LOCOMOTOR COMPENSATION CREATES A MISMATCH BETWEEN LABORATORY AND FIELD ESTIMATES OF ESCAPE SPEED IN LIZARDS: A CAUTIONARY TALE FOR PERFORMANCE-TO-FITNESS STUDIES

Different traits at different rates: The effects of dynamic strain rate on structural traits in biology

Phenotypic diversity is influenced by physical laws that govern how an organism's morphology relates to functional performance. To study comparative organismal biology, we need to quantify this diversity using biological traits (definable aspects of the morphology, behavior, and/or life history of an organism). Traits are often assumed to be immutable properties that need only be measured a single time in each adult. However, organisms often experience changes in their biotic and abiotic environments that can alter trait function. In particular, structural traits represent the physical capabilities of an organism and may be heavily influenced by the rate at which they are exposed to physical demands ('loads'). For instance, materials tend to become more brittle when loaded at faster rates which could negatively affect structures trying to resist those loads (e.g., brittle materials are more likely to fracture). In the following perspective piece, we address the dynamic properties of structural traits and present case studies that demonstrate how dynamic strain rates affect the function of these traits in diverse groups of organisms. First, we review how strain rate affects deformation and fracture in biomaterials and demonstrate how these effects alter puncture mechanics in systems such as snake strikes. Second, we discuss how different rates of bone loading affect the locomotor biomechanics of vertebrates and their ecology. Through these examinations of diverse taxa and ecological functions, we aim to highlight how rate-dependent properties of structural traits can generate dynamic form-function relationships in response to changing environmental conditions. Findings from these studies serve as a foundation to develop more nuanced ecomechanical models that can predict how complex traits emerge and, thereby, advance progress on outlining the Rules of Life.

Scaling of maneuvering performance in baleen whales: larger whales outperform expectations

Despite their enormous size, whales make their living as voracious predators. To catch their much smaller, more maneuverable prey, they have developed several unique locomotor strategies that require high energetic input, high mechanical power output and a surprising degree of agility. To better understand how body size affects maneuverability at the largest scale, we used bio-logging data, aerial photogrammetry and a high-throughput approach to quantify the maneuvering performance of seven species of free-swimming baleen whale. We found that as body size increases, absolute maneuvering performance decreases: larger whales use lower accelerations and perform slower pitch-changes, rolls and turns than smaller species. We also found that baleen whales exhibit positive allometry of maneuvering performance: relative to their body size, larger whales use higher accelerations, and perform faster pitch-changes, rolls and certain types of turns than smaller species. However, not all maneuvers were impacted by body size in the same way, and we found that larger whales behaviorally adjust for their decreased agility by using turns that they can perform more effectively. The positive allometry of maneuvering performance suggests that large whales have compensated for their increased body size by evolving more effective control surfaces and by preferentially selecting maneuvers that play to their strengths.

Field Observations of Body Temperature for the Wolf Spider, Rabidosa rabida (Araneae: Lycosidae), Differ From Reported Laboratory Temperature Preference Suggesting Thermoconforming Behavior

Temperature affects all aspects of ectotherm ecology, behavior, and physiology. Descriptions of thermal ecology are important for understanding ecology in changing thermal environments. Both laboratory and field estimates are important for understanding thermal ecology. Rabidosa rabida (Walckenaer 1837) (Araneae: Lycosidae) is a large wolf spider with some natural history, including laboratory estimates of thermal preference, tolerance, and performance, reported in the scientific literature. Laboratory tests suggest the active choice of temperature environment. To test published estimates of thermal ecology from the laboratory, we took body temperature measurements of mature spiders in the field nocturnally and diurnally using a FLIR camera in July 2019. We made comparisons between sexes and activity periods using field observations. We compared these measurements with the published values for thermal preferences and thermal maximum and with mean weather station data. Observed field temperatures differed significantly from published preference, but not from mean temperature from a local weather station. This suggests that this species is thermoconforming rather than actively thermoregulating. Reported thermal preference fell between the diurnal and nocturnal mean measurements closer to the diurnal than nocturnal temperatures. These field observations show how important it is to make field observations for physiology and thermal ecology. Maximum observed diurnal temperatures closely approached the published critical thermal maximum. We observed spiders performing behaviors such as hunting and feeding in conditions well above published thermal preference and near-critical high temperature. These observations suggest that R. rabida is thermoconforming in this limited period but does not rule out that they might thermoregulate in certain situations.

Pendulum-based measurements reveal impact dynamics at the scale of a trap-jaw ant

Small organisms can produce powerful, sub-millisecond impacts by moving tiny structures at high accelerations. We developed and validated a pendulum device to measure the impact energetics of microgram-sized trap-jaw ant mandibles accelerated against targets at 10⁵ m s^-2 Trap-jaw ants (Odontomachus brunneus; 19 individuals, 212 strikes) were suspended on one pendulum and struck swappable targets that were either attached to an opposing pendulum or fixed in place. Mean post-impact kinetic energy (energy from a strike converted to pendulum motion) was higher with a stiff target (21.0-21.5 µJ) than with a compliant target (6.4-6.5 µJ). Target mobility had relatively little influence on energy transfer. Mean contact duration of strikes against stiff targets was shorter (3.9-4.5 ms) than against compliant targets (6.2-7.9 ms). Shorter contact duration was correlated with higher post-impact kinetic energy. These findings contextualize and provide an energetic explanation for the diverse, natural uses of trap-jaw ant strikes such as impaling prey, launching away threats and performing mandible-powered jumps. The strong effect of target material on energetic exchange suggests material interactions as an avenue for tuning performance of small, high acceleration impacts. Our device offers a foundation for novel research into the ecomechanics and evolution of tiny biological impacts and their application in synthetic systems.

Convergent Evolution of Locomotory Modes in Euarchontoglires

The research of phenotypic convergence is of increasing importance in adaptive evolution. Locomotory modes play important roles in the adaptive evolution of species in the Euarchontoglires, however, the investigation of convergent evolution of the locomotory modes across diverse Euarchontoglire orders is incomplete. We collected measurements of three phalangeal indices of manual digit III, including metacarpal of digit III (MC3), manus proximal phalanx of digit III (MPP3), and manus intermediate phalanx of digit III (MIP3), from 203 individuals of 122 Euarchontoglires species representing arboreal (orders Scandentia, Rodentia, and Primates), terrestrial (orders Scandentia and Rodentia), and gliding (orders Dermoptera and Rodentia) locomotory modes. This data can be separated into seven groups defined by order and locomotory mode. Based on combination of the three phalangeal indices, the Principle component analyses (PCA), phylomorphospace plot, and C-metrics analyses clustered the arboreal species of Scandentia, Rodentia, and Primates together and the terrestrial species of Scandentia and Rodentia together, showing the convergent signal in evolution of the arboreal (C1 = 0.424, P < 0.05) and terrestrial (C1 = 0.560, P < 0.05) locomotory modes in Euarchontoglires. Although the gliding species from Dermoptera and Rodentia did not cluster together, they also showed the convergent signal (C1 = 0.563, P < 0.05). Our work provides insight into the convergent evolution of locomotory modes in Euarchontoglires, and reveals that these three indices contribute valuable information to identify convergent evolution in Euarchontoglires.

Functional and Environmental Constraints on Prey Capture Speed in a Lizard

Movement is an important component of animal behavior and determines how an organism interacts with its environment. The speed at which an animal moves through its environment can be constrained by internal (e.g., physiological state) and external factors (e.g., habitat complexity). When foraging, animals should move at speeds that maximize prey capture while minimizing mistakes (i.e., missing prey, slipping). We used experimental arenas containing obstacles spaced in different arrays to test how variation in habitat complexity influenced attack distance, prey capture speed, and foraging success in the Prairie Lizard. Obstacles spaced uniformly across arenas resulted in 15% slower prey capture speed and 30–38% shorter attack distance compared to arenas with no obstacles or with obstacles clustered in opposite corners of the arena. Prey capture probability was not influenced by arena type or capture speed, but declined with increasing attack distance. Similarly, the probability of prey consumption declined with attack distance across arena types. However, prey consumption probability declined with increasing prey capture speed in more open arenas but not in the cluttered arena. Foraging accuracy declined with increasing speed in more open arenas, and remained relatively constant when obstacles were in closer proximity. Foraging success was primarily constrained by intrinsic properties (speed-maneuverability tradeoff) when ample space was available, but environmental conditions had a greater impact on foraging success in “cluttered” habitats. This empirical test of theoretical predictions about optimal movement speeds in animals provides a step forward in understanding how animals select speeds in nature.

Morphological determinants of jumping performance in the Iberian green frog

Predation is one of the main selective forces in nature, frequently selecting potential prey for developing escape strategies. Escape ability is typically influenced by several morphological parameters, such as morphology of the locomotor appendices, muscular capacity, body mass, or fluctuating asymmetry, and may differ between sexes and age classes. In this study, we tested the relationship among these variables and jumping performance in 712 Iberian green frogs Pelophylax perezi from an urban population. The results suggest that the main determinant of jumping capacity was body size (explaining 48% of variance). Larger frogs jumped farther, but jumping performance reached an asymptote for the largest frogs. Once controlled by structural body size, the heaviest frogs jumped shorter distances, suggesting a trade-off between fat storage and jumping performance. Relative hind limb length also determined a small but significant percentage of variance (2.4%) in jumping performance—that is, the longer the hind limbs, the greater the jumping capacity. Juveniles had relatively shorter and less muscular hind limbs than adults (for a given body size), and their jumping performance was poorer. In our study population, the hind limbs of the frogs were very symmetrical, and we found no effect of fluctuating asymmetry on jumping performance. Therefore, our study provides evidence that jumping performance in frogs is not only affected by body size, but also by body mass and hind limb length, and differ between age classes.

Modeling escape success in terrestrial predator–prey interactions

Prey species often modify their foraging and reproductive behaviors to avoid encounters with predators; yet once they are detected, survival depends on out-running, out-maneuvering, or fighting off the predator. Though predation attempts involve at least two individuals—namely, a predator and its prey—studies of escape performance typically measure a single trait (e.g., sprint speed) in the prey species only. Here, we develop a theoretical model in which the likelihood of escape is determined by the prey animal’s tactics (i.e., path trajectory) and its acceleration, top speed, agility, and deceleration relative to the performance capabilities of a predator. The model shows that acceleration, top speed, and agility are all important determinants of escape performance, and because speed and agility are biomechanically related to size, smaller prey with higher agility should force larger predators to run along curved paths that do not allow them to use their superior speeds. Our simulations provide clear predictions for the path and speed a prey animal should choose when escaping from predators of different sizes (thus, biomechanical constraints) and could be used to explore the dynamics between predators and prey.

Maternal diet affects juvenile Carpetan rock lizard performance and personality

Differences in both stable and labile state variables are known to affect the emergence and maintenance of consistent interindividual behavioral variation (animal personality or behavioral syndrome), especially when experienced early in life. Variation in environmental conditions experienced by gestating mothers (viz. nongenetic maternal effects) is known to have significant impact on offspring condition and behavior; yet, their effect on behavioral consistency is not clear. Here, by applying an orthogonal experimental design, we aimed to study whether increased vitamin D3 content in maternal diet during gestation (vitamin-supplemented vs. vitamin control treatments) combined with corticosterone treatment (corticosterone-treated vs. corticosterone control treatments) applied on freshly hatched juveniles had an effect on individual state and behavioral consistency of juvenile Carpetan rock lizards (Iberolacerta cyreni). We tested the effect of our treatments on (a) climbing speed and the following levels of behavioral variation, (b) strength of animal personality (behavioral repeatability), (c) behavioral type (individual mean behavior), and (d) behavioral predictability (within-individual behavioral variation unrelated to environmental change). We found higher locomotor performance of juveniles from the vitamin-supplemented group (42.4% increase), irrespective of corticosterone treatment. While activity personality was present in all treatments, shelter use personality was present only in the vitamin-supplemented × corticosterone-treated treatment and risk-taking personality was present in corticosterone control treatments. Contrary to our expectations, behavioral type was not affected by our treatments, indicating that individual quality can affect behavioral strategies without affecting group-level mean behavior. Behavioral predictability decreased in individuals with low climbing speed, which could be interpreted as a form of antipredator strategy. Our results clearly demonstrate that maternal diet and corticosterone treatment have the potential to induce or hamper between-individual variation in different components of boldness, often in interactions.

Terrestrial locomotion of the Svalbard rock ptarmigan: comparing field and laboratory treadmill studies

Research into the terrestrial locomotion of birds is often based upon laboratory treadmill experiments. However, it is unclear how transposable these results are for birds moving in the wild. Here, using video recordings, we compared the kinematics of locomotion (stride frequency, stride length, stance phase, swing phase, duty factor) and speed range of Svalbard rock ptarmigan (Lagopus muta hyperborea) under field and laboratory treadmill conditions. Our findings indicate that the kinematics of walking and aerial running are conserved when moving on the treadmill and in the field. Differences, however, were found when grounded running under the two conditions, linked to substrate. Substrate effects were confirmed by analysing trials only moving over very hard snow. In line with laboratory treadmill energetic predictions, wild ptarmigan have a preferred speed during walking and to a lesser extent when aerial running but not when moving with a grounded running gait. The birds were also capable of a higher top speed in the field than that observed during treadmill studies. Our findings demonstrate that laboratory treadmill research provides meaningful information relevant to wild birds while highlighting the importance of understanding the substrate the animals are moving over.

Linking locomotor performance to morphological shifts in urban lizards

Urban habitats are drastically modified from their natural state, creating unique challenges and selection pressures for organisms that reside in them. We compared locomotor performance of Anolis lizards from urban and forest habitats on tracks differing in angle and substrate, and found that using artificial substrates came at a cost: lizards ran substantially slower and frequently lost traction on man-made surfaces compared to bark. We found that various morphological traits were positively correlated with sprint speed and that these same traits were significantly larger in urban compared to forest lizards. We found that urban lizards ran faster on both man-made and natural surfaces, suggesting similar mechanisms improve locomotor performance on both classes of substrate. Thus, lizards in urban areas may be under selection to run faster on all flat surfaces, while forest lizards face competing demands of running, jumping and clinging to narrow perches. Novel locomotor challenges posed by urban habitats likely have fitness consequences for lizards that cannot effectively use man-made surfaces, providing a mechanistic basis for observed phenotypic shifts in urban populations of this species.

Physiological mechanisms underlying animal social behaviour

Many species of animal live in groups, and the group represents the organizational level within which ecological and evolutionary processes occur. Understanding these processes, therefore, relies on knowledge of the mechanisms that permit or constrain group formation. We suggest that physiological capacities and differences in physiology between individuals modify fission-fusion dynamics. Differences between individuals in locomotor capacity and metabolism may lead to fission of groups and sorting of individuals into groups with similar physiological phenotypes. Environmental impacts such as hypoxia can influence maximum group sizes and structure in fish schools by altering access to oxygenated water. The nutritional environment determines group cohesion, and the increase in information collected by the group means that individuals should rely more on social information and form more cohesive groups in uncertain environments. Changing environmental contexts require rapid responses by individuals to maintain group coordination, which are mediated by neuroendocrine signalling systems such as nonapeptides and steroid hormones. Brain processing capacity may constrain social complexity by limiting information processing. Failure to evaluate socially relevant information correctly limits social interactions, which is seen, for example, in autism. Hence, functioning of a group relies to a large extent on the perception and appropriate processing of signals from conspecifics. Many if not all physiological systems are mechanistically linked, and therefore have synergistic effects on social behaviour. A challenge for the future lies in understanding these interactive effects, which will improve understanding of group dynamics, particularly in changing environments.This article is part of the themed issue 'Physiological determinants of social behaviour in animals'.

Locomotion, Energetics, Performance, and Behavior: A Mammalian Perspective on Lizards, and Vice Versa

SYNOPSIS

Animals are constrained by their abilities and by interactions with environmental factors, such as low ambient temperatures. These constraints range from physical impossibilities to energetic inefficiencies, and may entail trade-offs. Some of the constraints related to locomotion and activity metabolism can be illustrated through allometric comparisons of mammals and lizards, as representative terrestrial vertebrate endotherms and ectotherms, respectively, because these lineages differ greatly in aerobic metabolic capacities, resting energetic costs, and thermoregulatory patterns. Allometric comparisons are both useful and unavoidable, but "outlier" species (unusual for their clade) can also inform evolutionary scenarios, as they help indicate extremes of possible adaptation within mammalian and saurian levels of organization. We compared mammals and lizards for standard metabolic rate (SMR), maximal oxygen consumption during forced exercise (VO2max), net (incremental) cost of transport (NCT), maximal aerobic speed (MAS), daily movement distance (DMD), daily energy expenditure (DEE) during the active season, and the ecological cost of transport (ECT = percentage of DEE attributable to locomotion). (Snakes were excluded because their limbless locomotion has no counterpart in terrestrial mammals.) We only considered lizard SMR, VO2max, NCT, MAS, and sprint speed data if measured at 35-40 °C. On average, MAS is ∼7.4-fold higher in mammals, whereas SMR and VO2max are ∼6-fold greater, but values for all three of these traits overlap (or almost overlap) between mammals and lizards, a fact that has not previously been appreciated. Previous studies show that sprint speeds are similar for smaller mammals and lizards, but at larger sizes lizards are not as fast as some mammals. Mammals move ∼6-fold further each day than lizards, and DMD is by far the most variable trait considered here, but their NCT is similar. Mammals exceed lizards by ∼11.4-fold for DEE. On average for both lineages, the ECT is surprisingly low, somewhat higher for lizards, and positively allometric. If a lizard and mammal of 100 g body mass were both to move their entire DMD at their MAS, they could do so in ∼21 and 17 min, respectively, thus de-emphasizing the possible importance of time constraints. We conclude that ecological-energetic constraints related to locomotion are relatively more likely to occur in large, carnivorous lizards. Overall, our comparisons support the idea that the (gradual) evolution of mammalian endothermy did not necessarily require major changes in locomotor energetics, performance, or associated behaviors. Instead, we speculate that the evolution of thermoregulatory responses to low temperatures (e.g., shivering) may have been a key and "difficult" step in this transition.

Comparability and repeatability of three commonly used methods for measuring endurance capacity

Measures of endurance (time to exhaustion) have been used to address a wide range of questions in ecomorphological and physiological research, as well as being used as a proxy for survival and fitness. Swimming, stationary (circular) track running, and treadmill running are all commonly used methods for measuring endurance. Despite the use of these methods across a broad range of taxa, how comparable these methods are to one another, and whether they are biologically relevant, is rarely examined. We used Australian water dragons (Intellagama lesueurii), a species that is morphologically adept at climbing, swimming, and running, to compare these three methods of endurance and examined if there is repeatability within and between trial methods. We found that time to exhaustion was not highly repeatable within a method, suggesting that single measures or a mean time to exhaustion across trials are not appropriate. Furthermore, we compared mean maximal endurance times among the three methods, and found that the two running methods (i.e., stationary track and treadmill) were similar, but swimming was distinctly different, resulting in lower mean maximal endurance times. Finally, an individual's endurance rank was not repeatable across methods, suggesting that the three endurance trial methods are not providing similar information about an individual's performance capacity. Overall, these results highlight the need to carefully match a measure of performance capacity with the study species and the research questions being asked so that the methods being used are behaviorally, ecologically, and physiologically relevant.

Linking muscle metabolism and functional variation to field swimming performance in bluegill sunfish (Lepomis macrochirus)

Skeletal muscle has diverse mechanical roles during locomotion. In swimming fish, power-producing muscles work in concert with the accessory muscles of the fins which augment and control power transfer to the water. Although fin muscles represent a significant proportion of the locomotor muscle mass, their physiological properties are poorly characterized. To examine the relationship between muscle metabolism and the differing mechanical demands placed on distinct muscle groups, we quantified the aerobic and glycolytic capacities of the myotomal, pectoral and caudal muscles of bluegill sunfish. These were indicated by the activities of citrate synthase and lactate dehydrogenase, rate-limiting enzymes for aerobic respiration and glycolysis, respectively. The well-established roles of slow and fast myotomal muscle types in sustained and transient propulsive movements allows their use as benchmarks to which other muscles can be compared to assess their function. Slow myotomal muscle had the highest CS activity, consistent with meeting the high metabolic and mechanical power demands of body-caudal fin (BCF) swimming at the upper end of the aerobically supported speed range. The largest pectoral adductors and abductors had CS activities lower than the slow myotomal muscle, in line with their role supplying thrust for low-speed, low-power swimming. The metabolic capacities of the caudal muscles were surprisingly low and inconsistent with their activity during steady-state BCF swimming at high speeds. This may reflect adaptation to the observed swimming behavior in the field, which typically involved short bouts of BCF-propulsive cycles rather than sustained propulsive activity.

Run for your life, but bite for your rights? How interactions between natural and sexual selection shape functional morphology across habitats

A central issue in evolutionary biology is how morphology, performance, and habitat use coevolve. If morphological variation is tightly associated with habitat use, then differences in morphology should affect fitness through their effect on performance within specific habitats. In this study, we investigate how evolutionary forces mold morphological traits and performance differently given the surrounding environment, at the intraspecific level. For this purpose, we selected populations of the lizard Podarcis bocagei from two different habitat types, agricultural walls and dunes, which we expected to reflect saxicolous vs ground-dwelling habits. In the laboratory, we recorded morphological traits as well as performance traits by measuring sprint speed, climbing capacity, maneuverability, and bite force. Our results revealed fast-evolving ecomorphological variation among populations of P. bocagei, where a direct association existed between head morphology and bite performance. However, we could not establish links between limb morphology and locomotor performance at the individual level. Lizards from walls were better climbers than those from dunes, suggesting a very fast evolutionary response. Interestingly, a significant interaction between habitat and sex was detected in climbing performance. In addition, lizards from dunes bit harder than those from walls, although sexual differentiation was definitely the main factor driving variation in head functional morphology. Taking into account all the results, we found a complex interaction between natural and sexual selection on whole-organism performance, which are, in some cases, reflected in morphological variation.

Altitude influences thermal ecology and thermal sensitivity of locomotor performance in a toad-headed lizard

Population differentiation in ectotherm physiological performance may be driven by adapting to different thermal environments. In this study, we measured locomotor performance in two different altitude populations of the Qinghai toad-headed lizards (Phrynocephalus vlangalii) at different test temperatures to assess between-population differences in thermal sensitivity of sprint speed. Low-elevation lizards ran faster than high-elevation lizards at most test temperatures. Sprint speed varied with test temperature similarly between populations, but the thermal sensitivity (performance breadth) differed significantly. Low-elevation lizards had a lower optimal temperature (T_opt) for sprint speed and narrower performance breadth than high-elevation lizards as inferred from the thermal performance curves constructed for each individual. We also measured the body temperature of active lizards (T_b) in the field and selected temperature (T_sel) in the laboratory. Low-elevation lizards had a lower T_sel, and less variable T_b than high-elevation lizards. In both populations, T_sel was lower than T_opt for sprint speed, which was inconsistent with the prediction for a match between thermal preference and T_opt. Our results suggest that lower thermal sensitivity and weaker locomotor ability for high-elevation lizards may be an adaptive response to the local environmental conditions (e.g., greater thermal variability, higher food availability, and lower predator pressure).

Field swimming performance of bluegill sunfish, Lepomis macrochirus: implications for field activity cost estimates and laboratory measures of swimming performance

Mobility is essential to the fitness of many animals, and the costs of locomotion can dominate daily energy budgets. Locomotor costs are determined by the physiological demands of sustaining mechanical performance, yet performance is poorly understood for most animals in the field, particularly aquatic organisms. We have used 3‐D underwater videography to quantify the swimming trajectories and propulsive modes of bluegills sunfish (Lepomis macrochirus, Rafinesque) in the field with high spatial (1–3 mm per pixel) and temporal (60 Hz frame rate) resolution. Although field swimming trajectories were variable and nonlinear in comparison to quasi steady‐state swimming in recirculating flumes, they were much less unsteady than the volitional swimming behaviors that underlie existing predictive models of field swimming cost. Performance analyses suggested that speed and path curvature data could be used to derive reasonable estimates of locomotor cost that fit within measured capacities for sustainable activity. The distinct differences between field swimming behavior and performance measures obtained under steady‐state laboratory conditions suggest that field observations are essential for informing approaches to quantifying locomotor performance in the laboratory.

Field swimming behavior in largemouth bass deviates from predictions based on economy and propulsive efficiency

Locomotion is energetically expensive. This may create selection pressures that favor economical locomotor strategies, such as the adoption of low-cost speeds and efficient propulsive movements. For swimming fish, the energy expended to travel a unit distance, or cost of transport (COT), has a U-shaped relationship to speed. The relationship between propulsive kinematics and speed, summarized by the Strouhal number (St=fA/U, where f is tail beat frequency, A is tail tip amplitude in m and U is swimming speed in m s−1), allows for maximal propulsive efficiency where 0.2<St<0.4. Largemouth bass adopted field speeds that were generally below the range predicted to minimize their COT. This may reflect speed modulation to meet competing functional demands such as enabling effective prey detection and capture. St exceeded the optimal range for the lowest observed swimming speeds. Mechanical and physiological constraints may prevent adoption of efficient St during low-speed swimming.

Repeatability and correlation of physiological traits: Do ectotherms have a "thermal type"?

Across a range of taxa, individuals within a species differ in suites of correlated traits. These trait complexes, known as syndromes, can have dramatic evolutionary consequences as they do not evolve independently but rather as a unit. Current research focuses primarily on syndromes relating to aspects of behavior and life history. What is less clear is whether physiological traits also form a syndrome. We measured 10 thermal traits in the delicate skink, Lampropholis delicata, to test this idea. Repeatability was calculated and their across‐context correlations evaluated. Our results were in alignment with our predictions in that individual thermal traits varied consistently and were structured into a physiological syndrome, which we are referring to as the thermal behavior syndrome (TBS). Within this syndrome, lizards exhibited a “thermal type” with each being ranked along a cold–hot continuum. Hot types had faster sprint speeds and higher preferred body temperatures, whereas the opposite was true for cold types. We conclude that physiological traits may evolve as a single unit driven by the need to maintain optimal temperatures that enable fitness‐related behaviors to be maximized.

Microgeographic variation in locomotor traits among lizards in a human-built environment

Microgeographic variation in fitness-relevant traits may be more common than previously appreciated. The fitness of many vertebrates is directly related to their locomotor capacity, a whole-organism trait integrating behavior, morphology, and physiology. Because locomotion is inextricably related to context, I hypothesized that it might vary with habitat structure in a wide-ranging lizard, Podarcis erhardii, found in the Greek Cyclade Islands. I compared lizard populations living on human-built rock walls, a novel habitat with complex vertical structure, with nearby lizard populations that are naive to human-built infrastructure and live in flat, loose-substrate habitat. I tested for differences in morphology, behavior, and performance. Lizards from built sites were larger and had significantly (and relatively) longer forelimbs and hindlimbs. The differences in hindlimb morphology were especially pronounced for distal components—the foot and longest toe. These morphologies facilitated a significant behavioral shift in jumping propensity across a rocky experimental substrate. I found no difference in maximum velocity between these populations; however, females originating from wall sites potentially accelerated faster over the rocky experimental substrate. The variation between these closely neighboring populations suggests that the lizards inhabiting walls have experienced a suite of trait changes enabling them to take advantage of the novel habitat structure created by humans.

Skeletal muscle contractile function predicts activity and behaviour in zebrafish

Locomotion facilitates behaviour and its underlying physiological mechanisms may therefore impact behavioural phenotypes. Metabolism is often thought to modulate locomotion and behaviour, but empirical support for this suggestion is equivocal. Muscle contractile function is directly associated with locomotion. Here, we test the hypotheses that muscle mechanics determine locomotor performance and activity in zebrafish (Danio rerio) and thereby also affect risk-taking behaviour. We show that there is a mechanistic link between muscle performance and behaviour by manipulating muscle contractile properties, which caused proportional changes in critical sustained swimming performance and, in an open arena, voluntary swimming speed, the proportion of time fish were active, and the latency to move. We modelled the relationships between muscle contractile properties, swimming performance, activity and behaviour with a partial least-squares path model. The latent variable ‘muscle’, formed by isolated muscle force production, stress, fatigue resistance and activation and relaxation rates, had a significant positive effect on swimming performance (‘swim’ reflected in sustained and sprint speeds). Together, muscle and swim had a significant positive effect on activity, and explained 71.8% of variation in the distance moved, time active and maximum voluntary speed in an open field. Activity had a significant positive effect on boldness, explaining 76.0% of variation in latencies to move and to approach a novel object. Muscle contractile function determines voluntary movement and we suggest that exploration and dispersal are functions of physiological and mechanical optimisation. Boldness therefore may be partly explained by the greater likelihood of faster fish to move further and encounter novel objects and conspecifics more quickly as a result.

Body size, swimming speed, or thermal sensitivity? Predator-imposed selection on amphibian larvae

Background

Many animals rely on their escape performance during predator encounters. Because of its dependence on body size and temperature, escape velocity is fully characterized by three measures, absolute value, size-corrected value, and its response to temperature (thermal sensitivity). The primary target of the selection imposed by predators is poorly understood. We examined predator (dragonfly larva)-imposed selection on prey (newt larvae) body size and characteristics of escape velocity using replicated and controlled predation experiments under seminatural conditions. Specifically, because these species experience a wide range of temperatures throughout their larval phases, we predict that larvae achieving high swimming velocities across temperatures will have a selective advantage over more thermally sensitive individuals.

Results

Nonzero selection differentials indicated that predators selected for prey body size and both absolute and size-corrected maximum swimming velocity. Comparison of selection differentials with control confirmed selection only on body size, i.e., dragonfly larvae preferably preyed on small newt larvae. Maximum swimming velocity and its thermal sensitivity showed low group repeatability, which contributed to non-detectable selection on both characteristics of escape performance.

Conclusions

In the newt-dragonfly larvae interaction, body size plays a more important role than maximum values and thermal sensitivity of swimming velocity during predator escape. This corroborates the general importance of body size in predator–prey interactions. The absence of an appropriate control in predation experiments may lead to potentially misleading conclusions about the primary target of predator-imposed selection. Insights from predation experiments contribute to our understanding of the link between performance and fitness, and further improve mechanistic models of predator–prey interactions and food web dynamics.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0522-y) contains supplementary material, which is available to authorized users.

Puffed and bothered: Personality, performance, and the effects of stress on checkered pufferfish

Although consistent individual-level differences in behaviour are widespread and potentially important in evolutionary and ecological processes, relatively few studies focus on the physiological mechanisms that might underlie and regulate these individual-level differences in wild populations. We conducted experiments to determine whether checkered pufferfish (Sphoeroides testudineus), which were collected from a dynamic (in terms of depth and water temperature) tidal mangrove creek environment in The Bahamas, have consistent individual-level differences in locomotor activity and the response to a simulated predator threat, as well as swimming performance and puffing in response to stressors. The relationships between personality and performance traits were evaluated to determine whether they represented stress-coping styles or syndromes. Subsequently, a displacement study was conducted to determine how personality and performance in the laboratory compared to movements in the field. In addition, we tested whether a physiological dose of the stress hormone cortisol would alter individual consistency in behavioural and performance traits. We found that pufferfish exhibited consistent individual differences in personality traits over time (e.g., activity and the duration of a response to a threat) and that performance was consistent between the lab and the natural enclosure. Locomotor activity and the duration of startled behaviour were not associated with swimming and puffing performance. Locomotor activity, puffing performance, and swimming performance were not related to whether fish returned to the tidal creek of capture after displacement. Similarly, a cortisol treatment did not modify behaviour or performance in the laboratory. The results reveal that consistent individual-level differences in behaviour and performance were present in a population from a fluctuating and physiologically challenging environment but that such traits are not necessarily correlated. We also determined that certain individual performance traits were repeatable between the lab and a natural enclosure. However, we found no evidence of a relationship between exogenous cortisol levels and behavioural traits or performance in these fish, which suggests that other internal and external mechanisms may underlie the behaviours and performance tested.

Sprint sensitivity and locomotor trade-offs in green anole (Anolis carolinensis) lizards

How well an organism completes an ecologically relevant task – its performance – is often considered a key factor in determining individual fitness. Historically, ecomorphological studies have examined how morphological traits determine individual performance in a static manner, assuming that differential fitness in a population is due indirectly to differences in morphological traits that determine a simple measure of performance. This assumption, however, ignores many ecological factors that can constrain performance in nature, such as substrate variation and individual behavior. We examined some of these complexities in the morphology–performance–fitness paradigm, primarily the impact that substrate variation has on performance. We measured maximal sprint speed of green anole lizards on four substrates that varied in size and complexity and are used by or available to individuals in nature. Performance decreased significantly from a broad substrate to a narrow substrate, and lizards were three times slower on a complex substrate than the broadest substrate. We also detected trade-offs in running on substrates with different diameters and in cluttered versus uncluttered environments. Furthermore, morphological predictors of performance varied among substrates. This indicates that natural selection may act on different morphological traits, depending on which substrates are used by individuals, as well as an individual's ability to cope with changes in substrate rather than maximal capacities.

The life history of whole-organism performance

For almost 40 years, studies of whole-organism performance have formed a cornerstone of evolutionary physiology. Although its utility as a heuristic guide is beyond question, and we have learned much about morphological evolution from its application, the ecomorphological paradigm has frequently been applied to performance evolution in ways that range from unsatisfactory to inappropriate. More importantly, the standard ecomorphological paradigm does not account for tradeoffs among performance and other traits, nor between performance traits that are mediated by resource allocation. A revised paradigm that includes such tradeoffs, and the possible ways that performance and fitness-enhancing traits might affect each other, could potentially revivify the study of phenotypic evolution and make important inroads into understanding the relationships between morphology and performance and between performance and Darwinian fitness. We describe such a paradigm, and discuss the various ways that performance and key life-history traits might interact with and affect each other. We emphasize both the proximate mechanisms potentially linking such traits, and the likely ultimate factors driving those linkages, as well as the evolutionary implications for the overall, multivariate phenotype. Finally, we highlight several research directions that will shed light on the evolution and ecology of whole-organism performance and related life-history traits.

Evidence of circadian rhythm, oxygen regulation capacity, metabolic repeatability and positive correlations between forced and spontaneous maximal metabolic rates in lake sturgeon Acipenser fulvescens

Animal metabolic rate is variable and may be affected by endogenous and exogenous factors, but such relationships remain poorly understood in many primitive fishes, including members of the family Acipenseridae (sturgeons). Using juvenile lake sturgeon (Acipenser fulvescens), the objective of this study was to test four hypotheses: 1) A. fulvescens exhibits a circadian rhythm influencing metabolic rate and behaviour; 2) A. fulvescens has the capacity to regulate metabolic rate when exposed to environmental hypoxia; 3) measurements of forced maximum metabolic rate (MMR_F) are repeatable in individual fish; and 4) MMR_F correlates positively with spontaneous maximum metabolic rate (MMR_S). Metabolic rates were measured using intermittent flow respirometry, and a standard chase protocol was employed to elicit MMR_F. Trials lasting 24 h were used to measure standard metabolic rate (SMR) and MMR_S. Repeatability and correlations between MMR_F and MMR_S were analyzed using residual body mass corrected values. Results revealed that A. fulvescens exhibit a circadian rhythm in metabolic rate, with metabolism peaking at dawn. SMR was unaffected by hypoxia (30% air saturation (O_2sat)), demonstrating oxygen regulation. In contrast, MMR_F was affected by hypoxia and decreased across the range from 100% O_2sat to 70% O_2sat. MMR_F was repeatable in individual fish, and MMR_F correlated positively with MMR_S, but the relationships between MMR_F and MMR_S were only revealed in fish exposed to hypoxia or 24 h constant light (i.e. environmental stressor). Our study provides evidence that the physiology of A. fulvescens is influenced by a circadian rhythm and suggests that A. fulvescens is an oxygen regulator, like most teleost fish. Finally, metabolic repeatability and positive correlations between MMR_F and MMR_S support the conjecture that MMR_F represents a measure of organism performance that could be a target of natural selection.

Repeatability of movement behaviour in a wild salmonid revealed by telemetry

Movement estimates derived from sub-daily tracking of radio-tagged bull trout Salvelinus confluentus on the Columbia River, British Columbia, Canada, were used to test whether interindividual variation in behaviour was repeatable among contexts, both short- and long-term. Interindividual variation in S. confluentus behaviour was consistent across contexts. These findings emphasize the potential for telemetry as a tool in animal personality and temperament research.

Individual variation in thermal performance curves: swimming burst speed and jumping endurance in wild-caught tropical clawed frogs

The importance of studying individual variation in locomotor performance has long been recognized as it may determine the ability of an organism to escape from predators, catch prey or disperse. In ectotherms, locomotor performance is highly influenced by ambient temperature (Ta), yet several studies have showed that individual differences are usually retained across a Ta gradient. Less is known, however, about individual differences in thermal sensitivity of performance, despite the fact that it could represent adaptive sources of phenotypic variation and/or additional substrate for selection to act upon. We quantified swimming and jumping performance in 18 wild-caught tropical clawed frogs (Xenopus tropicalis) across a Ta gradient. Maximum swimming velocity and acceleration were not repeatable and individuals did not differ in how their swimming performance varied across Ta. By contrast, time and distance jumped until exhaustion were repeatable across the Ta gradient, indicating that individuals that perform best at a given Ta also perform best at another Ta. Moreover, thermal sensitivity of jumping endurance significantly differed among individuals, with individuals of high performance at low Ta displaying the highest sensitivity to Ta. Individual differences in terrestrial performance increased with decreasing Ta, which is opposite to results obtained in lizards at the inter-specific and among-individual levels. To verify the generality of these patterns, we need more studies on individual variation in thermal reaction norms for locomotor performance in lizards and frogs.

Choose your weapon: defensive behavior is associated with morphology and performance in scorpions

Morphology can be adaptive through its effect on performance of an organism. The effect of performance may, however, be modulated by behavior; an organism may choose a behavioral option that does not fully utilize its maximum performance. Behavior may therefore be decoupled from morphology and performance. To gain insight into the relationships between these levels of organization, we combined morphological data on defensive structures with measures of defensive performance, and their utilization in defensive behavior. Scorpion species show significant variation in the morphology and performance of their main defensive structures; their chelae (pincers) and the metasoma ("tail") carrying the stinger. Our data show that size-corrected pinch force varies to almost two orders of magnitude among species, and is correlated with chela morphology. Chela and metasoma morphology are also correlated to the LD50 of the venom, corroborating the anecdotal rule that dangerously venomous scorpions can be recognized by their chelae and metasoma. Analyses of phylogenetic independent contrasts show that correlations between several aspects of chela and metasoma morphology, performance and behavior are present. These correlations suggest co-evolution of behavior with morphology and performance. Path analysis found a performance variable (pinch force) to partially mediate the relationship between morphology (chela aspect ratio) and behavior (defensive stinger usage). We also found a correlation between two aspects of morphology: pincer finger length correlates with the relative "thickness" (aspect ratio) of the metasoma. This suggests scorpions show a trade-off between their two main weapon complexes: the metasoma carrying the stinger, and the pedipalps carrying the chelae.

Egg incubation effects generate positive correlations between size, speed and learning ability in young lizards

Previous studies have suggested that body size and locomotor performance are targets of Darwinian selection in reptiles. However, much of the variation in these traits may derive from phenotypically plastic responses to incubation temperature, rather than from underlying genetic variation. Intriguingly, incubation temperature may also influence cognitive traits such as learning ability. Therefore, we might expect correlations between a reptile's size, locomotor speed and learning ability either due to selection on all of these traits or due to environmental effects during egg incubation. In the present study, we incubated lizard eggs (Scincidae: Bassiana duperreyi) under 'hot' and 'cold' thermal regimes and then assessed differences in hatchling body size, running speed and learning ability. We measured learning ability using a Y-maze and a food reward. We found high correlations between size, speed and learning ability, using two different metrics to quantify learning (time to solution, and directness of route), and showed that environmental effects (incubation temperature) cause these correlations. If widespread, such correlations challenge any simple interpretation of fitness advantages due to body size or speed within a population; for example, survivors may be larger and faster than nonsurvivors because of differences in learning ability, not because of their size or speed.

The ontogeny of escape behavior, locomotor performance, and the hind limb in Sceloporus woodi

Flight initiation distance describes the distance at which an animal flees during the approach of a predator. This distance presumably reflects the tradeoff between the benefits of fleeing versus the benefits of remaining stationary. Throughout ontogeny, the costs and benefits of flight may change substantially due to growth-related changes in sprint speed; thus ontogenetic variation in flight initiation distance may be substantial. If escape velocity is essential for surviving predator encounters, then juveniles should either tolerate short flight initiation distances and rely on crypsis, or should have high flight initiation distances to remain far away from their predators. We examined this hypothesis in a small, short-lived lizard (Sceloporus woodi). Flight initiation distance and escape velocity were recorded on an ontogenetic series of lizards in the field. Maximal running velocity was also quantified in a laboratory raceway to establish if escape velocities in the field compared with maximal velocities as measured in the lab. Finally a subset of individuals was used to quantify how muscle and limb size scale with body size throughout ontogeny. Flight initiation distance increased with body size; larger animals had higher flight initiation distances. Small lizards had short flight initiation distances and remained immobile longer, thus relying on crypsis for concealment. Escape velocity in the field did not vary with body size, yet maximum velocity in the lab did increase with size. Hind limb morphology scaled isometrically with body size. Isometric scaling of the hind limb elements and its musculature, coupled with similarities in sprint and escape velocity across ontogeny, demonstrate that smaller S. woodi must rely on crypsis to avoid predator encounters, whereas adults alter their behavior via larger flight initiation distance and lower (presumably less expensive) escape velocities.

Performance, personality, and energetics: correlation, causation, and mechanism

The study of phenotypic evolution should be an integrative endeavor that combines different approaches and crosses disciplinary and phylogenetic boundaries to consider complex traits and organisms that historically have been studied in isolation from each other. Analyses of individual variation within populations can act to bridge studies focused at the levels of morphology, physiology, biochemistry, organismal performance, behavior, and life history. For example, the study of individual variation recently facilitated the integration of behavior into the concept of a pace-of-life syndrome and effectively linked the field of energetics with research on animal personality. Here, we illustrate how studies on the pace-of-life syndrome and the energetics of personality can be integrated within a physiology-performance-behavior-fitness paradigm that includes consideration of ecological context. We first introduce key concepts and definitions and then review the rapidly expanding literature on the links between energy metabolism and personality traits commonly studied in nonhuman animals (activity, exploration, boldness, aggressiveness, sociability). We highlight some empirical literature involving mammals and squamates that demonstrates how emerging fields can develop in rather disparate ways because of historical accidents and/or particularities of different kinds of organisms. We then briefly discuss potentially interesting avenues for future conceptual and empirical research in relation to motivation, intraindividual variation, and mechanisms underlying trait correlations. The integration of performance traits within the pace-of-life-syndrome concept has the potential to fill a logical gap between the context dependency of selection and how energetics and personality are expected to interrelate. Studies of how performance abilities and/or aspects of Darwinian fitness relate to both metabolic rate and personality traits are particularly lacking.