Hunting by day is a doddle when equipped with a sharp pair of eyes, but when the conditions are dim, even the keenest sight isn't going to help, which is where thermal imaging can come in handy. Pit vipers are equipped with specialised infrared-sensing pit organs that provide a thermal perspective of the serpent's surroundings, which can help the predator to pick off warm-blooded animals. Intrigued by the reptile's twin visual systems, Qin Chen, from the Chengdu Institute of Biology, China, explains that it was not clear how much significance the vipers attached to each system. ‘How the snakes combine the two into a picture has always fascinated us’, says Chen. Knowing that vipers that are equipped with tape blinds can strike successfully at warm and cold objects when viewed with their infrared sensor against an intermediate temperature background, Chen and his colleagues Guangzhan Fang and Yezhong Tang wondered how much of an emphasis the snakes put on their thermal view when the contrast between the warmth of their victim and its surroundings varies.
As Qin Chen had been bitten by a large pit viper (Protobothrops jerdonii) previously, the team chose to work with the smaller short-tailed pit viper (Gloydius brevicaudus), which is more docile and an adept hunter in bright and dim conditions. After collecting the vipers and transporting them back to Chengdu, Chen and Yang Liu built a drum-shaped arena lined with a heating film so that they could warm one side of the chamber while the other half remained at room temperature (26°C). The duo then measured the surface temperature of the mice (33°C) that would be pitted against the vipers, and set one half of the arena at that temperature (so that the mouse would blend in) while holding the other at 26°C (which the mouse should stand out brightly from) before filming individual vipers as they attacked the rodent either with or without the benefit of their eyes and thermal pit sensors.
Guided by both their eyes and infrared pits, the vipers successfully landed all nine of their attacks against the cooler background, although they missed two out of the 12 attacks against the 33°C walls. In contrast, when the vipers had to strike with only their pit organs for guidance, their success rate was significantly higher against the 26°C wall – landing 14 successful strikes – compared with the four that they managed to land as the mice scampered across the 33°C background, only two of which succeeded. However, when the snakes had to depend on vision alone, they still missed four of their 10 strikes against the cool cylinder wall, while only landing eight of the nine strikes at mice in front of the low-contrast 33°C background.
Chen and Liu then repeated the experiments, but this time they heated the warmer half up to 40°C while the cooler half remained at 26°C, to find out how the contrast of the rodent's body temperature against the background affected the snake's success. The snakes were 100% successful against the cooler background when guided by their thermal pits alone, although they only landed 55% of the strikes when the mice appeared as a dim thermal shape against the 40°C background.
‘Snakes prefer to target prey in front of backgrounds with positive thermal contrasts when only their pits are active’, says the team, and they suggest that the vipers are equally dependent on their thermal and visual senses as their success rate fell when the snakes were only able to use their eyes.