The cargo area is quite cool. AA doesn't allow pets in cargo in my area... they have restrictions as to min and max temperatures at all grounding points on a flight. Unfortunately for me, Texas is quite a bit higher than the max. As far as I have been able to tell, each airline (domestically anyway) seems to have their own restrictions about pets onboard and in cargo. International flights add another dimension and often involve a period of quarantine upon arrival.

My vet suggested treating the cat in a carrier like a baby in a car carrier... keep it covered with a thin blanket so they can't see much to cause additional stress. And after a moment or thirty of meowing they will usually settle down and sleep.