You're right - this is an unusual beaver-cutting pattern. There are probably two reasons: First, the tree is a very large hardwood rather than one of the smaller aspens, willows, birches, or poplars beavers normally favour. Second, the cuts seem to have been started by two - or possibly three - different animals at different times before the tree was abandoned.

"Beavers don't go after dense hardwoods until they start to run out of their preferred food trees," explains Mark Engstrom, a wildlife biologist at the Royal Ontario Museum in Toronto. "When they start on a large hardwood like this one, they are probably under some stress, and I'd be surprised if they didn't abandon this area soon."

Apparently, though, beavers don't give up until they've given it their best shot. Franco Mariotti, a biologist with Science North in Sudbury, Ont., points out that the dark chew mark on the lower left of the picture seems to be older than the rest of the pattern. He speculates that the older cut may have been made a season earlier than the rest. It's the living cambium (the innermost bark layers) that beavers are after for food, so a dead tree isn't any good to them. But in this case, the first cut didn't kill the tree, so either the same beaver or a different one came back to tackle it again. Also, the fresher cuts are on more than one level. They could have been made by different-sized beavers, or could have been made at different times of the year, both when the ground was bare and when there was enough snow to make the higher level accessible.

Now, however, it's clear from your picture that the tree is completely girdled. So it's unlikely, says Mariotti, that the beavers will be back to finish the job.