To understand how snakes evolved their infrared detection systems, a group of scientists led by Prof. David Julius at the University of California, San Francisco, searched for potential infrared-sensing proteins in the western diamondback rattlesnake. They looked in particular at genes active in the nerve cells that are connected to the pits, called trigeminal neurons.

They found one gene, known as TRPA1, that was 400-fold more active in rattlesnake trigeminal neurons than in other kinds of neurons. Moreover, they found that the TRPA1 gene was not highly active in the trigeminal neurons of snakes lacking pits. These two pieces of evidence suggested that TRPA1 might encode a protein involved in infrared sensing.

The TRPA1 protein was very familiar to the scientists. A few years earlier, Dr. Julius’s group had identified TRPA1 as the receptor that drives our response to the molecules that give wasabi its punch, as well as to other chemical irritants, like tear gas.

The TRPA1 gene encodes a type of receptor protein known as an ion channel. In humans and other mammals, when the protein is exposed to and binds specific chemicals, the channel opens, allowing ions to flow into nerve cells and setting off a sequence of events that produces a nerve impulse.

In pit vipers, however, Dr. Julius and his collaborators discovered that the TRPA1 has evolved to be especially heat-sensitive. While the receptor is not activated in most snakes by temperatures approaching 37 degrees Celsius (98 degrees Fahrenheit, our normal body temperature), the western diamondback rattlesnake TRPA1 receptor is stimulated around 27 degrees Celsius (80 degrees Fahrenheit), creating a “thermal image” of the heat source in the snake’s brain that is used to aim its strike. Pit vipers are not the only animals or even the only snakes to have evolved infrared sensing. Pythons and boas have also evolved heat-sensing pit organs on their faces, although of a different structure. Dr. Julius and his team found that TRPA1 was also highly expressed in the trigeminal neurons of python and boa pit organs, about 65-fold and 170-fold higher, respectively, than in the trigeminal neurons of other snakes lacking pits. Similarly, their TRPA1 receptors were 5 to 8 degrees Celsius more heat-sensitive than typical snakes.

In both groups of snakes, changes in the structure of the TRPA1 receptor, and the evolution of very high levels of expression in their sensory pits, endowed the animals with sensitive infrared detectors.