Why can some snakes sense heat without seeing it?

Pit vipers such as rattlesnakes, copperheads, and cottonmouths have specialized pit organs between the eyes and nostrils. These pits contain membranes packed with nerve endings that detect tiny temperature differences, effectively sensing infrared heat from prey. The system is so sensitive that snakes can target warm-blooded animals in near-total darkness. This ability evolved independently from vision and gives them a major advantage for nocturnal hunting. It is one of the clearest examples of a biological thermal sensor.