One of our Guild members had some yak yarn she'd spun on display today, so I asked her if she thought people would be interested in fibre direct from a UK yak. (At this time, I still hadn't noticed Fleecewife's prior enquiry, so hadn't realised that I and my other friends - besides FW - would be second in line!
)
Anyway, said Guild member said she understood that the yak could only survive at above 12,000 feet, and so she was sure these must be hybrids. As she understood it, the yak's system is so adapted to life at high altitude / extreme (low) temperatures that they simply can't function in a temperate climate at a low (to them) level.