Form-function relationships underlie rapid dietary changes in a lizard

Macroevolutionary changes such as variation in habitat use or diet are often associated with convergent, adaptive changes in morphology. However, it is still unclear how small-scale morphological variation at the population level can drive shifts in ecology such as observed at a macroevolutionary scale. Here, we address this question by investigating how variation in cranial form and feeding mechanics relate to rapid changes in diet in an insular lizard (Podarcis siculus) after experimental introduction into a new environment. We first quantified differences in the skull shape and jaw muscle architecture between the source and introduced population using three-dimensional geometric morphometrics and dissections. Next, we tested the impact of the observed variation in morphology on the mechanical performance of the masticatory system using computer-based biomechanical simulation techniques. Our results show that small differences in shape, combined with variation in muscle architecture, can result in significant differences in performance allowing access to novel trophic resources. The confrontation of these data with the already described macroevolutionary relationships between cranial form and function in these insular lizards provides insights into how selection can, over relatively short time scales, drive major changes in ecology through its impact on mechanical performance.

The relationship between bite force, morphology, and diet in southern African agamids

Background

Many animals display morphological and behavioural adaptations to the habitats in which they live and the resources they exploit. Bite force is an important whole-organism performance trait that allows an increase in dietary breadth, the inclusion of novel prey in the diet, territory and predatory defence, and is important during mating in many lizards.

Methods

Here, we study six species of southern African agamid lizards from three habitat types (ground-dwelling, rock-dwelling, and arboreal) to investigate whether habitat use constrains head morphology and bite performance. We further tested whether bite force and head morphology evolve as adaptations to diet by analysing a subset of these species for which diet data were available.

Results

Overall, both jaw length and its out-lever are excellent predictors of bite performance across all six species. Rock-dwelling species have a flatter head relative to their size than other species, possibly as an adaptation for crevice use. However, even when correcting for jaw length and jaw out-lever length, rock-dwelling species bite harder than ground-dwelling species. Diet analyses demonstrate that body and head size are not directly related to diet, although greater in-levers for jaw closing (positively related to bite force) are associated to an increase of hard prey in the diet. Ground-dwelling species consume more ants than other species.

Conclusions

Our results illustrate the role of head morphology in driving bite force and demonstrate how habitat use impacts head morphology but not bite force in these agamids. Although diet is associated with variation in head morphology it is only partially responsible for the observed differences in morphology and performance.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01859-w.

Morphology, locomotor performance and habitat use in southern African agamids

Understanding the relationships between form and function can help us to understand the evolution of phenotypic diversity in different ecological contexts. Locomotor traits are ecologically relevant as they reflect the ability of an organism to escape from predators, to catch prey or to defend territories. As such, locomotion provides a good model to investigate how environmental constraints may influence an organism’s performance. Here, we investigate the ecomorphological relationships between limb morphology, locomotor performance (sprint speed and endurance) and habitat use in six southern African agamid species. The investigated agamid species showed differences in hind limb and toe lengths. Both of these traits were further correlated with endurance capacity. This association was supported by stepwise multiple regression analyses. However, we demonstrate trade-offs in locomotor performance traits, suggesting that specialization towards speed comes at the detriment of endurance capacity. Overall, the single arboreal species studied had longer hind limbs, a higher exertion capacity and a higher mean speed. However, for a given hind limb length, the arboreal species was slower than the other habitat specialists. This study provides insights into the evolutionary mechanisms that have driven the morphological and functional evolution in southern African agamid lizards.

Broader head, stronger bite: In vivo bite forces in European eel Anguilla anguilla

This work examined three different phenotypes of the yellow-eel stage of the European eel Anguilla anguilla, broad-heads, narrow-heads and eels with an intermediate head shape. The aim was to see whether broad-headed A. anguilla, which generally consume harder, larger prey, such as crustaceans and fish, exerted greater bite force than the narrow-headed variant, which mainly consume soft, small prey such as chironomid larvae. It was found that in 99 yellow A. anguilla, in vivo bite force of broad-heads are higher compared with narrow-heads and intermediates.

Masticatory muscle architecture in a water-rat from Australasia (Murinae, Hydromys) and its implication for the evolution of carnivory in rodents

Murines are well known for their generalist diet, but several of them display specializations towards a carnivorous diet such as the amphibious Indo-Pacific water-rats. Despite the fact that carnivory evolved repeatedly in this group, few studies have investigated associated changes in jaw muscle anatomy and biomechanics. Here, we describe the jaw muscles and cranial anatomy of a carnivorous water-rat, Hydromys chrysogaster. The architecture of the jaw musculature of six specimens captured both on Obi and Papua were studied and described using dissections. We identified the origin and insertions of the jaw muscles, and quantified muscle mass, fiber length, physiological cross-sectional area, and muscle vectors for each muscle. Using a biomechanical model, we estimated maximum incisor and molar bite force at different gape angles. Finally, we conducted a 2D geometric morphometric analyses to compare jaw shape, mechanical potential, and diversity in lever-arm ratios for a set of 238 specimens, representative of Australo-Papuan carnivorous and omnivorous murids. Our study reveals major changes in the muscle proportions among Hydromys and its omnivorous close relative, Melomys. Hydromys was found to have large superficial masseter and temporalis muscles as well as a reduced deep masseter and zygomatico-mandibularis, highlighting major functional divergence among omnivorous and carnivorous murines. Changes in these muscles are also accompanied by changes in jaw shape and the lines of action of the muscles. A more vertically oriented masseter, reduced masseteric muscles, as well as an elongated jaw with proodont lower incisors are key features indicative of a reduced propalinality in carnivorous Hydromys. Differences in the fiber length of the masseteric muscles were also detected between Hydromys and Melomys, which highlight potential adaptations to a wide gape in Hydromys, allowing it to prey on larger animals. Using a biomechanical model, we inferred a greater bite force in Hydromys than in Melomys, implying a functional shift between omnivory and carnivory. However, Melomys has an unexpected greater bite force at large gape compared with Hydromys. Compared with omnivorous Melomys, Hydromys have a very distinctive low mandible with a well-developed coronoid process, and a reduced angular process that projects posteriorly to the ascending rami. This jaw shape, along with our mechanical potential and jaw lever ratio estimates, suggests that Hydromys has a faster jaw closing at the incisor, with a higher bite force at the level of the molars. The narrowing of the Hydromys jaw explains this higher lever advantage at the molars, which constitutes a good compromise between a wide gape, a reduced anterior masseteric mass, and long fiber lengths. Lever arms of the superficial and deep masseter are less favourable to force output of the mandible in Hydromys but more favourable to speed. Compared with the small input lever arm defined between the condyle and the angular process, the relatively longer mandible of Hydromys increases the speed at the expense of the output force. This unique combination of morphological features of the masticatory apparatus possibly has permitted Hydromys to become a highly successful amphibious predator in the Indo-Pacific region.

Darwin's finches and their diet niches: the sympatric coexistence of imperfect generalists

Adaptive radiation can be strongly influenced by interspecific competition for resources, which can lead to diverse outcomes ranging from competitive exclusion to character displacement. In each case, sympatric species are expected to evolve into distinct ecological niches, such as different food types, yet this expectation is not always met when such species are examined in nature. The most common hypotheses to account for the coexistence of species with substantial diet overlap rest on temporal variation in niches (often diets). Yet spatial variation in niche overlap might also be important, pointing to the need for spatiotemporal analyses of diet and diet overlap between closely related species persisting in sympatry. We here perform such an analysis by characterizing the diets of, and diet overlap among, four sympatric Darwin's ground finch species at three sites and over 5 years on a single Galápagos island (Santa Cruz). We find that the different species have broadly similar and overlapping diets - they are to some extent generalists and opportunists - yet we also find that each species retains some 'private' resources for which their morphologies are best suited. Importantly, use of these private resources increased considerably, and diet overlap decreased accordingly, when the availability of preferred shared foods, such as arthropods, was reduced during drought conditions. Spatial variation in food resources was also important. These results together suggest that the ground finches are 'imperfect generalists' that use overlapping resources under benign conditions (in space or time), but then retreat to resources for which they are best adapted during periods of food limitation. These conditions likely promote local and regional coexistence.

Is the whole more than the sum of its parts? Evolutionary trade-offs between burst and sustained locomotion in lacertid lizards

Trade-offs arise when two functional traits impose conflicting demands on the same design trait. Consequently, excellence in one comes at the cost of performance in the other. One of the most widely studied performance trade-offs is the one between sprint speed and endurance. Although biochemical, physiological and (bio)mechanical correlates of either locomotor trait conflict with each other, results at the whole-organism level are mixed. Here, we test whether burst (speed, acceleration) and sustained locomotion (stamina) trade off at both the isolated muscle and whole-organism level among 17 species of lacertid lizards. In addition, we test for a mechanical link between the organismal and muscular (power output, fatigue resistance) performance traits. We find weak evidence for a trade-off between burst and sustained locomotion at the whole-organism level; however, there is a significant trade-off between muscle power output and fatigue resistance in the isolated muscle level. Variation in whole-animal sprint speed can be convincingly explained by variation in muscular power output. The variation in locomotor stamina at the whole-organism level does not relate to the variation in muscle fatigue resistance, suggesting that whole-organism stamina depends not only on muscle contractile performance but probably also on the performance of the circulatory and respiratory systems.

Why the long face? A comparative study of feeding kinematics of two pipefishes with different snout lengths

This study showed that the mouth of Doryrhamphus dactyliophorus, a species with a relatively long snout, travels a greater distance compared with Doryrhamphus melanopleura, a species with a considerably shorter snout, allowing it to strike at prey that are farther away from the mouth. The long-snouted species also tended to reach significantly higher linear velocities of the mouth approaching the prey. On the other hand, D. melanopleura needed less time to capture its prey. A striking difference in prey-capture success was observed between species: D. melanopleura and D. dactyliophorus had a prey-capture success of 91 and 31%, respectively. The small prey size and the relatively large distance between eyes and prey are potential reasons why directing the mouth accurately to the prey is difficult in D. dactyliophorus, hence possibly explaining the lower prey-capture success in this long-snouted species.

Scaling and shear transformations capture beak shape variation in Darwin's finches

Evolution by natural selection has resulted in a remarkable diversity of organism morphologies that has long fascinated scientists and served to establish the first relations among species. Despite the essential role of morphology as a phenotype of species, there is not yet a formal, mathematical scheme to quantify morphological phenotype and relate it to both the genotype and the underlying developmental genetics. Herein we demonstrate that the morphological diversity in the beaks of Darwin's Finches is quantitatively accounted for by the mathematical group of affine transformations. Specifically, we show that all beak shapes of Ground Finches (genus Geospiza) are related by scaling transformations (a subgroup of the affine group), and the same relationship holds true for all the beak shapes of Tree, Cocos, and Warbler Finches (three distinct genera). This analysis shows that the beak shapes within each of these groups differ only by their scales, such as length and depth, which are genetically controlled by Bmp4 and Calmodulin. By measuring Bmp4 expression in the beak primordia of the species in the genus Geospiza, we provide a quantitative map between beak morphology and the expression levels of Bmp4. The complete morphological variation within the beaks of Darwin's finches can be explained by extending the scaling transformations to the entire affine group, by including shear transformations. Altogether our results suggest that the mathematical theory of groups can help decode morphological variation, and points to a potentially hierarchical structure of morphological diversity and the underlying developmental processes.

Chewing variation in lepidosaurs and primates

Mammals chew more rhythmically than lepidosaurs. The research presented here evaluated possible reasons for this difference in relation to differences between lepidosaurs and mammals in sensorimotor systems. Variance in the absolute and relative durations of the phases of the gape cycle was calculated from kinematic data from four species of primates and eight species of lepidosaurs. The primates exhibit less variance in the duration of the gape cycle than in the durations of the four phases making up the gape cycle. This suggests that increases in the durations of some gape cycle phases are accompanied by decreases in others. Similar effects are much less pronounced in the lepidosaurs. In addition, the primates show isometric changes in gape cycle phase durations, i.e. the relative durations of the phases of the gape cycle change little with increasing cycle time. In contrast, in the lepidosaurs variance in total gape cycle duration is associated with increases in the proportion of the cycle made up by the slow open phase. We hypothesize that in mammals the central nervous system includes a representation of the optimal chew cycle duration maintained using afferent feedback about the ongoing state of the chew cycle. The differences between lepidosaurs and primates do not lie in the nature of the sensory information collected and its feedback to the feeding system, but rather the processing of that information by the CNS and its use feed-forward for modulating jaw movements and gape cycle phase durations during chewing.

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.

Fightversusflight: physiological basis for temperature-dependent behavioral shifts in lizards

Previous studies have demonstrated that a behavioral shift from flight to aggressive behavior occurs at low temperatures in some lizards. Our data for the agamid lizard Trapelus pallida demonstrate how the effect of temperature on whole organism performance traits such as sprint speed (much lower performance at lower temperature) and bite force (largely independent of temperature) may explain the shift from flight to fight behavior with decreasing temperature. Moreover, our data hint at the physiological basis for this effect as isolated muscle power output, twitch and tetanus time traits,relevant to sprinting, appear to be strongly temperature-dependent muscle properties. Maximal muscle force production, on the other hand, appears largely independent of temperature. Unexpectedly, differences in the physiological properties of jaw versus limb muscle were observed that enhance the ability of the jaw muscle to generate maximal force at all temperatures tested. Thus our data show how behavioral responses may be determined by the limitations set by temperature on physiological processes.

Morphological integration and adaptation in the snake feeding system: a comparative phylogenetic study

A long-standing hypothesis for the adaptive radiation of macrostomatan snakes is that their enlarged gape--compared to both lizards and basal snakes--enables them to consume "large" prey. At first glance, this hypothesis seems plausible, or even likely, given the wealth of studies showing a tight match between maximum consumed prey mass and head size in snakes. However, this hypothesis has never been tested within a comparative framework. We address this issue here by testing this hypothesis in 12 monophyletic clades of macrostomatan snakes using recently published phylogenies, published maximum consumed prey mass data and morphological measurements taken from a large sample of museum specimens. Our nonphylogenetically corrected analysis shows that head width--independent of body size--is significantly related to mean maximum consumed prey mass among these clades, and this relationship becomes even more significant when phylogeny is taken into account. Therefore, these data do support the hypothesis that head shape is adapted to prey size in snakes. Additionally, we calculated a phylogenetically corrected morphological variance-covariance matrix to examine the role of morphological integration during head shape evolution in snakes. This matrix shows that head width strongly covaries with both jaw length and out-lever length of the lower jaw. As a result, selection on head width will likely be associated with concomitant changes in jaw length and lower jaw out-lever length in snakes.

The functional meaning of “prey size” in water snakes (Nerodia fasciata, Colubridae)

The evolutionary success of macrostomatan (enlarged-gape) snakes has been attributed to their ability to consume large prey, in turn made possible by their highly kinetic skulls. However, prey can be "large" in several ways, and we have little insight into which aspects of prey size and shape affect skull function during feeding. We used X-ray videos of broad-banded water snakes (Nerodia fasciata) feeding on both frogs and fish to quantify movements of the jaw elements during prey transport, and of the anterior vertebral column during post-cranial swallowing. In a sample of additional individuals feeding on both frogs and fish, we measured the time and the number of jaw protractions needed to transport prey through the buccal cavity. Prey type (fish vs. frog) did not influence transport kinematics, but did influence transport performance. Furthermore, wider and taller prey induced greater movements of most cranial elements, but wider prey were transported with significantly less anterior vertebral bending. In the performance trials, heavier, shorter, and wider prey took significantly more time and a greater number of jaw protractions to ingest. Thus, the functional challenges involved in prey transport depend not only upon prey mass, but also prey type (fish vs. frog) and prey shape (relative height, width and length), suggesting that from the perspective of a gape-limited predator, the difficulty of prey ingestion depends upon multiple aspects of prey size.

Evolution of bite force in Darwin's finches: a key role for head width

Studies of Darwin's finches of the Galapagos Islands have provided pivotal insights into the interplay of ecological variation, natural selection, and morphological evolution. Here we document, across nine Darwin's finch species, correlations between morphological variation and bite force capacity. We find that bite force correlates strongly with beak depth and width but only weakly or not at all with beak length, a result that is consistent with prior demonstrations of natural selection on finch beak morphology. We also find that bite force is predicted even more strongly by head width, which exceeds all beak dimensions in predictive strength. To explain this result we suggest that head width determines the maximum size, and thus maximum force generation capacity of finch jaw adductor muscles. We suggest that head width is functionally relevant and may be a previously unrecognized locus of natural selection in these birds, because of its close relationship to bite force capacity.

Omnivory in lacertid lizards: adaptive evolution or constraint?

Feeding specializations such as herbivory are an often cited example of convergent and adaptive evolution. However, some groups such as lizards appear constrained in the evolution of morphological specializations associated with specialized diets. Here we examine whether the inclusion of plant matter into the diet of omnivorous lacertid lizards has resulted in morphological specializations and whether these specializations reflect biomechanical compromises as expected if omnivores are constrained by functional trade-offs. We examined external head shape, skull shape, tooth structure, intestinal tract length and bite performance as previous studies have suggested correlations between the inclusion of plants into the diet and these traits. Our data show that omnivorous lacertid lizards possess modifications of these traits that allow them to successfully exploit plant material as a food source. Conversely, few indications of a compromise phenotype could be detected, suggesting that the evolution towards herbivory is only mildly constrained by functional trade-offs.

Frugivory in polychrotid lizards: effects of body size

As more data have become available on lizard diets in the past few decades, researchers have stressed the importance of lizards as pollinators and seed dispersers. Whereas large body size has been traditionally put forward as a major biological factor "allowing" herbivory and frugivory in lizards, a recent review of frugivory and seed dispersal by lizards showed that frugivory might be considered to be a typical island phenomenon, independent of body size. Here we show that frugivory is correlated with lizard body size among a group of syntopic Anolis species in Jamaica, with larger species eating more fruit. Additionally, the size of the fruits consumed is significantly related to lizard body size. Multiple regression analyses show that this is largely a pure body size effect as head shape or residual bite force are uncorrelated to overall fruit size. Moreover, we demonstrate that among polychrotid (Anolis-like) lizards in general, those that consume fruit are on average larger than those that do not. Lizards from the mainland were not significantly different in body size from island species. We thus suggest that fruit consumption in polychrotid lizards is mediated by large body size whether living on islands or not.