Yet when they open their mouth, it is a different matter. Koalas are loud. They bellow a loud belch-like call that seems disproportionately powerful and deep for their meagre stature. Males use these calls to boast of their sexual prowess and to try to seduce females into choosing them as a mate. It turns out that the male koala is the Barry White of the Australian outback. Though they seem docile and can sleep for up to 19 hours a day, spring has arrived Down Under and with the mating season now under way the male’s annual concert has begun again.
Why might koalas need to make such a racket? What would drive the evolution of such an unusual talent? It could be that their snort-like songs are some form of exaggeration used when competing with rival males. Rivals hearing the deep calls might think that they were about to face a giant and, in avoiding confrontation, never find out how diminutive the singer is. This advantage could, over time, favour deeper and deeper calls as each male attempted to outdo its competitors in bragging ability.
What started off as a lie eventually became an honest advertisement of the koala’s size and condition. If every male has a deep voice, then the one with the deepest is still the best. Larger males appear to have lower voices and to be able to holler for longer, so the females are probably able to gauge which males are bigger and, to them, more attractive, by the bellows alone. The tactic seems to work: the longer, louder and more guttural the song, the more groupies the baritone koala can acquire. It may explain why the males are much larger than females.
The volume could also be related to the need to be heard over a distance. Until recently it was assumed that only one prevailing alpha male had the opportunity for breeding. Research now suggests that the calls may be alluring enough to entice females out of their home range and on to the territory of other males. This could be a mechanism to avoid inbreeding and ensures that the genes of any one male do not come to dominate the population completely.
How they make these low resonating cries had been a mystery, until a recent study published in the Journal of Experimental Biology uncovered the strange workings of their vocal apparatus. It is down to their larynx, which, surprisingly, works in a manner very similar to the human voice box. The larynx is a remarkable organ involved in breathing, sound production and protecting the windpipe from filling with food. It houses the vocal cords, which are used to modulate the pitch and volume of the voice. The vocal tracts of animals, like the pipes of a musical instrument, have frequencies at which the movement of air inside naturally vibrates and amplifies sound. The longer the pipe, the lower and louder it resonates. So when it comes to making sound, size is everything – and this is why, in general, larger animals are louder animals.
But there is a way of making the vocal tract longer even in smaller animals. Like us, the koala has what is known as a descended larynx. Its position is lower in the body and it has a muscle connecting it to the sternum, which allows it to be pulled down deeper into the chest, creating a longer vocal tract: think of it like being able to alter the pitch of a trombone by moving the telescopic slider to make the instrument longer or shorter. The result is that, although the koala is barely the size of a large house cat, it sounds as big as a lion.
It had long been thought that we humans were the only animals to possess this vocal set-up, which we use for the complex machinations involved in producing speech. Then, about 10 years ago, scientists started to uncover similar sound-producing arrangements in red and fallow deer. This proved that these traits have evolved independently on several occasions.
Read more at The Telegraph
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