TOEFL Academic Passage Practice 038The Purpose of Forked Tongues in Snakes

The forked tongue of snakes has attracted attention for thousands of years, leading to a wide range of explanations. In many cultures, it has symbolized deceit or evil. Aristotle was the first person known to raise the question of its purpose, suggesting that the fork might intensify the sense of taste by doubling the sensation. By the early twentieth century, most scientists believed the tongue functioned as a tactile organ, used to probe the ground in a manner similar to how a blind person might use a cane.

In 1920, Browman introduced a theory that was widely accepted for decades. He proposed that when a snake draws its tongue back into its mouth, the tips of the fork enter two openings in the roof of the mouth, allowing chemical substances to reach a pair of vomeronasal organs (VNO). These organs, which are particularly developed in snakes, lizards, and many mammals, are part of a secondary olfactory system believed to detect pheromones—chemical messages that animals secrete to signal others of their species. According to Browman’s theory, the tongue gathers these chemical cues from the environment and transports them to the VNO. However, X-ray films taken in the 1980s revealed this was incorrect. When the tongue retracts, it enters a protective sheath and the tips do not contact the VNO openings. Instead, chemical particles are transferred to pads at the base of the mouth, and when the snake closes its mouth, the pads press the particles against the VNO openings.

If the tongue is not forked to fit into the VNO, then what might its shape accomplish? A researcher named Schwenk proposed a new hypothesis based on findings from behavior, ecology, sensory physiology, and neuroanatomy. He suggested that the forked tongue enables the snake to sense chemical cues at two separate points simultaneously, helping it detect slight differences in an odor trail. This capacity makes it easier for the animal to locate the edges of a chemical trail and follow it accurately—an ability that is important both for finding prey and locating mates.

This form of spatial chemical detection resembles other systems in which perception depends on input from two separated organs. For example, the brain determines the source of sounds by comparing what each ear hears. Likewise, using two eyes enables depth perception through stereoscopic vision.

Several pieces of evidence support the idea that forked tongues evolved to serve as chemical edge detectors that improve the tracking of odor trails. First, snakes and lizards spread the tines of their tongues apart when collecting odor particles, then bring them together when pulling the tongue back in. The farther apart the tines are when sampling, the better the animal can detect variations across the odor field. Second, lizard species that travel over wide areas in search of food generally have forked tongues, while those that do not forage widely usually lack this feature. Third, forked tongues have appeared independently in at least two separate reptile groups, suggesting that this trait has adaptive value. Finally, in snakes, each tine of the tongue is connected to a specific part of the opposite side of the brain, and those brain regions are connected to each other. This is similar to the arrangement found in the auditory centers of mammals and birds, where signals from both ears are compared to determine sound direction.

Other types of animals have also evolved paired chemical-sensing structures to locate food or mates. For example, male gypsy moths use their large, sensitive antennae to detect the pheromones of females from far away. Similarly, the ant nest beetle has spoon-like antennae extending from either side of its head that help it follow the scent trails of ants, which it preys upon.