Interesting link from Susan: 
http://www.nytimes.com/2010/07/02/science/02tibet.html?src=un&feedurl=http%3A%2F%2Fjson8.nytimes.com%2Fpages%2Fscience%2Findex.jsonp

The gist of the article is that Tibetans have evolved to compensate for
the lack of oxygen at their high altitude.   It occurred to me that
perhaps dogs who developed in Tibet might actually be at a disadvantage
when brought to our lower altitudes.  Okay, I'm no scientist, but I
can't help but wonder if maybe these dogs whose attributes seem to
indicate such strength, agility, etc., are perhaps weakened when we
remove them from their natural habitat.  
Susan