Modeling the Metabolic Energetics of Brief and Intermittent Locomotion in Lizards and Rodents

When locomotor activity is brief, physiological steady stateconditions are not attained. It is therefore difficult to modelthe energetic costs of intermittent activity using standardmethods. This difficulty is addressed by considering as reflectiveof the metabolic costs of activity not only the oxygen consumedduring the activity itself, but also the excess post-exerciseoxygen consumption (EPOC) and any excess metabolites persistingat the end of EPOC. This paper briefly reviews the metabolicevents associated with EPOC, and then examines how this approachcan be applied to address questions of how behavioral variablesassociated with locomotion (activity duration, intensity, frequency)can influence the energetic costs to the animal per unit distance.Using data for lizards, mice, and others, EPOC can be shownto be the major component of energetic costs when durationsare short, regardless of exercise intensity. Brief activityis much more expensive by this measure than is steady statelocomotion, regardless of phylogeny or body mass. Three studiesof intermittent locomotion provide evidence that brief behaviorscan be undertaken at lower metabolic costs than predicted fromsingle bouts of activity when repeated in a frequent, repeatedpattern. Metabolic savings appear greatest when the pause periodbetween behaviors is short relative to EPOC duration, the timefor organismal metabolic rate to return to pre-exercise levels,although longer pause periods may increase endurance.     

Effect of activity duration on recovery and metabolic costs in the desert iguana (Dipsosaurus dorsalis).

The majority of elevated O(2) consumption associated with short and vigorous activity occurs during recovery, thus an assessment of associated metabolic costs should also examine the excess post-exercise oxygen consumption (EPOC). This study examined O(2) uptake during exercise, EPOC and distance traveled during 5-, 15-, 60- and 300-s sprints at maximal treadmill intensity in Dipsosaurus (N=10; 74.3+/-2.1 g). EPOC (0.08, 0.14, 0.23 and 0.18 ml O(2) g(-1), respectively) was large (80-99% of total elevated O(2) consumption) and increased significantly between 5 and 60 s. The cost of activity (C(act); ml O(2) g(-1) x km(-1)), intended to reflect the total net costs associated with the activity, was calculated as the total elevated O(2) consumption per unit distance traveled. C(act) decreased with activity duration due to proportionally larger increases in distance traveled relative to EPOC volume, and is predicted by the equation C(act)=14.7 x activity duration (s)(-0.24). The inclusion of EPOC costs provides an ecologically relevant estimate of the total metabolic cost of locomotor activity. C(act) exceeds standard transport costs at all durations examined due to the addition of obligate recovery costs. The differences are large enough to impact energy budget analyses for ectotherms.     

Roles of lactate and catecholamines in the energetics of brief locomotion in an ectothermic vertebrate

We have investigated the magnitude and duration of excess post-exercise oxygen consumption (EPOC) in a lizard following a single bout of vigorous exercise of 5-60 s, common activity durations for many ectothermic vertebrates. Desert iguanas (Dipsosaurus dorsalis) were run for 5 s, 15 s, 30 s, or 60 s. Oxygen consumption (VO2) increased from 0.16 ml O2 g(-1) h(-1) at rest to 1.3-1.6 ml O2 g(-1) h(-1) during 5-60 s of running. EPOC duration increased with activity duration, ranging from 35-63 min. EPOC volume, the excess oxygen consumed post-exercise, doubled from 0.13 ml O2 g(-1) following 5 s of activity to 0.25 ml O2 g(-1) after 60 s. EPOC represented 91-98% of the total metabolic expense of the activity. EPOC durations were always shorter than the period required for lactate removal, illustrating that these two processes are not causally related. Alpha- and beta-adrenergic receptor blockade by phentolamine and propranolol had no effect on resting VO2 but depressed excess post-exercise oxygen consumption volumes 2540%. The extent of catechol stimulation post-exercise may be motivation or stimulus dependent. The data indicate that metabolic elevations post-exercise represent the majority of activity costs in lizards. The study suggests that EPOC of ectothermic vertebrates is sensitive to exercise duration and catecholamine release post-activity, even when activity periods are less than 60 s in duration.     

Intermittent locomotor activity that increases endurance also increases metabolic costs in the desert Iguana (Dipsosaurus dorsalis)

Intermittent activity, alternating bouts of activity and rest, can extend endurance relative to continuous locomotion. Utilizing a rapid fatiguing activity intensity (1.08 m s(-1)), Dipsosaurus dorsalis (n = 14) ran repeated bouts of varying durations (5, 15, or 30 s) interspersed with variable pause periods (100%, 200%, 400%, or 800% of the activity period) until exhausted. Total distance ran increased relative to continuous locomotion. The largest increases were seen when activity periods were limited to 5 s and pause periods were extended from 5 s to 20 s to 40 s (55, 118, and 193 m, respectively). To analyze these increases further, O(2) consumption was measured for six bouts of 5-s activity separated by either 5, 20, or 40 s (n = 8). The sum of elevated O(2) consumption during activity, pauses, and recovery increased significantly from 0.08 to 0.09 and 0.12 mL O(2) g(-1) as pause duration increased, primarily due to greater O(2) consumption during longer pause intervals. Postexercise recovery metabolism was a large cost (>57% of total) but did not differ among treatments. Overall, 40-s pauses were most expensive (absolutely and per unit distance) but provided the greatest endurance, likely due to further repletion of metabolites or removal of end products during the longer pause. In contrast, the shortest pause period was most economical but exhausted the animal most rapidly. Thus, a pattern of intermittent activity utilized by an animal may have energetic advantages that sometimes may be offset by behavioral costs associated with fatigue.     

How to climb a tree: lizards accelerate faster,but pause more,when escaping on vertical surfaces

Many species of lizards effectively traverse both two and three‐dimensional habitats. However, few studies have examined maximum locomotor performance on different inclines. Do maximum acceleration and velocity differ on a level and inclined surface? Do lizards pause more on an inclined surface? To address these questions, Sceloporus woodi lizards (N = 12) were run in the laboratory on a level trackway and a vertical tree trunk. This species is known to frequently utilize both vertical and horizontal aspects of its habitat. Average maximum acceleration on the vertical surface exceeded that on the level surface, although average maximum velocity exhibited the opposite pattern. The average number of pauses during level locomotion was lower compared to vertical locomotion. In addition, the average location of the first pause on the level surface was 0.51 m, which is farther than the average for vertical locomotion where the first pause was at 0.35 m. The combination of performance and pause data suggests that the relative lack of pausing during level locomotion allows individuals to reach higher maximum velocities on level surfaces because they accelerate over greater distances. The increased pausing when moving vertically could be a result of high energetic demands of vertical locomotion, or greater microhabitat complexity as a result of branching and/or refuges. The faster acceleration exhibited during vertical locomotion by S. woodi likely offsets the frequent pauses. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102 , 83–90.     

Energetic costs of bipedal and quadrupedal walking in Japanese macaques

We investigated the energetic costs of quadrupedal and bipedal walking in two Japanese macaques. The subjects were engaged in traditional bipedal performance for years, and are extremely adept bipeds. The experiment was conducted in an airtight chamber with a gas analyzer. The subjects walked quadrupedally and bipedally at fixed velocities (<5 km/hr) on a treadmill in the chamber for 2.5-6 min. We estimated energy consumption from carbon dioxide (CO2) production. While walking bipedally, energetic expenditure increased by 30% relative to quadrupedalism in one subject, and by 20% in another younger subject. Energetic costs increased linearly with velocity in quadrupedalism and bipedalism, with bipedal/quadrupedal ratios remaining almost constant. Our experiments were relatively short in duration, and thus the observed locomotor costs may include presteady-state high values. However, there was no difference in experimental duration between bipedal and quadrupedal trials. Thus, the issue of steady state cannot cancel the difference in energetic costs. Furthermore, we observed that switching of locomotor mode (quadrupedalism to bipedalism) during a session resulted in a significant increase of CO2 production. Taylor and Rowntree ([1973] Science 179:186-187) noted that the energetic costs for bipedal and quadrupedal walking were the same in chimpanzees and capuchin monkeys. Although the reason for this inconsistency is not clear, species-specific differences should be considered regarding bipedal locomotor energetics among nonhuman primates. Extra costs for bipedalism may not be great in these macaques. Indeed, it is known that suspensory locomotion in Ateles consumes 1.3-1.4 times as much energy relative to quadrupedal progression. This excess ratio surpasses the bipedal/quadrupedal energetic ratios in these macaques.     

Effects of tail loss on the movement patterns of the lizard, Psammodromus algirus

1. Many lizards use caudal autotomy as a defensive strategy. However, subsequent costs related to the alteration of locomotor abilities might decrease the fitness of individuals. In this paper, the movement patterns of spontaneously moving Psammodromus algirus lizards and their escape performance running at high speed were compared before and after tail loss. A control tailed group was also studied to assess the repeatability of locomotor patterns between trials.
2. Tail loss had a significant effect on spontaneous movement patterns. Tailless individuals moved at significantly slower speeds during bursts of locomotion, and distances moved within bursts were significantly reduced. The overall time spent pausing increased, and, as a result, overall speeds decreased to an even greater extent than burst speeds. However, mean durations of individual locomotor bursts and mean pause durations did not change significantly after tail loss.
3. Loss of the tail decreased mean stride length, although the positive relation between stride length and speed was retained.
4. Escape performance was also greatly affected; loss of the tail resulted in substantially reduced attained, maximal and overall escape speeds. These changes resulted in shorter escape distances (the time of the first pause after the initiation of the escape response) because the mean duration of escape responses did not change.
5. The relevance of these alterations for the ecology of this species, and how individuals may compensate for the costs of tail loss, favouring autotomy as an escape strategy, are discussed.     

Locomotor energetics in primates: gait mechanics and their relationship to the energetics of vertical and horizontal locomotion

All primates regularly move within three-dimensional arboreal environments and must often climb, but little is known about the energetic costs of this critical activity. Limited previous work on the energetics of incline locomotion suggests that there may be differential selective pressures for large compared to small primates in choosing to exploit a complex arboreal environment. Necessary metabolic and gait data have never been collected to examine this possibility and biomechanical mechanisms that might explain size-based differences in the cost of arboreal movement. Energetics and kinematics were collected for five species of primate during climbing and horizontal locomotion. Subjects moved on a treadmill with a narrow vertical substrate and one with a narrow horizontal substrate at their maximum sustainable speed for 10–20 min while oxygen consumption was monitored. Data during climbing were compared to those during horizontal locomotion and across size. Results show that climbing energetic costs were similar to horizontal costs for small primates (<0.5 kg) but were nearly double for larger species. Spatio-temporal gait characteristics suggest that the relationship between the cost of locomotion and the rate of force production changes between the two locomotor modes. Thus, the main determinants of climbing costs are fundamentally different from those during horizontal locomotion. These new results combining spatiotemporal and energetic data confirm and expand on our previous argument (Hanna et al.: Science 320 (2008) 898) that similar costs of horizontal and vertical locomotion in small primates facilitated the successful occupation of a fine-branch arboreal milieu by the earliest primates.     

Intermittent locomotion increases endurance in a gecko

Nocturnal geckos can actively forage at low temperatures. A low minimum cost of locomotion allows greater sustainable speeds by partially offsetting the decrease in maximal oxygen consumption (VO2max) associated with low nocturnal temperatures. The nocturnality hypothesis (Autumn et al. 1997) proposes that the reduced cost of continuous locomotion is a shared, derived characteristic that increases the capacity to sustain locomotion at low temperatures. Yet many lizards move intermittently at speeds exceeding those that elicit VO2max. We exercised the frog-eyed gecko, Teratoscincus przewalskii, continuously and intermittently on a treadmill. At an exercise speed of 0.90 km h-1 (270% maximum aerobic speed), lizards alternating a 15-s exercise period with a 30-s pause period exhibited a 1.7-fold increase in distance capacity (total distance traveled before fatigue) compared with lizards exercised continuously at the same average speed (0.30 km h-1). The average aerobic cost of intermittent exercise was not significantly different from VO2max. Locomoting intermittently could augment the increase in endurance resulting from the low minimum cost of continuous locomotion in nocturnal geckos. Intermittent behavior could increase the endurance of lizard movement in general.     

Contributions to elevated metabolism during recovery: dissecting the excess postexercise oxygen consumption (EPOC) in the desert iguana (Dipsosaurus dorsalis)

The excess postexercise oxygen consumption (EPOC), a measure of recovery costs, is known to be large in ectothermic vertebrates such as the desert iguana (Dipsosaurus dorsalis), especially after vigorous activity. To analyze the cause of these large recovery costs in a terrestrial ectotherm, Dipsosaurus were run for 15 s at maximal-intensity (distance 35.0+/-1.9 m; 2.33+/-0.13 m s(-1)) while O(2) uptake was monitored via open-flow respirometry. Muscle metabolites (adenylates, phosphocreatine, and lactate) were measured at rest and after 0, 3, 10, and 60 min of recovery. Cardiac and ventilatory activity during rest and recovery were measured, as were whole-body lactate and blood lactate, which were used to estimate total muscle activity. This vigorous activity was supported primarily by glycolysis (65%) and phosphocreatine hydrolysis (29%), with only a small contribution from aerobic metabolism (2.5%). Aerobic recovery lasted 43.8+/-4.6 min, and EPOC measured 0.166+/-0.025 mL O(2) g(-1). This was a large proportion (98%) of the total suprabasal metabolic cost of the activity to the animal. The various contributions to EPOC after this short but vigorous activity were quantified, and a majority of EPOC was accounted for. The two primary causes of EPOC were phosphocreatine repletion (32%-50%) and lactate glycogenesis (30%-47%). Four other components played smaller roles: ATP repletion (8%-13%), elevated ventilatory activity (2%), elevated cardiac activity (2%), and oxygen store resaturation (1%).     

Posture, gait and the ecological relevance of locomotor costs and energy-saving mechanisms in tetrapods

A reanalysis of locomotor data from functional, energetic, mechanical and ecological perspectives reveals that limb posture has major effects on limb biomechanics, energy-saving mechanisms and the costs of locomotion. Regressions of data coded by posture (crouched vs. erect) reveal nonlinear patterns in metabolic cost, limb muscle mass, effective mechanical advantage, and stride characteristics. In small crouched animals energy savings from spring and pendular mechanisms are inconsequential and thus the metabolic cost of locomotion is driven by muscle activation costs. Stride frequency appears to be the principal functional parameter related to the decreasing cost of locomotion in crouched animals. By contrast, the shift to erect limb postures invoked a series of correlated effects on the metabolic cost of locomotion: effective mechanical advantage increases, relative muscle masses decrease, metapodial limb segments elongate dramatically (as limbs shift from digitigrade to unguligrade designs) and biological springs increase in size and effectiveness. Each of these factors leads to decreases in the metabolic cost of locomotion in erect forms resulting from real and increasing contributions of pendular savings and spring savings. Comparisons of the relative costs and ecological relevance of different gaits reveal that running is cheaper than walking in smaller animals up to the size of dogs but running is more expensive than walking in horses. Animals do not necessarily use their cheapest gaits for their predominant locomotor activity. Therefore, locomotor costs are driven more by ecological relevance than by the need to optimize locomotor economy.     

The effects of meal size on postprandial metabolic response and post-exercise metabolic recovery process in juvenile black carp (Mylopharyngodon piceus)

The effects of meal size on the postprandial metabolic response and of digestion on the post-exercise metabolic recovery process were investigated in juvenile black carp (Mylopharyngodon piceus) . Experimental fish were forcedly fed with compound feed (meal sizes: 0.5%, 1% and 2% body weight). Then, the postprandial oxygen consumption rate and excess post-exercise oxygen consumption (EPOC) of the experimental fish were measured. Both the duration and the peak of oxygen consumption rate (PMR) increased with increasing meal size. The peak post-exercise metabolic rate of digesting fish were significantly higher, whereas EPOC magnitude and its duration were significantly smaller or (shorter) than those in the fasting fish. It is suggested that (1) this fish fulfills the increased energy demand during the digestive process by increasing PMR and by prolonging SDA duration with increasing meal size and (2) digesting fish might decrease their anaerobic exhaustive activity but increase the post-exercise recovery capacity.     

Lizard threat display handicaps endurance

Honest-signalling theory asserts that threat displays reliably advertise attributes that influence fighting success. Endurance, as measured by treadmill performance, predicts the outcome of agonistic interactions among lizards. If threat displays in lizards function to advertise endurance capacity then variation in threat displays should correlate with endurance. I tested this prediction for the duration of threat posturing in male side-blotched lizards (Uta stansburiana) and examined whether threat displays act as quality handicaps, reliable signals that expend the attribute that is advertised. Individual variation in the duration of threat posturing correlated with endurance, while an experimental reduction of endurance diminished the duration of threat posturing. As expected of a quality handicap, endurance fell below baseline after display production. A restriction of aerobic metabolism can account for this effect. In threat posturing, lateral compression of the thorax may interfere with respiration or with circulation, limiting aerobic metabolism and causing a compensatory increase in anaerobic metabolism, thereby generating lactate and diminishing locomotor capacity. Concentrations of lactate measured after display production were higher than baseline, consistent with the proposed mechanism. By restricting aerobic metabolism, the threat posture can act as a quality handicap, simultaneously advertising and expending the endurance capacity of displaying lizards.     

Effects of Microhabitat‐Dependent Predation Risk on Vigilance during Intermittent Locomotion in Psammodromus algirus Lizards

Animals should be able to adjust their behavior by tracking changes in predation risk level continuously. Many animals show a pattern of intermittent locomotion with short pauses that may increase detection and vigilance of predators. These locomotor patterns may depend on the microhabitat structure, which affect predation risk levels. We examined in detail in the laboratory the characteristics of spontaneous locomotion, scanning behavior, and the escape performance of Psammodromus algirus lizards moving in two different microhabitats (leaf litter patches and open sand areas). Results showed that in leaf litter, lizards moved at slower speed and had shorter bursts of locomotion both in distance and duration, than in sand substrates. This locomotor pattern allowed lizards to increase scanning rate and total time spent in vigilance behavior. When lizards were forced to flee, they escaped to longer distances and during more time in open sand areas, but lizards were able to attain similar escape speed in the two substrates. Lizards may be able to compensate the cost of moving between different microhabitats with different predation risk by behaviorally changing their locomotor and vigilance patterns. However, complex interactions between the visibility of lizards to predators and the ability of lizards to detect predators, together with the need of attending simultaneously to other conflicting demands, may lead to apparently non‐intuitive solutions in locomotor patterns and the rate of vigilance behavior.     

Metabolic response of bluegill to exercise at low water temperature: implications for angling conservation

The metabolic response of fish to exercise is highly dependent on environmental factors such as temperature. In addition to natural challenges that force exercise (foraging, avoiding predators, etc.), sportfish species are also subjected to exercise when they are hooked by anglers, leading to metabolic energy costs that may impact fitness. While several studies have examined the physiological response of fish to capture in warm conditions, little work has examined this response under cold winter conditions when fish are targeted by ice-anglers. To fill this gap, we examined the metabolic impacts of exercise duration and air exposure on bluegill, Lepomis macrochirus, at a temperature typical for ice angling. Thirty-two bluegill were subjected to a simulated angling session which included either a light (30 s) or exhaustive exercise procedure, followed by either 30 s or 4 min of air exposure. Fish were then assessed at 5 °C for the following metabolic metrics using intermittent-flow respirometry: standard metabolic rate (SMR), maximum metabolic rate (MMR), aerobic scope (AS), recovery time, and excess post-exercise oxygen consumption (EPOC). Fish exercised to exhaustion had higher EPOC compared to lightly exercised fish, however EPOC was not affected by air exposure time. No other metrics were impacted by air exposure or exercise duration. These results are directly applicable to physiological outcomes for fish captured by ice-anglers during the winter and suggest that both low temperatures and low durations of exercise serve to keep metabolic costs low for fish angled during the winter months.     

The movement patterns of lacertid lizards: speed, gait and pauses in Lacerta vivipara

Lacerta vivipara moving across an open space at their normal activity temperature alternate bursts of locomotion with short pauses which tend to occur at the extremes of the limb cycle, i.e. when individual limbs are maximally adducted or retracted and the spinal cord is maximally flexed in the lateral plane. The movement bursts and pauses in adult lizards have mean durations of 0–30 and 012 s, respectively, and within bursts the lizards move at a mean speed of 14–6 cm s^-1. Movement in juvenile lizards is 2–5 times faster (relative to body length) and the pauses are of longer duration (mean = 019 s), giving the locomotion of juveniles a more jerky appearance. Lizards which are chasing crickets increase the speed and the duration of locomo-tory bursts, although the pauses persist. Lizards which are searching for a previously perceived cricket increase pause duration (mean = 0–40 s). Lizards which are fleeing from a sudden disturbance move at almost twice (juveniles) or 3–7 times (adults) the speed of foraging animals: the pauses persist, although at much reduced frequency. Increases in speed result from increases in both stride length (Λ) and stride frequency (n); the ratio Λ/ n appears to remain constant at 006. The significance of these observations is discussed, although the functions of the pauses cannot yet be explained.     

Meeting energy budgets by modulation of behaviour and physiology in the eel (Anguilla anguilla L.)

Availability of energy for feeding, and the scope to accommodate the associated increase in oxygen demand (SDA: specific dynamic action) can, to a large degree, regulate the future feeding and energy availability of an animal. There is a fundamental conflict between locomotion and SDA within the physiological capacity of a mobile organism to respire sufficiently in order to simultaneously meet both requirements. This paper is a first attempt to integrate the costs of behaviour and physiology and produce a testable model of energy allocation in the eel. Total oxygen consumption (metabolic rate MO2) of the eel (Anguilla anguilla L.) was 109 micromol O2 x g(-1) x day(-1) with a cost of measured protein synthesis representing 49% of this value, and measured routine swimming (locomotor) activity representing approximately 34%. By allocating periods of reduced activity, the eel is able to develop a strategy to prudently meet the costs of feeding and temporally balance energy budgets (in terms of oxygen) by modulation of the behaviour and demands of physiology.     

Fiber‐type composition in the perivertebral musculature of lizards: Implications for the evolution of the diapsid trunk muscles

The perivertebral musculature of lizards is critical for the stabilization and the mobilization of the trunk during locomotion. Some trunk muscles are also involved in ventilation. This dual function of trunk muscles in locomotion and ventilation leads to a biomechanical conflict in many lizards and constrains their ability to breathe while running (“axial constraint”) which likely is reflected by their high anaerobic scope. Furthermore, different foraging and predator‐escape strategies were shown to correlate with the metabolic profile of locomotor muscles in lizards. Because knowledge of muscle's fiber‐type composition may help to reveal a muscle's functional properties, we investigated the distribution pattern of muscle fiber types in the perivertebral musculature in two small lizard species with a generalized body shape and subjected to the axial constraint (Dipsosaurus dorsalis, Acanthodactylus maculatus) and one species that circumvents the axial constraint by means of gular pumping (Varanus exanthematicus). Additionally, these species differ in their predator‐escape and foraging behaviors. Using refined enzyme‐histochemical protocols, muscle fiber types were differentiated in serial cross‐sections through the trunk, maintaining the anatomical relationships between the skeleton and the musculature. The fiber composition in Dipsosaurus and Acanthodactylus showed a highly glycolytic profile, consistent with their intermittent locomotor style and reliance on anaerobic metabolism during activity. Because early representatives of diapsids resemble these two species in several postcranial characters, we suggest that this glycolytic profile represents the plesiomorphic condition for diapsids. In Varanus, we found a high proportion of oxidative fibers in all muscles, which is in accordance with its high aerobic scope and capability of sustained locomotion. J. Morphol., 2013. © 2012 Wiley Periodicals, Inc.     

Integrating the Physiology, Mechanics and Behavior of Rapid Running Ghost Crabs: Slow and Steady Doesn't Always Win the Race

In 1979 Bliss predicted that, "land crabs are and will undoubtedlycontinue to be promising objects of scientific research." Studiesof rapid running ghost crabs support her contention and haveresulted in several general findings relating to locomotionand activity. 1) Energy exchange mechanisms during walking aregeneral and not restricted to quadrupedal and bipedal morphologies.2) "Equivalent gaits," such as trots and gallops, may existin 4-, 6- and 8-legged animals that differ greatly in leg andskeletal (i.e., exo- vs. endoskeletal) design. These findingssupport the hypothesis that terrestrial locomotion in many speciescan modeled by an inverted pendulum or spring-mass system. 3)An open circulatory system and chitin-covered gills do not necessarilylimit the rate at which oxygen consumption can be increasedor the factorial increase oxygen consumption over resting rates.4) Interspecific and intraspecific i.e., ontogenetic) scalingof sub-maximal oxygen consumption and maximal aerobic speedcan differ significantly. 5) Locomotion at speeds above themaximal aerobic speed requiring non-aerobic contributions maybe far more costly than can be predicted from aerobic costsalone. The cost transport may attain a minimum at less thanmaximum speed. 6) The speed which elicits maximal oxygen consumptionduring continuous exercise is attained at moderate walking speedsin crabs and probably other ectotherms. Speeds 15- to 20-foldfaster are possible, but cannot be sustained. 7) The low enduranceassociated with the low maximal oxygen consumption and maximalaerobic speed of ectotherms moving continuously can be increasedor decreased by altering locomotor behavior and moving intermittently.Ectotherms can locomote at high speeds and travel for considerabledistances or remain active for long periods by including restpauses. Alternatively, intense activity with extended exerciseperiods with short pause periods may actually reduce behavioralcapacity or work accomplished relative to continuous activityduring which the behavior is carried out at a lower intensitylevel without pauses.     

The Uses of Anaerobiosis by Amphibians and Reptiles

Amphibians and reptiles rely upon anaerobic glycolysis to supporttheir energetic requirements under a variety of circumstances.Although adult frogs derive most of the energy for muscle contractionduring intense, short-term locomotion from glycolysis, anurantadpoles have a very low rate of lactate formation during 30sec of burst swimming; instead, they rely largely on the useof phosphocreatine stores. Among squamate reptiles, the rateof lactate formation during vigorous exercise is largely relatedto the duration of activity and to body temperature. Recentstudies have shown that fossorial, limbless reptiles do notdiffer from surface-dwelling, quadrupedal species in the rateof glycolysis during intense activity. The energetics of locomotiondiffers significantly between swimming and running turtles;thus the site of activity influences the role of anaerobiosisin movement. Lactate levels increase in some frogs during callingand nest building and in some reptiles during prey capture andingestion. However, voluntary locomotion and diving by reptilesare rarelyaccompanied by an increase in lactate levels. Freshwaterturtles rely heavily on glycolysis during aquatic hibernation.Thus, it can be concluded that amphibians and reptiles derivea significant proportion of their energetic requirements fromanaerobic metabolism only under selected circumstances whenthe benefits outweigh the costs associated with the accumulationof lactate.