Snakes' Defensive Farts
When threatened, many snakes produce clear warning sounds. Some hiss, others rattle. But place a Sonoran Coral Snake in the same situation, and the best it can do is fart.
'Cloacal popping', as it is referred to in polite scientific circles, occurs in two rare and relatively small North American burrowing snakes--the Sonoran Coral Snake (Micruroides euryxanthus) and the Western Hook-nosed Snake (Gyalopion canum)--and was recently studied by Bruce Young and colleagues (Lafayette College, Pennsylvania).
Each pop lasts less than two-tenths of a second, may be repeated several times, and can be heard from up to two metres away. The pops sound very much like human farts, although slightly higher in pitch. When Young plays recordings of cloacal pops to his students, he has to swear that it is the snakes they are listening to, and not him!
The Sonoran Coral Snake produces fairly consistent, evenly spaced pops of low amplitude (about 50 decibels) and limited range in pitch or frequency (442-5523 Hz). The Western Hook-nosed Snake, however, produces more variable pops, initially quite loud (about 70 decibels) with a broad frequency range (359-15,178 Hz), but then tapering off to 50 decibels and narrowing in pitch.
To make the sound, the snakes contract their cloacal sphincter, forcing air (and any other material that happens to be there) out. Multiple pops are created by relaxing the sphincter, drawing air in, and contracting again. The different acoustic properties of the two species can be explained by the different sizes of their cloacae and sphincters (small cloaca and large sphincter in the Western Hook-nosed Snake; and vice versa for the Sonoran Coral Snake). A large sphincter, for example, would produce initially loud noises but tire quickly, leading to the Western Hook-nosed Snake's diminuendo.
Young and colleagues point out that the only requirements for cloacal popping seem to be a cloaca and associated musculature (plus, of course, the inclination!). All snakes have the physical features, and the researchers therefore believe the behaviour may be more common than has been reported. They also note that those snakes known to fart in the face of adversity occupy the same geographic range, and suggest the behaviour evolved in response to a common predator.
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