Scientists have figured out how koalas are able to produce a mating call that is so low-pitched it ought to come from a creature the size of an elephant.
The vocalizations are made with a previously unknown vocal organ that researchers have dubbed “velar vocal folds,” according to a study published Monday by the journal Current Biology. When stretched out, this pair of membranes can become more more than five times as long as the koala’s vocal cords. That length allows the animals to produce the low-frequency sound overheard during the breeding season. (You can listen to the decidedly un-cuddly sound here.)
An international team of researchers discovered the membranes by dissecting the larynges of 10 male koalas. At first, all they found were the usual vocal cords, whose average length was 9.8 millimeters. That size should allow the koalas to produce a sound no lower than 51.0 hertz (a frequency that’s between the lowest G and G-sharp on a piano keyboard).
But male koalas looking for some action are known to bellow at a much lower frequency that averages 27.1 Hz, about a full octave lower than the 51-Hz sound. Koalas usually weigh about 18 pounds, and a 27.1-Hz sound is “more typical of an animal the size of an elephant,” the researchers wrote.
They kept looking and came upon the longer membranes in the back of the mouth. They were “heavily wrinkled,” which suggested they could be stretched out when needed but otherwise kept in a relaxed state that would not interfere with normal breathing. Based on their shape and position, they were used by koalas to make sounds by sucking in air through their nostrils, the researchers surmised.
In their wrinkled state, the velar vocal folds measured 32.8 millimeters on average. When fully stretched, they extended to 51.1 millimeters, the researchers reported. At that length, it would be possible to produce a bellow of only 9.8 Hz, according to the study.
The folds were also heavier than the vocal cords, allowing the koalas to project their bellows farther than normal vocalizations.
To test whether these membranes really could make the low-frequency mating call, the researchers mimicked the inhalation process using three male koala cadavers. They sucked air through the pharynx and larynx and used a small video camera to film the velar vocal folds in motion.
It worked. The scientists produced 250 bellow sounds of frequencies ranging from 6 Hz to 133 Hz. It wasn’t just the math that lined up. “The sharp pulses produced by the velar vocal folds in our experiment look and sound remarkably like those produced in live animals,” the testers reported. (Click here to watch a video of the membranes in action.)
The bellowing anatomy is only the second known case of a mammal using a specialized organ to make sound without the aid of the larynx. The other case is the way toothed whales use a structure called “phonic lips” to make clicking sounds for the purpose of echolocation.
The research team wrote that female koalas have been known to produce low-frequency bellows as well, though much less often than males. Further studies should examine whether female koalas possess velar vocal folds too, they wrote.