The functions of laryngeal air sacs in primates: a new hypothesis.
A possible function of laryngeal air sacs in apes and gibbons was investigated by examining the relationships between air sac distribution, call rate, call duration and body weight in a phylogenetic context. The results suggest that lack of sacs in the smaller gibbons and in humans is a derived feature. Call parameters in primates, such as rate and duration, scaled to resting breathing rate (and therefore to body weight) only in species without air sacs, which appear to modify these relationships. Apes and larger gibbons may be able to produce fast extended call sequences without the risk of hyperventilating because they can re-breathe exhaled air from their air sacs. Humans may have lost air sacs during their evolutionary history because they are able to modify their speech breathing patterns and so reduce any tendency to hyperventilate.