I flew with my dog in February this year, so I was worried about the cold weather restrictions, too. Delta (who we flew with) told me that as long as the temperatures in the departure city and the arrival city (plus any stops along the way) were both above 10 degrees Fahrenheit, then all would be well. Fortunately, it was, so I didn't have to worry.
Of course, I don't know if this applies across the board. I don't have any clue how Delta's cargo holds compare to other airlines', but that was our experience.
For what it's worth, if you have to look at other options (not sure if Air Canada is an approve carrier on an approved route), I just want to let you know that our experience with Delta was a fantastic and very easy one, so if they're an option, it would be worth considering.