Did australopithecines croak?20 Sep 2006
I am especially intrigued by the detailed morphology of the hyoid bone in the throat of the fossil. Does the open space in the body of the hyoid mean that A. afarensis had air sacs in its neck? In the absence of large canines, these air sacs might have been a way in which males established a dominance hierarchy, and females judged the quality of a potential mate.
Most apes have air sacs associated with their larynx, and these were lost sometime during human evolution, because we lack them. So it wouldn't be especially surprising if australopithecines retained this ape-like character.
Hewitt, MacLarnon and Jones (2002) suggested that the loss of air sacs in human evolution was pertinent to fine control of breathing related to language:
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.
If this were true, there would really be no reason to expect australopithecines to differ from chimpanzees and gorillas in this regard. One might even imagine call sequences to be more important to them considering the possibility of higher predation (and the adaptive value of alarm calls) in more open habitats.
I don't think there is any need to infer a different function in australopithecines than in apes, based on the anatomy. Here is what Alemseged et al. wrote about it:
The hyoid of DIK-1-1 is only the second example in the hominin fossil record, and this element was previously unknown for any species earlier than Neanderthals. Its similarities with Pan and Gorilla hyoids suggest that the bulla-shaped body is the primitive condition for African apes and humans, rather than the more shallow, bar-like body shown by modern humans and Pongo. The bulla-shaped body almost certainly reflects the presence of laryngeal air sacs characteristic of African apes. However, the function of these structures is not well understood.
Fitch and Hauser (2003) wrote about ape laryngeal air sacs thusly:
A related possibility, the "accessory lung" hypothesis, is proposed here for the laryngeal air sacs of the great apes. Chimpanzees, orangutans and gorillas all have voluminous air sacs (6 liters in orangs, Schön-Ybarra 1995), that can be inflated with air from the lungs. The air sacs connect to the larynx via a long thin-walled channel that opens directly above the vocal membranes and vocal folds. The air sacs extend into the subdermal space in the pectoral region, and are overlain by the sheetlike platysma muscle. Thus, an ape could inflate the air sacs via lung pressure, and then forcibly deflate them by tensing the platysma and other pectoral muscles (or by pounding the chest, as in Gorilla). This anatomy suggests that great ape air sacs may act as "accessory lungs", providing an additional source of expiratory air flow and thus of energy into the source. This hypothesis seems more plausible than that offered by Negus (1949), who suggested that ape air sacs act as storage sites for oxygen during vigorous activity. Because the sacs are inflated with exhaled air that has already been in the lungs, and thus will be low in oxygen and high in CO2, such an air reserve would be of dubious respiratory value (Fitch and Hauser 1995) (Fitch and Hauser 2003:95).
Well, we're not looking at a 6-liter air sac for australopithecines -- but then it is odd that the orang and human hyoids should be similar considering the huge air sac in orangs and the lack of any in humans.
Aside from the problem of figuring out how apes actually use these laryngeal air sacs, I guess there's not much of a story here. But I'm beginning to appreciate the lack of surprises!
Alemseged Z, Spoor F, Kimbel WH, Bobe R, Geraads D, Reed D, Wynn JG. 2006. A juvenile early hominid skeleton from Dikika, Ethiopia. Nature 443:296-301. Abstract
Fitch WT, Hauser MD. 2003. Unpacking "honesty": vertebrate vocal production and the evolution of acoustic signals. In Acoustic Communication, edited by Simmons AM, Popper AN, Fay RR, Springer-Verlag, New York. pp. 65-137.
Hewitt G, MacLarnon A, Jones KE. 2002. The functions of laryngeal air sacs in primates: a new hypothesis. Folia Primatologica 73:70-94. DOI link
Wood B. 2006. A precious little bundle. Nature 443:278-281. Full text (free)