Variations in cost of transport and their ecological consequences: a review

Movement is essential in the ecology of most animals, and it typically consumes a large proportion of individual energy budgets. Environmental conditions modulate the energetic cost of movement (cost of transport, COT), and there are pronounced differences in COT between individuals within species and across species. Differences in morphology affect COT, but the physiological mechanisms underlying variation in COT remain unresolved. Candidates include mitochondrial efficiency and the efficiency of muscle contraction-relaxation dynamics. Animals can offset increased COT behaviourally by adjusting movement rate and habitat selection. Here, we review the theory underlying COT and the impact of environmental changes on COT. Increasing temperatures, in particular, increase COT and its variability between individuals. Thermal acclimation and exercise can affect COT, but this is not consistent across taxa. Anthropogenic pollutants can increase COT, although few chemical pollutants have been investigated. Ecologically, COT may modify the allocation of energy to different fitness-related functions, and thereby influence fitness of individuals, and the dynamics of animal groups and communities. Future research should consider the effects of multiple stressors on COT, including a broader range of pollutants, the underlying mechanisms of COT and experimental quantifications of potential COT-induced allocation trade-offs.

Temperature effects on the contractile performance and efficiency of oxidative muscle from a eurythermal versus a stenothermal salmonid

We compared the thermal sensitivity of oxidative muscle function between the eurythermal Atlantic salmon (Salmo salar) and the more stenothermal Arctic char (Salvelinus alpinus; which prefers cooler waters). Power output was measured in red skeletal muscle strips and myocardial trabeculae, and efficiency (net work/energy consumed) was measured for trabeculae, from cold (6°C) and warm (15°C) acclimated fish at temperatures from 2 to 26°C. The mass-specific net power produced by char red muscle was greater than in salmon, by 2-to 5-fold depending on test temperature. Net power first increased, then decreased, when the red muscle of 6°C-acclimated char was exposed to increasing temperature. Acclimation to 15°C significantly impaired mass-specific power in char (by ∼40-50%) from 2 to 15°C, but lessened its relative decrease between 15 and 26°C. In contrast, maximal net power increased, and then plateaued, with increasing temperature in salmon from both acclimation groups. Increasing test temperature resulted in a ∼3- to 5-fold increase in maximal net power produced by ventricular trabeculae in all groups, and this effect was not influenced by acclimation temperature. Nonetheless, lengthening power was higher in trabeculae from warm-acclimated char, and char trabeculae could not contract as fast as those from salmon. Finally, the efficiency of myocardial net work was approximately 2-fold greater in 15°C-acclimated salmon than char (∼15 versus 7%), and highest at 20°C in salmon. This study provides several mechanistic explanations as to their inter-specific difference in upper thermal tolerance, and potentially why southern char populations are being negatively impacted by climate change.

Energetics of optimal undulatory swimming organisms

Energy consumption is one of the primary considerations in animal locomotion. In swimming locomotion, a number of questions related to swimming energetics of an organism and how the energetic quantities scale with body size remain open, largely due to the difficulties with modeling and measuring the power production and consumption. Based on a comprehensive theoretical framework that incorporates cyclic muscle behavior, structural dynamics and swimming hydrodynamics, we perform extensive computational simulations and show that many of the outstanding problems in swimming energetics can be explained by considering the coupling between hydrodynamics and muscle contraction characteristics, as well as the trade-offs between the conflicting performance goals of sustained swimming speed U and cost of transport COT. Our results lead to three main conclusions: (1) in contrast to previous hypotheses, achieving optimal values of U and COT is independent of producing maximal power or efficiency; (2) muscle efficiency in swimming, in contrast to that in flying or running, decreases with increasing body size, consistent with muscle contraction characteristics; (3) the long-standing problem of two disparate patterns of longitudinal power output distributions in swimming fish can be reconciled by relating the two patterns to U-optimal or COT-optimal swimmers, respectively. We also provide further evidence that the use of tendons in caudal regions is beneficial from an energetic perspective. Our conclusions explain and unify many existing observations and are supported by computational data covering nine orders of magnitude in body size.

Moderate and sustained exercise modulates muscle proteolytic and myogenic markers in gilthead sea bream (Sparus aurata)

Swimming activity primarily accelerates growth in fish by increasing protein synthesis and energy efficiency. The role of muscle in this process is remarkable and especially important in teleosts, where muscle represents a high percentage of body weight and because many fish species present continuous growth. The aim of this work was to characterize the effects of 5 wk of moderate and sustained swimming in gene and protein expression of myogenic regulatory factors, proliferation markers, and proteolytic molecules in two muscle regions (anterior and caudal) of gilthead sea bream fingerlings. Western blot results showed an increase in the proliferation marker proliferating cell nuclear antigen (PCNA), proteolytic system members calpain 1 and cathepsin D, as well as vascular endothelial growth factor protein expression. Moreover, quantitative real-time PCR data showed that exercise increased the gene expression of proteases (calpains, cathepsins, and members of the ubiquitin-proteasome system in the anterior muscle region) and the gene expression of the proliferation marker PCNA and the myogenic factor MyoD in the caudal area compared with control fish. Overall, these data suggest a differential response of the two muscle regions during swimming adaptation, with tissue remodeling and new vessel formation occurring in the anterior muscle and enhanced cell proliferation and differentiation occurring in the caudal area. In summary, the present study contributes to improving the knowledge of the role of proteolytic molecules and other myogenic factors in the adaptation of muscle to moderate sustained swimming in gilthead sea bream.

Resolving shifting patterns of muscle energy use in swimming fish

Muscle metabolism dominates the energy costs of locomotion. Although in vivo measures of muscle strain, activity and force can indicate mechanical function, similar muscle-level measures of energy use are challenging to obtain. Without this information locomotor systems are essentially a black box in terms of the distribution of metabolic energy. Although in situ measurements of muscle metabolism are not practical in multiple muscles, the rate of blood flow to skeletal muscle tissue can be used as a proxy for aerobic metabolism, allowing the cost of particular muscle functions to be estimated. Axial, undulatory swimming is one of the most common modes of vertebrate locomotion. In fish, segmented myotomal muscles are the primary power source, driving undulations of the body axis that transfer momentum to the water. Multiple fins and the associated fin muscles also contribute to thrust production, and stabilization and control of the swimming trajectory. We have used blood flow tracers in swimming rainbow trout (Oncorhynchus mykiss) to estimate the regional distribution of energy use across the myotomal and fin muscle groups to reveal the functional distribution of metabolic energy use within a swimming animal for the first time. Energy use by the myotomal muscle increased with speed to meet thrust requirements, particularly in posterior myotomes where muscle power outputs are greatest. At low speeds, there was high fin muscle energy use, consistent with active stability control. As speed increased, and fins were adducted, overall fin muscle energy use declined, except in the caudal fin muscles where active fin stiffening is required to maintain power transfer to the wake. The present data were obtained under steady-state conditions which rarely apply in natural, physical environments. This approach also has potential to reveal the mechanical factors that underlie changes in locomotor cost associated with movement through unsteady flow regimes.

Lactate kinetics of rainbow trout during graded exercise: do catheters affect the cost of transport?

Changes in lactate kinetics as a function of exercise intensity have never been measured in an ectotherm. Continuous infusion of a tracer is necessary to quantify rates of lactate appearance (Ra) and disposal (Rd), but it requires double catheterization, which could interfere with swimming. Using rainbow trout, our goals were to: (1) determine the potential effects of catheters and blood sampling on metabolic rate (O2), total cost of transport (TCOT), net cost of transport (NCOT) and critical swimming speed (Ucrit), and (2) monitor changes in lactate fluxes during prolonged, steady-state swimming or graded swimming from rest to Ucrit. This athletic species maintains high baseline lactate fluxes of 24 μmol kg(-1) min(-1) that are only increased at intensities >2.4 body lengths (BL) s(-1) or 85% Ucrit. As the fish reaches Ucrit, Ra is more strongly stimulated (+67% to 40.4 μmol kg(-1) min(-1)) than Rd (+41% to 34.7 μmol kg(-1) min(-1)), causing a fourfold increase in blood lactate concentration. Without this stimulation of Rd during intense swimming, lactate accumulation would double. By contrast, steady-state exercise at 1.7 BL s(-1) increases lactate fluxes to ~30 μmol kg(-1) min(-1), with a trivial mismatch between Ra and Rd that only affects blood concentration minimally. Results also show that the catheterizations and blood sampling needed to measure metabolite kinetics in exercising fish have no significant impact on O2 or TCOT. However, these experimental procedures affect locomotion energetics by increasing NCOT at high speeds and by decreasing Ucrit.

Increased ventricular stiffness and decreased cardiac function in Atlantic cod (Gadus morhua) at high temperatures

We employed the work loop method to study the ability of ventricular and atrial trabeculae from Atlantic cod to sustain power production during repeated contractions at acclimation temperatures (10°C) and when acutely warmed (20°C). Oxygen tension (Po2) was lowered from 450 to 34% air saturation to augment the thermal stress. Preparations worked under conditions simulating either a large stroke volume (35 contractions/min rate, 8-12% muscle strain) or a high heart rate (70 contractions/min, 2-4% strain), with power initially equal under both conditions. The effect of declining Po2 on power was similar under both conditions but was temperature and tissue dependent. In ventricular trabeculae at 10°C (and atria at 20°C), shortening power declined across the full range of Po2 studied, whereas the power required to lengthen the muscle was unaffected. Conversely, in ventricular trabeculae at 20°C, there was no decline in shortening power but an increase in lengthening power when Po2 fell below 100% air saturation. Finally, when ventricular trabeculae were paced at rates of up to 115 contractions/min at 20°C (vs. the maximum of 70 contractions/min in vivo), they showed marked increases in both shortening and lengthening power. Our results suggest that although elevated heart rates may not impair ventricular power as they commonly do isometric force, limited atrial power and the increased work required to expand the ventricle during diastole may compromise ventricular filling and hence, stroke volume in Atlantic cod at warm temperatures. Neither large strains nor high contraction rates convey an apparent advantage in circumventing this.

Effects of temperature on power output and contraction kinetics in the locomotor muscle of the regionally endothermic common thresher shark (Alopias vulpinus)

The common thresher shark (Alopias vulpinus) is a pelagic species with medially positioned red aerobic swimming musculature (RM) and regional RM endothermy. This study tested whether the contractile characteristics of the RM are functionally similar along the length of the body and assessed how the contractile properties of the common thresher shark compare with those of other sharks. Contractile properties of the RM were examined at 8, 16 and 24 °C from anterior and posterior axial positions (0.4 and 0.6 fork length, respectively) using the work loop technique. Experiments were performed to determine whether the contractile properties of the RM are similar along the body of the common thresher shark and to document the effects of temperature on muscle power. Axial differences in contractile properties of RM were found to be small or absent. Isometric twitch kinetics of RM were ~fivefold slower than those of white muscle, with RM twitch durations of about 1 s at 24 °C and exceeding 5 s at 8 °C, a Q(10) of nearly 2.5. Power increased approximately tenfold with the 16 °C increase in temperature, while the cycle frequency for maximal power only increased from about 0.5-1.0 Hz over this temperature range. These data support the hypothesis that the RM is functionally similar along the body of the common thresher shark and corroborate previous findings from shark species both with and without medial RM. While twitch kinetics suggest the endothermic RM is not unusually temperature sensitive, measures of power suggest that the RM is not well suited to function at cool temperatures. The cycle frequency at which power is maximized appeared relatively insensitive to temperature in RM, which may reflect the relatively cooler temperature of the thresher RM compared to that observed in lamnid sharks as well as the relatively slow RM phenotype in these large fish.

Optimal shape and motion of undulatory swimming organisms

Undulatory swimming animals exhibit diverse ranges of body shapes and motion patterns and are often considered as having superior locomotory performance. The extent to which morphological traits of swimming animals have evolved owing to primarily locomotion considerations is, however, not clear. To shed some light on that question, we present here the optimal shape and motion of undulatory swimming organisms obtained by optimizing locomotive performance measures within the framework of a combined hydrodynamical, structural and novel muscular model. We develop a muscular model for periodic muscle contraction which provides relevant kinematic and energetic quantities required to describe swimming. Using an evolutionary algorithm, we performed a multi-objective optimization for achieving maximum sustained swimming speed U and minimum cost of transport (COT)--two conflicting locomotive performance measures that have been conjectured as likely to increase fitness for survival. Starting from an initial population of random characteristics, our results show that, for a range of size scales, fish-like body shapes and motion indeed emerge when U and COT are optimized. Inherent boundary-layer-dependent allometric scaling between body mass and kinematic and energetic quantities of the optimal populations is observed. The trade-off between U and COT affects the geometry, kinematics and energetics of swimming organisms. Our results are corroborated by empirical data from swimming animals over nine orders of magnitude in size, supporting the notion that optimizing U and COT could be the driving force of evolution in many species.

Thunniform swimming: muscle dynamics and mechanical power production of aerobic fibres in yellowfin tuna (Thunnus albacares)

We studied the mechanical properties of deep red aerobic muscle of yellowfin tuna (Thunnus albacares), using both in vivo and in vitro methods. In fish swimming in a water tunnel at 1-3 L s(-1) (where L is fork length), muscle length changes were recorded by sonomicrometry, and activation timing was quantified by electromyography. In some fish a tendon buckle was also implanted on the caudal tendon to measure instantaneous muscle forces transmitted to the tail. Between measurement sites at 0.45 to 0.65 L, the wave of muscle shortening progressed along the body at a relatively high velocity of 1.7 L per tail beat period, and a significant phase shift (31+/-4 degrees ) occurred between muscle shortening and local midline curvature, both suggesting red muscle power is directed posteriorly, rather than causing local body bending, which is a hallmark of thunniform swimming. Muscle activation at 0.53 L was initiated at about 50 degrees of the tail beat period and ceased at about 160 degrees , where 90 degrees is peak muscle length and 180 degrees is minimum length. Strain amplitude in the deep red fibres at 0.5 L was +/-5.4%, double that predicted from midline curvature analysis. Work and power production were measured in isolated bundles of red fibres from 0.5 L by the work loop technique. Power was maximal at 3-4 Hz and fell to less than 50% of maximum after 6 Hz. Based on the timing of activation, muscle strain, tail beat frequencies and forces in the caudal tendon while swimming, we conclude that yellowfin tuna, like skipjack, use their red muscles under conditions that produce near-maximal power output while swimming. Interestingly, the red muscles of yellowfin tuna are slower than those of skipjack, which corresponds with the slower tail beat frequencies and cruising speeds in yellowfin.