I know DL refuses to take pets in the hold at all during the summer months for this reason. Don't know if UA has a similar ban or they take them only if the temps are reasonable.

This reminds me of a perhaps apocraphyl, but funny story:

The baggage handlers are emptying the cargo hold on a hot Houston afternoon, and find a pet carrier with a dead dog inside. They feel so bad that they go to a local pet store and pool their money and buy a new dog that looks like the dead one.

When the owner comes to claim the dog, she looks inside the carrier and says immediately "that's not my dog". When asked how she knows this, she said "My dog is dead--I was shipping him back here to be buried."