Differences in muscle mechanics underlie divergent optimality criteria between feeding and locomotor systems

Does the unusual phenomenon of sustained force circumvent the speed-endurance trade-off in the jaw muscle of the southern alligator lizard (Elgaria multicarinata)?

The jaw muscles of the southern alligator lizard, Elgaria multicarinata, are used in prolonged mate-holding behavior, and also to catch fast prey. In both males and females, these muscles exhibit an unusual type of high endurance known as sustained force in which contractile force does not return to baseline between subsequent contractions. This phenomenon is assumed to facilitate the prolonged mate-holding observed in this species. Skeletal muscle is often subject to a speed-endurance trade-off. Here, we determined the isometric twitch, tetanic and isotonic force-velocity properties of the jaw muscles at ∼24°C as metrics of contractile speed and compared these properties with a more typical thigh locomotory muscle to determine whether endurance by sustained force allows for circumvention of the speed-endurance trade-off. The specialized jaw muscle was generally slower than the more typical thigh muscle: time to peak twitch force, twitch 90% relaxation time (P<0.01), and tetanic 90% and 50% relaxation times (P<0.001) were significantly longer, and force-velocity properties were significantly slower (P<0.001) in the jaw than the thigh muscle. However, there seemed to be greater effects on relaxation rates and shortening velocity than on force rise times: there was no effect of muscle on time to peak, or 50% of tetanic force. Hence, the jaw muscle of the southern alligator lizard does not seem to circumvent the speed-endurance trade-off. However, the maintenance of force rise times despite slow relaxation, potentially enabled by the presence of hybrid fibers, may allow this muscle to meet the functional demand of prey capture.

The dual function of prokinesis in the feeding and locomotor systems of parrots

Prokinesis, a mode of avian cranial kinesis involving motion between the neurocranium and upper beak, has long been investigated in biomechanical analyses of avian feeding and drinking. However, the modern avian beak is also used for non-feeding functions. Here, we investigate the dual function of prokinesis in the feeding and locomotor systems of the rosy-faced lovebird (Agapornis roseicollis). Lovebirds and other parrots utilize their beak both during feeding and as a third limb during vertical climbing. Thus, we experimentally measured both force-generating potential and movement of the rosy-faced lovebird mandible and maxilla (via prokinetic flexion of the craniofacial hinge) during tripedal climbing and mandibular/maxillary adduction. We found that whereas the maxilla is primarily responsible for generating force during locomotion, the mandible is primarily responsible for generating force during forceful jaw adduction, hinting at a remarkable capacity to alter prokinetic function with differing neuromuscular control. The ability of the prokinetic apparatus to perform functions with competing optimality criteria via modulation of motor control illustrates the functional plasticity of the avian cranial kinesis and sheds new light on the adaptive significance of cranial mobility.

Do salamanders chew? An X-ray reconstruction of moving morphology analysis of ambystomatid intraoral feeding behaviours

Chewing is widespread across vertebrates, including mammals, lepidosaurs, and ray-finned and cartilaginous fishes, yet common wisdom about one group-amphibians-is that they swallow food whole, without processing. Earlier salamander studies lacked analyses of internal kinematics of the tongue, analyses of muscle function, and sampled few individuals, which may have caused erroneous conclusions. Specifically, without tongue and food kinematics, intraoral behaviours are difficult to disambiguate. We hypothesized that ambystomatid salamanders use diverse intraoral behaviours, including chewing, and tested this hypothesis with biplanar videofluoroscopy, X-ray reconstruction of moving morphology, and fluoromicrometry. We generated musculoskeletal kinematic profiles for intraoral behaviours in Axolotls (Ambystoma mexicanum), including three-dimensional skeletal kinematics associated with feeding, for gape, cranial and pectoral girdle rotations, and tongue translations. We also measured muscle fibre and muscle-tendon unit strains for six muscles involved in generating skull, jaw and tongue kinematics (adductor mandibulae, depressor mandibulae, geniohyoid, sternohyoid, epaxialis and hypaxialis). A principal component analysis recovered statistically significant differences between behaviour cycles, classified based on food movements as either chewing or transport. Thus, our data suggest that ambystomatid salamanders use a previously unrecognized diversity of intraoral behaviours, including chewing. Combined with existing knowledge, our data suggest that chewing is a basal trait for tetrapods and jaw-bearing vertebrates. This article is part of the theme issue 'Food processing and nutritional assimilation in animals'.

Energetic costs of feeding in 12 species of small-bodied primates

There are no comparative, empirical studies of the energetic costs of feeding in mammals. As a result, we lack physiological data to better understand the selection pressures on the mammalian feeding apparatus and the influence of variables such as food geometric and material properties. This study investigates interspecific scaling of the net energetic costs of feeding in relation to body size, jaw-adductor muscle mass and food properties in a sample of 12 non-human primate species ranging in size from 0.08 to 4.2 kg. Net energetic costs during feeding were measured by indirect calorimetry for a variety of pre-cut and whole raw foods varying in geometric and material properties. Net feeding costs were determined in two ways: by subtracting either the initial metabolic rate prior to feeding or subtracting the postprandial metabolic rate. Interspecific scaling relationships were evaluated using pGLS and OLS regression. Net feeding costs scale negatively relative to both body mass and jaw-adductor mass. Large animals incur relatively lower feeding costs indicating that small and large animals experience and solve mechanical challenges in relation to energetics in different ways. This article is part of the theme issue 'Food processing and nutritional assimilation in animals'.

Got rhythm? Rhythmicity differences reflect different optimality criteria in feeding and locomotor systems

Evolutionary analyses of joint kinematics and muscle mechanics suggest that, during cyclic behaviors, tetrapod feeding systems are optimized for precise application of forces over small displacements during chewing, whereas locomotor systems are more optimized for large and rapid joint excursions during walking and running. If this hypothesis is correct, then it stands to reason that other biomechanical variables in the feeding and locomotor systems should also reflect these divergent functions. We compared rhythmicity of cyclic jaw and limb movements in feeding and locomotor systems in 261 tetrapod species in a phylogenetic context. Accounting for potential confounding variables, our analyses reveal higher rhythmicity of cyclic movements of the limbs than of the jaw. Higher rhythmicity in the locomotor system corroborates a hypothesis of stronger optimization for energetic efficiency: deviation from the limbs' natural frequency results in greater variability of center of mass movements and limb inertial changes, and therefore more work by limb muscles. Relatively lower rhythmicity in the feeding system may be a consequence of the necessity to prevent tooth breakage and wear, the greater complexity of coordination with tongue movements, and/or a greater emphasis on energy storage in elastic elements rather than the kinetics of limb movement.

The effects of skull flattening on suchian jaw muscle evolution

Jaw muscles are key features of the vertebrate feeding apparatus. The jaw musculature is housed in the skull whose morphology reflects a compromise between multiple functions, including feeding, housing sensory structures, and defense, and the skull constrains jaw muscle geometry. Thus, jaw muscle anatomy may be suboptimally oriented for the production of bite force. Crocodylians are a group of vertebrates that generate the highest bite forces ever measured with a flat skull suited to their aquatic ambush predatory style. However, basal members of the crocodylian line (e.g., Prestosuchus) were terrestrial predators with plesiomorphically tall skulls, and thus the origin of modern crocodylians involved a substantial reorganization of the feeding apparatus and its jaw muscles. Here, we reconstruct jaw muscles across a phylogenetic range of crocodylians and fossil suchians to investigate the impact of skull flattening on muscle anatomy. We used imaging data to create 3D models of extant and fossil suchians that demonstrate the evolution of the crocodylian skull, using osteological correlates to reconstruct muscle attachment sites. We found that jaw muscle anatomy in early fossil suchians reflected the ancestral archosaur condition but experienced progressive shifts in the lineage leading to Metasuchia. In early fossil suchians, musculus adductor mandibulae posterior and musculus pterygoideus (mPT) were of comparable size, but by Metasuchia, the jaw musculature is dominated by mPT. As predicted, we found that taxa with flatter skulls have less efficient muscle orientations for the production of high bite force. This study highlights the diversity and evolution of jaw muscles in one of the great transformations in vertebrate evolution.