Szent Istvan University, Wildlife Ecology 
2006-05-14 

Caroline Lillrud, Sofia Nälgård

Thierry Aubin, Pierre Jouventin and Christophe Hildebrand. Penguins use the two-voice system to recognize each other.

 

We have read an article about two-voice communication system concerning penguins.

The sound producing structure in birds is the syrinx, located at the junction of the bronchi. This is a two part organ, the two branches producing sound independently which means that many birds have two acustic sources. In song-birds the sounds are not activated simultaneoulsy whereas in penguins the sound is beating simultaneously for identification of each other. This is of high importance since the penguin is a non nesting bird.

A study has bean made regarding the emperor penguin (Aptenodites forsteri), whick have no attachment zone and walks freely on the ice and is thereby dependent of individual recognition. Their display-call consist of “syllables” which is highly stereotyped within the individual, and highly variable between the individuals, and allows individual recognition and sexing. The two syringeal structures are in contrast to many singing birds activated simultaneously, equals two-vocie signal.

Some playback experiments were performed to investigate the coding system. This was done by altering or removing one of the two voices, with the help of a special tape-recorder with frequency with at range from 30-20000 Hz. The experiment involved 32 adults and their respective mates or chicks. They used three sets of signals at 15 minutes intervalls regarding adults during incubation and chicks during rearing. Each set was two separate signals with three seconds of separation. First signal corresponded to natural display call of the mate of the adult or the parent to the chick. Rather than using a natural call they constructed a synthetic signal which allowed a tight control over the structure of the signal. The natural call frequencies were 350 Hz and 450 Hz.

In the measurment of 23 emperor penguins, the lower voice frequency was estimated to 371 Hz, and the upper voice frequency was 432 Hz, which was very close to their measurments.  The control signal was played out among the adults and chicks, and all of the adults and chicks called in reply. The lower frequecy did not have such strong effect on the adults. But still all except two replied. Regarding the chicks, all called in reply to the lower frequency sound.

When one voice was experimentally surpresses, neither adults nor chicks made a responce. The hign frequency calls is highly dependent on surroundings. High frequencies cannot propagate  more than a few meters in a dense population, which means that the pengiuns more effectively relies on the low frequcncy channels.

But still the two frequencies in interaction generates a beat which creates individual identity, which allows recognition of partners, mates or parents.

The emperor penguing is not the only species using the two- voice system, but it is not as  explorated in the other species.