How do animals (and humans) know when it’s hot or cold

It seems our bodies are riddled with various temperature sensors — the most researched of these being a group of proteins called transient receptor potential (TRP) channels located on sensory neurons all over our and animals’ bodies. Each is tuned to a specific temperature and opens when that temperature is reached; the sensory neurons immediately send signals to the brain, telling it whether it’s getting too hot, too cold, or when it is just right! The TRP channels are tuned specifically for the comfort of each animal. In warm-blooded animals, the channel TR P8 triggers if it begins getting too cold, for us that trigger point is around 24 degrees Celsius, (which, is probably why it is recommended for air-conditioners) but for something like a frog it may be 14 degrees Celsius. Some fish that swim happily in near-freezing water may not have a TR P8 channel at all so that they are unaware that they’re swimming in icy water!

At the other end of the scale, the TR PVi channel alerts us if we are in danger of burning up — when for example, we have eaten capsaicin (from where capsicum comes). For us and mice, the upper limit is 42 degrees Celsius and 38 degrees C for frogs. Some animals, like the 13-striped ground squirrel of the US, have a huge tolerance range: They can withstand both icy conditions and sizzling desert sun — their temperature triggers are set very low and very high.

Thus, these animals (and we, in our comfort zone) simply don’t feel the cold or the heat. So there’s no need to feel sorry for a camel plodding placidly across the burning desert, or Emperor penguins huddled together having a team meeting in a blizzard. It’s even suspected that people living in the northern latitudes have their TR P8 triggers set lower than those in the tropics. This is why they roam around in t-shirts when they come to New Delhi in winter while the rest of us huddle in down jackets!

These temperature sensors can be extremely sensitive. Those on the tips of the antennae of flies can sense temperatures independently and can tell the difference of a 0.1 degree C between one antenna and the other, making the insect swerve (usually) towards the warmer source. This phenomenon has been called thermotaxis.

Some insects, like fire-chaser beetles, actively seek out the heat — usually from forest fires — and can pick it up from miles away. They lay their eggs on smouldering tree trunks, knowing their larvae would have fresh crisp food to eat and be safe from predators who would dare not approach the area.

Now ‘heat’ is nothing but infrared rays born of agitated molecules and atoms, which are mostly invisible to us (except when things get ‘red’ or ‘white’ hot), but which we can very much feel. It are these invisible infrared rays that the beetle’s extraordinarily sensitive sensors pick up, guiding them to the source. These sensors, located between the middle pair of legs, just beneath the wings, clump like tiny bunches of berries, each one (about 70 in total) filled with fluid. They are connected to neurons. When they sense the heat, they get agitated and try to expand, but can’t because the little ‘berry’ is tough and unyielding, so the fluid presses against the neuron, triggering it off and signalling to the beetle, that the heat is on! There is still some debate around the fact whether the beetles can actually sense the heat from a fire that may be miles away while sitting still on a leaf or twig, or it needs to fly — wherein the vibration of its wings may kind of waft the infra-red rays to its sensors — rather in the manner of a chef waving in the aroma of a stew towards his nose, I would imagine.

It’s amazing how finely tuned these systems can get. Vampire bats, which have heat-seeking pits around their mouths, could likely be confused when sensing the temperature of a prospective victim because it is warm-blooded and its own TR Vi number is probably the same as their victim’s. So, the TR Vi number around its heat-sensitive pits has been reset to 31 degrees C instead of the normal 42 degrees C so that it can sense accurately where its victim is and where to bite it!

With global broiling going strong, our TR Vi sensors have been screaming ‘Mayday!’ all summer, and we still refuse to get our act together. What are we waiting for? To spontaneously combust?