OK...i bow to your superior knowledge. I think once you have shot 200 or so the differences stand out.
They would - if you started with no knowledge of beavers, and decided that any beaver you shot that looked one way was Eurasian, and any beaver that looked another way was American.
Sorry - but with all due respect to you mate, who may well be an extremely thoughtful and observant man, unless he had learned what the two looked like by examining definitely known specimens of the two species, all he's likely to be doing is noticing variation in the population, and assigning big ones to American and small ones to Eurasian. Even then - anyone who actually works with them professionally will tell you that it's not something you can have any certainty about without genetic samples.
