Furry Friends and their Lullaby

 

The weather got cold, and I missed my furry friends, cats again. We went to a local cat café to meet furry cuties.

And to talk about cat, purring is the most fascinating thing. I remember especially in cold nights; purring is the best lullaby I have ever heard, a simple low frequency vibration with repeating beat. Although you can always hear such delightful sounds whenever you stroke your furry friends, the mechanism of purring has been surrounded in the mystery. Here, I want to share the efforts of scientists conducted interesting study to crack secrets of purring.

According to these studies, our feline friends have pads embedded within their vocal cords, which add an extra layer of fatty tissue that allows them to vibrate at low frequencies. The article was published in ‘Current Biology’. Larynx or voice box (Hereafter I use voice box because this is easier word for laypersons) of cats doesn’t need any input from the brain to produce purr.

Scientists involved this research said that their study was very first step to scientifically explain such phenomena because until now many theories were devised but they were not tested at all. Scientists have been puzzled because domestic cats were small, but still can produce the low frequency vocalization with 20~30 Hz frequency band. As a matter of fact, such low frequencies are usually only observed in much larger animals like elephants, which have longer vocal cords.

Now it is well known that most mammal vocalizations including cat noises such as meowing and hissing are produced similar way: a signal from the brain causes the vocal cords to press together, and the flow of air through the voice box causes the cord to knock against each other hundreds of times per second, producing sound. This process is called ‘flow induced self-sustained oscillation and is passive phenomena. Once the vocal cords start to vibrate, no further neural input is required to keep them going.

In the 1970s, some scientists proposed that purring of cat was different. They believed that purring is sort of ‘active muscle contraction’ meaning that domestic cats actively contract and relax their voice box muscles about 30 times per second to purr. The idea, based on measurements of electrical activity in the voice box muscles in purring cat and has been common explanation for cat purring ever since.

The new study challenges this hypothesis. To conduct the work, scientists participated the research removed the voice boxes from eight domestic cats that had been euthanized because of terminal disease and were investigated with full consent of their owners. The scientists pinched the vocal cords together and pumped warm, humidified air through them. By isolating the voice box this way to guarantee any sound produced was occurring without muscle contractions or any input from the brain.

The team was able to produce purring in all the voice boxes and they were surprised. From this study, they found out that all eight voice boxes produced self-sustained oscillation at frequency between 25 and 30 Hz without any active neural control.

However, in my opinion, this is the experiment using vocal organs taken out from the donated cats and real mechanism of purring taking place in living cats could be different. But it is an interesting result providing some hints of how our feline friends can sing peaceful and soothing songs for us.