Coal miners once carried caged canaries down into the tunnels. If methane or carbon monoxide leaked into the shafts, the gases would kill the bird and alert the miners to get the hell out.
Pablo functions as our little canary. Each night we lug him upstairs (And I do mean lug! Pablo will not climb stairs.), plop him on the bed, and observe him. If he’s panting after a few minutes, we know it’s too hot and switch on the AC. If he settles down, breathing easy, we know we’re okay just with the fan.