Bite Force Capacity Research Articles

Functionally mediated cranial allometry evidenced in a genus of rock-wallabies.

In assessments of skeletal variation, allometry (disproportionate change of shape with size) is often corrected to examine size-independent variation for hypotheses relating to function. However, size-related trade-offs in functional demands may themselves be an underestimated driver of mammalian cranial diversity. Here, we use geometric morphometrics alongside dental measurements to assess craniodental allometry in the rock-wallaby genus Petrogale (all 17 species, 370 individuals). We identified functional aspects of evolutionary allometry that can be both extensions of, and correlated negatively with, static or ontogenetic allometric patterns. Regarding constraints, larger species tended to have relatively smaller braincases and more posterior orbits, the former of which might represent a constraint on jaw muscle anatomy. However, they also tended to have more anterior dentition and smaller posterior zygomatic arches, both of which support the hypothesis of relaxed bite force demands and accommodation of different selective pressures that favour facial elongation. By contrast, two dwarf species had stouter crania with divergent dental adaptations that together suggest increased relative bite force capacity. This likely allows them to feed on forage that is mechanically similar to that consumed by larger relatives. Our results highlight a need for nuanced considerations of allometric patterns in future research of mammalian cranial diversity.

Morphological determinants of bite force capacity in insects: a biomechanical analysis of polymorphic leaf-cutter ants.

The extraordinary success of social insects is partially based on division of labour, i.e. individuals exclusively or preferentially perform specific tasks. Task preference may correlate with morphological adaptations so implying task specialization, but the extent of such specialization can be difficult to determine. Here, we demonstrate how the physical foundation of some tasks can be leveraged to quantitatively link morphology and performance. We study the allometry of bite force capacity in Atta vollenweideri leaf-cutter ants, polymorphic insects in which the mechanical processing of plant material is a key aspect of the behavioural portfolio. Through a morphometric analysis of tomographic scans, we show that the bite force capacity of the heaviest colony workers is twice as large as predicted by isometry. This disproportionate 'boost' is predominantly achieved through increased investment in muscle volume; geometrical parameters such as mechanical advantage, fibre length or pennation angle are likely constrained by the need to maintain a constant mandibular opening range. We analyse this preference for an increase in size-specific muscle volume and the adaptations in internal and external head anatomy required to accommodate it with simple geometric and physical models, so providing a quantitative understanding of the functional anatomy of the musculoskeletal bite apparatus in insects.

Trait differences among discrete morphs of a color polymorphic lizard, Podarcis erhardii.

Color polymorphism defies evolutionary expectations as striking phenotypic variation is maintained within a single species. Color and other traits mediate social interactions, and stable polymorphism within a population is hypothesized to be related to correlational selection of other phenotypic traits among color morphs. Here, we report on a previously unknown throat color polymorphism in the Aegean Wall Lizard (Podarcis erhardii) and examine morph-correlated differences in traits important to social behavior and communication: maximum bite force capacity and chemical signal profile. We find that both sexes of P. erhardii have three color morphs: orange, yellow, and white. Moreover, orange males are significantly larger and tend to bite harder than yellow and white males. Although the established color polymorphism only partially matches the observed intraspecific variation in chemical signal signatures, the chemical profile of the secretions of orange males is significantly divergent from that of white males. Our findings suggest that morph colors are related to differences in traits that are crucial for social interactions and competitive ability, illustrating the need to look beyond color when studying polymorphism evolution.

Skull variation in a shovel-headed amphisbaenian genus, inferred from the geometric morphometric analysis of five South American Leposternon species.

Amphisabenia is a group of squamates adapted for a fossorial lifestyle. The skull is the animal's the main digging tool, and can present one of four principal shapes. The shovel-headed shape is considered to be the most specialized for digging. The South American genus Leposternon presents a shovel-headed morphotype, and is widely distributed on this continent. The general shovel-headed skull pattern may vary considerably, even within the same genus, and we hypothesized that this variation may be influenced primarily by body size and geographical factors. This study investigated the variation in skull size and shape among five Leposternon species, and examined the potential relationship between this variation and the size of the specimens and bioclimatic variables, through a geometric morphometric approach. Significant morphological variation was found among the species, and was also related systematically to body size and the geographical distribution of the specimens. As even subtle differences in the skull size or shape may represent significant modification in bite force and digging capacity and digging speed, the cranial variation found among the Leposternon species and specimens may have a direct influence on their diet and locomotor performance. Our results, together with direct observations of some of these species, suggest that shovel-headed amphisbaenians may be able to penetrate different soil types under a range of climatic conditions, especially considering the ample, but often sympatric distribution of the species studied here.

Beyond size – morphological predictors of bite force in a diverse insectivorous bat assemblage from Malaysia

Summary Bite force is used to investigate feeding performance in a variety of vertebrates. In all taxa studied, bite force is strongly correlated with body and head size. Studies of bite force in bats have largely centred on neotropical species with a particular focus on species that maximize dietary differences. Little is known about the bite force of bats from the Old World tropics, nor of variation in bite force within diverse assemblages of obligate insectivores. Moreover, factors other than size are poorly known but may be important in driving interspecific differences in bite force, and thereby diet. Here, we examine the correlation between morphological variation and bite force of 35 species of insectivorous bats from a single palaeotropical assemblage. We confirmed the overall relationship between size and bite force across species, but found that bite force is predicted more strongly by head length than body mass or forearm length. From the combined action of jaw muscles and muscle‐bone mechanisms, bats generate a mechanical advantage that creates pressure during biting. We calculated the size‐independent mechanical advantage for each of five mandible lever systems (three delineated by the temporalis muscle and two delineated by the masseter muscle) operating through three function points (molar, canine and incisor). Size‐independent mechanical advantage of the suprazygomatic portion of the temporalis muscle at the molar function point was the only significant predictor of size‐independent maximum bite force across all species. Within families, the size‐independent mechanical advantage of the superficial portion of the masseter muscle plays a significant role in predicting size‐independent maximum bite force in both the Rhinolophidae and Vespertilionidae. For the family Hipposideridae, however, size‐independent mechanical advantage showed no role in predicting size‐independent maximum bite force, suggesting that size really matters in predicting the maximum bite force capacity for this family.

Sex‐specific differences in ecomorphological relationships in lizards of the genus Gallotia

Summary Males and females often differ from one another in phenotypic traits due to differential investment in traits relevant to the fitness of each sex. However, how differences in sexually dimorphic traits affect ecologically relevant performance traits and whether these are correlated with variation in ecology remains poorly understood. Here, we test the co‐evolution of head shape, bite force capacity and diet in male and female lizards (Gallotia) from the Canary Islands, known to be sexually dimorphic. We collected data on bite force and head size and shape for both sexes of all seven extant species on all seven islands of the archipelago (ten evolutionary‐independent lineages). Moreover, we collected diet data for five out of the seven species (eight lineages). Our results show that the evolution of head morphology is associated with the evolution of bite force in both sexes. However, only in females is the evolution of head morphology and bite force associated with the evolution of diet. In males, head morphology and bite force are decoupled from the evolution of diet. In conjunction with the male head shape characterized by a broad rostrum, this suggests that head shape and bite force may be evolving principally under sexual selection in males. Our data thus suggest that head morphology and associated functional traits may evolve under different selective pressures in the two sexes.

Parentage analyses suggest female promiscuity and a disadvantage for athletic males in the colour-polymorphic lizard Podarcis melisellensis

Although laboratory measurements of whole- animal performance have become a standard tool in evolu- tionary biology, if and how interindividual variation in perfor- mance translates into differential fitness remains poorly un- derstood. Particularly rare are studies that have connected performance to mating and reproductive success in the field. Inthisstudy, we use DNA microsatelliteparentage analysesto study the fitness gradient in a colour-polymorphic lizard, Podarcis melisellensis. We report on two surprising findings. First, contrary to our expectations, individual sprint speed and bite force capacity correlated negatively, not positively, with male mating and reproductive success. Second, we found an unexpected degree of promiscuity in females. Also, contrary to traditional parental investment theory, the variation in mating success and reproductive success was as high in fe- males as in males. Our results call for a better integration of whole-animal performance and life history traits, and for a reconsideration of the ideas on the likeliness of sexual selec- tion acting on female phenotypes.

An integrative approach to detect subtle trophic niche differentiation in the sympatric trawling bat species Myotis dasycneme and Myotis daubentonii

Bats are well known for species richness and ecological diversity, and thus, they provide a good opportunity to study relationships and interaction between species. To assess interactions, we consider distinct traits that are probably to be triggered by niche shape and evolutionary processes. We present data on the trophic niche differentiation between two sympatric European trawling bat species, Myotis dasycneme and Myotis daubentonii, incorporating a wide spectrum of methodological approaches. We measure morphological traits involved in foraging and prey handling performance including bite force, weightlifting capacity and wing morphology. We then measure resulting prey consumption using both morphological and molecular diet analyses. These species closely resemble each other in morphological traits, however, subtle but significant differences were apparent in bite force and lift capacity, which are related to differences in basic body and head size. Both morphological and molecular diet analyses show strong niche overlap. We detected subtle differences in less frequent prey items, as well as differences in the exploitation of terrestrial and aquatic-based prey groups. Myotis dasycneme feeds more on aquatic prey, like Chironomidae and their pupal stages, or on the aquatic moth Acentria ephemerella. Myotis daubentonii feeds more on terrestrial prey, like Brachycera, or Coleoptera. This suggests that these bats use different microhabitats within the habitat where they co-occur.

Is dietary niche breadth linked to morphology and performance in Sandveld lizardsNucras(Sauria: Lacertidae)?

The functional characteristics of prey items (such as hardness and evasiveness) have been linked with cranial morphology and performance in vertebrates. In lizards particularly, species with more robust crania generally feed on harder prey items and possess a greater bite force, whereas those that prey on evasive prey typically have longer snouts. However, the link between dietary niche breadth, morphology, and performance has not been explicitly investigated in lizards. The southern African genus Nucras was used to investigate this link because the species exhibit differing niche breadth values and dietary compositions. A phylogeny for the genus was established using mitochondrial and nuclear markers, and morphological clusters were identified. Dietary data of five Nucras species, as reported previously, were used in correlation analyses between cranial shape (quantified using geometric morphometrics) and dietary niche breadth, and the proportion of hard prey taken and bite force capacity. Dietary niche breadth and the proportion of hard prey eaten were significantly related to cranial shape, although not once phylogeny was accounted for using a phylogenetic generalized least squares regression. The proportion of evasive prey eaten was a significant predictor of forelimb length when phylogeny was taken into account. We conclude that, in Nucras, the percentage of evasive prey taken co-evolves with forelimb morphology, and dietary niche breadth co-evolves with cranial shape. However, although head width is correlated with the proportion of hard prey eaten, this appears to be the result of shared ancestry rather than adaptive evolution. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110, 674–688.

Proximate determinants of bite force capacity in the mouse lemur

AbstractBoth mating system and diet are thought to drive inter‐individual variation in bite force. Although previously published data suggest that bite force variation may be driven by variation in morphology (e.g. head morphology, body size, muscle size), age and physiology (e.g. fluctuating plasma testosterone levels) in some vertebrates, this remains untested in primates. Here, we explore the proximal determinants of bite force capacity in the grey mouse lemur Microcebus murinus. Our results show that in male grey mouse lemurs, bite force measurements are repeatable across a 1‐month period. Yet, bite forces were independent of fluctuation plasma testosterone levels. Head dimensions and body mass were all positively correlated with bite force. Among these, head width was the best predictor of bite force as has been observed for other vertebrates. Unexpectedly, age was highly significantly and positively correlated with bite force. Whereas older animals generally bit harder, the oldest age group (5.5 years) showed a decline in bite force capacity. These results suggest that bite force in the grey mouse lemur is mostly determined by morphology and age, yet is independent of variation in testosterone. Future studies including a broader age range and animals of different sexes would be of interest to better understand the variation in bite force in this small lemur.

An experimental test of the effect of signal size and performance capacity on dominance in the green anole lizard

Summary1. Many animals use signals to resolve disputes over resources. Some signals act as reliable indicators of other traits, such as whole‐organism performance or body condition, which may also be important for resolving disputes. Because of the correlations inherent in reliable signals, it is challenging to determine which variables are directly relevant for resolving aggressive interactions.2. We examined the relationships among dewlap size, bite force and condition, all traits that may be important to conflict resolution in male green anole lizards (Anolis carolinensis). Using a large sample of wild‐caught animals, we showed significant positive correlations between dewlap size and maximum bite force capacity, when each trait is corrected for its correlation with body size.3. We tested the relative importance of each trait to the outcome of interactions in a subset of our sample. We staged dominance encounters between size‐matched male green anoles after surgically reducing the dewlap size of one competitor.4. We show that reducing the size of the dewlap does not significantly change the outcome of staged interactions. Rather, males with higher values of bite force capacity were more likely to win fights. We hypothesize that during close‐proximity aggressive interactions, male green anoles use more direct means of assessing one another and that dewlap size functions as a signal of bite force primarily during long‐distance territorial displays. Body condition was correlated with bite force, but did not differ significantly between winners and losers.5. Our results show how an experimental approach can decouple reliable signals from their correlated traits to test which factors influence male contest resolution.

Sexual dimorphism, body size, bite force and male mating success in tuatara

287..292 Sexual dimorphisms in body size and head size are common among lizards and are often related to sexual selection on male fighting capacity (organismal performance) and territory defence. However, whether this is generally true or restricted to lizards remains untested. Here we provide data on body and head size, bite performance and indicators of mating success in the tuatara (Sphenodon punctatus), the closest living relative to squamates, to explore the generality of these patterns. First, we test whether male and female tuatara are dimorphic in head dimensions and bite force, independent of body size. Next, we explore which traits best predict bite force capacity in males and females. Finally, we test whether male bite force is correlated with male mating success in a free-ranging population of tuatara (Sphenodon punctatus). Our data confirm that tuatara are indeed dimorphic in head shape, with males having bigger heads and higher bite forces than females. Across all individuals, head length and the jaw closing in-lever are the best predictors of bite force. In addition, our data show that males that are mated have higher absolute but not relative bite forces. Bite force was also significantly correlated to condition in males but not females. Whereas these data suggest that bite force may be under sexual selection in tuatara, they also indicate that body size may be the key trait under selection in contrast to what is observed in squamates that defend territories or resources by biting. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100, 287‐292.

Effects of testosterone on morphology, performance and muscle mass in a lizard

Because sexual selection pressures are high in sexually dimorphic organisms, morphological, physiological and performance traits are often studied in a sexual selection context. The proximate mechanisms underlying evolutionary change in these traits, however, remain largely unstudied. Here, we examined the role of steroids in shaping morphology and physiological performance in males of a sexually dimorphic lizard (Gallotia galloti). We compared morphology and physiological performance of males with experimentally elevated testosterone levels to sham-operated males. Before surgery, inter-individual variation in plasma testosterone levels correlated positively with bite force capacity. Administration of exogenous testosterone resulted in an increase of the mass of both jaw closing and locomotory muscles compared with sham-operated individuals, but the responsiveness varied considerably among muscle groups. In contrast to our expectations, the dramatic testosterone-induced changes in muscle masses did not result in concordant changes in bite force performance or sprint speed.

The mystery of the missing toes: extreme levels of natural mutilation in island lizard populations

Summary 1. Ecological interactions that involve aggressive confrontations between animals are important in shaping the evolution of morphology, behaviour and life history. However, as such confrontations are rarely witnessed, direct quantification of the intensity of these processes in natural populations is notoriously difficult. While the utilization of the frequency of non‐lethal injuries is fraught with difficulties, it may provide information concerning different types of interaction, such as predation, intraspecific aggression and interspecific interference competition. 2. In this paper, we report on an exceptionally large difference in toe loss incidence between two populations of Podarcis sicula lizards living on small, neighbouring islands in the Adriatic Sea. We caught 900 lizards and recorded the number and location of missing toes. Subsequently, we investigated five non‐mutually exclusive hypotheses concerning differences in bite force capacity, bone strength, predation intensity, average age and intraspecific aggression that may provide proximate explanations for the observed differences in injury frequencies. 3. Bite force differences differed considerably between the populations, but bone strength was found to be stronger in the populations with a higher frequency of natural scars. Predation pressure clearly differed between the populations, but we found higher injury rates under predation relaxation. 4. Our results indicate that density and consequently an increased intraspecific competition is the most likely explanation for the observed high frequencies of injuries. We suggest that the intensity of toe amputation between lizard populations may be a useable indirect indication for the intensity of intraspecific competition. 5. This study shows how a combination of morphological, physiological, behavioural and ecological measurements can be used to test assumptions implicit to alternative explanations of an observed phenomenon. Such tests can reveal how likely each of these explanations is, even if the processes leading to the phenomenon are difficult to observe directly.

The fitness advantage of a high-performance weapon

Weapons used in combat between males are usually attributed to sexual selection, which operates via a fitness advantage for males with weapons of better ‘quality’. Because the performance capacity of morphological traits is typically considered the direct target of selection, Darwin’s intrasexual selection hypothesis can be modified to predict that variation in reproductive success should be explained by variation in performance traits relevant to combat. Despite such a straightforward prediction, tests of this hypothesis are conspicuously lacking. We show that territorial male collared lizards with greater bite-force capacity sire more offspring than weaker biting rivals but exhibit no survival advantage. We did not detect stabilizing or disruptive selection on bite-force capacity. Taken together, these results support the hypothesis that superior weapon performance provides a fitness advantage through increased success in male contests. Sexual selection on weapon performance therefore appears to be a force driving the evolution and maintenance of sexual dimorphism in head shape. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 96, 840‐845.

Relationships between hormones, physiological performance and immunocompetence in a color-polymorphic lizard species, Podarcis melisellensis

Species with alternative phenotypes offer unique opportunities to investigate hormone–behavior relationships. We investigated the relationships between testosterone, corticosterone, morphology, performance, and immunity in a population of lizards ( Podarcis melisellensis) which exhibits a color polymorphism. Males occur in three different color morphs (white, yellow, orange), providing an opportunity to test the idea of morphs being alternative solutions to the evolutionary challenges posed on the link between hormones, morphology, performance, and immunity. Morphs differed in bite force capacity, with orange males biting harder, and in corticosterone levels, with yellow males having lower levels than orange. However, morphs did not differ in testosterone levels or in the immunological parameters tested. At the individual level, across morphs, testosterone levels predicted size-corrected bite force capacity, but no relation was found between hormone levels and immunity. Our results do not support the testosterone-based polymorphism hypothesis and reject the hypothesis of a trade-off between testosterone and immunity in this species, but provide a mechanistic link between testosterone and a sexually selected performance trait.

Force–velocity trade‐off in Darwin's finch jaw function: a biomechanical basis for ecological speciation?

1 Biomechanical trade-offs have been proposed to constrain trajectories of evolutionary diversification. In songbirds, however, one such trade-off may facilitate diversification; adaptations that enhance bite force capacity are assumed to constrain vocal performance by hampering velocities of beak gape modulations required for vocal resonance tracking during song production. Resulting divergence in vocal mating signals may thus generate mating isolation between groups that eat foods of differing size and hardness. 2 We tested for a force–velocity trade-off in jaw function in Darwin's finches, by measuring bite forces and jaw movements during song production in birds on Santa Cruz Island. Bite force and speed of jaw closing varied broadly in our sample, and were negatively correlated both within and among species. Moreover, these correlations were largely independent of overall body size and phylogenetic relationships. 3 Adaptations to varying food types thus appear to drive divergence not only in beak size and bite force, but also in jaw closing velocity and vocal performance capacity. These results support a biomechanical link between adaptive divergence and mating signal divergence, the two key features that were assumed to have driven this radiation.

Is Extreme Bite Performance Associated with Extreme Morphologies in Sharks?

As top predators in many oceanic communities, sharks are known to eat large prey and are supposedly able to generate high bite forces. This notion has, however, largely gone untested due to the experimental intractability of these animals. For those species that have been investigated, it remains unclear whether their high bite forces are simply a consequence of their large body size or the result of diet-related adaptation. As aquatic poikilotherms, sharks can grow very large, making them ideal subjects with which to investigate the effects of body size on bite force. Relative bite-force capacity is often associated with changes in head shape because taller or wider heads can, for example, accommodate larger jaw muscles. Constraints on bite force in general may also be released by changes in tooth shape. For example, more pointed teeth may allow a predator to penetrate prey more effectively than blunt, pavementlike teeth. Our analyses show that large sharks do not bite hard for their body size, but they generally have larger heads. Head width is the best predictor of bite force across the species included in our study as indicated by a multiple regression model. Contrary to our predictions, sharks with relatively high bite forces for their body size also have relatively more pointed teeth at the front of the tooth row. Moreover, species including hard prey in their diet are characterized by high bite forces and narrow and pointed teeth at the jaw symphysis.

Morphological and mechanical determinants of bite force in bats: do muscles matter?

Bats are one of the most diverse groups of mammals and have radiated into a wide variety of trophic niches. Accordingly, the cranial structure in bats is unusually variable among mammals and thought to reflect specializations for feeding and echolocation. However, recent analyses of cranial structure, feeding behavior and bite force across a wide range of bats suggest that correlations between morphology and performance and/or ecology are not as clearcut as previously thought. For example, most of the variation in bite force across a wide range of phyllostomid bats was explained by differences in body size rather than specific cranial traits. However, remarkably little is known about the muscular components that are responsible for generating the actual bite forces. We have tested which aspects of the cranial muscular system are good predictors of bite force across a wide range of species using a modeling approach. Model calculations of bite force show good correspondence with in vivo data suggesting that they can be used to estimate performance of the cranial system. Moreover, our data show that bite force is strikingly well explained by differences in temporalis muscle mass, temporalis fiber length and masseter muscle mass. Moreover, our data show that evolutionary changes in bite force capacity in bats are associated with evolutionary changes in relative m. temporalis mass and absolute skull length.

Sexual Size Dimorphisms and Bite Force in The Northern Alligator Lizard, Elgaria Coerulea

Abstract Bite force capacity is a directly related performance correlate of head shape and is an integrative measure of performance in capturing and handling prey, fighting, and mating, especially for males. We investigated head shape and bite force dimorphisms in a small semifossorial lizard (Elgaria coerulea) that exhibits females-larger sexual size dimorphism (FL-SSD) in some populations but not in others. Specifically, we explored how body size, head shape, and bite force compare between the sexes relative to any dimorphisms in body or head size in a coastal population of E. coerulea. Female larger-SSD was confirmed for this population, but it contrasted with males-larger sexual dimorphism in head size (ML-SDHS). Males also had greater bite force than females of similar size. The secretive habit of E. coerulea hampers observations of behavioral interactions among conspecifics. However, it is expected that lizards with FL-SSD will have ML-SDHS if males with greater bite force win fights with other male...