Architectonic constraints on the hyoid's optimal starting position for suction feeding of fish

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Abstract
During suction feeding teleost fish have to start mouth opening prior to other expansion movements of the head such as operculo-suspensorium abduction. The distribution of the input force over the various expansion movements is determined by the position of the hyoid in the expansion apparatus. Based on a three-dimensional (3-D) kinematic model of this apparatus it can be calculated at which positions of the hyoid operculo-suspensorial abduction is precluded. For 73 cichlid species from various African lakes and covering a wide array of feeding types and adult sizes, it is demonstrated that these optimal positions for the onset of suction feeding can be attained or closely approached by species whose regular diet only requires suction feeding but not by species whose regular diet is dominated by items requiring forceful biting. It is argued that the suboptimality of the biters is due to an architectonic constraint, viz. an increase in head width necessary to accommodate their enlarged m. adductor mandibulae. Although it is theoretically feasible to optimize the model's parameters for every head width, the biters apparently have not achieved such an adaptive change. As these parameters also feature in the execution of other functions, it is likely that conflicting demands on their optimal value overrule their optimization for the starting position of the hyoid of biters. The results hold for cichlids of independently evolved species flocks and therefore concern general rules for biter-sucker transformations in cichlids.