American alligators Alligator mississippiensis undergo major transformations in morphology and ecology during development. These include several thousand-fold changes in body mass, modified snout and dental proportions, and shifts in diet from small, delicate foodstuffs to the inclusion of increasingly larger, more robust prey. How these changes in anatomical form contribute to actual physical performance and niche use is largely unknown. In the present study, bite-force measurements for 41 specimens of A. mississipiensis, were made throughout ontogeny (hatchling-older adults) using a series of precision force transducers. How this performance indicator scaled with respect to cranial and whole-body measurements was determined. Bite-force production throughout development was contrasted with ontogenetic changes in trophic ecology. The influences of this performance measure on these changes were then analysed. The results showed a 800-fold range (12-9452 N) of bite forces with values positively correlating with increases in body size. Scaling of biting forces through ontogeny showed positive allometry with respect to body mass, head length, jaw length, snout-vent length and total length. These patterns may be attributable to allometric growth of individual skeletal elements (and associated musculature), and/or progressive fusion and ossification of skull and jawbones during development. The overall pattern of force increase throughout ontogeny did not vary in association with major shifts in diet. Notably, the bite-force values for adult A. mississippiensis are the highest measured for any living animal and represent the first measures for a large crocodilian. Additionally, these data provide the first documentation of how bite force changes during ontogeny in a reptile. By bridging the rich morphological and ecological databases for these animals, this study opens the door to a comprehensive understanding of feeding in A. mississippiensis. Furthermore, it provides groundwork for standardized comparative studies of feeding among crocodilian, reptilian, or other gnathostome vertebrates.

BackgroundCrocodilians have dominated predatory niches at the water-land interface for over 85 million years. Like their ancestors, living species show substantial variation in their jaw proportions, dental form and body size. These differences are often assumed to reflect anatomical specialization related to feeding and niche occupation, but quantified data are scant. How these factors relate to biomechanical performance during feeding and their relevance to crocodilian evolutionary success are not known.Methodology/Principal FindingsWe measured adult bite forces and tooth pressures in all 23 extant crocodilian species and analyzed the results in ecological and phylogenetic contexts. We demonstrate that these reptiles generate the highest bite forces and tooth pressures known for any living animals. Bite forces strongly correlate with body size, and size changes are a major mechanism of feeding evolution in this group. Jaw shape demonstrates surprisingly little correlation to bite force and pressures. Bite forces can now be predicted in fossil crocodilians using the regression equations generated in this research.Conclusions/SignificanceCritical to crocodilian long-term success was the evolution of a high bite-force generating musculo-skeletal architecture. Once achieved, the relative force capacities of this system went essentially unmodified throughout subsequent diversification. Rampant changes in body size and concurrent changes in bite force served as a mechanism to allow access to differing prey types and sizes. Further access to the diversity of near-shore prey was gained primarily through changes in tooth pressure via the evolution of dental form and distributions of the teeth within the jaws. Rostral proportions changed substantially throughout crocodilian evolution, but not in correspondence with bite forces. The biomechanical and ecological ramifications of such changes need further examination.

Neuromechanics seeks to understand how muscles, sense organs, motor pattern generators, and brain interact to produce coordinated movement, not only in complex terrain but also when confronted with unexpected perturbations. Applications of neuromechanics include ameliorating human health problems (including prosthesis design and restoration of movement following brain or spinal cord injury), as well as the design, actuation and control of mobile robots. In animals, coordinated movement emerges from the interplay among descending output from the central nervous system, sensory input from body and environment, muscle dynamics, and the emergent dynamics of the whole animal. The inevitable coupling between neural information processing and the emergent mechanical behavior of animals is a central theme of neuromechanics. Fundamentally, motor control involves a series of transformations of information, from brain and spinal cord to muscles to body, and back to brain. The control problem revolves around the specific transfer functions that describe each transformation. The transfer functions depend on the rules of organization and operation that determine the dynamic behavior of each subsystem (i.e., central processing, force generation, emergent dynamics, and sensory processing). In this review, we (1) consider the contributions of muscles, (2) sensory processing, and (3) central networks to motor control, (4) provide examples to illustrate the interplay among brain, muscles, sense organs and the environment in the control of movement, and (5) describe advances in both robotics and neuromechanics that have emerged from application of biological principles in robotic design. Taken together, these studies demonstrate that (1) intrinsic properties of muscle contribute to dynamic stability and control of movement, particularly immediately after perturbations; (2) proprioceptive feedback reinforces these intrinsic self-stabilizing properties of muscle; (3) control systems must contend with inevitable time delays that can simplify or complicate control; and (4) like most animals under a variety of circumstances, some robots use a trial and error process to tune central feedforward control to emergent body dynamics.

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In territorial polygynous taxa, reproductive success reflects phenotypic variation. Using Crotaphytus collaris, a sexually dimorphic lizard in which males use the head (i.e., jaws and associated musculature) as a weapon when territorial interactions escalate to fights, we tested the hypothesis that weapon performance (i.e., bite force) is a better predictor of fitness than body or weapon size. Bite-force performance predicted the number of female home ranges overlapped, estimated mating success, and potential reproductive output. However, no body or weapon size measure correlated with these estimates of fitness, and only one weapon dimension (head width) correlated with bite force. These results indicate that weapon performance has far stronger effects on fitness than body or weapon size, likely because it directly influences fight outcomes. As such, it is desirable that the use of morphology as a proxy for performance and its presumed extensions to fitness be based on empirical morphology-performance relationships.