COMMUNICATION AND SIGNALING: PENGUINS USE THE TWO- VOICE SYSTEM TO RECOGNIZE EACH OTHER Spencer Hildie Sara Wang

Why study the emperor penguin?  The genus Aptenodytes is comprised of the emperor penguin and the king penguin  Both are non-nesting species, carrying their egg on their feet  Consequently, penguins cannot use landmarks to find their mates or parents; in addition, experiments have demonstrated that penguins cannot visually identify their partner  Penguins must identify the call of their mates or parents against the louder background noise of the colony (“the cocktail party effect”)  This makes the genus ideal to study individual acoustic identification

A brief explanation of terms  Amplitude modulation: works by varying the strength of the transmitted signal in relation to the information being sent (i.e. adjusting “loudness”/ amplitude of the wave)  Beat: an interference between two sounds of slightly different frequencies, perceived as periodic variations in volume whose rate is the difference between the two frequencies

Vocalizations of the emperor penguin  Emperor penguins activate both branches of the syrinx at the same time, unlike many other birds, which creates a beat pattern which varies between individuals

The sounds of the emperor penguin  Emperor penguin chick and adult Emperor penguin chick and adult  The call is highly stereotyped within individuals and highly variable between individuals: the temporal pattern of silences and syllables allow for recognition and sexing  The call is comprised of two simultaneous series of harmonically related bands of slightly different frequencies, a “two-voice” signal, creating a beat  What is the function of this double-frequency structure?

Methods  Recording and playback material  Display calls of penguins were recorded  Experimental signals were broadcast; for propagation tests, the signals were rerecorded  Sound synthesis and analysis  Recordings were digitized to measure the beat values accurately

Methods: experiments  Playback  3 sets of signals (control, low-pass two voices, low-pass one voice; all natural) played back at 15 minute intervals to marked adults during incubation and to marked chicks during rearing  Suppression of one of the two voices was done in equal proportions for each bird (upper or lower)  Upper frequencies had to be removed since two voices were difficult to distinguish and separate  Temporal pattern preserved  Order of presentation randomized  Recording was played at a natural sound pressure at 7 m, the natural calling distance of an adult  To evaluate the intensity of the response, researchers distinguished responses (calls in response to signal and non-responses (no reaction)

Methods: experiments  Propagation  Constructed synthetic signals, which allowed them to exert tight control over the structure of the signal and thus measure only the variables they wished to analyze  Synthesized an amplitude modulation generated by the beats of two frequencies (350 Hz and 450 Hz); these values correspond to those of the fundamental frequencies of a natural call  Synthesized corresponding true amplitude modulation with a carrier frequency of 400 Hz  Signals were 5 seconds each, broadcast in the center of a colony with a normal density of birds on a day without wind; relative positions of speaker and microphone were chosen to simulate adult-chick or male-female search situations

Results: playback with 1 or 2 voices Little difference between adults and chicks Control = response Low frequency = response Low frequency, one voice suppressed = no response Therefore, 2 voice system necessary for distinguishing individuals over a distance

Results: propagation of beats  Beats: Mean period duration remains constant  Amplitude modulation: Decreases as distance increases  Use period duration to recognize individuals  Therefore, 2 voice system more efficient over long distances

Results: propagation of beats a) True amp. mod. at 1 m without intervening bodies. b) Amp. mod. generated by beats of 350 and 450 Hz. c) True amp. mod. at 8m after propagation through penguin colony. d) Amp. mod. generated by beats after propagation.

Discussion: the 2-coding system  Amplitude modulation: syllables and silences  Interaction of complex sound with harmonics  Fast  Conveys position and identity  Temporal pattern  Beat modulation  Beats  Interaction of 2-voice system  Slower  Conveys identity  Carries over distance and through bodies

Final Thoughts  Sharp amplitude variations due to silence-syllable succession and beats generated by low frequencies propagate well and resist degradation  Evolutionary advantage in  Noisy, crowded environments  No landmarks  No nests  The two-voice system recognition is not limited to one species. Further research: king penguins