic wasting disease. Cougar eat almost nothing but white-tailed deer, which have small stationary ranges.

The variation they saw in the hydrogen-deuterium ratio on the map matched the pattern seen in precipitation, while the higher proportion of carbon-13 in the east of the state were attributed to the presence of cornfields there. Corn is a so-called C4 plant, building carbon dioxide into sugars via a four-carbon intermediate compound. This tends to give the plant a higher carbon-13 to carbon-12 ratio than C3 plants.

The scientists then compared the cougar’s claw with the maps. With either just the hydrogen or the carbon information, it would have been impossible to trace the animal’s route. But by overlaying the maps, and thereby narrowing down the possible locations for each time step, Hénaux was able to infer that the cougar followed the Missouri or Elkhorn River from the Black Hills of South Dakota. “By combining different isotopes, you can have very precise information about local areas in Nebraska,” she says.

Isotope tracker

According to Hobson, most tracking has been done on a global or continental scale, determining where migratory birds overwinter, for example. “The work with the cougar revealed that this could be at the state scale; I think that was an exciting breakthrough,” says Hobson.

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Hénaux now has a few other road-killed cougars to try the technique on. One of the animals was radio collared, so its actual movement records can be used to calibrate the time between two points on its claw. Hénaux says the technique could be used with other large mammals—both as part of an anti-poaching efforts and to help land managers plan where wildlife corridors should be set up.

“Stable isotopes are pretty amazing for what they can track and tell us about the environment,” says Robert Michener, manager of the Boston University Stable Isotope Laboratory in Massachusetts. “Using hydrogen isotopes to track migration is a relatively new and hot field.”

However, Michener cautions that it is not “a magic bullet”. “There are many situations where isotopes alone won’t work," he says. "We’re trying to use isotopes to track migrating bats in the northeast, for instance, and the picture is pretty muddled.” 

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