Yukon lemmings don't seem to fit the standard lemming pattern.

From Scandinavia to the Mackenzie Delta, the small rodents generally undergo a population cycle three to five years in length. But west of the Mackenzie Delta, in the Yukon, the pattern is different.

At least, it appears to be different, says Scott Gilbert of Yukon College, who has been studying Yukon lemmings for several years.

His work is part of a larger study co-ordinated by Dr. Charles Krebs of the University of British Columbia. Lemming populations are being followed at several sites across the Arctic from Cambridge Bay to the northern Yukon. By comparing changes in numbers each year at these sites Krebs hopes to learn whether lemming populations change in synchrony over large distances.

"Our fieldwork during past years has identified a cycle in lemming numbers in western portions of the Northwest Territories," Gilbert says. "However, we have not detected any pattern in changes in lemming populations in the northern Yukon."

The Yukon has two species of lemmings found in tundra areas and both are similar in size. They weigh about 50 grams (less than a single CD) and are about 10 cm long (a bit less than the width of a CD case).

The brown lemming is usually found in moist areas of tundra, while the varying lemming prefers drier areas. The varying lemming's name refers to its fur, which changes colour during the year, from white in winter to grayish brown in summer.

In the western Northwest Territories, lemmings follow the same pattern seen further east. Their numbers are relatively low for one or two years. Then they undergo a winter and summer of rapid population increase. One or, on rare occasions, two summers of high density are followed by a population crash. Then the cycle starts again.

The scientists use a standardized system of trapping to track the changes in population size. East of the Mackenzie Delta the lemmings were at peak numbers in 1993 and at a very low ebb in 1994. In 1995, the numbers were beginning to recover, Gilbert says, and in 1996 they were back at peak levels.

On the Yukon side of the Delta, the picture was different. In 1995, when eastern lemming populations were climbing, the capture rate of lemmings in the Yukon was still dropping. Yukon levels were still low in 1996, when the Northwest Territories lemmings were back at a population peak.

"We still can't tell if Yukon lemming populations follow some sort of cycle over several years," says Gilbert. "However it is clear that population trends differ on opposite sides of the Mackenzie River Delta."

In an effort to get a clearer idea of lemming population patterns in the Yukon, Gilbert and his colleagues are trying a new approach.

"Local knowledge suggests that lemming populations occasionally reach high numbers on Herschel Island, so we set out to try to identify historical lemming peaks by using vegetation analysis," Gilbert says.

Besides trapping rodents, as they have done in the past, the scientists have collected samples of willow that have scars at the base of the stems.

"During high density years, small mammals browse on willow and debark sections of the lower stem, which creates a scar," Gilbert explains. "The year that the scar was inflicted can be determined by carefully cross-sectioning the stem at the site of the scar and reading the growth rings."

The technique of reading the willow growth rings was developed by an ecologist in Sweden. Gilbert hopes that using this method will give a long-term view of how lemming populations have been changing in the last 20 to 30 years.

For more information about lemming studies in the Yukon, contact Scott Gilbert at the Renewable Resources Management Program at Yukon College, 668-8776.