I found Delta has a temperature restriction.
http://www.delta.com/content/www/en...needs/pets/pet-requirements-restrictions.html
Weather Requirements
To keep your pet safe, Delta observes the following seasonal restrictions on pet travel.
Year-round Restrictions
Delta will not accept animals as checked baggage during extreme weather, if at any point (origin/transit/destination) during the animal's trip the high temperature for the day is forecast to be: (no exceptions to the temperature restriction)
- Below 10°F (-12C)
- Above 85°F (29.4C)
And Westjet I really can't decipher:
http://www.westjet.com/en-ca/travel-info/pets?sc_ref=direct
It says "We love Canada but, sadly, heat is not an issue". I have no idea what that means, other than it may be a joke about lack of hot weather in Canada. Other countries (mostly tropical) seem to have restrictions for May 15 to Oct 31, which I guess is related to hot temperatures.