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Lower-level predictors and behavioral correlates of maximal aerobic capacity and sprint speed among individual lizards
Abstract
The standard paradigm of organismal biology views lower-level traits (e.g., aspects of physiology) as determining organismal performance abilities (e.g., maximal sprint speed), which in turn constrain behavior (e.g., social interactions). However, few studies have simultaneously examined all three levels of organization. We used focal observations to record movement behaviors and push-up displays in the field for adult male Sceloporus occidentalis lizards during the breeding season. We then captured animals, measured aspects of physiology, morphology, performance, and counted ecto- and endoparasites as potential predictors of sprint speed and maximal oxygen consumption (VO2max). Field behaviors were statistically repeatable, but not strongly so. Sprint speed and VO2max were repeatable using residuals from regressions on body mass (speed: r=0.70; VO2max: r=0.88). Both calf (standardized partial regression [path] coefficient B=0.53) and thigh (B=-0.37) muscle masses (as residuals from regressions on body mass) were significant predictors of sprint speed; hemoglobin concentration (B=0.42) was a predictor of VO2max. In turn, VO2max predicted the maximum number of 4-legged push-ups per bout (B=0.39). In path analysis, log likelihood ratio tests indicated no direct paths from lower-level traits to behavior, supporting the idea that morphology, in the broad sense, only affects behavior indirectly through measures of performance. Our results show that inter-individual variation in field behaviors can be related to performance abilities, which in turn reflect differences in morphology and physiology, although not parasite load. Given the low repeatability of field behaviors, some of the relationships between behavior and performance may be stronger than suggested by our results.
... Trials were excluded if lizards ran at an angle, made a U-turn, moved slowly or stopped (Losos et al., 2002;McElroy et al., 2012). From the three trials at each temperature, we selected the fastest 0.15 m racetrack interval (i.e. between two successive sensors) for each lizard as the estimate of its maximum sprinting ability at that temperature (Crowley, 1985;Bonino et al., 2011;Albuquerque et al., 2023). ...
Gravid female lizards often experience reduced thermal preferences and impaired locomotor performance. These changes have been attributed to the physical burden of the clutch, but some authors have suggested that they may be due to physiological adjustments. We compared the thermal biology and locomotor performance of the lizard Liolaemus wiegmannii one week before and one week after oviposition. We found that gravid females had a thermal preference 1°C lower than that of non-gravid females. This was accompanied by a change in the thermal dependence of maximum running speed. The thermal optimum for locomotor performance was 2.6°C lower before oviposition than after. At relatively low temperatures (22 and 26°C), running speeds of females before oviposition were up to 31% higher than for females after oviposition. However, at temperatures above 26°C, females achieved similar maximum running speeds (∼1.5 m/s) regardless of reproductive stage. The magnitude of the changes in thermal parameters and locomotor performance of L. wiegmannii females was independent of relative clutch mass (clutches weighed up to 89% of post-oviposition body mass). This suggests that the changes are not simply due to the clutch mass, but are also due to physiological adjustments. L. wiegmannii females simultaneously adjusted their own physiology in a short period in order to improve locomotor performance and allocated energy for embryonic development during late gravid stage. Our findings have implications for understanding the mechanisms underlying life histories of lizards on the fast extreme of the slow-fast continuum, where physiological exhaustion could play an important role.
Animal communication can be complex, often involving multiple static and dynamic traits. The extent to which these traits are correlated can elucidate their function as either redundant or multiple messages. Using the agamid lizard, Psammophilus dorsalis , as a model system we examined patterns of trait expression and the role of steroid hormones in mediating these traits during social interactions. We staged male-male interactions in the lab and measured the repertoire of display behaviour and colours, which change dynamically in the visible and ultraviolet ranges in different body regions. Additionally, we measured testosterone and corticosterone levels before and after the social trials.
Our results show that within behaviour and colour trait categories, components were strongly correlated within individuals, suggesting either a shared physiological pathway or redundant information content. However, across trait categories, correlation patterns varied. The chromatic contrast of the (yellow) dorsal region of lizards was correlated with both body size and level of aggression, whereas the size of UV patches was correlated with body size only. We also found a negative association between baseline corticosterone levels, body size and dorsal yellow chromatic contrast, suggesting a mechanistic link between these traits. However, social interaction induced testosterone and corticosterone levels were uncorrelated with the expression of the dynamic behavioural and colour displays during the social interactions itself. Notably, the intensity of colour and behavioural displays of males were matched by their opponents. Overall, our results suggest that multiple signalling traits can ensure both redundancy as well as provide multiple messages to receivers, thus improving the robustness of information transfer, particularly during competitions which have high fitness consequences.
Trade-offs and constraints are inherent to life, and studies of these
phenomena play a central role in both organismal and evolutionary
biology. Trade-offs can be defined, categorized, and studied in at
least six, not mutually exclusive, ways. (1) Allocation constraints are
caused by a limited resource (e.g., energy, time, space, essential
nutrients), such that increasing allocation to one component necessarily
requires a decrease in another (if only two components
are involved, this is referred to as the Y-model, e.g., energy devoted
to size versus number of offspring). (2) Functional conflicts occur
when features that enhance performance of one task decrease
performance of another (e.g., relative lengths of in-levers and outlevers,
force-velocity trade-offs related to muscle fiber type composition).
(3) Shared biochemical pathways, often involving integrator
molecules (e.g., hormones, neurotransmitters, transcription
factors), can simultaneously affect multiple traits, with some effects
being beneficial for one or more components of Darwinian fitness
(e.g., survival, age at first reproduction, fecundity) and others detrimental.
(4) Antagonistic pleiotropy describes genetic variants that
increase one component of fitness (or a lower-level trait) while
simultaneously decreasing another. (5) Ecological circumstances
(or selective regime) may impose trade-offs, such as when foraging
behavior increases energy availability yet also decreases survival.
(6) Sexual selection may lead to the elaboration of (usually male)
secondary sexual characters that improve mating success but
handicap survival and/or impose energetic costs that reduce other
fitness components. Empirical studies of trade-offs often search
for negative correlations between two traits that are the expected
outcomes of the trade-offs, but this will generally be inadequate if
more than two traits are involved and especially for complex physiological
networks of interacting traits. Moreover, trade-offs often
occur only in populations that are experiencing harsh environmental
conditions or energetic challenges at the extremes of
phenotypic distributions, such as among individuals or species
that have exceptional athletic abilities. Trade-offs may be (partially)
circumvented through various compensatory mechanisms,
depending on the timescale involved, ranging from acute to evolutionary.
Going forward, a pluralistic view of trade-offs and constraints,
combined with integrative analyses that cross levels of biological
organization and traditional boundaries among disciplines,
will enhance the study of evolutionary organismal biology.
Keywords: adaptation, allocation, antagonistic, biomechanics,
constraint, energetics, evolution, genetic correlation, hormones,
integrator molecules, locomotion, plasticity, pleiotropy, selection
experiments, sexual selection.
Three decades ago, interactions between evolutionary biology and physiology gave rise to evolutionary physiology. This caused comparative physiologists to improve their research methods by incorporating evolutionary thinking. Simultaneously, evolutionary biologists began focusing more on physiological mechanisms that may help to explain constraints on and trade-offs during microevolutionary processes, as well as macroevolutionary patterns in physiological diversity. Here we argue that evolutionary physiology has yet to reach its full potential, and propose new avenues that may lead to unexpected advances. Viewing physiological adaptations in wild animals as potential solutions to human diseases offers enormous possibilities for biomedicine. New evidence of epigenetic modifications as mechanisms of phenotypic plasticity that regulate physiological traits may also arise in coming years, which may also represent an overlooked enhancer of adaptation via natural selection to explain physiological evolution. Synergistic interactions at these intersections and other areas will lead to a novel understanding of organismal biology.
Parasites may impair host behavior in ways that reduce host fitness, especially when access to territories or mates becomes disrupted. Western fence lizards (Sceloporus occidentalis) are a key host to western blacklegged ticks (Ixodes pacificus). Males are highly aggressive during the mating season, competing with rivals through displays of color badges, pushups, and other behaviors. We hypothesized that experimental tick infestation diminishes the performance of male western fence lizards in intrasexual contests, via either blood loss, damage to sensory structures, or both. We infested adult males with larval ticks for 7 to 10 days, staged contests between infested and quality-matched control males, and measured their behavior in enclosure arenas. Infested lizards were less aggressive and exhibited decreased hematocrit, compared to non-infested animals. We found no relationships between aggression and either body size or blue ventral badge color traits, for either infested or uninfested males. There was also no effect of tick attachment location and hemoparasite infection on host contest behavior. This is the first demonstration of the impact of I. pacificus parasitism on intraspecific interactions of western fence lizards and suggests that tick infestation has substantial impacts on lizard fitness. Because I. pacificus rely heavily on these lizards for blood meals and dispersal, these impacts could also influence the abundance of ticks and the pathogens they vector.
Significance statement
Parasitism frequently impairs host behavior. We investigated the effect of blacklegged tick parasitism on western fence lizard contest behavior using a lab-based manipulation experiment. We demonstrated that tick infestation reduced lizard aggressiveness, which may lead to territory loss and reduced mating opportunities in the wild. We found no evidence that host body size or coloration are correlated with aggression. This study adds to a growing body of evidence that tick parasitism is costly for lizards, while demonstrating the usefulness of manipulation experiments to understand host-parasite interactions.
It is generally accepted that parasites exert negative effects on their hosts and that natural selection favors specific host responses that mitigate this impact. It is also known that some components of the host immune system often co-evolve with parasite antigens resulting in a host-parasite arms race. In addition to immunological components of the anti-parasitic response, host behavioral responses are also important in this arms race and natural selection may favor avoidance strategies that preclude contact with parasites, or shifts in the host’s thermoregulatory strategy to combat active infections (e.g., behavioral fever). Ticks are widespread parasites with direct and indirect costs on their vertebrate hosts. Their saliva provokes hemolysis in the blood of their hosts and can transmit a plethora of tick-borne pathogens. We enquired whether tick infestation by Ixodes pacificus can provoke a thermoregulatory response in Sceloporus occidentalis. For this, we compared the thermoregulatory behavior of tick-infested lizards against tick-infested lizards co-infected with two different species of coccidians (Lankesterella occidentalis and Acroeimeria sceloporis). After this, lizards were kept in individual terraria with a basking spot and fed ad libitum. We found that tick-infested lizards sought cooler temperatures in proportion to their tick load, and this response was independent of the co-infection status by L. occidentalis. This was consistent in April and June (when tick loads were significantly lower) and suggests a conservative strategy to save energy which might have been selected to overcome tick infestations during phenological peaks of this parasite. However, this behavior was not observed in lizards co-infected with A. sceloporis, suggesting that co-infection with this intestinal parasite prompt lizards to be active. Cost of tick infestation was confirmed because housed lizards lost weight at a constant ratio to initial tick load, independently of other infections. The broader implications of these findings are discussed in the context of climate change.
Superior locomotor performance confers advantages in terms of male combat success, survival and fitness in a variety of organisms. In humans, investment in increased performance via the exercise response is also associated with numerous health benefits, and aerobic capacity is an important predictor of longevity. Although the response to exercise is conserved across vertebrates, no studies have tested whether non-human animals that invest in increased athletic performance through exercise realize a survival advantage in nature. Green anole lizards respond to exercise training, and enhanced performance drives trade-offs with reproduction and immunocompetence. We released sprint-trained, endurance-trained and untrained-control male and female green anole lizards into an isolated, urban island in New Orleans, LA, USA and monitored their survival. Sedentary controls realized a significant survivorship advantage compared to trained lizards. Our results suggest that locomotor capacity is currently optimized to maximize survival in green anoles, and that forcing additional investment in performance moves them into a suboptimal phenotypic space relative to their current environmental demands.
Butterflies display extreme variation in wing shape associated with tremendous ecological diversity. Disentangling the role of neutral versus adaptive processes in wing shape diversification remains a challenge for evolutionary biologists. Ascertaining how natural selection influences wing shape evolution requires both functional studies linking morphology to flight performance, and ecological investigations linking performance in the wild with fitness. However, direct links between morphological variation and fitness have rarely been established. The functional morphology of butterfly flight has been investigated but selective forces acting on flight behaviour and associated wing shape have received less attention. Here, we attempt to estimate the ecological relevance of morpho-functional links established through biomechanical studies in order to understand the evolution of butterfly wing morphology. We survey the evidence for natural and sexual selection driving wing shape evolution in butterflies, and discuss how our functional knowledge may allow identification of the selective forces involved, at both the macro-and micro-evolutionary scales. Our review shows that although correlations between wing shape variation and ecological factors have been established at the macro-evolutionary level, the underlying selective pressures often remain unclear. We identify the need to investigate flight behaviour in relevant ecological contexts to detect variation in fitness-related traits. Identifying the selective regime then should guide experimental studies towards the relevant estimates of flight performance. Habitat, predators and sex-specific behaviours are likely to be major selective forces acting on wing shape evolution in butterflies. Some striking cases of morphological divergence driven by contrasting ecology involve both wing and body morphology, indicating that their interactions should be included in future studies investigating co-evolution between morphology and flight behaviour.
Mortality through predation is often selective, particularly at life‐history bottlenecks. While many studies have looked at the importance for survival of specific prey characteristics in isolation, few have looked at a broad array of attributes and how they relate to survival in a realistic context.
Our study measures 18 morphological, performance and behavioural traits of a juvenile damselfish that have been hypothesized as important for prey survival, and examines how they relate to survival in the field immediately after settlement. These attributes included size, relative false eye‐spot size, fast‐start escape response kinematics, thigmotaxis, laterality, and space use and activity in the field.
Using conditional inference trees, we identify the most important drivers out of a reduced suite of 13 characters. Fast‐start response latency, boldness, feeding rates and two measures of activity were found to significantly contribute to survival. Morphological variables and most laboratory measures of performance appeared to contribute little to survival.
Results suggest selection works on a suite of characters associated with boldness. Bold and active fish are those that will be best able to learn using public information, but because of the relative naiveté of newly metamorphosed fishes, speed to react to a strike from an unknown predator is of critical importance.
Findings substantiate the ecomorphological paradigm by suggesting that selection on behaviour modifies the correlations of morphological and performance variables with survival probabilities, since behaviour modifies performance capabilities by making them specific to context.
A plain language summary is available for this article.
Integrative behavioral studies show that the interplay between individual physiology and social behavior influences the ecology of the species, ultimately affecting individual fitness. Particularly in lizards, color polymorphism is associated with differential behaviors and reproductive strategies, which are evident in mature males during the mating season. Dominant males generally have greater endurance, higher body temperature, and larger bodies than submissive males, so they can acquire and defend larger territories and have greater access to females for mating. We studied whether the color morphs observed in males of one of the world’s southernmost reptiles, Liolaemus sarmientoi, are related to behavioral variation during agonistic interactions, thermal physiology, morphology, and/or locomotor stamina. Liolaemus sarmientoi males exhibit three color morphs: red (RR), red–yellow (RY), and yellow (YY). These lizards exhibit subtle behavioral displays and we did not observe stamina differences among morphs. However, we found that RR males are more aggressive than YY males during agonistic encounters. In addition, greater body temperature change during trials, higher field body temperatures, and greater head sizes of RR males compared to RY or YY indicate that RR is a dominant morph, which may influence their ability to acquire and defend territory and tactics for achieving reproductive success.
Scientific and public interest in host manipulation by parasites has surged over the past few decades, resulting in an exponential growth of cases where potential behavioral manipulation has been identified. However, these studies dwarf the number of genuine attempts to elucidate the mechanistic processes behind this behavioral manipulation. Ultimately, this imbalance has slowed progress in the study of the mechanisms underlying host manipulation. As it stands, research suggests that the mechanisms of host manipulation fall into three categories: immunological, genomic/proteomic and neuropharmacological, and forth potential category: symbioant-mediated manipulation. After exploration of the literature pertaining to these four pathways, four major trends become evident. First and foremost, there is a severe disconnect between the observed molecular and behavioral shifts in a parasitized host. Indeed, very rarely a study demonstrates that molecular changes observed in a host are the result of active manipulation by the resident parasite, or that these molecular changes directly result in behavioral manipulation that increases the parasite's fitness. Secondly, parasites may often employ multiple pathways in unison to achieve control over their hosts. Despite this, current scientific approaches usually focus on each manipulation pathway in isolation rather than integrating them. Thirdly, the relative amount of host-parasite systems yet to be investigated in terms of molecular manipulation is staggering. Finally, as a result of the aforementioned trends, guiding mechanisms or principles for the multiple types of behavioral manipulation are yet to be found. Researchers should look to identify the manipulative factors required to generate the molecular changes seen in hosts, while also considering the “multi-pronged” approach parasites are taking to manipulate behavior. Assessing gene expression and its products during transitional periods in parasites may be a key methodological approach for tackling these recent trends in the host manipulation literature.
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.
The mechanistic foundations of performance trade-offs are clear: because body size and shape constrains movement, and muscles vary in strength and fibre type, certain physical traits should act in opposition with others (e.g. sprint versus endurance). Yet performance trade-offs are rarely detected, and traits are often positively correlated. A potential resolution to this conundrum is that within-individual performance trade-offs can be masked by among-individual variation in ‘quality’. Although there is a current debate on how to unambiguously define and account for quality, no previous studies have partitioned trait correlations at the within- and among-individual levels. Here, we evaluate performance trade-offs among and within 1369 elite athletes that performed in a total of 6418 combined-events competitions (decathlon and heptathlon). Controlling for age, experience and wind conditions, we detected strong trade-offs between groups of functionally similar events (throwing versus jumping versus running) occurring at the among-individual level. We further modelled individual (co)variation in age-related plasticity of performance and found previously unseen trade-offs in throwing versus running performance that manifest through ageing. Our results verify that human performance is limited by fundamental genetic, environmental and ageing constraints that preclude the simultaneous improvement of performance in multiple dimensions. Identifying these constraints is fundamental to understanding performance trade-offs and predicting the ageing of motor function. © 2017 The Author(s) Published by the Royal Society. All rights reserved.
The endocrine mechanisms causing variation and plasticity in life history traits (e.g., development time, mass at birth/hatching,
rate of postnatal growth, age or size at sexual maturity, litter or clutch size, annual survival, and lifespan) or fitness
(annual or lifetime reproductive success) have recently garnered considerable interest. We review three issues facing studies
that quantify relationships between endocrine traits and life histories or measures of fitness and describe possible solutions
using insights from evolutionary ecology. We focus in particular on the steroid hormones glucocorticoids that are involved
in the vertebrate neuroendocrine stress response. First, context-dependent associations between endocrine traits and life
histories or fitness are widespread, and therefore, it is important to quantify how intrinsic or extrinsic factors modify
these relationships. Second, studies in evolutionary endocrinology may aspire to quantify patterns of natural selection on
endocrine traits, but this may not tell us how they influence fitness. Studies that also identify the actual targets of selection
that the endocrine traits are influencing will be very useful. Third, environmental or intrinsic factors can cause co-variance
between endocrine traits and life histories or fitness. This is problematic for interpreting the potential evolutionary consequences
of selection on endocrine traits, but it can also produce divergent answers for relationships between endocrine traits and
life histories or fitness depending upon whether the data are analyzed in an among- or within-year framework. Future long-term
studies following uniquely marked individuals over their lifetime (longitudinal individual-based approach) in combination
with experimental manipulations of the endocrine traits or environmental factors influencing both endocrine traits and life
histories or fitness may help to produce new insights in evolutionary endocrinology despite these issues. This is an ambitious
endeavor, and we briefly review some of the key issues facing such long-term studies and experimental manipulations of endocrine
traits.
Despite the complexity of nature, most comparative studies of phenotypic evolution consider selective pressures in isolation. When competing pressures operate on the same system, it is commonly expected that trade-offs will occur that will limit the evolution of phenotypic diversity, however, it is possible that interactions amongst selective pressures may promote diversity instead. We explored the evolution of locomotor performance in lizards in relation to possible selective pressures using the Ornstein-Uhlenbeck process. Here, we show that a combination of selection based on foraging mode and predator escape is required to explain variation in performance phenotypes. Surprisingly, habitat use contributed little explanatory power. We find that it is possible to evolve very different abilities in performance which were previously thought to be tightly correlated, supporting a growing literature that explores the many-to-one mapping of morphological design. While we generally find the expected trade-off between maximal exertion and speed, this relationship surprisingly disappears when species experience selection for both performance types. We conclude that functional integration need not limit adaptive potential, and that an integrative approach considering multiple major influences on a phenotype allows a more complete understanding of adaptation and the evolution of diversity. This article is protected by copyright. All rights reserved.
Courtship displays are typically thought to have evolved via female choice, whereby females select mates based on the characteristics of a display that is expected to honestly reflect some aspect of the male's quality. Honesty is typically enforced by mechanistic costs and constraints that limit the level at which a display can be performed. It is becoming increasingly apparent that these costs may be energetic costs involved in the production of dynamic, often repetitive displays. A female attending to such a display may thus be assessing the physical fitness of a male as an index of his quality. Such assessment would provide information on his current physical quality as well as his ability to carry out other demanding activities, qualities with which a choosy female should want to provision her offspring. In the current study we use courtship interactions in the Cuban burrowing cockroach, Byrsotria fumigata to directly test whether courtship is associated with a signaler's performance capacity. Males that had produced courtship displays achieved significantly lower speeds and distances in locomotor trials than non-courting control males. We also found that females mated more readily with males that produced a more vigorous display. Thus, males of this species have developed a strategy where they produce a demanding courtship display, while females choose males based on their ability to produce this display. Courtship displays in many taxa often involve dynamic repetitive actions and as such, signals of stamina in courtship may be more widespread than previously thought.
Trade-offs are a common focus of study in evolutionary biology and in studies of locomotor physiology and biomechanics. A previous comparative study of 12 species of European lacertid lizards found a statistically significant negative correlation between residual locomotor speed and stamina (controlling for variation in body size), consistent with ideas about trade-offs in performance based on variation in muscle fiber type composition and other subordinate traits. To begin examining the generality of this finding in other groups of squamates, we measured maximal sprint running speed on a high-speed treadmill and endurance at 1.0 km/h (0.28 m/s) in 14 species of North American phrynosomatid lizards, plus a sample of nine additional species to encompass some of the broadscale diversity of lizards. We used both conventional and phylogenetically informed regression analyses to control for some known causes of performance variation (body size, stockiness, body temperature) and then computed residual performance values. We found no evidence for a trade-off between speed and endurance among the 14 phrynosomatids or among the 23 species in the extended data set. Possible explanations for the apparent difference between lacertids and phrynosomatids are discussed.
Activity metabolism is supported by phosphorylated reserves (adenosine triphosphate, creatine phosphate), glycolytic, and aerobic metabolism. Because there is no apparent variation between vertebrate groups in phosphorylated reserves or glycolytic potential of skeletal muscle, variation in maximal metabolic rate between major vertebrate groups represents selection operating on aerobic mechanisms. Maximal rates of oxygen consumption in vertebrates are supported by increased conductive and diffusive fluxes of oxygen from the environment to the mitochondria. Maximal CO2 efflux from the mitochondria to the environment must be matched to oxygen flux, or imbalances in pH will occur. Among vertebrates, there are a variety of modes of locomotion and vastly different rates of metabolism supported by a variety of cardiorespiratory architectures. However, interclass comparisons strongly implicate systemic oxygen transport as the rate-limiting step to maximal oxygen consumption for all vertebrate groups. The key evolutionary step that accounts for the approximately 10-fold increase in maximal oxygen flux in endotherms versus ectotherms appears to be maximal heart rate. Other variables such as ventilation, pulmonary/gill, and tissue diffusing capacity, have excess capacity and thus are not limiting to maximal oxygen consumption. During maximal activity, the ratio of ventilation to respiratory system blood flow is remarkably similar among vertebrates, and CO2 extraction efficiency increases while oxygen extraction efficiency decreases, suggesting that the respiratory system provides the largest resistance to maximal CO2 flux. Despite the large variation in modes of activity and rates of metabolism, maximal rates of oxygen and CO2 flux appear to be limited by the cardiovascular and respiratory systems, respectively. © 2015 American Physiological Society. Compr Physiol 5:1677-1703, 2015.
Home range is defined as the area traversed during normal daily activities, such as foraging, avoiding predators, and social or antagonistic behaviors. All else being equal, larger home ranges should be associated with longer daily movement distances and/or higher average movement speeds. The maximal rate of oxygen consumption (V̇O2max) generally sets an upper limit to the intensity of work (e.g., speed of locomotion) that an animal can sustain without fatigue. Therefore, home range area and V̇O2max are predicted to evolve in concert (coadapt). We gathered literature data on home range and V̇O2max for 55 species of mammals. We computed residuals from log-log (allometric) regressions on body mass with two different regression models: ordinary least squares (OLS) and phylogenetic generalized least squares (PGLS). Residuals were weakly positively related for both the OLS (r = 0.278, one-tailed P < 0.05) and PGLS (r = 0.210, P > 0.05) regressions. For V̇O2max, the PGLS regression model had a slightly higher likelihood than the OLS model, but the situation was reversed for home range area. In addition, for both home range area and V̇O2max, models that fit better than either OLS or PGLS were obtained by modeling residual variation with the Ornstein-Uhlenbeck process to mimic stabilizing selection (RegOU), indicating that phylogenetic signal is present in both size-adjusted traits, consistent with findings of previous studies. (However, residuals from the RegOU models cannot be tested for correlation due to mathematical complexities.) We conclude that the best estimate of the residual correlation is probably somewhere between these two values reported above. Possible reasons for the low correlation between residual home range area and V̇O2max are discussed.
Whereas measures of maximal performance are important for defining the functional operating space of organisms, it is not clear what determines the levels of submaximal performance used in nature. In this study, I address the following two questions. What influence does the habitat have in determining performance levels in nature? Why do animals not perform at levels more similar to their performance limits? To do this, I studied clinging performance of lizards (Sceloporus occidentalis) living in an abandoned quarry. Results indicate that two habitat characteristics play an important role in determining clinging performance: microhabitat type and sun exposure. On average, lizards in this study used perches sloped about 45° elevated 68 cm above the ground. The slope of the perch used by lizards is similar to the average rock perch (44°) found in the quarry. The height of the perch used by lizards is similar to the average sun exposed perch (70 cm) in the quarry. Thus, it appears that lizards' thermoregulatory preference for high, sunny, rock basking sites has greatly influenced their choice of perch. Examination of laboratory measures of clinging performance revealed that lizards were capable of clinging to perches approaching 1.00° from horizontal. However, in nature lizards only used perches sloped up to 80°. Experiments using dead lizards revealed that friction alone allowed lizards to cling to perches up to 75°. The level of maximal voluntary performance observed in nature may result from the friction of the lizards' ventral surface.
Ectoparasites infest many lizard species, but the physiological consequences of infestation in free-living lizards have not been studied. We examined male western fence lizards, Sceloporus occidentalis, infested with ticks, Ixodes pacificus, in northern California. Many lizards in this population were also infected with malaria, Plasmodium mexicanum. We found that the effects of nymphal ticks on blood composition and body condition (relative body weight) depended on the age of the lizard host and whether the lizard was also infected with malaria. Adult male lizards, but not juveniles, infested with ticks had lower hematocrits than uninfested lizards. Although the reduction in hematocrit that accompanies the mean level of tick infestation probably does not substantially affect the lizards, heavy infestations (>5 ticks per lizard) may significantly limit the aerobic capacity and behavior of the lizard host. Lizards parasitized with both ticks and malaria had significantly lower body condition than lizards with one or neither parasite.
Whole-animal physiological performance is highly polygenic and highly plastic, and the same is generally true for the many subordinate traits that underlie performance capacities. Quantitative genetics therefore provides an appropriate framework for the analysis of physiological phenotypes and can be used to infer the microevolutionary processes that have shaped patterns of trait variation within and among species. When specific genes are known to contribute to variation in physiological traits, analyses of intraspecific polymorphism and interspecific divergence can reveal molecular mechanisms of functional evolution and can provide insights into the possible adaptive significance of observed sequence changes. Here we explain how the tools and theory of quantitative genetics, population genetics, and molecular evolution can inform our understanding of mechanism and process in physiological evolution. For example, lab-based studies of polygenic inheritance can be integrated with field-based studies of trait variation and survivorship to measure selection in the wild, thereby providing direct insights into the adaptive significance of physiological variation. Analyses of quantitative genetic variation in selection experiments can be used to probe interrelationships among traits and the genetic basis of physiological trade-offs and constraints. We review approaches for characterizing the genetic architecture of physiological traits, including linkage mapping and association mapping, and systems approaches for dissecting intermediary steps in the chain of causation between genotype and phenotype. We also discuss the promise and limitations of population genomic approaches for inferring adaptation at specific loci. We end by highlighting the role of organismal physiology in the functional synthesis of evolutionary biology.
Copyright © 2015, American Journal of Physiology - Regulatory, Integrative and Comparative Physiology.
The behavioral time budget of the western fence lizard, Sceloporus occidentalis, was recorded in northern California, where fence lizards are frequently infected by the malarial parasite, Plasmodium mexicanum. Because malarial infection results in pathologies that could affect the host's behavior in nature, time budgets of malarious and noninfected animals were compared. Malarious and noninfected lizards utilized the microhabitat in similar ways, except that infected animals perched more often in shade. Fully 94% of the time lizards were observed, they spent perched, unmoving and apparently visually scanning the local area. Although behavior patterns differed seasonally and between sexes and age classes, they were very similar in infected and noninfected lizards. Noninfected adult males, though, spent more time engaged in social behaviors than did malarious males. Social interactions appear strenuous, occasionally lasting several minutes. Such social activities by the lizard may depend on rapid oxygen consumption that is disrupted by Plasmodium infection. Number of burst runs and length of runs were similar for both infected and noninfected animals; most lasted less than 1 sec, and the longest covered 6 m. In laboratory experiments, recovery from burst runs was rapid (less than 1 min/sec of running), probably negating any ecological consequences of an effect of the parasite on rate of metabolic recovery by infected lizards. The behavior of malarious lizards, compared to noninfected animals, can be explained by the effects of Plasmodium infection on the host's energy metabolism, as measured in the laboratory. These results demonstrate that estimating the virulence of parasitic infection requires both laboratory studies and observation on the host in a natural setting.
1. Review of published ecological studies using path analysis suggests that path analysis is often misused and collinearity is a moderate to severe problem in a majority of the analyses. 2. Multiple regression is shown to be related to path analysis, and thus collinearity, which is a well-known problem in regression analysis, can also be a problem in path analysis. Collinearity occurs when independent variables are highly correlated and causes estimates of standardized partial regression coefficients and of path coefficients to be less precise, less accurate and prone to rounding error. 3. A review of 24 path analyses in 12 papers revealed incorrectly calculated path coefficients in 13 cases and problems with collinearity in 15 cases. Estimates of path coefficients are biased and lack precision if there is collinearity. 4. An additional 40 path analyses could not be evaluated because of incomplete information. Most studies lack sufficient sample size to justify the use of path analysis. Problems associated with the use of categorical variables, which were used in six cases, are unappreciated. 5. These problems suggest that path coefficients may often mislead ecologists about the relative importance of ecological processes.
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For almost 40 years, studies of whole-organism performance have formed a cornerstone of evolu-tionary 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 fre-quently been applied to performance evolution in ways that range from unsatisfactory to inappropri-ate. 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.
Animals undergo significant weight change due to a variety of causes. Autotomy, the voluntary shedding of an appendage in response to a predator stimulus, provides an effective model for measuring the effects of rapid weight change on locomotor behavior and the responses to more gradual weight gain, particularly in lizards capable of both autotomizing and regenerating their tail. Although the general effects of autotomy on locomotor performance are commonly explored, we investigated changes in locomotor mechanics associated with tail loss and long-term regeneration for the first time by measuring morphology, 3D kinematics, and ground reaction forces (GRFs) in the leopard gecko, Eublepharis macularius. Tail autotomy resulted in a 13% anterior shift in the center of mass (CoM), which only partially recovered after full regeneration of the tail. Although no changes in body or forelimb kinematics were evident, decreases in hindlimb joint angles signify a more sprawled posture following autotomy. Changes in hindlimb GRFs resulted in an increase in weight-specific propulsive force, without a corresponding change in locomotor speed. Hindlimb kinematics and GRFs following autotomy recovered to pre-autotomy values as the tail regenerated. These results suggest an active locomotor response to tail loss that demonstrates the causal relationships between morphological, kinematic, and force variation.
Evolutionary physiology strives to understand how the function and integration of physiological systems influence the way in which organisms evolve. Studies of the O2 transport pathway – the integrated physiological system that transports O2 from the environment to mitochondria – are well suited to this endeavour. We consider the mechanistic underpinnings across the O2 pathway for the evolution of aerobic capacity, focusing on studies of artificial selection and naturally selected divergence among wild populations of mammals and fish. We show that evolved changes in aerobic capacity do not require concerted changes across the O2 pathway and can arise quickly from changes in one or a subset of pathway steps. Population divergence in aerobic capacity can be associated with the evolution of plasticity in response to environmental variation or activity. In some cases, initial evolutionary divergence of aerobic capacity arose exclusively from increased capacities for O2 diffusion and/or utilization in active O2-consuming tissues (muscle), which may often constitute first steps in adaptation. However, continued selection leading to greater divergence in aerobic capacity is often associated with increased capacities for circulatory and pulmonary O2 transport. Increases in tissue O2 diffusing capacity may augment the adaptive benefit of increasing circulatory O2 transport owing to their interactive influence on tissue O2 extraction. Theoretical modelling of the O2 pathway suggests that O2 pathway steps with a disproportionately large influence over aerobic capacity have been more likely to evolve, but more work is needed to appreciate the extent to which such physiological principles can predict evolutionary outcomes.
Parasites are ubiquitous components of biological systems that have evolved in multiple independent lineages during the history of life, resulting in a diversity of taxa greater than that of their free-living counterparts. Extant host-parasite associations are the result of tight reciprocal adaptations that allow parasites to exploit specific biological features of their hosts to ensure their transmission, survival, and maintenance of viable populations. As a result, parasites may affect host physiology, morphology, reproduction or behaviour, and they are increasingly recognized as having significant impacts on host individuals, populations, communities and even ecosystems. Although this is usually acknowledged by parasite ecologists, fish ecologists often ignore parasitism in their studies, often acting as though their systems are free of parasites. However, the effects of parasites on their hosts can alter variables routinely used in fish ecology, ranging from the level of individual fish (e.g. condition factors) to populations (e.g. estimates of mortality and reproductive success) or communities (e.g. measures of interspecific competition or the structure and functioning of food webs). By affecting fish physiology, parasites can also interfere with measurements of trophic levels by means of stable isotope composition, or have antagonistic or synergistic effects with host parameters normally used as indicators of different sources of pollution. Changes in host behaviour induced by parasites can also modify host distribution patterns, habitat selection, diet composition, sexual behaviour, etc., with implications for the ecology of fish and of their predators and prey. In this review, we summarise and illustrate the likely biases and erroneous conclusions that one may expect from studies of fish ecology that ignore parasites, from the individual to the community level. Given the impact of parasites across all levels of biological organisation, we show that their omission from the design and analyses of ecological studies poses real risks of flawed interpretations for those patterns and processes that ecologists seek to uncover.
Trade‐offs in performance expression occur because animals must perform multiple whole‐organism performance tasks that place conflicting demands on shared underlying morphology. Although not always detectable within populations, such trade‐offs may be apparent when analyzed at the level of the individual, particularly when all of the available data are taken into account as opposed to only maximum values. Detection of performance trade‐offs is further complicated in species where sexual dimorphism drives performance differences between males and females, leading potentially to differing patterns of trade‐offs within each sex. We tested for within‐ and between‐individual trade‐offs among three whole‐organism performance traits (sprint speed, endurance, and bite force) in adult male and female Anolis carolinensis lizards using all of the measured performance data. Sprinting and endurance did not trade‐off among individuals in either sex, but we found a significant negative among‐individual relationship between sprint speed and bite force in females only, likely driven by the mechanical burden of larger than optimal heads imposed on females through intralocus sexual conflict. We also found evidence for marked within‐individual plasticity in male bite force, but no within‐individual trade‐offs between any traits in either sex. These data offer new insight into the sex‐specific nature of performance trade‐offs and plasticity and, ultimately, into the constraints on multivariate performance evolution.
Behavioral ecologists have hypothesized that among-individual differences in resting metabolic rate (RMR) may predict consistent individual differences in mean values for costly behaviors or for behaviors that affect energy intake rate. This hypothesis has empirical support and presently attracts considerable attention, but, notably, it does not provide predictions for individual differences in (a) behavioral plasticity or (b) unexplained variation (residual variation from mean individual behavior, here termed predictability). We outline how consideration of aerobic maximum metabolic rate (MMR) and particularly aerobic scope (= MMR − RMR) can be used to simultaneously make predictions about mean and among- and within-individual variation in behavior. We predict that while RMR should be proportional to an individual’s mean level of sustained behavioral activity (one aspect of its personality), individuals with greater aerobic scope will also have greater scope to express behavioral plasticity and/or greater unpredictability in behavior (=greater residual variation). As a first step toward testing these predictions, we analyze existing activity data from selectively bred lines of mice that differ in both daily activity and aerobic scope. We find that replicate high-scope mice are more active on average and show greater among-individual variation in activity, greater among-individual variation in plasticity, and greater unpredictability. These data provide some tentative first support for our hypothesis, suggesting that further research on this topic would be valuable.
The foraging mode of lizards has been a central theme in guiding research in lizard biology for three decades. Foraging mode has been shown to be a pervasive evolutionary force molding the diet, ecology, behavior, anatomy, biomechanics, life history, and physiology of lizards. This 2007 volume reviews the knowledge on the effects of foraging mode on these and other organismal systems to show how they have evolved, over a wide taxonomic survey of lizard groups. The reviews presented here reveal the continuous nature of foraging strategies in lizards and snakes, providing the reader with a review of the field, and will equip researchers with fresh insights and directions for the sit-and-wait vs. wide foraging paradigm. This will serve as a reference book for herpetologists, evolutionary biologists, ecologists and animal behaviorists. é Cambridge University Press 2007.
Trachydosaurus rugosus is a large, slow-moving Australian skink. The maximal sprint velocity of this species averages only 2.7 km/h and is apparently limited by short limbs that have relatively little muscle mass. Standard metabolic rate at 35 C is 0.073 ± 0.009 ml O₂/(g·h), and the maximal rate of O₂ consumption is 0.722 ± 0.019 ml O₂/(g·h). The maximal aerobic speed is 0.67 km/h, and endurance at 1.0 km/h is 23 ± 7 min. The net cost of locomotion () is 0.921 ml O₂/(g·km) (95% confidence limit [C.L.]: 1. 133, 0.709). This value is tabulated together with all other data available for lizards, and a new allometric equation describing net cost of locomotion as a function of body mass is presented. For all lizards, (ml O₂/(g·km) = 4.22 . The locomotory and energetic capacities of Trachydosaurus describe an unusually slow lizard with limited stamina.
To understand why organisms function the way that they do, we must understand how evolution shapes physiology. This requires knowledge of how selection acts on physiological traits in nature. Selection studies in the wild allow us to determine how variation in physiology causes variation in fitness, revealing how evolution molds physiology over evolutionary time. Manipulating phenotypes experimentally in a selection study shifts the distribution of trait variation in a population to better explore potential constraints and the adaptive value of physiological traits. There is a large database of selection studies in the wild on a variety of traits, but very few of those are physiological traits. Nevertheless, data available so far suggest that physiological traits, including metabolic rate, thermal physiology, whole-organism performance, and hormone levels, are commonly subjected to directional selection in nature, with stabilizing and disruptive selection less common than predicted if physiological traits are optimized to an environment. Selection studies on manipulated phenotypes, including circulating testosterone and glucocorticoid levels, reinforce this notion, but reveal that trade-offs between survival and reproduction or correlational selection can constrain the evolution of physiology. More studies of selection on physiological traits in nature that quantify multiple traits are necessary to better determine the manner in which physiological traits evolve and whether different types of traits (dynamic performance vs. regulatory) evolve differently. (C) 2016 American Physiological Society.
Introduction, Everything that an animal does, from feeding to escaping predation, is influenced by underlying physiological traits. In this chapter, I focus on the insights gained by examining physiology in the context of lizard foraging mode. I will discuss apparent relationships between physiological traits and foraging mode and identify areas where we might expect to uncover explanatory correlates in the future. Understanding these relationships allows us to learn more about the evolution of lizard foraging and the related evolutionary processes and selective factors that act on underlying physiological components. Many biologists have addressed the importance of evolutionary and comparative physiology (see, for example, Prosser, 1950; Diamond, 1993; Garland and Carter, 1994; Natochin and Chernigovskaya, 1997; Feder et al., 2000). Physiology can be rather broadly defined to include many integrated and hierarchical suborganismal traits that manifest in organismal function. An incomplete list of these traits and processes includes enzyme activity, membrane selectivity, establishment of ion gradients, ATP production, cellular respiration, lung ventilation, aerobic capacity, Q10 effects, lactate buffering, pH balance, sprint speed, digestive efficiency, and many other processes involved in homeostasis that ultimately contribute to the survival and reproduction (Darwinian fitness) of organisms (see Table 3.1 for a list of physiology-related traits likely to inform the study of foraging modes). Importantly, these myriad suborganismal traits do not work in isolation, nor are they typically involved in only one aspect of organismal function (see, for example, Bennett, 1989; Garland and Losos, 1994; Rose and Lauder, 1996b). é Cambridge University Press 2007.
The speed of the quadrupedal gecko Phyllodactylus marmoratus nearly doubled after autotomy, a result contrary to earlier work on bipedal lizards. -from Author
In this study, we investigated the territorial behavior of Sceloporus occidentalis by focusing on three questions: do individuals of S. occidentalis (1) exhibit site-fidelity, (2) defend the site with aggressive behavior, and (3) maintain exclusive use of the defended area? Spatial distributions and aggressive behavior were studied in two natural populations. We also used a small arena to observe aggressive behavior in paired interactions. Repeatability analysis revealed that individuals of S. occidentalis maintain home ranges of similar areas and in similar locations across years. While lizards use pushup displays to delimit territories that are approximately 61% of total home range area, overt aggression (e.g., chases, bites) was infrequent. Furthermore, lizards maintained exclusive use of only a small proportion of their defended areas. We discuss our results in the context of what it means to be "territorial".
Publisher Summary This chapter is dedicated to describing citrate synthase. The assay of citrate synthase is performed by coupling it to the transacetylase reaction. The disappearance of acetyl phosphate is followed by a hydroxamate method and the formation of citrate by the pentabromoacetone method. The malate dehydrogenase catalyzed reaction is used to generate the oxaloacetate for the citrate synthase reaction. Another method for assaying citrate synthase uses 14 C-acetyl-CoA and measures its incorporation in 14 C-citrate, which is isolated as a silver salt. Citrate synthase can be followed by measuring the appearance of the free SH group of the released CoASH; three such methods are discussed in the chapter. One method is to measure the oxidation of the CoASH by dichlorophenol- indophenol, which is accompanied by a decrease in absorbancy at 578 mμ. Another method measures the CoASH polarographically. The third method measures SH by the use of 5, 5’-dithiobis-(2-nitrobenzoate) (DTNB) (Ellman's reagent).
We examined the sprinting and jumping capabilities of eight West Indian Anolis species during three natural activities (escape from a predator, feeding, and undisturbed activity). We then compared these field data with maximal performance under optimal laboratory conditions to answer three questions: (1) Has maximal (i.e., laboratory) sprinting and jumping performance coevolved with field performance among species? (2) What proportion of their maximum capabilities do anoles sprint and jump in different ecological contexts? (3) Does a relationship exist between maximal sprinting and jumping ability and the proportion of maximal performance used in these contexts? Among species, maximal speed is tightly positively correlated with sprinting performance during both feeding and escape in the field. Sprinting speed during escape closely matches maximal sprinting ability (i.e., about 90% of maximum performance). By contrast, sprinting performance during undisturbed activity is markedly lower (about 32% of maximum) than maximal sprinting performance. Sprinting ability during feeding is intermediate (about 71% of maximum) between field escape and field undisturbed activity. In contrast to sprinting ability, jumping ability is always substantially less than maximum (about 40% of maximum during feeding and undisturbed activity). A negative relationship exists among species between maximal speed and the proportion to which species sprint to their maximal abilities during field escape.
Territorial behavior of Sceloporus jarrovi was observed in Arizona during 3 years. Male aggression, activity and home range size increased and overlap between home ranges decreased as the fall breeding season approached. Most animals defended the entire home range. Males shifted their positions to maximize overlap with females in the fall. Females were less aggressive, occupied smaller areas and shifted positions less.
The social behavior patterns of both natural and enclosed populations of Sceloporus undulatus and Cnemidophorus sexlineatus were observed in Oklahoma. During challenge display, the male Sceloporus compresses his trunk, extends his dewlap, orients laterally towards his opponent and performs push-ups. The push-up series follows a species-specific sequence of units of movement: double, single, followed by a series of doubles, performed to a specific cadence in time. The dominant male is the most active, challenging and chasing other males, but tolerating females. Subordinate males retreat or assume a submissive posture. Female aggression is indicated by postural changes accompanied by sidle-hopping. Sex recognition depends upon the response to a challenge. A courting male nods rapidly while following a female. During copulation, the male maintains a biting hold on the shoulder of the female. A dominant male readily challenges his image in a mirror. Under crowded conditions of an enclosure dominance may be indicated by one individual supplanting another at a favorable lookout. Sexual arousal in the male Cnemidophorus is indicated by cloacal rubbing. During courtship the male follows the female and may walk astraddle her. During copulation, the male maintains a biting hold on the flank of the female as his body arches across her. The dominant males aggressively chase all others and will attempt to mate with submissive males. The behavior patterns of these two lizards show differences relative to adaptation to their respective habitat niches.
Ωnyx is a free software environment for creating and estimating structural equation models (SEM). It provides a graphical user interface that facilitates an intuitive creation of models, and a powerful back end for performing maximum likelihood estimation of parameters. Path diagrams in Ωnyx can be exported to OpenMx, lavaan, and Mplus to allow an easy transition to script-based SEM software. Also, OpenMx scripts can be transformed into a path diagrams in Ωnyx. In addition to the graphical user interface, Ωnyx provides a maximum likelihood optimizer that finds multiple minima and does not require the user to provide starting values. To our knowledge, Ωnyx is the only SEM program that provides multiple optima. This article describes both the graphical and conceptual features of Ωnyx, including the optimization approach, and demonstrates two hands-on modeling examples.
The lengths and diameters of the limb segments of 105 monitor lizards from 22 species were measured on preserved museum specimens in order to determine whether limb proportions vary in relation to snout-vent length (used as an indicator of overall body size). Scaling exponents (slopes of allometric equations) were estimated for log-transformed species' mean values, using both conventional nonphylogenetic statistics as well as the method of phylogenetically independent contrasts. Both methods gave essentially the same results. All limb segment lengths and diameters scale with exponents exceeding 1.0; thus, larger species of monitors tend to have larger limbs relative to their snout-vent length. Foot length, however, decreases relative to total hindlimb length in larger species. Measures of limb segment diameters scale with greater exponents than do limb lengths; thus, larger species also tend to have relatively thicker limbs. The empirical results on limb shape are consistent with predictions derived from biomechanical models.
Physiological capacities may constrain behavioural options and hence have important ecological consequences. We tested the hypo-thesis that social dominance is related to capacities for locomotor performance in territorial male lizards (Sceloporus occidentalis Baird & Girard). We first measured the maximal sprint speed and stamina of individual lizards in the laboratory. Pairs of size-matched males were then placed into a novel laboratory arena and allowed to compete for access to a basking site under a heat lamp. The lizard that physically controlled the basking site was judged the 'winner' of a domi-nance interaction (winner vs loser status was confirmed by quantitative scoring of behaviour). Winners of these dyadic encounters had signifi-cantly higher sprint speeds in 14 of 20 cases, with winners averaging 16.5% faster than losers. Stamina, however, did not differ between winners and losers.
Although the hind limb usually is considered to pro vide the propulsive force in lizard locomotion, no study to date has analyzed motor or kinematic patterns of the lizard hind limb during running for more than one stride for a single individual. Quantitative electromyography and kinematic data are used to describe the motor pat terns of 11 muscles of the hind limb used during quadru pedal running in the lizard Sceloporus clarki. Basic kine matic patterns of hind-limb and axial movements are de scribed briefly, and motor patterns are quantified by averaging electromyographical patterns from nine cons ecutive strides during which the lizard was running at 0.83 rn/s. Kinematics and muscle functions are discus sed in light of hypotheses presented in the literature. Many functional hypotheses based on gross observatio nal studies are not supported by quantitative electromyo graphical and kinematic data. These preliminary results indicate that axial bending, limb retraction, crural exten sion, and plantar flexion of the foot have important syn ergistic contributions to generating force during the limb cycle. Thus, extensive kinematic and electromyographi cal studies are needed to probe the functional details of hind-limb locomotion in sprawling vertebrates.
The signalling function of displays broadcast when animals are distant from conspecifics can be difficult to determine. I tested the extent to which visually transmitted broadcast displays given by free-ranging territorial male collared lizards signalled same-sex rivals or females. One test involved recording the frequency of broadcast displays, aggressive contests with rivals, and courtship encounters with females during ten reproductive seasons when local sex ratios varied markedly. The frequency of broadcast displays decreased as the ratio of male competitors to females increased. The frequency with which males initiated contests with rivals was not related to the ratio of competitors to females, whereas the frequency of courtship interactions decreased with sex ratio because there were fewer females to court. The behaviour of males that defended territories during two successive seasons showed a similar pattern. Broadcast display frequency was positively correlated with courtship frequency, but not with the frequency of contests with rivals. Lastly, individual males gave more broadcast displays during focal observations when they also engaged in courtship encounters with females than other observations when they engaged in aggressive conflicts with rival males. Although these results do not reject the possibility that broadcast displays may also signal male rivals, they support a major role of these displays in advertisement to females. © 2013 The Linnean Society of London