Carlos A. Navas

The evolution of thermal physiology in ectotherms

Biologists usually refer to mammals and birds as homeotherms, but these animals universally experience regional and temporal variations in body temperature. These variations could represent adaptive strategies of heterothermy, which in turn would favor genotypes that function over a wide range of temperatures. This coadaptation of thermoregulation and thermosensitivity has been studied extensively among ectotherms, but remains unexplored among endotherms. In this review, we apply classical models of thermal adaptation to predict variation in body temperature within and among populations of mammals and birds. We then relate these predictions to observations generated by comparative and experimental studies. In general, optimality models can explain the qualitative effects of abiotic and biotic factors on thermoregulation. Similar insights should emerge when using models to predict variation in the thermosensitivity of endotherms, but the dearth of empirical data on this subject precludes a rigorous analysis at this time. Future research should focus on the selective pressures imposed by regional and temporal heterothermy in endotherms.

Tradeoffs and the evolution of thermal reaction norms

Tradeoffs have played a prominent role in the development of theories describing the evolution of reaction norms. Different classes of tradeoffs are known to constrain the evolution of phenotypes, but current theories incorporate only a subset of these tradeoffs. Consequently, these theories cannot account for some of the variation in reaction norms that has been observed within and among species. Empirical studies of thermal reaction norms for physiological and life historical traits have shown that different proximate mechanisms can produce similar reaction norms. As a consequence, certain tradeoffs can be circumvented when the fitness costs imposed by these tradeoffs are severe. We argue that a unified theory that includes all classes of tradeoffs would provide a better understanding of the mechanisms that drive the evolution of reaction norms.

Coadaptation: A Unifying Principle in Evolutionary Thermal Biology*

Over the last 50 yr, thermal biology has shifted from a largely physiological science to a more integrated science of behavior, physiology, ecology, and evolution. Today, the mechanisms that underlie responses to environmental temperature are being scrutinized at levels ranging from genes to organisms. From these investigations, a theory of thermal adaptation has emerged that describes the evolution of thermoregulation, thermal sensitivity, and thermal acclimation. We review and integrate current models to form a conceptual model of coadaptation. We argue that major advances will require a quantitative theory of coadaptation that predicts which strategies should evolve in specific thermal environments. Simply combining current models, however, is insufficient to understand the responses of organisms to thermal heterogeneity; a theory of coadaptation must also consider the biotic interactions that influence the net benefits of behavioral and physiological strategies. Such a theory will be challenging to develop because each organism’s perception of and response to thermal heterogeneity depends on its size, mobility, and life span. Despite the challenges facing thermal biologists, we have never been more pressed to explain the diversity of strategies that organisms use to cope with thermal heterogeneity and to predict the consequences of thermal change for the diversity of communities.

Herpetological diversity along Andean elevational gradients: links with physiological ecology and evolutionary physiology

A well-defined macroecological pattern is the decline in biodiversity with altitude. However, this decline is taxa-specific. For example, amphibians are more diverse than squamates at extreme elevations in the tropical Andes, but this pattern is reversed at extreme elevations in the southern latitudes. Several ecophysiological and evolutionary factors may be related to this difference. At high-elevations in southern latitudes temperature differs dramatically among seasons and dry soils dominate, characteristics that appear to favor lizard physiological ecology. Tropical high altitudes, in contrast, are humid and offer abundant and diverse water resources. These characteristics allow for a richer anuran community but might complicate lizard egg development through temperature and oxygen constrains. Differences in strategies of thermal adaptation might also modulate diversity patterns. The thermal physiology of anurans is extremely labile so that behavioral and physiological performance is maintained despite an altitudinal decrease in field body temperature. Lizards, in contrast, exhibit a conservative thermal physiology and rely on behavioral thermoregulation to face cold and variable temperatures. Both, lizard behavioral strategies and anuran physiological adjustments seem equally efficient in allowing ecological success and diversification for both groups in the tropics up to approximately 3000 m. At higher elevations physiological thermal adaptation is required, and lizards are ecologically constrained, perhaps at various ontogenetic stages. Patterns of biodiversity along environmental clines can be better understood through a physiological approach, and can help to refine and propose hypotheses in evolutionary physiology.

Metabolic Physiology, Locomotor Performance, and Thermal Niche Breadth in Neotropical Anurans

The metabolic physiology of five high-elevation anurans in the Colombian Andes (Atelopus sp. nov., Atelopus muisca, Colostethus subpunctatus, Eleutherodactylus bogotensis, and Hyla labialis) was analyzed for correlates with ife in an extreme and variable environment. I compared populations of these species at 3,500 m and 2,900 m and contrasted the results with those from lowland congenerics from Panama (Atelopus varius, Colostethus talamancae, Colostethus flotator, Eleutherodactylus diastema, Hyla microcephala, and Hyla ebraccata). I collected data on the effects of temperature on locomotor performance, the thermal sensitivity of resting and activity metabolism, and the capacity for metabolic acclimation. All five high-elevation species were able to swim through a broad temperature range, whereas low-elevation species showed little ability to move at low temperatures. None of the species studied exhibited a capacity for metabolic acclimation. However, activity metabolic rates and aerobic metabolic scopes at low temperatures were always higher in the high-elevation frogs. Apparently, similar physiological traits that facilitate activity at the low and variable temperatures that characterize the high-elevation environment evolved independently in the four genera studied.

Urban Physiology: City Ants Possess High Heat Tolerance

Urbanization has caused regional increases in temperature that exceed those measured on a global scale, leading to urban heat islands as much as 12°C hotter than their surroundings. Optimality models predict ectotherms in urban areas should tolerate heat better and cold worse than ectotherms in rural areas. We tested these predications by measuring heat and cold tolerances of leaf-cutter ants from South Americas largest city (São Paulo, Brazil). Specifically, we compared thermal tolerances of ants from inside and outside of the city. Knock-down resistance and chill-coma recovery were used as indicators of heat and cold tolerances, respectively. Ants from within the city took 20% longer to lose mobility at 42°C than ants from outside the city. Interestingly, greater heat tolerance came at no obvious expense of cold tolerance; hence, our observations only partially support current theory. Our results indicate that thermal tolerances of some organisms can respond to rapid changes in climate. Predictive models should account for acclimatory and evolutionary responses during climate change.

How important are skeletal muscle mechanics in setting limits on jumping performance

SUMMARY Jumping is an important locomotor behaviour used by many animals. The power required to perform a jump is supplied by skeletal muscle. The mechanical properties of skeletal muscle, including the power it can produce, are determined by its composition, which in turn reflects trade-offs between the differing tasks performed by the muscle. Recent studies suggest that muscles used for jumping are relatively fast compared with other limb muscles. As animals get bigger absolute jump performance tends to increase, but recent evidence suggests that adult jump performance may be relatively independent of body size. As body size increases the relative shortening velocity of muscle decreases, whereas normalised power output remains relatively constant. However, the relative shortening velocity of the fastest muscle fibre types appears to remain relatively constant over a large body size range of species. It appears likely that in many species during jumping, other factors are compensating for, or allowing for, uncoupling of jumping performance from size-related changes in the mechanical properties of muscle. In some species smaller absolute body size is compensated for by rapid development of locomotor morphology to attain high locomotor performance early in life. Smaller animal species also appear to rely more heavily on elastic storage mechanisms to amplify the power output available from skeletal muscle. Adaptations involving increased relative hindlimb length and relative mass of jumping muscles, and beneficial alteration of the origin and/or insertion of jumping muscles, have all been found to improve animal jump performance. However, further integrative studies are needed to provide conclusive evidence of which morphological and physiological adaptations are the most important in enhancing jump performance.

Morphological and physiological specialization for digging in amphisbaenians, an ancient lineage of fossorial vertebrates.

SUMMARY Amphisbaenians are legless reptiles that differ significantly from other vertebrate lineages. Most species dig underground galleries of similar diameter to that of the animal. We studied the muscle physiology and morphological attributes of digging effort in the Brazilian amphisbaenid Leposternon microcephalum (Squamata; Amphisbaenia), which burrows by compressing soil against the upper wall of the tunnel by means of upward strokes of the head. The individuals tested (<72 g) exerted forces on the soil of up to 24 N. These forces were possible because the fibres of the longissimus dorsi, the main muscle associated with burrowing, are highly pennated, thus increasing effective muscle cross-sectional area. The muscle is characterized by a metabolic transition along its length: proximal, medial and distal fibres are fast contracting and moderately oxidative, but fibres closer to the head are richer in citrate synthase and more aerobic in nature. Distal fibres, then, might be active mainly at the final step of the compression stroke, which requires more power. For animals greater than a given diameter, the work required to compress soil increases exponentially with body diameter. Leposternon microcephalum, and probably some other highly specialized amphisbaenids, are most likely constrained to small diameters and can increase muscle mass and effective muscle cross-sectional area by increasing body length, not body diameter.

Dishonest Signals of Strength in Male Slender Crayfish (Cherax dispar) during Agonistic Encounters

Many animals resolve disputes without combat by displaying signals of potential strength during threatening displays. Presumably, competitors use each others displays to assess their relative strengths, and current theory predicts that these signals of strength should generally be honest. We tested this prediction by investigating the relationships among morphology, performance, and social dominance in males of the slender crayfish Cherax dispar. Crayfish routinely use their enlarged front claws (chelae) for both intimidation and fighting, making this species ideal for studying the honesty of weapon size. We evaluated five competing models relating morphological and physiological traits to dominance during paired competitive bouts. Based on the best model, larger chelae clearly resulted in greater dominance; however, chela strength had no bearing on dominance. Thus, displays of chela size were dishonest signals of strength, and the enlarged chelae of males seemingly function more for intimidation than for fighting. In addition, an analysis of the performance of isolated chela muscle showed that muscle from male crayfish produced only half the force that muscle from female crayfish produced ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape