Is dietary niche breadth linked to morphology and performance in Sandveld lizards Nucras (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.     

Insularity affects head morphology,bite force and diet in a Mediterranean lizard

Island environments differ with regard to numerous features from the mainland and may induce large‐scale changes in most aspects of the biology of an organism. In this study, we explore the effect of insularity on the morphology and performance of the feeding apparatus, a system crucial for the survival of organisms. To this end, we examined the head morphology and feeding ecology of island and mainland populations of the Balkan green lizard, Lacerta trilineata. We predicted that head morphology, performance and diet composition would differ between sexes and habitats as a result of varying sexual and natural selection pressures. We employed geometric morphometrics to test for differences in head morphology, measured bite forces and analysed the diet of 154 adult lizards. Morphological analyses revealed significant differences between sexes and also between mainland and island populations. Relative to females, males had larger heads, a stronger bite and consumed harder prey than females. Moreover, island lizards differed in head shape, but not in head size, and, in the case of males, demonstrated a higher bite force. Islanders had a wider food niche breadth and included more plant material in their diet. Our findings suggest that insularity influences feeding ecology and, through selection on bite force, head morphology. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 112 , 469–484.     

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

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.     

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

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 variation. 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.     

Gape and bite force in the rodents Onychomys leucogaster and Peromyscus maniculatus: Does jaw‐muscle anatomy predict performance?

Compared with the deer mouse, Peromyscus maniculatus, the grasshopper mouse, Onychomys leucogaster, exhibits modifications in its jaw‐muscle architecture that promote wide gapes and large bite forces at wide gapes to prey upon large vertebrate prey. In this study, we determine whether jaw‐muscle anatomy predicts gape and biting performance in O. leucogaster, and we also assess the influence of gape on bite force in the two species. Although O. leucogaster has an absolutely longer jaw, which facilitates larger gapes, maximum passive gape is similar in both species, averaging ～12.5 mm. Thus, when scaled to jaw length, O. leucogaster has a smaller maximum passive gape. These results suggest that predatory behaviors of O. leucogaster may not require remarkably large gapes. On the other hand, both absolute and relative bite forces exerted by O. leucogaster are significantly larger than those of P. maniculatus. The largest bite forces in both species occur at 5.0 mm of gape at the incisors, or 40% of maximum gape. Although bite force in both species decreases at larger gapes, O. leucogaster does maintain a larger percentage of maximum bite force at gapes larger than 40% of maximum passive gape. Therefore, although structural modifications in the masticatory apparatus of O. leucogaster may constrain gape, they may help to maintain bite force at large gapes. These results suggest that increases in gape differentially influence the length‐tension properties of the jaw muscles in the two species. Finally, these results highlight the importance of considering the effect of muscle stretch on force production in comparative studies of bite force. As a first approximation, it appears that gapes of 40–50% of maximum gape in rodents optimizes bite force production at the incisors. J. Morphol., 2009. © 2009 Wiley‐Liss, Inc.     

Push and bite: trade‐offs between burrowing and biting in a burrowing skink (Acontias percivali)

Trade‐offs are thought to be important in constraining evolutionary divergence, as they may limit phenotypic diversification. Limbless animals that burrow head‐first have been suggested to be evolutionarily constrained in the development of a large head size and sexual head shape dimorphism because of potential trade‐offs associated with burrowing. Here we use an acontiine skink (Acontias percivali) to test for the existence of trade‐offs between traits thought to be important in burrowing (speed and force). As head size dimorphism has been shown to be limited in acontiine lizards, thus suggesting constraints on head size and shape, we additionally explore the potential for trade‐offs between burrowing and biting. Our data show that A. percivali uses a burrowing style different from those previously described for caecilians and amphisbaenians, which relies on the use of extensive lateral and dorsoventral head movements. Our data also show that animals use their entire bodies to generate force, as peak force was determined by total length only. Additionally, both bite force and the time needed to burrow into the substrate were principally determined by relative head width, suggesting a trade‐off between biting and burrow speed. Performance data were indeed suggestive of a correlation between bite force and the time needed to burrow, but additional data are needed to confirm this pattern. In summary, our data suggests that trade‐offs may exist, and may have been of crucial importance in shaping the evolution of head shape in A. percivali, and burrowing lizards more generally. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102 , 91–99.     

Comparative bite forces and canine bending strength in feline and sabretooth felids: implications for predatory ecology

The sabretooth felids were widespread across much of the world in the Late Tertiary, and appear to have been an important group of large predators. Owing to the substantially different skull morphology of derived sabretooths compared with extant felids, there has been considerable debate over the killing mode, bite forces, and bending strength of the large upper canines, and over the implications of these characteristics on feeding ecology. Debates have, however, usually been based on indirect comparisons of force vectors. In this paper, I provide assessments of the estimated force output from the jaw adductor muscles, based on estimates of muscle cross-sectional areas and force vectors, along with canine bending strengths, in a variety of sabretooth felids, in comparison with extant felids. In general, sabretoothed felids had moderately powerful bites, albeit with less jaw adductor power for their body sizes compared with extant felids, sometimes markedly so. Less derived sabrecats appear to have had proportionally higher bite forces than derived forms. The length of the upper canines seemingly compromised their bending strength at any given body size, and again this was most marked in derived forms. However, compared with estimated jaw adductor forces, the canines of sabrecats appear, if anything, to have been stronger than those of extant conical-toothed felids. It has previously been suggested that large sabretoothed felids hunted large prey with a canine shearing bite, powered in part by the jaw adductors and in part by the muscles of the upper neck–occipital region. The present results of canine bending strengths versus the predicted bite force from the jaw adductors supports this suggestion.  © 2007 The Linnean Society of London, Zoological Journal of the Linnean Society , 2007, 151 , 423–437.     

Some Like It Hot: Camera Traps Unravel the Effects of Weather Conditions and Predator Presence on the Activity Levels of Two Lizards

It is generally assumed that favourable weather conditions determine the activity levels of lizards, because of their temperature-dependent behavioural performance. Inactivity, however, might have a selective advantage over activity, as it could increase survival by reducing exposure to predators. Consequently, the effects of weather conditions on the activity patterns of lizards should be strongly influenced by the presence of predators. Using remote camera traps, we test the hypothesis that predator presence and weather conditions interact to modulate daily activity levels in two sedentary cordylid lizards, Karusasaurus polyzonus and Ouroborus cataphractus. While both species are closely related and have a fully overlapping distribution, the former is a fast-moving lightly armoured lizard, whereas the latter is a slow-moving heavily armoured lizard. The significant interspecific difference in antipredator morphology and consequently differential vulnerability to aerial and terrestrial predators, allowed us to unravel the effects of predation risk and weather conditions on activity levels. Our results demonstrate that K. polyzonus is predominantly active during summer, when ambient temperatures are favourable enough to permit activity. In contrast, a peak in activity during spring was observed in O. cataphractus, with individuals being inactive during most of summer. While favourable weather conditions had a strong effect on the activity levels of K. polyzonus, no such relationship was present in O. cataphractus. Contrary to our hypothesis, the presence of terrestrial predators does not seem to affect daily activity levels or alter the influence of weather conditions on activity levels. We conclude that inactivity in O. cataphractus appears to be related to seasonal differences in vulnerability to predators, rather than the presence of predators, and highlight the importance of additional selective pressures, such as food abundance, in determining the species’ activity levels.     

Functional and ecological relevance of intraspecific variation in body size and shape in the lizard Podarcis melisellensis (Lacertidae)

Within populations, individual animals may vary considerably in morphology and ecology. The degree to which variation in morphology is related to ecological variation within a population remains largely unexplored. We investigated whether variation in body size and shape among sexes and age classes of the lizard Podarcis melisellensis translates in differential whole-animal performance (sprint speed, bite force), escape and prey attack behaviour in the field, microhabitat use and diet. Male and female adult lizards differed significantly in body size and head and limb proportions. These morphological differences were reflected in differences in bite strength, but not in sprint speed. Accordingly, field measurements of escape behaviour and prey attack speed did not differ between the sexes, but males ate larger, harder and faster prey than females. In addition to differences in body size, juveniles diverged from adults in relative limb and head dimensions. These shape differences may explain the relatively high sprint and bite capacities of juvenile lizards. Ontogenetic variation in morphology and performance is strongly reflected in the behaviour and ecology in the field, with juveniles differing from adults in aspects of their microhabitat use, escape behaviour and diet.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 251–264.     

Skull morphology and functionality of extant Felidae (Mammalia: Carnivora): a phylogenetic and evolutionary perspective

Felids morphology and ecological role as hypercarnivores are quite constant, despite considerable body size variation among species. Skull morphological and functional features of 34 extant cat species were evaluated under a phylogenetic framework of the Felidae. Twenty skull measurements were analysed through Principal Component Analysis to assess the species morphofunctional spaces. Force indexes were obtained from static equilibrium equations to infer jaw mechanics. Correlations between morphological, functional, and ecological traits were tested by phylogenetically independent contrasts. In spite of the general cat‐like pattern, specific features on the skulls allowed differentiation among groups. Acinonyx jubatus, for instance, showed a shorter and shallower temporal fossa than other big cats, and their bite functionality is marked by an increased contribution of the masseteric system. A morphofunctional dichotomy between Neotropical and Eurasian/African small cats was detected, and is associated with the major transversal axes of the skulls. According to the contrast analyses, the skull size is correlated with the bite force and prey size, but it is uncorrelated with the variations on jaw mechanics (from temporalis or masseter muscle optimizations). Also, there was no correlation between functional differences on jaw muscles and the ratio of prey weight to cat weight. The efficiency of the jaw apparatus among cats is quite consistent; therefore, the different evolutionary trends of jaw mechanics seem to be caused by the casuistic fixation of phenotypical variations, rather than by specific adaptative selections. © 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011, 161 , 414–462.     

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.     

Tooth root morphology as an indicator for dietary specialization in carnivores (Mammalia: Carnivora)

The Carnivora occupy a wide range of feeding niches in concordance with the enormous diversity in their skull and dental form. It is well established that differences in crown morphology are linked to variations in the material properties of the foods ingested and masticated. However, how tooth root form is related to dietary specialization is less well known. In the present study, we investigate the relationship between tooth root morphology and dietary specialization in terrestrial carnivores (canids, felids, hyaenids, and ursids). We specifically address the question of how variation in tooth root surface area is related to bite force potentials as one of the crucial masticatory performance parameters in feeding ecology. We applied computed tomography imaging to reconstruct and quantify dental root surface area in 17 extant carnivore species. Moreover, we computed maximal bite force at several tooth positions based on a dry skull model and assessed the relationship of root surface area to skull size, maximal bite force, food properties, and prey size. We found that postcanine tooth root surface areas corrected for skull size serve as a proxy for bite force potentials and, by extension, dietary specialization in carnivores. Irrespective of taxonomic affinity, species that feed on hard food objects have larger tooth roots than those that eat soft or tough foods. Moreover, carnivores that prey on large animals have larger tooth root surface areas. Our results show that tooth root morphology is a useful indicator of bite force production and allows inferences to be made about dietary ecology in both extant and extinct mammals. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105, 456–471.     

Ecological consequences of ontogenetic changes in head shape and bite performance in the Jamaican lizard Anolis lineatopus

It has been documented extensively that body size affects the physiology and musculoskeletal function of organisms. However, less well understood is how body size affects the ecology of organisms through its effects on physiology and performance. We explored the effects of body size on morphology and performance in different ontogenetic classes and sexes of a common Anolis lizard ( A. lineatopus ). Next, we tested whether these morphological and performance differences may affect functional aspects of the diet such as prey size and prey hardness. Our data showed that males, females and juveniles differ significantly in head size, head shape and bite force. Multiple regression models indicated that head shape and bite force are significantly correlated to prey size and hardness. Yet juveniles had relatively large heads and bit disproportionately hard for their size, allowing them to eat prey as large as those of females. However, for a given prey size, males and females ate more robust prey than did juveniles. Additionally, males ate relatively harder prey than did juveniles. These data suggest that: (1) body size affects the dietary ecology of animals through its effect on head size and bite force; (2) changes in head morphology independent of changes in overall size also have important effects on performance and diet.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 89 , 443–454.     

Dangerous food: lacking venom and constriction,how do snake‐like lizards (Lialis burtonis,Pygopodidae) subdue their lizard prey?

Snakes are renowned for their ability to subdue and swallow large, often dangerous prey animals. Numerous adaptations, including constriction, venom, and a strike-and-release feeding strategy, help them avoid injury during predatory encounters. Burton's legless lizard ( Lialis burtonis Gray, Pygopodidae) has converged strongly on snakes. It is functionally limbless and feeds at infrequent intervals on relatively large prey items (other lizards) capable of inflicting a damaging bite. However, L. burtonis possesses neither venom glands, nor the ability to constrict prey. We investigated how L. burtonis subdues its prey without suffering serious retaliatory bites. Experiments showed that lizards modified their strike precision according to prey size; very large prey were always struck on the head or neck, preventing them from biting. In addition, L. burtonis delayed swallowing large lizards until they were incapacitated, whereas smaller prey were usually swallowed while still struggling. Lialis burtonis also displays morphological adaptations protecting it from prey retaliation. Its long snout prevents prey from biting, and it can retract its lidless eyes out of harm's way while holding onto a food item. The present study further clarifies the remarkable convergence between snakes and L. burtonis , and highlights the importance of prey retaliatory potential in predator evolution.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 91 , 719–727.     

Correlations between lizard cranial shape and diet: a quantitative, phylogenetically informed analysis

Although the relationship between dietary and phenotypic specialization has been well documented for many vertebrate groups, it has been stated that few such general trends can be established for lizards. This is often thought to be due to the lack of dietary specialization in many lizards. For example, many species that are reported to be insectivorous may also consume a variety of plant materials, and the reverse is often true as well. In this study, we investigate whether a correlation exists between general cranial form and dietary niche in lizards. Additionally, we test previously proposed hypotheses suggesting that herbivorous lizards should be larger bodied than lizards with other diets. Our data indicate that lizards specializing in food items imposing different mechanical demands on the feeding system show clear patterns of morphological specialization in their cranial morphology. True herbivores (diet of fibrous and tough foliage) are clearly distinguished from omnivorous and carnivorous lizards by having taller skulls and shorter snouts, likely related to the need for high bite forces. This allows herbivores to mechanically reduce relatively less digestible foliage. Carnivores have relatively longer snouts and retroarticular processes, which may result in more efficient capture and processing of elusive prey. When analysed in an explicit phylogenetic context, only snout length and skull mass remained significantly different between dietary groups. The small number of differences in the phylogenetic analyses is likely the result of shared evolutionary history and the relative paucity of independent origins of herbivory and omnivory in our sample. Analyses of the relationship between diet and body size show that on average herbivores have a larger body size than carnivores, with omnivores intermediate between the two other dietary groups. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 86 , 433–466.     

Size-related changes in cranial morphology affect diet in the catfish Clariallabes longicauda

Within the catfish family Clariidae, species exist with different degrees of jaw adductor hypertrophy. This jaw adductor hypertrophy has been related to bite performance, in turn suggesting a link to dietary specialization. Thus, an increase in the degree of hypertrophy will likely be reflected in an increase in the amount of hard prey in the diet. In the present study, we examine the ontogenetic scaling of cranial structure and diet in a species of catfish with a moderate degree of jaw adductor hypertrophy, Clariallabes longicauda . Additionally, we investigate whether the observed changes in the morphology of the feeding system during growth are linked to changes in diet. The fish examined demonstrate a strong positively allometric growth of the jaw adductors, of head height and of maximal head width, suggesting that larger fish can feed on larger and harder prey. Dietary data confirm these hypotheses and reveal an increase in maximal prey size consumed, the proportion of large prey in the diet, and average prey hardness during ontogeny. Moreover, the observed changes in the proportion of large prey consumed and prey hardness are correlated with an increase in lower jaw width and maximal head width, respectively. An increase in the amount of evasive prey in the diet with fish size is correlated with an increase in hyoid length. In summary, not only size dependent, but also size-independent variation of the feeding system was associated with ontogenetic changes in diet in C. longicauda .  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 323–334.     

Sexual dimorphism of head size in Gallotia galloti: testing the niche divergence hypothesis by functional analyses

1. Two often cited hypotheses explaining sexual head size dimorphism in lizards are: sexual selection acting on structures important in intrasexual competition, and reduction of intersexual competition through food niche separation.
2. In this study some implicit assumptions of the latter hypothesis were tested, namely that an increase in gape distance and bite force should accompany the observed increase in head size. These assumptions are tested by recording bite forces, in vivo , for lizards of the species Gallotia galloti . In this species, male lizards have significantly larger heads than female conspecifics of similar snout–vent length.
3. Additionally, the average force needed to crush several potential prey species was determined experimentally and compared with the bite force data. This comparison clearly illustrates that animals of both sexes can bite much harder than required for most insect food items, which does not support the niche divergence hypothesis. The apparent 'excess' bite force in both sexes might be related to the partially herbivorous diet of the animals.
4. To unravel the origin of differences between sexes in bite capacity, the crushing phase of biting was modelled. The results of this model show different strategies in allocation of muscle tissue between both sexes. The origin of this difference is discussed and a possible evolutionary pathway of the development of the sexual dimorphism in the species is provided.     

Feeding ecology of North American gopher snakes (Pituophis catenifer, Colubridae)

Studies of food relations are important to our understanding of ecology at the individual, population and community levels. Detailed documentation of the diet of large‐bodied, widespread snakes allows us to assess size‐dependent and geographical variation in feeding preferences of gape‐limited predators. Furthermore, with knowledge of the food habits of sympatric taxa we can explore possible causes of interspecific differences in trophic niches. The feeding ecology of the North American gopher snake, Pituophis catenifer, was studied based on the stomach contents of more than 2600 preserved and free‐ranging specimens, and published and unpublished dietary records. Of 1066 items, mammals (797, 74.8%), birds (86, 8.1%), bird eggs (127, 11.9%), and lizards (35, 3.3%) were the most frequently eaten prey. Gopher snakes fed upon subterranean, nocturnal and diurnal prey. The serpents are primarily diurnal, but can also be active at night. Therefore, gopher snakes captured their victims by actively searching underground tunnel systems, retreat places and perching sites during the day, or by pursuing them or seizing them while they rested at night. Gopher snakes of all sizes preyed on mammals, but only individuals larger than 40 and 42 cm in snout–vent length took bird eggs and birds, respectively, possibly due to gape constraints in smaller serpents. Specimens that ate lizards were smaller than those that consumed mammals or birds. Gopher snakes raided nests regularly, as evidenced by the high frequency of nestling mammals and birds and avian eggs eaten. Most (332) P. catenifer contained single prey, but 95 animals contained 2–35 items. Of the 321 items for which direction of ingestion was determined, 284 (88.5%) were swallowed head‐first, 35 (10.9%) were ingested tail‐first, and two (0.6%) were taken sideways. Heavier gopher snakes took heavier prey, but heavier serpents ingested prey with smaller mass relative to snake mass, evidence that the lower limit of prey mass did not increase with snake mass. Specimens from the California Province and Arid Deserts (i.e. Mojave, Sonoran and Chihuahuan Deserts) took the largest proportion of lizards, whereas individuals from the Great Basin Desert consumed a higher percentage of mammals than serpents from other areas, and P. catenifer from the Great Plains ate a greater proportion of bird eggs. Differences in prey availability among biogeographical regions and unusual circumstances of particular gopher snake populations may account for these patterns. Gopher snakes have proportionally longer heads than broadly sympatric Rhinocheilus lecontei (long‐nosed snake), Charina bottae (rubber boa) and Lampropeltis zonata (California mountain kingsnake), which perhaps explains why, contrary to the case in P. catenifer, the smaller size classes of those three species do not eat mammals. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 77 , 165–183.     

An analysis of the relative roles of plasticity and natural selection in the morphology and performance of a lizard (<Emphasis Type="Italic">Urosaurus ornatus</Emphasis>)

Evolutionary ecologists have devoted substantial attention to understanding which factors dictate processes of mortality within populations. Our goal was to understand the dynamics of natural selection on two performance traits (bite force and sprint speed) and associated morphological variables. We first quantified performance and morphology for a sample of marked tree lizards (Urosaurus ornatus) at the middle of the breeding season. We then sampled the same population in the nonbreeding season to determine which of the original lizards survived, and we also remeasured morphological and performance variables for surviving lizards. We found evidence for directional selection favoring fast sprinters in male lizards, but also a nonsignificant stabilizing trend that disfavored the very fastest lizards. However, we also detected substantial seasonal plasticity in bite force and head width, suggesting that an analysis of selection on only preselection (breeding season) values may be overly simplistic. Urosaurus males and females with low bite forces (and narrow heads) in the breeding season generally increased their bite forces and head widths during the nonbreeding season. In contrast, lizards that were initially strong biters in the breeding season diminished in head width and declined dramatically in bite force (up to about 35%). We suggest that seasonal plasticity could act as a retarding force for selection on performance, and could dampen seasonal and year-to-year fluctuations in selective pressures. We argue that this phenomenon may be particularly likely for performance traits important for social interactions related to breeding, such as bite force.     

Feeding behaviour and bite force of sabretoothed predators

The feeding behaviour of extinct sabretoothed predators (machaeroidines, nimravids, barbourofelids, machairodonts and thylacosmilines) is investigated using beam theory. Because bite force applied along the mandible should be proportional to the external dimension of the mandibular corpus, patterns of variation in these dimensions at interdental gaps will reflect the adaptation of the jaw to specific loads, related to killing methods. Comparison of the mandibular force profiles of sabretooths to those of extant conical‐toothed carnivorans of known feeding behaviour reveals that sabretooths had a powerful bite, as strong or stronger than extant felids of similar mandibular length. Loads exerted at the lower canine were better constrained in the sagittal plane than in extant conical‐toothed carnivorans, indicating that prey was efficiently restrained when the sabre bite was delivered. The mandibular symphysis is generally better buttressed dorsoventrally in dirk‐toothed sabretooths than in scimitar‐toothed sabretooths, implying different killing strategies for the two ecomorphs: dirktooths delivered powerful sabre bites on prey they restrained with their forelimbs, while scimitartooths delivered slashing sabre bites and may have used their incisor battery to subdue their prey. The mandibular symphysis of Smilodon fatalis is less buttressed dorsoventrally than that of other dirk‐toothed sabretooths, possibly as a consequence of the greater torsional stresses induced while feeding rapidly on carcasses in response to intense competition. The mandibular symphysis of Thylacosmilus atrox is better buttressed dorsoventrally in juveniles than in adults, suggesting that young marsupial sabretooths underwent an extended period of parental care as typically observed in modern felids and inferred for eutherian sabretooths. Finally, machaeroidines and the nimravid Nimravus brachyops are exceptional in exhibiting a degree of dorsoventral buttressing of the mandibular symphysis that is intermediate between advanced sabretooths and conical‐toothed felids but similar to the extant Neofelis nebulosa, suggesting that the latter taxon may be close to the ancestral condition of a new sabretooth radiation. © 2005 The Linnean Society of London, Zoological Journal of the Linnean Society, 2005, 145 , 393–426.