You're probably familiar with the use of canary during coal mining. At the slightest trace of poisonous gas the canary dies and the miners are warned. Well my story involves a small animal that was bred over the course of thousands of years to serve as a living bioindicator. The animal is highly sensitive to changes in the environment and gives off different signals to tell the humans what the environment is like. Is it too hot, cold, dry, humid, acidic or basic? The animal accurately displays all of these. Or just dies, in which case they reproduce quick enough to be replaceable. The animal is basically a sacrificial pet. It serves as a food taster, anything it can eat we can eat.
The thing is pretty much living litmus paper, giving off changes in colour or other signals. What I haven't got figured out though is how the thing gives off overlapping signals for different stimuli. For example if its hot and dry, the signal would be different from when it's hot and wet. Or when the ph of the ground is too high how do you know it's not a bacterial infection? The creatures form changes depending on where it lives, so would it have distinct signals to display everything at once?
It has to be visual, physical changes, otherwise if the creature dies there's no telling what was wrong.
Edit: [To be more specific the critter displays temperature, humidity, Ph as well as the presence of bacteria and pollutants.]