EVOLUTION OF PIT ORGANS

According to the phylogenetic tree below, the pit organ of pit vipers originated in Southeast Asia. Other than that, genomic information for pit vipers tends to be scarce at best.

(A. Phylogenetic relationships and geographic distribution of viper taxa and presence (+) or absence (-) of behavioral thermoregulation mediated by thermal radiation cues. The asterisk denotes the origin of the facial pit. Phylogenetic relationships of the genera enclosed by the broken box are shown in greater detail in B. Image courtesy of Aaron R. Krochmal)

Researchers have recently discovered the molecular origins responsible for the pit vipers' ability to sense the varying temperature of infrared radiation. These warmth-activated cation channels are called transient receptor potential ankyrin 1 or TRPA1s (Panzano, 2010). TRPA1 ion channels are found in the trigeminal ganglia, which innervate the pit organs (Panzano, 2010). This same ion channel is found in humans but it has a different use. In humans, it is the protein responsible for the burning sensation you experience after consuming wasabi or other noxious stimuli (Yokoyama, 2010). According to the phylogenetic tree below, there are three mutations at positions 330, 391, and 434 within the amino acid sequence for these proteins in rattlesnakes (crotalines), which seem to be responsible for this change in function (Yokoyama et al., 2011). These mutations are believed to have occurred by Darwinian natural selection by "the acquisition of infrared vision" (Yokoyama et al., 2011).

(The numbers next to the different nodes indicate clustering percentage supports generated by bootstrap analysis. Three amino acids in TRPA1 that are suspected to enable infrared detection are boxed. The numerical column headings specify the amino acid positions of the rattlesnake TRPA1.--Figure courtesy of Shozo Yokoyama)

(The molecular structures of ankyrin receptor. The three orange balls illustrate amino acid sites 330, 391, and 434 [from left to right]--Image courtesy of Shozo Yokoyama)

SUGGESTED THEORIES FOR EVOLUTION

While there is limited factual data on the evolution of the pit organ, there are many theories based off of discoveries for why vipers developed the pit organ. Pit vipers comprise 75% of the Viperidae, which suggests that the evolution of the pit organ has been critical in its survival (Greene, 1992). The following are suggested theories:

• The evolution of the pit organ must have happened in the presence of "large targets with strong temperature contrast" (Bakken, 2007). The ability to see such targets evidently provided an advantage to these snakes.

• Climates of lethally high temperatures pressured the development of the pit organ to allow the pit viper to seek refuge (Krochmal et al., 2004) The acquisition of the pit organs has been correlated with the pit vipers' ability to behaviorally thermoregulate (Krochmal et al., 2004).

• Dietary changes may have led to the evolution of the pit organ so that the pit vipers can continue to find and identify prey items (Cooper, 2008).

• The evolution of defensive displays led to the evolution of facial pits so that the pit viper could be more efficient in confrontations (Krochmal, 2004).