Male koalas live separately from groups of females and are fairly sedentary, which can make mating a challenge. In addition to scents, they call to their future mate using a really deep noise that sounds like a mixture of croaking, grunting, and burping. This call has baffled researchers because the noise is about 20 times lower in pitch than the koala should be able to  produce, based on its size. After extensive study, the answer has finally been revealed: koalas have two sets of vocal cords, unlike any other land mammal. The details of this research come from Benjamin D. Charlton of the University of Sussex and were published in Current Biology.

Vocal cords are pairs of lip-shaped folded membranes that work much like guitar strings. Thinner, shorter strings produce higher sounds while lower sounds come from thicker, longer strings. As air goes through them, it produces vibrations that produce pitch based on the size of the cords. These membranes are located within the larynx, and because the size of the larynx is related to the size of the animal, large animals make deep vocalizations while small animals produce higher sounds. Because adult koalas weigh about 18 pounds (8 kilograms), it would be expected that their vocal cords would be thinner and produce much higher pitched sounds. The tones coming out of these marsupials would be more fitting if it were the size of an elephant. 

During the course of the research, the scientists dissected 10 male koalas in order to understand how that deep, resonant mating call was made. They were shocked to discover a second set of vocal cords that were much longer and heavier than the primary set. Whales have similar structures, but no other land mammal has a vocal setup like this.

The second set of vocal cords is being described as “velar cords” because they are above the larynx, by the nasal and oral cavities. This area is called the velum, but is probably better known as the soft palate. Actually, the sound made by koalas is sort of analogous to the snoring sound made by humans on the soft palate, though our vela are not actually intended to produce noise. 

Future research will seek to show if this special call actually provides clues to the female about the health and quality of the male, or if it’s just a really unique way of getting their attention.

So what kind of sound does the koala make because of the two sets of vocal cords? Check this out: