Consumption of earthworms by Yellowstone grizzly bears

There are very few records, primarily from Eurasia, of bears consuming earthworms (class Oligocheata). Sign of grizzly bears (Ursus arctos horribilis) eating earthworms was observed at 20 sites during a radiotelemetry study of the Yellowstone population, 1977-92, and on 14 additional occasions during direct observations, 1985-96. Relative frequency and intensity of earthworm consumption peaked during April and May and declined thereafter. No'consumption of earthworms was detected after June. In general, soils were wet to saturated from snowmelt at the time they were excavated by bears to consume earthworms. At fine scales, earthworm use was positively associated with nearness to a forest edge and with cover of bluegrasses (Poa spp.). Excavations tended to be larger in total size during months when regional precipitation was greater; depths averaged 0.9 din. These depths together With other site characteristics suggest thatthe bears consumed anecic species of earthworm, most likely non-natives of the family Lumbricidae. At two sites, we estimated that 120-170 earthworms m(-2) were available to the bears, totaling 4,000-7,900 for the entire feeding sites. This potentially amounted to 0.9-1.8 kg of worms, comprised of 60-70% protein (dry weight). Earthworms are a unique high-quality bear food that may have been overlooked in other areas. Given the continuing invasion of previously-glaciated boreal and montane regions in North America by non-native earthworms, consumption of worms by bears may become more widespread.