The role of cranial kinesis in birds.

In birds, the ability to move the upper beak relative to the braincase has been the subject of many functional morphological investigations, but in many instances the adaptive significance of cranial kinesis remains unclear. Alternatively, cranial kinesis may be considered a consequence of the general design of the skull, rather than an adaptive trait as such. The present study reviews some results related to the mechanism and functional significance of cranial kinesis in birds. Quantitative three-dimensional X-ray has shown that in skulls morphologically as divers as paleognaths and neognaths the mechanism for elevation of the upper beak is very similar. One of the mechanisms proposed for avian jaw movement is a mechanical coupling of the upper and the lower jaw movement by the postorbital ligament. Such a mechanical coupling would necessitate upper beak elevation. However, independent control of upper and lower jaw has been shown to occur during beak movements in birds. Moreover, kinematic modeling and force measurements suggests that the maximum extensibility of collagen, in combination with the short distance of the insertion of the postorbital ligament to the quadrato-mandibular articulation do not constitute a block to lower jaw depression. The lower jaw ligaments serve to limit the maximal extension of the mandibula. It is suggested here that cranial kinesis in avian feeding may have evolved as a consequence of an increase in eye size. This increase in size led to a reduction of bony bars in the lateral aspect of the skull enabling the transfer of quadrate movement to the upper jaw. The selective forces favoring the development of a kinetic upper beak in birds may be subtle and act in different ecological contexts. Simultaneous movement of the upper and lower jaw not only increases the velocity of beak movements, but with elevated upper beak also less force is required to open the lower jaw. However, the penalty of increased mobility of elements in a lightweight skull and a large eye is potential instability of skull elements during biting, smaller bite forces and limitations on joint reaction forces. Such a lightly built, kinetic skull may have evolved in animals that feed on small plant material or insects. This type of food does not require the resistance of large external forces on the jaws as in carnivores eating large prey.     

Implications of predatory specialization for cranial form and function in canids

The shape of the cranium varies widely among members of the order Carnivora, but the factors that drive the evolution of differences in shape remain unclear. Selection for increased bite force, bite speed or skull strength may all affect cranial morphology. We investigated the relationship between cranial form and function in the trophically diverse dog family, Canidae, using linear morphometrics and finite element (FE) analyses that simulated the internal and external forces that act on the skull during the act of prey capture and killing. In contrast to previous FE-based studies, we compared models using a newly developed method that removes the effects of size and highlights the relationship between shape and performance. Cranial shape varies among canids based on diet, and different selective forces presumably drove evolution of these phenotypes. The long, narrow jaws of small prey specialists appear to reflect selection for fast jaw closure at the expense of bite force. Generalists have intermediate jaw dimensions and produce moderate bite forces, but their crania are comparable in strength to those of small prey specialists. Canids that take large prey have short, broad jaws, produce the largest bite forces and possess very strong crania. Our FE simulations suggest that the remarkable strength of skulls of large prey specialists reflect the additional ability to resist extrinsic loads that may be encountered while struggling with large prey items.     

New observations on the skull of Archaeopteryx

Although skeletal remains of the iconic oldest known avialian Archaeopteryx have been known for almost 150 years, several aspects of the cranial anatomy of this taxon have remained enigmatic, mainly because of the strongly flattened and often fractured and incomplete nature of available skull materials. New investigation of the skulls of the recently described, excellently preserved tenth (Thermopolis) and the seventh (Munich) specimens revealed several previously unrecognized characters and helps to resolve some problematic issues. Thus, the nasal of Archaeopteryx shows a lateral notch for the lacrimal, as is found in many other saurischian dinosaurs, the maxilla clearly participates in the margin of the external nares, and there seems to be a pneumatic foramen in the lacrimal, comparable to the lacrimal fenestra found in many non-avian theropods. In the braincase, Archaeopteryx shows pneumatic features reminiscent of non-avian theropods, including a ventral basisphenoid recess and an anterior tympanic recess that is laterally incised into the basisphenoid/prootic. Most importantly, however, the postorbital process of the jugal shows a facet for the suture with the postorbital, thus resolving the question of whether Archaeopteryx had a closed postorbital bar. A new reconstruction of the skull of Archaeopteryx is presented, making the skull of this taxon even more theropod-like than previously recognized. Furthermore, the closed postorbital bar and the configuration of the bones of the skull roof cast serious doubt on claims that an avian-style cranial kinesis was present in this taxon.     

Functional implications of dicynodont cranial suture morphology

Cranial suture morphology of Lystrosaurus and the generalized dicynodont Oudenodon was investigated to determine the strain environment during mastication, which in turn may indicate a difference in cranial function between the two taxa. Finite element (FE) analysis indicated that less strain accumulated in the cranium of Lystrosaurus during orthal bite simulations than in Oudenodon. Despite the overall difference in strain magnitude, moderate to high FE‐predicted strain accumulated in similar areas of the cranium of both taxa. The suture morphology in these cranial regions of Lystrosaurus and Oudenodon was investigated further by examination of histological sections and supplemented by observations of serial sections and computed tomography (CT) scans. The predominant type of strain from selected blocks of finite elements that contain sutures was determined, enabling comparison of suture morphology to strain type. Drawing from strain‐suture correlations established in extant taxa, the observed patterns of sutural morphology for both dicynodonts were used to deduce cranial function. The moderate to high compressive and tensile strain experienced by the infraorbital bar, zygomatic arch, and postorbital bar of Oudenodon and Lystrosaurus may have been decreased by small adjustive movements at the scarf sutures in those regions. Disparities in cranial suture morphology between the two taxa may reflect differences in cranial function. For instance, the tongue and groove morphology of the postorbital‐parietal suture in Oudenodon could have withstood the higher FE‐predicted tensile strain in the posterior skull roof. The scarf premaxilla‐nasal suture of Lystrosaurus provided an additional region of sutural mobility in the anterior surface of the snout, suggesting that Lystrosaurus may have employed a different biting regime than Oudenodon. The morphology of several sutures sampled in this study correlated with the FE‐predicted strain, although other cranial functional hypotheses remain to be tested. J. Morphol., 2010. © 2010 Wiley‐Liss, Inc.     

Skeletal heterochrony is associated with the anatomical specializations of snakes among squamate reptiles

Snakes possess a derived anatomy, characterized by limb reduction and reorganization of the skull and internal organs. To understand the origin of snakes from an ontogenetic point of view, we conducted comprehensive investigations on the timing of skeletal elements, based on published and new data, and reconstructed the evolution of the ossification sequence among squamates. We included for the first time Varanus, a critical taxon in phylogenetic context. There is comprehensive delay in the onset of ossification of most skeletal elements in snakes when compared to reference developmental events through evolution. We hypothesize that progressing deceleration accompanied limb reduction and reorganization of the snake skull. Molecular and morphological studies have suggested close relationship of snakes to either amphisbaenians, scincids, geckos, iguanids, or varanids. Likewise, alternative hypotheses on habitat for stem snakes have been postulated. Our comprehensive heterochrony analyses detected developmental shifts in ossification for each hypothesis of snake origin. Moreover, we show that reconstruction of ancestral developmental sequences is a valuable tool to understand ontogenetic mechanisms associated with major evolutionary changes and test homology hypotheses. The “supratemporal” of snakes could be homolog to squamosal of other squamates, which starts ossification early to become relatively large in snakes.     

Dietary hardness, loading behavior, and the evolution of skull form in bats

The morphology and biomechanics of the vertebrate skull reflect the physical properties of diet and behaviors used in food acquisition and processing. We use phyllostomid bats, the most diverse mammalian dietary radiation, to investigate if and how changes in dietary hardness and loading behaviors during feeding shaped the evolution of skull morphology and biomechanics. When selective regimes of food hardness are modeled, we found that species consuming harder foods have evolved skull shapes that allow for more efficient bite force production. These species have shorter skulls and a greater reliance on the temporalis muscle, both of which contribute to a higher mechanical advantage at an intermediate gape angle. The evolution of cranial morphology and biomechanics also appears to be related to loading behaviors. Evolutionary changes in skull shape and the relative role of the temporalis and masseter in generating bite force are correlated with changes in the use of torsional and bending loading behaviors. Functional equivalence appears to have evolved independently among three lineages of species that feed on liquids and are not obviously morphologically similar. These trends in cranial morphology and biomechanics provide insights into behavioral and ecological factors shaping the skull of a trophically diverse clade of mammals.     

The mammalian postorbital bar as a torsion-resisting helical strut

The mammalian skull is asymmetrically loaded during mastication because most of these animals chew on only one side at a time. This loading regime tends to twist the braincase relative to the rostral, tooth bearing part of the skull at the zone of potential weakness between the orbits. This torsional effect is exaggerated, and a postorbital bar is present, in those animals with very large masseter and pterygoid muscles. The lines of action of these muscles are oriented at large angles to the long axis of the skull in lateral view, providing large components of force that twist the skull segments relative to one another. When the temporalis is the dominant muscle, the torsional effect is usually less important, and the bar is absent, because this muscle acts at a smaller angle to the skull axis. The postorbital bar exhibits the predicted three dimensional spatial orientation required to resist torsional forces: it is a segment of an imaginary 45° helix that is wound around the skull, if the skull is idealized as a cylinder. This orientation is significant because, in general, maximum compressive and tensile shear stresses lie along 45° helices on a cylinder loaded in torsion; to resist torsion, material should be placed far from the axis of torsion and along a helix oriented at 45° to the deforming forces. Each half of a supraorbital ridge is also a segment of a 45° helix that is perpendicular to the helix passing through the postorbital bar. This model suggests that the postorbital bar is loaded in compression on the chewing side and in tension on the non-chewing side; the supraorbital ridge is loaded in tension on the chewing side and in compression on the non-chewing side.     

Convergence and remarkably consistent constraint in the evolution of carnivore skull shape

Phenotypic similarities between distantly related marsupials and placentals are commonly presented as examples of convergence and support for the role of adaptive evolution in shaping morphological and ecological diversity. Here we compare skull shape in a wide range of carnivoran placentals (Carnivora) and nonherbivorous marsupials using a three-dimensional (3-D) geometric morphometric approach. Morphological and ecological diversity among extant carnivorans is considerably greater than is evident in the marsupial order Dasyuromorphia with which they have most commonly been compared. To examine convergence across a wider, but broadly comparable range of feeding ecologies, a dataset inclusive of nondasyuromorphian marsupials and extinct taxa representing morphotypes no longer present was assembled. We found support for the adaptive paradigm, with correlations between morphology, feeding behavior, and bite force, although skull shape better predicted feeding ecology in the phylogenetically diverse marsupial sample than in carnivorans. However, we also show that remarkably consistent but differing constraints have influenced the evolution of cranial shape in both groups. These differences between carnivorans and marsupials, which correlate with brain size and bite force, are maintained across the full gamut of morphologies and feeding categories, from small insectivores and omnivores to large meat-specialists.     

Rotational feeding in caecilians: putting a spin on the evolution of cranial design

Caecilians are a poorly known group of amphibians with a highly derived skull and cranial musculature that has evolved in response to their specialized head-first burrowing lifestyle. They possess a unique jaw-closing system, which is shown to be capable of generating considerable bite forces for its head width (1.09+/-0.34 and 0.62+/-0.31 N for Schistometopum thomense and Boulengerula taitanus, respectively). However, comprehensive dietary studies indicate that there is no need for large bite forces, since most caecilians appear to be generalist predators of subterranean macrofauna. Here, we demonstrate, based on in vivo external and X-ray video recordings of animals feeding, that long-axis body rotations are used independent of prey size by these two species of caeciliid caecilians when feeding underground. Further, we show that individuals are capable of generating a substantial spinning force, which is greater than their bite force (1.35+/-0.26 and 1.02+/-0.18 N, respectively). These observations shed light on the functional and the evolutionary significance of several unique features of the cranial design in derived caecilians; spinning may allow the individuals to judge prey size and subsequently reduce oversized prey within gape limits.     

Mechanical Analysis of Feeding Behavior in the Extinct “Terror Bird” Andalgalornis steulleti (Gruiformes: Phorusrhacidae)

The South American phorusrhacid bird radiation comprised at least 18 species of small to gigantic terrestrial predators for which there are no close modern analogs. Here we perform functional analyses of the skull of the medium-sized (∼40 kg) patagornithine phorusrhacid Andalgalornis steulleti (upper Miocene–lower Pliocene, Andalgalá Formation, Catamarca, Argentina) to assess its mechanical performance in a comparative context. Based on computed tomographic (CT) scanning and morphological analysis, the skull of Andalgalornis steulleti is interpreted as showing features reflecting loss of intracranial immobility. Discrete anatomical attributes permitting such cranial kinesis are widespread phorusrhacids outgroups, but this is the first clear evidence of loss of cranial kinesis in a gruiform bird and may be among the best documented cases among all birds. This apomorphic loss is interpreted as an adaptation for enhanced craniofacial rigidity, particularly with regard to sagittal loading. We apply a Finite Element approach to a three-dimensional (3D) model of the skull. Based on regression analysis we estimate the bite force of Andalgalornis at the bill tip to be 133 N. Relative to results obtained from Finite Element Analysis of one of its closest living relatives (seriema) and a large predatory bird (eagle), the phorusrhacid''s skull shows relatively high stress under lateral loadings, but low stress where force is applied dorsoventrally (sagittally) and in “pullback” simulations. Given the relative weakness of the skull mediolaterally, it seems unlikely that Andalgalornis engaged in potentially risky behaviors that involved subduing large, struggling prey with its beak. We suggest that it either consumed smaller prey that could be killed and consumed more safely (e.g., swallowed whole) or that it used multiple well-targeted sagittal strikes with the beak in a repetitive attack-and-retreat strategy.     

Intraspecific variation in the skull morphology of the black caiman Melanosuchus niger (Alligatoridae,Caimaninae)

Melanosuchus niger is a caimanine alligatorid widely distributed in the northern region of South America. This species has been the focus of several ecological, genetic and morphological studies. However, morphological studies have generally been limited to examination of interspecific variation among extant species of South American crocodylians. Here, we present the first study of intraspecific variation in the skull of M. niger using a two‐dimensional geometric morphometric approach. The crania of 52 sexed individuals varying in size were analysed to quantify shape variation and to assign observed shape changes to different types of intraspecific variation, that is, ontogenetic variation and sexual dimorphism. Most of the variation in this species is ontogenetic variation in snout length, skull depth, orbit size and the width of the postorbital region. These changes are correlated with bite force performance and probably dietary changes. However, a comparison with previous functional studies reveals that functional adaptations during ontogeny seem to be primarily restricted to the postrostral region, whereas rostral shape changes are more related to dietary shifts. Furthermore, the skulls of M. niger exhibit a sexual dimorphism, which is primarily size‐related. The presence of non‐size‐related sexual dimorphism has to be tested in future examinations.     

Scaling of bite force in the blacktip shark Carcharhinus limbatus

Although bite force is a frequently studied performance measure of feeding ecology, changes in bite force over ontogeny have rarely been investigated. Biting by the blacktip shark Carcharhinus limbatus was theoretically modeled over ontogeny to investigate the scaling of bite force, the morphological basis of the observed scaling relationship, the ecological consequences of ontogenetic changes in performance, and whether cranial morphometrics can be used as an accurate proxy for bite force. Theoretical bite force, which was positively allometric with respect to total length (TL), ranged from 32 N (61 cm TL) to 423 N (152 cm TL) at the anterior tips of the jaws and from 107 (61 cm TL) to 1083 N (152 cm TL) at the posterior teeth. This observation is attributed to positive allometry in the mechanical advantage of the jaw-adducting mechanism and the cross-sectional area of all four jaw-adducting muscles. Theoretical bite force was accurately predicted by cranial morphometrics including prebranchial length and head width as well. Although positive allometry of bite force in C. limbatus would seem to indicate an ecological necessity for this phenomenon, dietary analyses do not necessarily indicate any ontogenetic shift in prey types requiring larger bite forces. The positively allometric increase in theoretical bite force may be associated with numerous other selective pressures including maintenance of an apical position within the ecosystem.     

Are Cranial Biomechanical Simulation Data Linked to Known Diets in Extant Taxa? A Method for Applying Diet-Biomechanics Linkage Models to Infer Feeding Capability of Extinct Species

Performance of the masticatory system directly influences feeding and survival, so adaptive hypotheses often are proposed to explain craniodental evolution via functional morphology changes. However, the prevalence of “many-to-one” association of cranial forms and functions in vertebrates suggests a complex interplay of ecological and evolutionary histories, resulting in redundant morphology-diet linkages. Here we examine the link between cranial biomechanical properties for taxa with different dietary preferences in crown clade Carnivora, the most diverse clade of carnivorous mammals. We test whether hypercarnivores and generalists can be distinguished based on cranial mechanical simulation models, and how such diet-biomechanics linkages relate to morphology. Comparative finite element and geometric morphometrics analyses document that predicted bite force is positively allometric relative to skull strain energy; this is achieved in part by increased stiffness in larger skull models and shape changes that resist deformation and displacement. Size-standardized strain energy levels do not reflect feeding preferences; instead, caniform models have higher strain energy than feliform models. This caniform-feliform split is reinforced by a sensitivity analysis using published models for six additional taxa. Nevertheless, combined bite force-strain energy curves distinguish hypercarnivorous versus generalist feeders. These findings indicate that the link between cranial biomechanical properties and carnivoran feeding preference can be clearly defined and characterized, despite phylogenetic and allometric effects. Application of this diet-biomechanics linkage model to an analysis of an extinct stem carnivoramorphan and an outgroup creodont species provides biomechanical evidence for the evolution of taxa into distinct hypercarnivorous and generalist feeding styles prior to the appearance of crown carnivoran clades with similar feeding preferences.     

Effects of gape and tooth position on bite force and skull stress in the dingo (Canis lupus dingo) using a 3-dimensional finite element approach

Models of the mammalian jaw have predicted that bite force is intimately linked to jaw gape and to tooth position. Despite widespread use, few empirical studies have provided evidence to validate these models in non-human mammals and none have considered the influence of gape angle on the distribution of stress. Here using a multi-property finite element (FE) model of Canis lupus dingo, we examined the influence of gape angle and bite point on both bite force and cranial stress. Bite force data in relation to jaw gape and along the tooth row, are in broad agreement with previously reported results. However stress data showed that the skull of C. l. dingo is mechanically suited to withstand stresses at wide gapes; a result that agreed well with previously held views regarding carnivoran evolution. Stress data, combined with bite force information, suggested that there is an optimal bite angle of between 25 degrees and 35 degrees in C. l. dingo. The function of these rather small bite angles remains unclear.     

The bony labyrinth of Platecarpus (Squamata: Mosasauria) and aquatic adaptations in squamate reptiles

Mosasaurs were among the last marine reptiles that lived before the Cretacesous–Paleogene extinction. Little is known about the sensory evolution of mosasaurs in relation to their aquatic lifestyle. In this study, the braincase of Platecarpus was CT-scanned and virtual models were constructed showing the bony labyrinth — or the inner ear — a sensory apparatus for balance and hearing. The virtual inner ear consists of the semicircular canals, vestibule, and cochlea. Compared with extant squamates, Platecarpus resembles sea snakes in having a small vestibule with a flat dorsal surface, but it differs from non-mosasaurian squamates in having rounded semicircular canals. Phylogenetic linear regression analysis supports a linear relationship, independent from phylogeny, between the length of the three semicircular canals and the length of the skull. The semicircular canals of Platecarpus are shorter than predicted, but the fossil data fell within the 95% prediction interval calculated from the extant data and the skull length of Platecarpus. Although size reduction of the bony labyrinth has been associated with aquatic adaptions in mammals, our results suggest that in squamates, semicircular canal size is related to skull size rather than habitat preference.     

Mechanical structure and function of the craniofacial skeleton of the domestic dog.

Masticatory habit is a major factor determining the morphology of the craniofacial skeleton. The craniofacial skeleton essentially comprises a series of bony stress-bearing bridges forming a structural framework. The structural framework of the skull of dog has been described as a rigid trestle-like structure; it can be illustrated by mechanically removing nonresistant areas of bone. It is then found that a framework is produced which is partially rigid (cranium) and partly flexible (rostrum). It is postulated that the flexibility of the rostrum acts to absorb shock and it is suggested that the primate postorbital bar is developed in response to craniofacial morphology which increases compressive bite forces.     

Functional basis for sexual differences in bite force in the lizard Anolis carolinensis

In many species of lizards, males attain greater body size and have larger heads than female lizards of the same size. Often, the dimorphism in head size is paralleled by a dimorphism in bite force. However, the underlying functional morphological basis for the dimorphism in bite force remains unclear. Here, we test whether males are larger, and have larger heads and bite forces than females for a given body size in a large sample of Anolis carolinensis . Next, we test if overall head shape differs between the sexes, or if instead specific aspects of skull shape can explain differences in bite force. Our results show that A. carolinensis is indeed dimorphic in body and head size and that males bite harder than females. Geometric morphometric analyses show distinct differences in skull shape between males and females, principally reflecting an enlargement of the jaw adductor muscle chamber. Jaw adductor muscle mass data confirm this result and show that males have larger jaw adductors (but not jaw openers) for a given body and head size. Thus, the observed dimorphism in bite force in A. carolinensis is not merely the result of an increase in head size, but involves distinct morphological changes in skull structure and the associated jaw adductor musculature.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 111–119.     

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.     

Foraging strategies,craniodental traits,and interaction in the bite force of Neotropical frugivorous bats (Phyllostomidae: Stenodermatinae)

1. Bats in the family Phyllostomidae exhibit great diversity in skull size and morphology that reflects the degree of resource division and ecological overlap in the group. In particular, the subfamily Stenodermatinae has high morphological diversification associated with cranial and mandibular traits that are associated with the ability to consume the full range of available fruits (soft and hard).
2. We analyzed craniodental traits and their relationship to the bite force in 343 specimens distributed in seven species of stenodermatine bats with two foraging strategies: nomadic and sedentary frugivory. We evaluated 19 traits related to feeding and bite force in live animals by correcting bite force with body size.
3. We used a generalized linear model (GLM) and post hoc tests to determine possible relationships and differences between cranial traits, species, and sex. We also used Blomberg''s K to measure the phylogenetic signal and phylogenetic generalized least‐squares (PGLS) to ensure the phylogenetic independence of the traits.
4. We found that smaller nomadic species, A. anderseni and A. phaeotis , have a similar bite force to the large species A. planirostris and A. lituratus; furthermore, P. helleri registered a bite force similar to that of the sedentary bat, S. giannae. Our study determined that all the features of the mandible and most of the traits of the skull have a low phylogenetic signal. Through the PGLS, we found that the diet and several cranial features (mandibular toothrow length, dentary length, braincase breadth, mastoid breadth, greatest length of skull, condylo‐incisive length, and condylo‐canine length) determined bite force performance among Stenodermatiane.
5. Our results reinforce that skull size is a determining factor in the bite force, but also emphasize the importance of its relationships with morphology, ecology, and phylogeny of the species, which gives us a better understanding of the evolutionary adaptions of this highly diverse Neotropical bat group.

     

Bite performance in clariid fishes with hypertrophied jaw adductors as deduced by bite modeling

Within clariid fishes several cranial morphologies can be discerned. Especially within anguilliform representatives an increase in the degree of hypertrophy of the jaw adductors occurs. The hypertrophy of the jaw adductors and skeletal modifications in the cranial elements have been linked to increased bite force. The functional significance of this supposed increase in bite force remains obscure. In this study, biomechanical modeling of the cranial apparatus in four clariid representatives showing a gradual increase in the hypertrophy of the jaw adductors (Clarias gariepinus, Clariallabes melas, Channallabes apus, and Gymnallabes typus) is used to investigate whether bite force actually increased. Static bite modeling shows that the apparent hypertrophy results in an increase in bite force. For a given head size, the largest bite forces are predicted for C. apus, the lowest ones for C. gariepinus, and intermediate values are calculated for the other species. In addition, also in absolute measures differences in bite force remain, with C. apus biting distinctly harder than C. gariepinus despite its smaller head size. This indicates that the hypertrophy of the jaw adductors is more than just a correlated response to the decrease in absolute head size. Further studies investigating the ecological relevance of this performance difference are needed.