A longer-term record of diet is provided by the animals' fur, which grows from midsummer to late autumn, and so contains a ratio of heavy isotopes that reflects the amount of salmon consumed during that period. The team found that Alaskan grizzly bears trapped in the wild had low blood plasma levels of heavy isotopes in early summer-before the return of spawning salmon. But hair samples from the same bears contained high levels of the isotopes, indicating that they rely heavily on salmon for at least part of the year. Analysis of bones and hair from the long-dead animals revealed that up to 90 per cent of the carbon and nitrogen in their diets had come from salmon. Even bears which are 700 to 800 miles from the ocean had isotope signatures similar to coastal bears.
Grizzlies can live without salmon-all the surviving populations in the inland Rocky Mountains do.
But taken together with the abundance of grizzlies described by early settlers along the Columbia River, Robbins and Hilderbrand's findings suggest that bears flourish where there is a healthy salmon population. Hilderbrand is continuing his work with Alaskan grizzlies, trapping bears as they emerge from hibernation in spring, in midsummer before the salmon runs, and in autumn after they have fed heavily on salmon. He measures isotope signatures in blood and hair samples and collects information on changes in body weight and proportion of body fat in the bears. They need nutrient-rich food in large quantities to reproduce successfully and to prepare for hibernation. The salmon return just when the grizzlies need them most.