no. 3607 KINESIS OF AVIAN SKULL — ZUSI 23 



Functions of kinesis. — Bock (1964, pp. 25-31) discussed six 

 possible functions of kinesis: (1) maintaining the mandible in the 

 closed position, (2) gaping, (3) maintenance of the primary axis of 

 orientation of the bill, (4) faster closing of the jaws, (5) more wide- 

 spread and even distribution of jaw muscle attachment, and (6) 

 shock absorbing. Of these, I believe that the first is doubtful because 

 it requires the hypothesis of strict coupling of the jaws for its explana- 

 tion and because the ligament does not hold the lower jaw closed 

 (at least in Gallus domestic us) . The third and sixth are probably of 

 widespread importance — the third because many species rely on rapid 

 grasping of tiny food items or of moving prey and the last because 

 of the light construction of the jaws and palate in birds. 



I believe that a primary advantage of kinesis is that it provides 

 increased possibilities for diversity of manipulation by the jaws. 

 Although birds lack teeth, they nevertheless must perform many 

 manipulative tasks with the bill such as the capture, holding, orienta- 

 tion, manipulation, and swallowing of food, nest-building, preening, 

 defence, and other activities. Two fundamental features of skull 

 construction in birds serve, in combination with kinesis, to enhance 

 the diversity of manipulation. 



First, the tomial edges of the upper jaw are drawn forward when 

 raised and backward when lowered (fig. 126) because the flexible 

 point of attachment of the upper jaw and cranium lies above the 

 plane of the tomia. In most birds the articulation of the lower 

 jaw lies below the tomial plane and there is a similar forward and 

 backward motion of the tomium during depression and adduction 

 of the lower jaw. A food item being constricted by the jaws, there- 

 fore, is wedged backward toward the throat rather than pushed 

 forward as it would be by a straight pair of tongs (fig. 12a). Immo- 

 bility of the upper jaw would reduce considerably this effect. Un- 

 fortunately, I can say nothing about its significance for the living 

 bird although its existence can scarcely be doubted. The effect 

 is greatest in relatively short and deep bills, where it may provide 

 more effective seed-cracking forces or help to keep seeds from 

 slipping forward in the bill during nibbling. 



A second type of manipulation is made possible by the anterior 

 placement of the flexible cranial attachment of the upper jaw relative 

 to the articulation of the lower jaw (fig. 13). The upper jaw thereby 

 rotates about a shorter radius than the lower, and the tomial edges 

 can be opposed in a greater variety of ways than would be possible 

 otherwise, especially if the upper jaw can be retracted below the 

 resting position as discussed by Beecher (1951b, p. 413) and Yudin 

 (1965, p. 68). Bock (1964, pp. 23, 24) argued against such retraction 

 in a coupled skull, but I have shown here that coupling can be over- 



