Snakes react to airborne sound

Although snakes lack external ears and a middle eardrum, they are still able to perceive sound. Hearing for them occurs though sound-induced head vibrations picked up by small bones (the homologue of the mammalian stapes) attached to their jawbone. Although hearing appears to be less important to the reptiles than vision or taste, new evidence suggests that it may still play some part in their behavior toward prey and avoidance of predators.

Christina Zdenek , a postdoctoral fellow at the University of Queensland in Australia, and her colleagues have now quantified the reactions of various freely moving snakes to different kinds of sound. They conducted more than 300 experimental trials on 19 snakes from five genera and monitored the effects of particular sounds in three different ranges, 0–150 Hz, 150–300 Hz, and 300–450 Hz. The sound, more specifically, was pink (1/f) noise, filtered to have equal energy per octave. And as a control, each snake was also exposed to silence in separate trials.

Of the three frequency bands, only the first produced ground vibrations, as measured by an accelerometer. The different ranges meant the researchers were able to test hearing via both ground vibrations picked up through a snake’s belly and airborne vibrations heard through its inner ear. The researchers quantified specific snake behaviors elicited in response, including body freezing, tongue flicks, hissing, and movement toward or away from the source of the sound. To analyze their results, Zdenek and her colleagues converted the sum of the behaviors for each trial into a binary variable for inclusion in a probability model.

The snakes’ behaviors turned out to be strongly correlated with their genus. Only woma pythons, one of which is pictured above, tended to move much in response to sound and toward its source, whereas taipans, brown snakes, and death adders tended to move away from it. That may be a consequence of the python’s natural behavior: As a large nocturnal snake with fewer predators than smaller species, it may not need to be as cautious. Taipans, in contrast, are active foragers that chase prey down during the day, so they were more likely to exhibit defensive behavior when sound was played, says Zdenek.

As for their responses to different frequency ranges, the death adders moved far away from speakers emitting either 0–150 Hz or 150–300 Hz frequencies. It is possible that the lower-jaw profile of those snakes, which naturally flattens along the ground, gives them superior sensitivity to vibrations that are transmitted to the inner ear. That sensitivity may have allowed them to sense ground vibrations in the 150–300 Hz range that the researchers’ accelerometer could not measure.

At least within a frequency range of 0–450 Hz, the data reveal that the snakes that were tested respond to airborne and ground-borne sounds. Academically, those results may improve our limited understanding of snake behavior and guide future research. More practically, they might help us all avoid snakebite. (C. N. Zdenek et al., PLoS One 18, e0281285, 2023 .)