AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 90:513-514 (1993)

Notes and Comments

Reply to Drs. Ravosa and Hylander

Walter Stalker Greaves Departments of Oral Biology, Anatomy and Cell Biology, and Biological Sciences, The University of Illinois at Chicago, Chicago, Illinois 60612

Ravosa and Hylander (1993) have done what you hope someone will do after you write a theoretical paper, that is, attempt to test your hypothesis. Unfortunately, they have taken an excessively narrow view by testing for only a single behavior and com- pletely ignoring all others. The result is that little has been tested. Apparently I did not make the implications of my analysis suffi- ciently clear. Perhaps this short reply will remedy that situation.

Describing more explicitly how I con- ducted the study may help to make my ideas on the symphysis a little clearer. My general approach is to find some common feature in a group of animals. The fact that all have the feature suggests to me that it must be im- portant to the animals and that an explana- tion for the feature can be developed. The fused symphysis is just such a case; I take it that it must be important because so many primates have it. At the same time however, the open or non-fused symphysis is present in almost all other mammals, suggesting that an open symphysis must be important to mammals in general.

Therefore, I was initially struck by how many mammals do not have a fused sym- physis (1988). This suggested, as a working hypothesis, that there might be something “wrong” with a fused symphysis; does a fused symphysis itself cause problems of some kind? With this idea in mind, it fol- lowed that the function of the fused symphy- sis, where it does occur, might be to solve a problem that most other mammals do not have and that it is present in primates in spite of possible disadvantages. Thus, from

this point of view, a fused symphysis should have nothing to do with resisting forces typ- ical of mastication because most mammals resist those forces without fusing the sym- physis.

One disadvantage of a fused symphysis might be that once it is fused it must be buttressed to resist the forces common to the symphyseal joint, fused or unfused. Hy- lander has shown in previous papers that the symphysis is buttressed but he did not see that as a disadvantage (i.e., a metabolic cost). But as is clear from most other mam- mals where the symphysis is not fused, ex- tra, metabolically expensive bone is not nec- essary to resist masticatory forces. Liga- ments, cartilaginous pads, inter-fingering bony processes (joined to each other with short ligaments, like teeth are to the jaw), as well as other features, apparently do a su- perb job of resisting all the forces encoun- tered by the chewing and biting jaws of most mammals, including transferring large bal- ancing-side muscle forces. Does mastication in primates require a different (and perhaps more metabolically expensive) system than is required in most other mammals? I as- sumed not and concluded that the fused symphysis in primates is necessary for something other than dealing with standard forces produced by mastication (i.e., some- thing important mainly to primates.)

Ravosa and Hylander clearly see a con- nection between mastication and symphy- seal fusion: “. . . a fused symphysis is best able to resist structural failure due to in- creased symphyseal stress resulting from increased recruitment of balancing-side jaw-muscle force during mastication (my italics).” Once fused, the symphysis has to be strong enough to resist the forces it encoun- ters. But if it does not have to be fused in order to resist masticatory forces (as is the case in most mammals), then demonstrating

Received September 1,1992; accepted December 18,1992

0 1993 WILEY-LISS, INC.

514 W.S. GREAVES

the existence of buttressing does not come to grips with what I think is the more intrigu- ing question of why the symphysis is fused in the first place.

My note (1988) offered a different function for fusion in primates: it precludes the loss of fully half of the force of the jaw muscles when the incisors bite at short inter-occlusal distances. Interestingly, while Ravosa and Hylander describe this mechanism without adverse comment, they then completely ig- nore it. Presumably they do so because they inferred that my note, in large part, referred to “incisal crushing of small hard ob- jects. . . presumably. . . mainly seeds” which they viewed as a masticatory func- tion. They then spend most of their paper discussing whether or not seeds or other small hard objects are eaten and if the inci- sors are involved. But this is a straw man. In my 1988 note where I described how I thought the system might work, I implied but did not list other possible uses of the incisors such as grooming, fighting, strip- ping bark, opening rinds, biting insects, bit- ing twigs, tearing food, eating or preparing small items in general, and other significant things primates might do with their incisors that might be equivalent to humans biting their fingernails, cutting thread or twine af- ter sewing, and removing knots from ropes-all when these teeth were not quite in contact but still were applying large bite forces.

Since Ravosa and Hylander restricted their analysis to a single behavior (biting small hard objects with the incisors) and avoided discussing my hypothesis, they, not surprisingly, found it wanting. But in my view my hypothesis has not yet been tested. We have to ask if many of the above func- tions, or any other similar function of the incisors that might be important to primates (Hiiemae and Kay, 1972) would be signifi-

cantly degraded (or not possible at all) if only half of the available bite force could be used. As examples, consider biting your own fingernail o r nipping of€ a hangnail with your incisors. I can readily imagine that these tasks would be far more difficult if only half our bite force were available.

I should also address their reference to the fossil lemurs. They point out that some of these animals had a fused symphysis but diminutive or even no upper incisors. They then state that “the evidence from subfossil Malagasy lemurs provides no convincing support for the incisor-crushing model of symphyseal fusion.” But this example is not relevant because my model specifically re- quires cases where a fused symphysis is paired with upper and lower incisors.

In short, the issue is not crushing hard objects with the incisors but rather the limi- tation of force at incisors working a t short inter-occlusal distances. I don’t think Ra- vosa and Hylander dealt with my hypothesis nor have they come to grips with why they think primates fuse their symphyses to ac- complish what most mammals do without fusion.

I hope the above has clarified this issue. Perhaps this exchange will encourage fur- ther examination by Ravosa and Hylander as well as others interested in this problem.

ACKNOWLEDGMENTS I thank M.L. Greaves for many improve-

ments in the manuscript.

LITERATURE CITED Greaves WS (1988) A functional consequence of an ossi-

fied mandibular symphysis. Am. J. Phys. Anthropol. 77:53-56.

Hiiemae KM, and Kay RF (1972) Trends in the evolution of primate mastication. Nature 240t486-487.

Ravosa MJ, and Hylander WL (1993) Functional signifi- cance of an ossified mandibular symphysis: A reply. Am. J. Phys. Anthropol. 90:509-512.