Bite Performance Research Articles

Divergent Skull Morphology Supports Two Trophic Specializations in Otters (Lutrinae).

Variation in terrestrial mammalian skull morphology is known to constrain feeding performance, which in turn influences dietary habits and ultimately fitness. Among mustelids, otters have evolved two feeding specializations: underwater raptorial capture of prey (mouth-oriented) and capture of prey by hand (hand-oriented), both of which have likely associations with morphology and bite performance. However, feeding biomechanics and performance data for otters are sparse. The first goal of this study was to investigate the relationships between feeding morphology and bite performance among two mouth-oriented piscivores (Pteronura brasiliensis and Lontra canadensis) and two hand-oriented invertebrate specialists (Enhydra lutris and Aonyx cinerea). Since other vertebrate taxa that are mouth-oriented piscivores tend to possess longer skulls and mandibles, with jaws designed for increased velocity at the expense of biting capability, we hypothesized that mouth-oriented otters would also possess long, narrow skulls indicative of high velocity jaws. Conversely, hand-oriented otters were expected to possess short, blunt skulls with adaptations to increase bite force and crushing capability. Concomitant with these skull shapes we hypothesized that sea otters would possess a greater mandibular bluntness index, providing for a greater mechanical advantage compared to other otter species investigated. A second goal was to examine morphological variation at a finer scale by assessing variation in cranial morphology among three sea otter subspecies. Since diet varies among these subspecies, and their populations are isolated, we hypothesized that the magnitude of mandibular bluntness and concomitant mechanical advantage, as well as occlusal surface area would also vary within species according to their primary food source (fish versus hard invertebrates). Functional expectations were met for comparisons among and within species. Among species the phylogeny suggests a deeply rooted transition to alternative foraging types. Yet within foraging types alternative species were also strongly variable, suggesting either selective differences in the extent or nature of realized foraging mode, or an accumulation of non-adaptive changes during the long independent evolutionary history. At the finest scale, variation among subspecies indicates that trophic adaptation occurred rapidly, making it interesting that we happened to find both deeply and shallowly-rooted transformations associated with diet type in otter species and subspecies.

Ecological explanations to island gigantism: dietary niche divergence, predation, and size in an endemic lizard.

Although rapid evolution of body size on islands has long been known, the ecological mechanisms behind this island phenomenon remain poorly understood. Diet is an important selective pressure for morphological divergence. Here we investigate if selection for novel diets has contributed to the multiple independent cases of island gigantism in the Skyros wall lizard (Podarcis gaigeae) and if diet, predation, or both factors best explain island gigantism. We combined data on body size, shape, bite force, and realized and available diets to address this. Several lines of evidence suggest that diet has contributed to the island gigantism. The larger islet lizards have relatively wider heads and higher bite performance in relation to mainland lizards than would be expected from size differences alone. The proportions of consumed and available hard prey are higher on islets than mainland localities, and lizard body size is significantly correlated with the proportion of hard prey. Furthermore, the main axis of divergence in head shape is significantly correlated with dietary divergence. Finally, a model with only diet and one including diet and predation regime explain body size divergence equally well. Our results suggest that diet is an important ecological factor behind insular body size divergence, but could be consistent with an additional role for predation.

Feeding biomechanics of the cownose ray, Rhinoptera bonasus, over ontogeny.

Growth affects the performance of structure, so the pattern of growth must influence the role of a structure and an organism. Because animal performance is linked to morphological specialization, ontogenetic change in size may influence an organism's biological role. High bite force generation is presumably selected for in durophagous taxa. Therefore, these animals provide an excellent study system for investigating biomechanical consequences of growth on performance. An ontogenetic series of 27 cownose rays (Rhinoptera bonasus) were dissected in order to develop a biomechanical model of the feeding mechanism, which was then compared with bite forces measured from live rays. Mechanical advantage of the feeding apparatus was generally conserved throughout ontogeny, while an increase in the mass and cross-sectional area of the jaw adductors resulted in allometric gains in bite force generation. Of primary importance to forceful biting in this taxon is the use of a fibrocartilaginous tendon associated with the insertion of the primary jaw adductor division. This tendon may serve to redirect muscle forces anteriorly, transmitting them within the plane of biting. Measured bite forces obtained through electrostimulation of the jaw adductors in live rays were higher than predicted, possibly due to differences in specific tension of actual batoid muscle and that used in the model. Mass-specific bite forces in these rays are the highest recorded for elasmobranchs. Cownose rays exemplify a species that, through allometric growth of bite performance and morphological novelties, have expanded their ecological performance over ontogeny.

Differential growth of naturally and sexually selected traits in an Anolis lizard

AbstractSexual dimorphism may be indicative of differential natural and/or sexual selection pressures acting on males and females. Although similar degrees of adult sexual dimorphism may be observed among populations and species, underlying physiological, developmental and ecological processes driving growth during the juvenile stages may vary. Unraveling how and when differences arise during ontogeny is important to our understanding of the evolutionary patterns observed among adults. In addition, growth patterns may differ among traits within a single individual. Anolis lizards vary greatly in the degree of adult sexual size and shape dimorphism. Ultimate hypotheses (e.g. sexual selection, habitat partitioning) have been put forward to explain this variation, but proximate mechanisms operating during the juvenile stages remain largely unknown. Herein, we aim at elucidating the proximate mechanisms of sexual dimorphism in head and limb shape, performance and dewlap size in Anolis baracoae, a crown‐giant anole. We used a longitudinal approach and raised 23 captive‐bred juvenile A. baracoae lizards through adulthood. By quantifying body size, head and limb dimensions, dewlap size and bite performance, we obtained growth trajectories that were compared between the sexes and among traits. Our results show that the growth of head and limb shape is similar in males and females, whereas dewlap size and bite performance diverge at a specific age. Based on these results, we hypothesize that, whereas selection favors a similar head and limb shape in males and females, selection pressures on dewlap size and bite performance likely differ between the sexes. In addition, dewlap size starts diverging around the time the animals reach sexual maturity, whereas bite performance diverges much earlier in (sexually) immature males.

Morphological integration and functional modularity in the crocodilian skull.

We explored the morphological organization of the skull within Crocodylidae, analyzing functional and phylogenetic interactions between its 2 constituent functional modules: the rostrum and the postrostrum. We used geometric morphometrics to identify localized shape changes, focusing on the differences between the major clades of the crown-group Crocodylia: Alligatoridae and Crocodylidae. We used published bite performance data to correlate rostral function with postrostral morphology. The skull modules appear more integrated within Alligatoridae than within Crocodyliade. Phylogenetic effects on shape variation are more evident in Alligatoridae than in Crocodylidae, where functional parameters concerning the rostral morphology are proportionally more important than phylogeny. Long-snouted species are characterized by low structural performance, which is significantly associated with a reduction of the pterygoid-quadrate cranial nipper, suggesting that the nipper is important for the ingestion of large food items in generalist species. This functional association is coupled with a significant evolutionary allometry at the clade level, while Alligatoridae and Crocodylidae show different degrees of evolutionary allometry for their entire shape and rostrum. The postrostrum is more conservative than the rostrum in terms of morphospace occupation, evolutionary allometry and phylogenetic signal.

The anatomy of a shrike bite: bill shape and bite performance in Loggerhead Shrikes

We examined the relationship between bill morphology and bite performance in Loggerhead Shrikes (Lanius ludovicianus), small passerines with raptorial bills. Shrikes feed on arthropods and vertebrates, and our aim was to understand how upper bill shape and jaw performance are integrated to meet the demands of their phenotypically diverse prey. We used geometric morphometrics to quantify variation in the shape of the upper bill, which houses the hooked tip and tomial ‘teeth,’ and took measurements of bite force and pressure (force per unit bill contact area) of wild-caught Loggerhead Shrikes throughout California, USA. We expected that longer bill hooks and tomial teeth would covary positively with bite force, because both hooks and teeth, as well as force, ought to be important for capturing, subduing, and dispatching vertebrate prey. Shrikes, however, with relatively longer, narrower culmens and longer hook tips produced lower bite forces than those with relatively stouter culmens and shorter hook tips. There was no significant relationship, however, between upper bill shape and bite pressure. Our results suggest that shrikes may achieve functional equivalence in bite pressure (force per unit of bill contact area) through variation in the amount of force applied by bills of different shapes. In this way, bill shape may be decoupled from force production, but without any direct costs to overall bite performance. This decoupling, in turn, may provide shrikes with greater phenotypic flexibility to meet the varied demands of their arthropod and vertebrate prey. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 112, 485–498.

Sexual Dimorphism in Bite Performance Drives Morphological Variation in Chameleons

Phenotypic performance in different environments is central to understanding the evolutionary and ecological processes that drive adaptive divergence and, ultimately, speciation. Because habitat structure can affect an animal’s foraging behaviour, anti-predator defences, and communication behaviour, it can influence both natural and sexual selection pressures. These selective pressures, in turn, act upon morphological traits to maximize an animal’s performance. For performance traits involved in both social and ecological activities, such as bite force, natural and sexual selection often interact in complex ways, providing an opportunity to understand the adaptive significance of morphological variation with respect to habitat. Dwarf chameleons within the Bradypodion melanocephalum-Bradypodion thamnobates species complex have multiple phenotypic forms, each with a specific head morphology that could reflect its use of either open- or closed-canopy habitats. To determine whether these morphological differences represent adaptations to their habitats, we tested for differences in both absolute and relative bite performance. Only absolute differences were found between forms, with the closed-canopy forms biting harder than their open-canopy counterparts. In contrast, sexual dimorphism was found for both absolute and relative bite force, but the relative differences were limited to the closed-canopy forms. These results indicate that both natural and sexual selection are acting within both habitat types, but to varying degrees. Sexual selection seems to be the predominant force within the closed-canopy habitats, which are more protected from aerial predators, enabling chameleons to invest more in ornamentation for communication. In contrast, natural selection is likely to be the predominant force in the open-canopy habitats, inhibiting the development of conspicuous secondary sexual characteristics and, ultimately, enforcing their overall diminutive body size and constraining performance.

Frequency domain response estimation of underwater acoustic channel and noise estimation for single-carrier frequency domain equalization

Single-carrier frequency domain equalization (SC-FDE) has drawn increasing researches because of its lower computational complexity and better performance of mitigating intersymbol interference (ISI) as compared with the time domain equalization. The practical SC-FDE systems, however, require the knowledge of the channel frequency domain response and the noise power to calculate filter coefficients of the frequency domain equalizer. Improper channel and noise estimations have been shown to result in error floor at high signal-to-noise ratio (SNR), which may impede the utilization of SC-FDE in the high-rate underwater acoustic (UWA) communications. In this paper, we discuss the possible influence of inaccurate noise estimation on the error floor at high SNR, and propose a joint estimation scheme for the channel time domain and frequency domain responses and also the noise power based on Chu sequence. The channel energy concentration region is determined by the threshold and the sparse channel impulse response (CIR) of UWA channels, used in denoising process of the CIR and the noise power computation. The bite error ratio performance of the SC-FDE variation versus the threshold is simulated via the UWA models, and accurate noise power estimation is shown to be desirable to improve the SC-FDE performance. Numerical examples also indicate that our joint estimation scheme can eliminate the error floor and improve the performance of SC-FDE at high SNR.

Scaling of bite performance with head and carapace morphometrics in green turtles (Chelonia mydas)

Adult green turtles (Chelonia mydas) are unusual relative to other sea turtles in that they are predominately herbivorous. This herbivorous diet is reflected in the serrated morphology of their beak, bite performance and the relative morphometrics of their heads. Recent bite performance data in loggerhead turtles (Caretta caretta), which are known for their durophagous capability, have demonstrated that bite force is correlated with head morphometrics. The objective of this study was to characterize bite force in green turtles and correlate bite performance with head and carapace morphometrics. We predicted that maximum bite force in green turtles would be less in magnitude relative to loggerheads, but would be positively correlated with head morphometrics. Therefore, mass, straight carapace length, straight carapace width, greatest head width, height, and length were collected with bite force from free-ranging green turtles from Punta Abreojos, Baja California Sur, Mexico and Otsuchi, Japan. Subjects ranged from 10.9 to 48kg, with straight carapace length and width ranged from 40.6cm to 71.9cm (mean=56.3±8.5cm) and from 33.5cm to 55.9cm (mean=44.8±5.7cm), respectively. A bite force apparatus was used to collect bite performance from subjects. The maximum bite force was 303N. Mean head width, head height, and head length were 8.7±1.2, 8.2±1.1, and 11.5±1.5cm, respectively. Bite force was lower in magnitude than reported for loggerhead turtles as predicted but still strong enough to process algal and plant matter, as well as crush many hard prey items. Simultaneous measurements of body and head size, and the use of non-linear reduced major axis regression, show that bite force scaled isometrically relative to body size and head size. Simple correlation showed that all logged transformed morphometrics were good predictors of logged bite performance, but an AICc-based weighted regression showed that body mass, followed by head width and head height, were better predictors of bite force than carapace size.

Biting Performance and Skull Biomechanics of a Chisel Tooth Digging Rodent (Ctenomys tuconax; Caviomorpha; Octodontoidea)

Biting performance is a key factor in vertebrate groups possessing particular food habits. In subterranean rodents that use the incisors as a digging tool, apart from requirements related to gnawing abrasive diets, the force exerted at the incisors tips must be sufficient to break down soils that are often exceedingly compact. The subterranean genus Ctenomys diversified in the southern portion of South America closely associated with the relatively open environments that characterize that region. This genus is considered a "claw and chisel tooth digger," that is, during the excavation of their galleries, the animals break down the soil with both the fore-claws and the incisors. We report here measurements of in vivo bite force in one of the largest species of the genus, C. tuconax, which occupies highland grasslands with compacted soils. We document the combined use of claws and incisors observed under field conditions, also providing measurements of soil compaction in the habitat occupied by this species. We report estimates of bite force at the level of the incisors and cheek teeth calculated from the physiological cross-sectional area of jaw muscles. To this aim, anatomical and biomechanical analyses of the mandibular apparatus were performed in preserved specimens. We found that C. tuconax bites with a higher force than expected for a mammal of its size. To assess anatomical correlates of biting performance, the morphology of the skull and jaw, and incisor second moment of area were compared with those of other caviomorph rodents with different lifestyle.

Cranial myology and bite force performance of Erlikosaurus andrewsi: a novel approach for digital muscle reconstructions

The estimation of bite force and bite performance in fossil and extinct animals is a challenging subject in palaeontology and is highly dependent on the reconstruction of the cranial myology. Furthermore, the morphology and arrangement of the adductor muscles considerably affect feeding processes and mastication and thus also have important dietary and ecological ramifications. However, in the past, the reconstruction of the (cranial) muscles was restricted to the identification of muscle attachment sites or simplified computer models. This study presents a detailed reconstruction of the adductor musculature of the Cretaceous therizinosaur Erlikosaurus andrewsi based on a stepwise and iterative approach. The detailed, three-dimensional models of the individual muscles allow for more accurate measurements of the muscle properties (length, cross-section, attachment angle and volume), from which muscle and bite force estimates are calculated. Bite force estimations are found to be the lowest at the tip of the snout (43-65 N) and respectively higher at the first (59-88 N) and last tooth (90-134 N) position. Nevertheless, bite forces are comparatively low for E. andrewsi, both in actual numbers as well as in comparison with other theropod dinosaurs. The results further indicate that the low bite performance was mainly used for leaf-stripping and plant cropping, rather than active mastication or chewing processes. Muscle and thus bite force in E. andrewsi (and most likely all therizinosaurs) is considerably constrained by the cranial anatomy and declines in derived taxa of this clade. This trend is reflected in the changes of dietary preferences from carnivory to herbivory in therizinosaurs.

Estimating maximum bite performance in Tyrannosaurus rex using multi-body dynamics

Bite mechanics and feeding behaviour in Tyrannosaurus rex are controversial. Some contend that a modest bite mechanically limited T. rex to scavenging, while others argue that high bite forces facilitated a predatory mode of life. We use dynamic musculoskeletal models to simulate maximal biting in T. rex. Models predict that adult T. rex generated sustained bite forces of 35 000-57 000 N at a single posterior tooth, by far the highest bite forces estimated for any terrestrial animal. Scaling analyses suggest that adult T. rex had a strong bite for its body size, and that bite performance increased allometrically during ontogeny. Positive allometry in bite performance during growth may have facilitated an ontogenetic change in feeding behaviour in T. rex, associated with an expansion of prey range in adults to include the largest contemporaneous animals.

The ontogenetic scaling of bite force and head size in loggerhead sea turtles (Caretta caretta): implications for durophagy in neritic, benthic habitats

Ontogenetic studies of vertebrate feeding performance can help address questions relevant to foraging ecology. Feeding morphology and performance can either limit access to food resources or open up new trophic niches in both aquatic and terrestrial systems. Loggerhead sea turtles are long-lived vertebrates with complex life histories that are marked by an ontogenetic shift from an oceanic habitat to a coastal neritic habitat, and a transition from soft oceanic prey to hard, benthic prey. Although considered durophagous and strong biters, bite performance has not been measured in loggerheads, nor has the ontogeny of bite performance been characterized. In the present study, we collected measurements of bite force in loggerhead turtles from hatchlings to adults. When subadults reach the body size at which the ontogenetic shift occurs, their crushing capability is great enough for them to consume numerous species of hard benthic prey of small sizes. As loggerheads mature and bite performance increases, larger and harder benthic prey become accessible. Loggerhead bite performance eventually surpasses the crushing capability of other durophagous carnivores, thereby potentially reducing competition for hard benthic prey. The increasing bite performance and accompanying changes in morphology of the head and jaws are likely an effective mechanism for resource partitioning and decreasing trophic competition. Simultaneous measurements of body and head size and the use of non-linear reduced major axis regression show that bite force increases with significant positive allometry relative to body size (straight carapace length, straight carapace width and mass) and head size (head width, height and length). Simple correlation showed that all recorded morphometrics were good predictors of measured bite performance, but an AICc-based weighted regression showed that body size (straight carapace width followed by straight carapace length and mass, respectively) were more likely predictors of bite force than head size morphometrics (head width and head length).

Functional morphology and bite performance of raptorial chelicerae of camel spiders (Solifugae)

Solifugae are an understudied group of relatively large arachnids with well over 1000 species distributed on almost all major continents. These highly active predators utilize their large chelicerae for feeding, defense, burrowing and mating. We investigated the differences in cheliceral morphology and performance of two ecologically divergent species from North Africa; the cursorial Galeodes sp. and the burrowing Rhagodes melanus. Morphological data show differences in aspect ratio between the two species. Bite force measurements show Rhagodes (N=11) to be a much stronger biter than Galeodes (N=8), in terms of both absolute maximum force (Rhagodes 5.63 N, Galeodes 2.12 N) and force relative to cheliceral size. Synchrotron microtomographs of one specimen for each species reveal large differences in physiological cross-sectional area (PCSA) and estimated muscle stress, resulting in a much higher muscle stress in Rhagodes. This species also showed a longer muscle fiber length. Muscle volume and PCSA were found to differ between the two chelicerae in the two scanned specimens. Whereas Rhagodes reflects this morphological asymmetry in having a higher bite force in the right chelicera, Galeodes shows no such bias.

Morphological innovation, diversification and invasion of a new adaptive zone

How ecological opportunity relates to diversification is a central question in evolutionary biology. However, there are few empirical examples of how ecological opportunity and morphological innovation open new adaptive zones, and promote diversification. We analyse data on diet, skull morphology and bite performance, and relate these traits to diversification rates throughout the evolutionary history of an ecologically diverse family of mammals (Chiroptera: Phyllostomidae). We found a significant increase in diversification rate driven by increased speciation at the most recent common ancestor of the predominantly frugivorous subfamily Stenodermatinae. The evolution of diet was associated with skull morphology, and morphology was tightly coupled with biting performance, linking phenotype to new niches through performance. Following the increase in speciation rate, the rate of morphological evolution slowed, while the rate of evolution in diet increased. This pattern suggests that morphology stabilized, and niches within the new adaptive zone of frugivory were filled rapidly, after the evolution of a new cranial phenotype that resulted in a certain level of mechanical efficiency. The tree-wide speciation rate increased non linearly with a more frugivorous diet, and was highest at measures of skull morphology associated with morphological extremes, including the most derived Stenodermatines. These results show that a novel stenodermatine skull phenotype played a central role in the evolution of frugivory and increasing speciation within phyllostomids.

Relationships between head morphology, bite performance and ecology in two species of Podarcis wall lizards

Understanding the relationship between form and function is central to our comprehension of how phenotypic diversity evolves. Traits involved in multiple activities, such as social interactions and ecological resource use, are under the influence of different evolutionary forces potentially acting in opposite directions. Such systems provide the opportunity of understanding how potential constraints on morphological variation may influence whole-organism performance. In this study we examined morphology and bite performance in two closely related species of Podarcis wall lizards with divergent microhabitat preferences, to investigate how natural and sexual selection interact to shape the evolution of head traits. Our results show that although head morphology is markedly different between species and sexes, only sexes differ in bite force, indicating that the ecological differentiation between species is reflected in their morphology but does not constrain performance. Rather, the modification of the relative size of head components between species and a shift in the form-function relationship provide a potential explanation of how equal performance is attained by different morphological configurations. Geometric morphometrics provide a clear, biomechanically meaningful image of how this is achieved and show a bisexual pattern of head shape-bite force association in both species. This, together with a strong allometry of head size on body size and head shape on head size, provides indirect morphological evidence for the importance of sexual selection in shaping morphological and functional patterns. Finally, our findings suggest that the differences observed between species and sexes in head traits and bite performance are not reflected in their dietary ecology, implying that if trophic niche segregation between groups occurs, the reasons behind it are not primarily related to head morphology and functional variation.

Diet, morphology and performance in two chameleon morphs: do harder bites equate with harder prey?

AbstractEcologically induced morphological variation has been identified as a mainstay in evolutionary theory. Species that inhabit different habitats are likely to display morphological and functional differences related to the exploitation of different dietary resources available in each habitat within limits imposed by trade‐offs. Here, we examine two populations of the Cape Dwarf Chameleon, Bradypodion pumilum, from fynbos (heathland) and woodland to investigate whether head morphology and bite performance are related to diet within and between populations. Stomach contents are compared with prey availability to test whether chameleons are selective with respect to prey size, hardness and evasiveness. Our data show that for adult chameleons from the fynbos (Kogelberg; n = 44), mean and maximum prey size are tightly correlated with head morphology and performance. In woodland habitat (Stellenbosch; n = 52), only maximum prey size is correlated with head morphology and performance. Fynbos chameleons showed no preference with respect to prey hardness, while those from woodland ate less hard and/or sedentary prey than available, thus preferring items that were soft and/or evasive. Finally, fynbos chameleons have a diet of sedentary and/or evasive prey similar in proportions to that available. Our results suggest that diet is not directly related to selection on the head morphology and biting performance of B. pumilum in woodland habitat, but that it may be important for selection in fynbos due to a reduction in overall prey availability.

Sex- and age-related morphofunctional differences in skulls ofTayassu pecariandPecari tajacu(Artiodactyla: Tayassuidae)

Tayassuids exhibit little sexual dimorphism. Sex-related craniomandibular variation was tested in 2 tayassuid species with regard to morphology and function. We analyzed skull features from 61 adult and 37 subadult white-lipped peccaries (Tayassu pecari) and 49 adult collared peccaries (Pecari tajacu) of both sexes. Seventeen skull measurements were used for the morphological characterization, and from these, indexes were obtained for the functional analysis of bite performance (temporal and masseter muscular systems) and head elevation (semispinalis capitis/complexus muscular system). Skull sexual dimorphism of the 2 species and 2 age groups were evaluated independently through discriminant function analysis (DFA), principal component analysis (PCA), and univariate methods. The DFA characterized a slight secondary sexual dimorphism in adult T. pecari and P. tajacu skulls and provided sex classification functions. However, no sexual variation was detected in skull function of adults in either species. Nevertheless, subadult males and females of T. pecari displayed marked morphological and functional skull variation. Subadult females displayed greater skull robustness, and their force indexes indicate an enhancement on bite strength and in head elevation. This could suggest ecomorphological dissimilarities between subadult male and female T. pecari. The sexual precocity of the females reported in the literature could be the main determinant of the marked morphofunctional variation among the T. pecari subadults. These sexual differences in skull morphology seem to become reduced as males mature.

Sex-specific evolution of bite performance in Liolaemus lizards (Iguania: Liolaemidae): the battle of the sexes

461..475 Although differential selective pressures on males and females of the same species may result in sex-specific evolutionary trajectories, comparative studies of adaptive radiations have largely neglected within-species varia- tion. In this study, we explore the potential effects of natural selection, sexual selection, or a combination of both, on bite performance in males and females of 19 species of Liolaemus lizards. More specifically, we study the evolution of bite performance, and compare evolutionary relationships between the variation in head morphology, bite performance, ecological variation and sexual dimorphism between males and females. Our results suggest that in male Liolaemus, the variation in bite force is at least partly explained by the variation in the degree of sexual dimorphism in head width (i.e. our estimate of the intensity of sexual selection), and neither bite force nor the morphological variables were correlated with diet (i.e. our proxy for natural selection). On the contrary, in females, the variation in bite force and head size can, to a certain extent, be explained by variation in diet. These results suggest that whereas in males, sexual selection seems to be operating on bite performance, in the case of females, natural selection seems to be the most likely and most important selective pressure driving the variation in head size. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101, 461-475. ADDITIONAL KEYWORDS: diet - ecomorphology - interspecific variation - natural selection - sexual differences - sexual selection.

Comparison between in vivo and theoretical bite performance: Using multi-body modelling to predict muscle and bite forces in a reptile skull

In biomechanical investigations, geometrically accurate computer models of anatomical structures can be created readily using computed-tomography scan images. However, representation of soft tissue structures is more challenging, relying on approximations to predict the muscle loading conditions that are essential in detailed functional analyses. Here, using a sophisticated multi-body computer model of a reptile skull (the rhynchocephalian Sphenodon), we assess the accuracy of muscle force predictions by comparing predicted bite forces against in vivo data. The model predicts a bite force almost three times lower than that measured experimentally. Peak muscle force estimates are highly sensitive to fibre length, muscle stress, and pennation where the angle is large, and variation in these parameters can generate substantial differences in predicted bite forces. A review of theoretical bite predictions amongst lizards reveals that bite forces are consistently underestimated, possibly because of high levels of muscle pennation in these animals. To generate realistic bites during theoretical analyses in Sphenodon, lizards, and related groups we suggest that standard muscle force calculations should be multiplied by a factor of up to three. We show that bite forces increase and joint forces decrease as the bite point shifts posteriorly within the jaw, with the most posterior bite location generating a bite force almost double that of the most anterior bite. Unilateral and bilateral bites produced similar total bite forces; however, the pressure exerted by the teeth is double during unilateral biting as the tooth contact area is reduced by half.