A common belief suggests that all venomous snakes can be identified by their elliptical or “slit” pupils, while non-venomous snakes possess round pupils. This widespread notion presents an oversimplified view of snake identification. The reality is more complex, and relying solely on eye shape can lead to dangerous misidentifications. Understanding the nuances of snake characteristics provides a more accurate approach to distinguishing between different species.
The Truth About Snake Eye Shapes
Many venomous snakes, particularly those belonging to the Elapidae family, have round pupils. Examples include cobras, mambas, kraits, and taipans found in Australia. Even coral snakes, native to North and South America, possess round pupils, directly contradicting this common rule.
Conversely, numerous non-venomous snakes exhibit elliptical or vertical slit pupils. Boas and pythons often have slit pupils. Some colubrid species, such as brown tree snakes and cat-eyed snakes, also display this pupil shape despite being non-venomous.
Understanding Pupil Shape Variation
Pupil shape in snakes is primarily an adaptation linked to their activity patterns and hunting strategies, rather than their venom. Snakes active during the day, known as diurnal species, typically have round pupils. Many active foragers also tend to have round pupils, providing them with a stable and wide field of vision.
Snakes with vertical slit pupils are often nocturnal or crepuscular, active at night or at dawn/dusk. This pupil shape offers wide light control, allowing the snake to constrict its pupil to a narrow slit in bright conditions to prevent glare and open wide in low light to maximize intake. Vertical pupils also provide enhanced depth perception for detecting horizontal movement, beneficial for ambush predators.
Beyond Eye Shape: Reliable Identification Cues
Since eye shape is unreliable, other physical characteristics can offer more helpful clues for snake identification. Many venomous snakes, particularly pit vipers (e.g., rattlesnakes, copperheads, cottonmouths), have a distinctly triangular head due to large venom glands. However, harmless snakes can mimic this by flattening their heads when threatened, making this cue unreliable.
Pit vipers also have heat-sensing pits. These specialized organs, located between the eye and nostril, detect infrared radiation from warm-blooded prey, even in darkness. Boas and pythons also have heat-sensitive pits, but theirs are smaller and located along their lips. The scales on the underside of a snake’s tail can also provide a clue: most vipers have a single row of scales (subcaudals) from the vent to the tail tip, while most non-venomous snakes and elapids have a double row.
Color patterns can sometimes be indicative, but mimicry requires caution. The rhyme for identifying coral snakes in the United States (“red touches yellow, kills a fellow; red touches black, venom lack”) refers to colored bands. However, this rhyme is region-specific and doesn’t apply globally, as some non-venomous snakes mimic this pattern, and some coral snakes don’t fit it. No single characteristic guarantees accurate identification. The safest approach is to avoid approaching or handling any snake if its species is uncertain. Consulting local wildlife experts is advisable for definitive identification.