no 3607 KINESIS OF AVIAN SKULL — ZUSI 25 



quadrate articulation. It thus appears that retraction of the upper 

 jaw beyond the resting position is one of the capabilities of avian 

 kinesis. Its importance lies in the potential for increased diversity 

 of manipulation by the bird's beak. 



In studying the functional significance of the palatine process of 

 the premaxilla, Bock (1960) postulated that there are two methods of 

 seed-cracking used by heavy-billed finches. The first he called "the 

 nutcracker method," characterized by a kinetic upper jaw and equal 

 development of the adductors of the mandible and the retractors of 

 the palate. He ascribed this method to the emberizine finches and 

 richmondenine finches. By contrast he claimed that the cardueline 

 finches employ "the vise method," which he describes (p. 427) as 

 follows : 



In the specialized carduelines, the upper jaw has lost its mobility; it is a nearly 

 stationary block against which the mandible presses. Heavy bosses of bone 

 (the lateral flanges) and rhamphotheca distribute the shocks associated with the 

 cracking of the seed evenly to all parts of the braincase. Only the adductor 

 muscles are well developed in the cardueline finches; in fact, the muscles associated 

 only with the movement of the upper jaw have become small and are on the 

 verge of becoming functionless. 



We have seen that the evening grosbeak — one of the heavy-billed 

 cardueline finches — has a kinetic upper jaw that operates even in the 

 absence of contraction by its prime protractors, the M. protractor 

 quad rati et pterygoidei. In order to exert a seed-cracking force, the 

 upper jaw must be moved in opposition to the adduction of the lower 

 jaw or held stationary while force is exerted by both jaws, requiring 

 in either case a powerful M. pterygoideus. It thus seems doubtful that 

 the vice and nutcracker methods of jaw action are of general validity 

 although certain species may have become specialized in one direction 

 or the other (e.g., Coccothraustes [Sims, 19551). Kinesis may be im- 

 portant to seed-cracking forms for manipulation of seeds and for the 

 application of biting forces from different angles as explained above. 



The role of the depressor mandibulae in avian kinesis can at present 

 only be inferred from a knowledge of its functional-anatomical prop- 

 erties. My findings suggest that through linkages of various kinds 

 the depressor mandibulae may serve to open both jaws simultaneously 

 when strong protraction is not required, whether or not a postorbital 

 ligament is present. In those species with a strong postorbital liga- 

 ment and/or interlocking condyles of the jaw articulation and espe- 

 cially in those in which the muscle angles far forward from its insertion 

 and in which the retroarticular process of the mandible is well de- 

 veloped, the depressor may have a more precise and powerful effect 

 in opening both jaws. Further investigations of these problems 

 through comparative morphology, physiology, and behavior may be 



