Fewer caribou calves are being born and more of them are dying in West Greenland as a result of a warming climate, a U.S. biologist says. ((Eric Post/Penn State))

Fewer caribou calves are being born and more of them are dying in West Greenland as a result of a warming climate, says a U.S. biologist who believes the mammal may serve as an indicator species for climate change.

Penn State biology professor Eric Post based his conclusions on data collected since 1993 showing that the timing of peak food availability no longer corresponds to the timing of caribou births. He said the number of calves surviving into adulthood decreased by roughly 75 per cent.

The study, which was conducted in collaboration with the University of Aarhus in Denmark, will be published in the July 12, 2008, issue of the Philosophical Transactions of the Royal Society of London.

Post's team studied the timing of calving by caribou and the timing of the growth of plants they rely on for food, finding what scientists call a "trophic mismatch." That means caribou are now giving birth just after foraging plants have reached their peak productivity and have begun to decline in nutritional value.

Throughout the long Arctic winter, when there is no plant growth, caribou dig through snow to find lichens. But come spring, they switch to grazing on the new growth of willows, sedges, and flowering tundra herbs.

As the birth season approaches, they are cued by increasing day length to migrate into areas where this newly emergent food is plentiful.

But this routine, which worked for millennia, is out of kilter. Now when pregnant females arrive at the calving grounds, they find that the plants on which they depend already peaked.  Post said the plants, whose growth is tied to temperature and not daylight, are peaking much earlier in response to rising temperatures. 

"Spring temperatures at our study site in West Greenland have risen by more than 4 C over the past few years," Post said in a release. "As a result, the timing of plant growth has advanced, but calving has not."

Trophic mismatches have been documented in birds. The most famous example is the study of Dutch birds and their caterpillar prey, highlighted in former U.S. vice-president Al Gore's film, An Inconvenient Truth. Until now, the mismatch hadn't been observed in terrestrial mammals.

"The rapidity with which this mismatch has developed is eye-opening, to say the least," said Post.