2. Acoustic and perceptual effects of air sacs Bart de Boer University of Amsterdam

3. The air sac Siamang Brown Howler monkey Orang Utan Humans

4. Air sac prevalence All apes have air sacs But humans don’t Why? –Speech?

5. The anatomy of air sacs “siamang” “howler monkey” (subhyoid)

6. The function of air sacs? Speculation Acoustics: –Resonators (e. g. Avril, 1963; Schön, 1971) –Impedance matchers (e.g. Fitch & Hauser, 1995) –Suppressing resonances (Haimoff, 1983) Non-acoustic –Accidental byproduct (Brandes, 1932) –Rebreathing air (Negus, 1949) –Preventing hyperventilation (Hewitt et al., 2002)

7. Why is this even important? For the evolution of speech Correlation air sacs – hyoid bone –Bones fossilize!

8. Shape correlation When an air sac is present (at least in primates) the hyoid has a cup-shaped front (the bulla) Chimpanzee (Avril 1962) Human ( http://www.anatomyatlases.org/atlasofanatomy/ plate01/08hyoidbone.shtml) Brown howler monkey

9. Fossil evidence Four fossil hyoid bones are known –Neanderthal (60 KyA) –2x H. heidelbergensis (530 KyA) –Australopithecus afarensis (3.3 MyA) Dikika baby, A. afarensis (Alamseged et al. 2006) Neanderthal (Arensburg et al 1989) H. Heidelbergensis (Martínez et al. 2008)

10. Fossil conclusion The latest common ancestor of Neanderthals and modern humans did not have air sacs Australopithecus afarensis did (+ possibility to find evidence for intermediate ancestors)

11. Implications for speech? Did air sacs disappear because of speech? But we need to be sure about their acoustic effect!

12. An acoustic model Important elements: –The neck –The cavity –The wall –Radiation Can be analyzed as an electrical circuit

13. The acoustic effect A new peak at ~the resonance of the air sac Original formants get shifted up and closer together [a] [ə][ə] [y]

14. Perception issues 1005001000150020002500300035004000 10 -5 10 0 5 Frequency (Hz) Rel. Power [y] F s1 F1F1 F s2 F2F2 F3F3 F s3 F4F4 We could measure distances in formant space –But which formants do we take? –Does the air sac resonance take over the role of F1? –And how about F2 – F2’?

15. Perception experiment Expose subjects to stimuli, and let them classify –[a]/[ə], [a]/[y], [ə]/[y], [a]/[ə]/[y] –(Two or three - unforced choice) –With and without air sac Iteratively find noise level at which performance halfway between perfect and chance

16. Example trial

17. Perception experimental results * *

18. Conclusion Apes have air sacs, humans don’t –Neanderthals don’t –Australopithecines do –Lost in evolution –Why? Change spectrum of speech –Can hear the difference less well Lost because of speech Hypothesis: –Neanderthals could speak, Australopithecines not –(proto-) speech is at least 500 000 years old

19. Analysis