Shhh! Snakes Hear You

Welcome back. It’s been nearly 10 years since I blogged about snakes (Afraid of Snakes?, Snake Photo Addendum), though not as long since they shared top billing with other animals (Lure-Using Animals Addendum, The Wildlife Trade). Given the common fear of snakes, I suspect you didn’t mind the wait. Sorry, the time is up. I’ve got a study you’ll definitely find of interest.

The study begins with people thinking snakes are deaf, that they rely on ground vibrations and sight. Well, evidence suggests that snakes can hear despite their lack of ears, mainly via sound-induced head vibrations. How they respond to sound is unclear.

A team of researchers affiliated with The University of Queensland, the Australian Reptile Academy and Queensland University of Technology built on that evidence to document snakes’ response.

Snakes and Sounds
While their study was not the first to investigate snakes’ hearing, most earlier studies focused on one species and tested partially anesthetized snakes, snakes confined in a hanging basket, or both. The recently published study measured the likelihood and variety of behavioral responses of 19 free-moving snakes to three different sounds.

The 19 snakes were captive-bred, adult Australian snakes representing five genera (Acanthophis, Aspidites, Hoplocephalus, Oxyuranus, Pseudonaja) from six species, with different life histories and hunting strategies. They varied in size (0.35–2.2 m), morphological body shape and foraging mode (active foragers, ambush predators, arboreal species and constrictor feeders).

Species of snakes studied for behavioral response to airborne sound (modified from Fig.2, journals.plos.org/plosone/article?id=10.1371/journal.pone.0281285).

The three sound frequency ranges tested were: 0–150 Hz, which produced ground vibrations, 150–300 Hz, which did not produce ground vibrations, and 300–450 Hz, which also did not produce ground vibrations. (Human hearing is most sensitive at 2,000–5,000 Hz.)

Experimental Trials
The researchers conducted 304 trials in a 4.9 x 4.9 m sound-proof room at 27–28ºC. A 2.4 x 2.4 m testing area in the room’s center enabled free movement of even the longest snake.

Diagram of sound-proof room for testing snake response to sound (not to scale) (Fig. 1, journals.plos.org/plosone/article?id=10.1371/journal.pone.0281285).

The room had three speakers on each of two opposite sides, facing a bottomless timber box in the testing area. The three sounds were played from the speakers on one side or the other at a time. Each was calibrated to 85 dB at the room’s center. An accelerometer was placed on the floor to measure possible ground vibrations.

Prior to beginning the trials, each snake was given 5 minutes to explore the room. The trials included a 30-second settling period before the box was raised by an overhead pulley system to reveal the snake in the testing arena. The snakes were presented either a 30-second period of sound or the control, no sound. In all, each snake was exposed to four replicates of each sound and no sound.

The researchers recorded nine behaviors, including the distinct defensive and cautious behaviors: cautionary exploration, freezing, head jerks, hissing, lower jaw drops, periscoping and fixation. They converted the sum of these for each trial into a binary variable if at least one defensive/cautious behavior occurred. This binary variable was modeled as a response variable in probability models.

Nine snake behaviors recorded in testing response to sound (Table 1, journals.plos.org/plosone/article?id=10.1371/journal.pone.0281285).

Wrap Up
The study found that Australian snakes respond to airborne and groundborne sounds and that the behavioral responses differed significantly with genus as well as with sound frequency.

(A) Three snake genera’s mean probability of exhibiting defensive or cautious behavior in no-sound control trials. (B) Change in probability of exhibiting defensive or cautious behaviors from the control to three sound frequencies; values above zero indicate increase in probability, and vice versa; faded points had intervals crossing zero suggesting no change in probability from control (from Fig. 3, journals.plos.org/plosone/article?id=10.1371/journal.pone.0281285).

The researchers judge that the common perception of snakes being deaf likely derives from a combination of (1) snakes’ limited hearing ability (regarding frequency range and sensitivity) and (2) peoples’ limited ability to notice and interpret many subtle snake behaviors.

They hope their results improve the limited understanding of snake behavior and help humans deter snakes and snakebites. Thanks for stopping by.

P.S.
Study of snake response to sounds in PLOS ONE journal: journals.plos.org/plosone/article?id=10.1371/journal.pone.0281285
Article on study on EurekAlert! website: www.eurekalert.org/news-releases/979666