Animal Senses #6: Heat

In physics, cold is the absence of heat. Not so in biology, says Ed Yong in Immense World:

“(Cold) is a different sense in its own right.”

Animals use a variety of sensors, the most studied one so far are a class of proteins called TRP channels. They are found throughout the body on the surface. Some are tuned to hot temperatures, others to cold temperatures. The trigger for these TRP channels isn’t only temperature – certain chemicals can trigger them too. This is why chillis “burn” and mints “cool”.

 

When we think of extremophiles (creatures that live in extreme conditions), we focus on the adaptations they have to survive like heat-reflecting hair or anti-freeze in their blood.

“But such adaptations would be useless if an animal’s sensory system were constantly screaming at it, triggering feelings of pain.”

Extremophiles tweak their senses “to like it”. Even in case of humans, one TRP genetic adaptation is common amongst people living at high altitudes – does it give them higher tolerance to cold? Possible, but scientists don’t know for sure.

 

The fire-chasing beetle is drawn to high heat. Their target? Forest fires! Why? Because they mate in the remains of the burnt area; and their larvae can feast on the burnt trees which are now too weak to defend themselves. But forest fires are rare and unpredictable, and these beetles need to detect them from far. Under its wing pits are about 140 tiny spheres, each filled with a fluid and enclosing the tip of a pressure-sensitive neuron. When heat hits the sphere, the fluid heats and expands. But it can’t expand outwards since the spheres are hard. Instead it expands inwards thereby triggering the enclosed neuron (signal). Imagine how sensitive the spheres must be to detect fires miles away.

 

Warm-blooded animals, as we know, maintain a steady body temperature. But this ability also makes them easy to track. Via their heat signature.

“Their unwavering body heat made them perpetually blaring beacons, which parasites could use to find hosts, and especially blood vessels.”

Many snakes have evolved to detect heat. That is why they can even hunt at night.

“A blind rattlesnake can kill mice as effectively as a sighted individual.”

Scientists found that two organs of such snakes (eyes and the heat seeking pits) eventually feed the same part of the brain. The snake, therefore, probably “sees” a fused “image” of visual and heat information of its surroundings. But the pits, unlike eyes, give a very blurry image and there is no lens to focus the incoming infrared. Some squirrels, when confronted by the rattlesnake, raise their tails and pump blood into them, thereby warming it up.

“This would increase the thermal silhouette and make them more intimidating to a heat-sensing predator.”

The evolutionary cat and mouse game goes on eternally.