The influence of habitat on travel speed, intermittent locomotion, and vigilance in a diurnal rodent

We studied effects of habitat structure on routine travel velocities, intermittent locomotion, and vigilance by the degu (Octodondegus), a diurnal rodent of central Chile. We predicted thattravel speed, pauses during locomotion, and vigilance wouldbe greater in open (riskier) than in shrub (safer) habitats.Video recordings of marked individuals in the wild were used to measure speed and other variables of spontaneous locomotionnot triggered by predatory attack or any other noticeable stimulusduring nonforaging periods. Time spent vigilant while foragingwas also measured. Because degus use bare-ground runways fordistant movements (e.g., between burrow openings and/or foodpatches), data on locomotion decisions were not confounded by effects of obstructive vegetation cover and/or resource abundance.When moving across the habitat between different feeding places,degus showed an intermittent pattern of locomotion, interruptingrunning events with short pauses. As predicted, travel speedand the duration of pauses between locomotion bursts were significantlygreater in open habitats. Further, the duration of locomotionbursts between feeding sites or between feeding sites and burrowswas significantly longer in open habitats. Our assumption that pauses and velocities are independent decisions was supportedby the lack of correlation between pauses and speeds duringlocomotion events. During foraging, degus devoted more timeto vigilance in open than in shrub habitats. The static positionadopted by degus during pauses, the speeds attained during movements, and the concordance between pausing behavior andvigilance across habitats suggest that pausing has an antipredatoryrole and is not limited to orientation and/or physiologicalrecovery. Our results support the view that degus perceivehigher predation risk in open areas and that flexible movement behavior reflects an adaptive antipredator response.     

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 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.     

Influence of predation risk and plant structure on vigilance and intermittent locomotion in <Emphasis Type="Italic">Microcavia australis</Emphasis> (Rodentia,Caviidae)

The aim of this study was to analyze and compare vigilance behavior and intermittent locomotion at two sites (El Leoncito and Ñacuñán, Argentina) that differ in predation risk, plant structure, and plant resource availability. Subjects were lesser cavies (Microcavia australis), a social species that is semi-fossorial, diurnal, and native to South America. Continuous focal sampling was conducted during the day, at times of food shortage, food abundance, and reproduction from 2003 to 2005. The proportion of time spent vigilance was significantly higher at Ñacuñán, where vigilance peaked at midday and reached a minimum in the evening. This midday peak of vigilance at Ñacuñán was associated with a midday peak of danger from raptors as indicated by a raptor activity peak at that time. In contrast, both vigilance and predator activity at El Leoncito were constant through the day. Records of intermittent locomotion and number and duration of pauses in locomotion were significantly higher at El Leoncito, a difference that may have been due to the need for greater vigilance while moving across areas of less protective cover at this site.     

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.     

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.     

Balancing requirements for stability and maneuverability in cetaceans

The morphological designs of animals represent a balance betweenstability for efficient locomotion and instability associatedwith maneuverability. Morphologies that deviate from designsassociated with stability are highly maneuverable. Major featuresaffecting maneuverability are positions of control surfacesand flexibility of the body. Within odontocete cetaceans (i.e.,toothed whales), variation in body design affects stabilityand turning performance. Position of control surfaces (i.e.,flippers, fin, flukes, peduncle) provides a generally stabledesign with respect to an arrow model. Destabilizing forcesgenerated during swimming are balanced by dynamic stabilizationdue to the phase relationships of various body components. Cetaceanswith flexible bodies and mobile flippers are able to turn tightlyat low turning rates, whereas fast-swimming cetaceans with lessflexibility and relatively immobile flippers sacrifice smallturn radii for higher turning rates. In cetaceans, body andcontrol surface mobility and placement appear to be associatedwith prey type and habitat. Flexibility and slow, precise maneuveringare found in cetaceans that inhabit more complex habitats, whereashigh-speed maneuvers are used by cetaceans in the pelagic environment.     

Lipoprotein lipase activity in white adipose tissue of rats subjected to exercise--rest cycles

This study was conducted to determine serum lipid levels and the activity of lipoprotein lipase in epididymal white adipose tissue of rats undergoing exercise training. During the 8-week period of treatment, one group of rats was kept sedentary and the remaining animals were exercise trained either continually (1 h of daily treadmill running) or intermittently (alternate weeks of daily running and inactivity). Exercise training, either continual or intermittent, decreased postprandial serum total and high-density lipoprotein cholesterol concentrations, which returned to sedentary levels in the intermittently trained animals following a week of rest. Lipoprotein lipase activity in whole epididymal adipose pad was lower in rats trained continually than in the sedentary group at the end of the treatment. The intermittent training program elicited large fluctuations in both the specific (per milligram of protein) and total (per tissue) activity of lipoprotein lipase in white adipose tissue. During rest periods, enzyme activity rose to levels that were higher than those of sedentary rats, whereas lipase activity was below that of sedentary animals following a week of running. In the last exercise--rest cycle, body weight gain of the intermittently trained rats was nearly abolished during the week of running, but it increased above that of sedentary animals during weeks of rest. The present results suggest that the modulation of lipoprotein lipase activity in white adipose tissue is one of the adaptations that take place to accommodate the fluctuations in the rate of energy deposition that occur in the rat during an intermittent training program.     

Long-term creatine intake is beneficial to muscle performance during resistance training

Vandenberghe, K., M. Goris, P. Van Hecke, M. Van Leemputte,L. Vangerven, and P. Hespel. Long-term creatine intake isbeneficial to muscle performance during resistance training. J. Appl. Physiol. 83(6):2055-2063, 1997.The effects of oral creatine supplementation onmuscle phosphocreatine (PCr) concentration, muscle strength, and bodycomposition were investigated in young female volunteers(n = 19) during 10 wk ofresistance training (3 h/wk). Compared with placebo, 4 days ofhigh-dose creatine intake (20 g/day) increased(P < 0.05) muscle PCr concentration by 6%. Thereafter, this increase was maintained during 10 wk of training associated with low-dose creatine intake (5 g/day).Compared with placebo, maximal strength of the muscle groups trained,maximal intermittent exercise capacity of the arm flexors, and fat-free mass were increased 20-25, 10-25, and 60% more(P < 0.05), respectively, duringcreatine supplementation. Muscle PCr and strength, intermittent exercise capacity, and fat-free mass subsequently remained at a higherlevel in the creatine group than in the placebo group during 10 wk ofdetraining while low-dose creatine was continued. Finally, on cessationof creatine intake, muscle PCr in the creatine group returned to normalwithin 4 wk. It is concluded that long-term creatine supplementationenhances the progress of muscle strength during resistance training insedentary females.

     

Functions of Intermittent Locomotion in Mustached Tamarins (<Emphasis Type="Italic">Saguinus mystax</Emphasis>)

Many animals interrupt their moving with brief pauses, which appear to serve several different functions. We examined the function of such intermittent locomotion in wild living mustached tamarins (Saguinus mystax), small arboreal New World primates that form mixed-species groups with saddleback tamarins (Saguinus fuscicollis). We investigated how different environmental and social factors affect pausing during locomotion and used these data to infer the function of this behavior. As measures of intermittent locomotion, we used percentage of time spent pausing and pause rate. We considered 3 possible functions that are not mutually exclusive: increased endurance, route planning, and antipredator vigilance. Mustached tamarins spent on average (mean ± SE) 55.1 ± 1.0% of time pausing, which makes effective resource exploitation more time consuming and needs to be outweighed by correspondingly large benefits. Percentage of time spent pausing decreased in larger mixed-species groups vs. smaller mixed-species groups and decreased with height and in monkeys carrying infants. It was not affected by sex, age, spatial arrangement, or single-species group size. Pause rate increased in individuals traveling independently compared to those traveling in file, but was not affected by other factors. The group size effect in mixed-species groups lends support to the notion that pausing during locomotion is an antipredator tactic that can be reduced in the increased safety of larger groups, but other results suggest that additional functions, particularly route planning, are also of great importance. Benefits in terms of predator confusion and group movement coordination are also likely to play a role and remain a topic for further research.     

Supplemental intermittent-day heat training and the lactate threshold

Heat acclimation over consecutive days has been shown to improve aerobic-based performance. Recently, it has been suggested that heat training can improve performance in a temperate environment. However, due to the multifactorial training demands of athletes, consecutive-day heat training may not be suitable. The current study aimed to investigate the effect of brief (8×30 min) intermittent (every 3–4 days) supplemental heat training on the second lactate threshold point (LT₂) in temperate and hot conditions. 21 participants undertook eight intermittent-day mixed-intensity treadmill exercise training sessions in hot (30 °C; 50% relative humidity [RH]) or temperate (18 °C; 30% RH) conditions. A pre- and post-incremental exercise test occurred in temperate (18 °C; 30% RH) and hot conditions (30 °C; 50% RH) to determine the change in LT₂. The heat training protocol did not improve LT₂ in temperate (Effect Size [ES]±90 confidence interval=0.10±0.16) or hot (ES=0.26±0.26) conditions. The primary finding was that although the intervention group had a change greater than the SWC, no statistically significant improvements were observed following an intermittent eight day supplemental heat training protocol comparable to a control group training only in temperate conditions. This is likely due to the brief length of each heat training session and/or the long duration between each heat exposure.     

Experimental hydrodynamics of fish locomotion: functional insights from wake visualization

Despite enormous progress during the last twenty years in understandingthe mechanistic basis of aquatic animal propulsion—a taskinvolving the construction of a substantial data base on patternsof fin and body kinematics and locomotor muscle function—thereremains a key area in which biologists have little information:the relationship between propulsor activity and water movementin the wake. How is internal muscular force translated intoexternal force exerted on the water? What is the pattern offluid force production by different fish fins (e.g., pectoral,caudal, dorsal) and how does swimming force vary with speedand among species? These types of questions have received considerableattention in analyses of terrestrial locomotion where forceoutput by limbs can be measured directly with force plates.But how can forces exerted by animals moving through fluid bemeasured? The advent of digital particle image velocimetry (DPIV)has provided an experimental hydrodynamic approach for quantifyingthe locomotor forces of freely moving animals in fluids, andhas resulted in significant new insights into the mechanismsof fish propulsion. In this paper we present ten "lessons learned"from the application of DPIV to problems of fish locomotionover the last five years. (1) Three-dimensional DPIV analysisis critical for reconstructing wake geometry. (2) DPIV analysisreveals the orientation of locomotor reaction forces. (3) DPIVanalysis allows calculation of the magnitude of locomotor forces.(4) Swimming speed can have a major impact on wake structure.(5) DPIV can reveal interspecific differences in vortex wakemorphology. (6) DPIV analysis can provide new insights intothe limits to locomotor performance. (7) DPIV demonstrates thefunctional versatility of fish fins. (8) DPIV reveals hydrodynamicforce partitioning among fins. (9) DPIV shows that wake interactionamong fins may enhance thrust production. (10) Experimentalhydrodynamic analysis can provide insight into the functionalsignificance of evolutionary variation in fin design.     

Locomotion patterns in two South American gymnophthalmid lizards: Vanzosaura rubricauda and Procellosaurinus tetradactylus

We quantified gait and stride characteristics (velocity, frequency, stride length, stance and swing duration, and duty factor) in the bursts of locomotion of two small, intermittently moving, closely related South American gymnophthalmid lizards: Vanzosaura rubricauda and Procellosaurinus tetradactylus. They occur in different environments: V. rubricauda is widely distributed in open areas with various habitats and substrates, while P. tetradactylus is endemic to dunes in the semi-arid Brazilian Caatinga. Both use trot or walking trot characterised by a lateral sequence. For various substrates in a gradient of roughness (perspex, cardboard, sand, gravel), both species have low relative velocities in comparison with those reported for larger continuously moving lizards. To generate velocity, these animals increase stride frequency but decrease relative stride length. For these parameters, P. tetradactylus showed lower values than V. rubricauda. In their relative range of velocities, no significant differences in stride length and frequency were recorded for gravel. However, the slopes of a correlation between velocity and its components were lower in P. tetradactylus on cardboard, whereas on sand this was only observed for velocity and stride length. The data showed that the difference in rhythmic parameters between both species increased with the smoothness of the substrates. Moreover, P. tetradactylus shows a highly specialised locomotor strategy involving lower stride length and frequency for generating lower velocities than in V. rubricauda. This suggests the evolution of a central motor pattern generator to control slower limb movements and to produce fewer and longer pauses in intermittent locomotion.     

The Behavioral Ecology of Intermittent Locomotion

Most physiological and ecological approaches to animal locomotionare based on steady state assumptions, yet movements of manyanimals are interspersed with pauses lasting from millisecondsto minutes. Thus, pauses, along with changes in the durationand speed of moves, form part of a dynamic system of intermittentlocomotion by which animals adjust their locomotor behaviorto changing circumstances. Intermittent locomotion occurs ina wide array of organisms from protozoans to mammals. It isfound in aerial, aquatic and terrestrial locomotion and in manybehavioral contexts including search and pursuit of prey, matesearch, escape from predators, habitat assessment and generaltravel. In our survey, animals exhibiting intermittent locomotionpaused on average nearly 50% of their locomotion time (range6–94%). Although intermittent locomotion is usually expectedto increase energetic costs as a result of additional expenditurefor acceleration and deceleration, a variety of energetic benefitscan arise when forward movement continues during pauses. Endurancealso can be improved by partial recovery from fatigue duringpauses. Perceptual benefits can arise because pauses increasethe capacity of the sensory systems to detect relevant stimuli.Several processes, including velocity blur, relative motiondetection, foveation, attention and interference between sensorysystems are probably involved. In animals that do not pause,alternative mechanisms for stabilizing the perceptual fieldare often present. Because movement is an important cue forstimulus detection, pauses can also reduce unwanted detectionby an organism's predators or prey. Several models have attemptedto integrate energetic and perceptual processes, but many challengesremain. Future advances will require improved quantificationof the effects of speed on perception.     

Hypothalamic autostimulation in rats at low frequencies of stimulation on the basis of locomotion]

A study was made of the hypothalamic autostimulation of rats with the impulse stimulation frequency of from 30 to 7 cycles/sec. The animals' brain was stimulated in Skinner's chamber and on the basis of locomotion. The duration of a series of impulses established by the animal on the basis of locomotion grew with reduction of the frequency of stimulation impulses. The pauses between the series of impulses and the duration of the series were also prolonged with a voltage reduction. With the help of stimulation frequency of 7 cycles it was possible to choose such a current that the sum of the total duration of pauses lay within 0 divided by 10% of the total experimental time. The duration of individual pauses was less than 0.5 sec.     

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.     

Age and gender interactions in physiological functional capacity: insight from swimming performance

Tanaka, Hirofumi, and Douglas R. Seals. Age and genderinteractions in physiological functional capacity: insight from swimming performance. J. Appl.Physiol. 82(3): 846-851, 1997.One experimentalapproach to studying the effects of aging on physiological functionalcapacity in humans is to analyze the peak exercise performance ofhighly trained athletes with increasing age. To gain insight into therelationships among age, gender, and exercise task duration with use ofthis model, we performed a 5-yr (1991-1995) retrospective analysisof top freestyle performance times from the US Masters SwimmingChampionships. Regression analysis showed that in both men and womenendurance swimming performance (i.e., 1,500 m) declined linearly frompeak levels at age 35-40 yr until ~70 yr of age, whereuponperformance declined exponentially thereafter. In both genders, thevariability among the top 10 winning times in each 5-yr age intervalincreased markedly with advancing age. Compared with the 1,500-mfreestyle, performance in the 50-m freestyle (short-duration task)showed only a modest decline until ages 75 and 80 yr in women and men,respectively. The rate and magnitude of the declines in both short- andlong-duration swimming performance with age were significantly(P < 0.05) greater in women than in men. In the women, the percent decline in swimming performance over a50-yr age period from the 19- to 24-yr to the 69- to 74-yr age groupsbecame progressively greater from the shortest distance (50 m) to thetwo longest distances (800 and 1,500 m), whereas in men, no differenceswere observed in the magnitude of performance decline with age amongthe five longest distance events (i.e., 100-1,500 m). The percentgender difference in performance throughout the age range studiedbecame progressively smaller (P < 0.05) with increasing distance from 50 m (19 ± 1%) to 1,500 m (11 ± 1%). The findings in this cross-sectional study indicate that from peak levels at age 35-40 yr, physiological functionalcapacity, as assessed by swimming performance, decreases linearly until ~70-80 yr of age, whereupon the decline becomes exponential.Moreover, the rate of decline with advancing age appears to beassociated with event duration and gender.

     

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.     

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.     

Effects of Intermittent Desiccation on Nutrient Economy and Growth of Two Ecologically Contrasted Mosses

The mossesBrachythecium rutabulum (Hedw.) B., S. & G. andPseudoscleropodiumpurum (Hedw.) Fleisch. were cultivated for more than 50 d ina growth cabinet with or without weekly drying interludes of24 h. Some plants also received applications of a dilute NPKnutrient solution at weekly intervals. The continuously hydratedplants showed appreciably more biomass production than thosereceiving intermittent desiccation. Desiccation led to somebleaching of the green tissues inB. rutabulum but not inP. purumwhich appeared more desiccation-tolerant. NPK addition causeda further significant growth stimulation in continuously hydratedplants, but not in intermittently desiccatedB. rutabulum. Pseudoscleropodiumpurum showed NPK-induced growth stimulation even when intermittentlydesiccated. Net uptake of N was similar in desiccated and hydratedplants in both species. Considerable net uptake of P and K⁺occurredin continuously wetB. rutabulum , but uptake was much reducedin intermittently desiccated plants. Net uptake of P and K⁺byP.purum was similar in desiccated and hydrated samples. IntracellularK⁺, leaked from the cells during the desiccation treatment,was retained by cation exchange on the negatively charged cellwalls in both species. Levels of intracellular K⁺and Mg²⁺inthe new growth were maintained at the expense of the pool ofexchangeable cations. The growth stimulation and the net uptakeof nutrients under intermittent desiccation was greatest whenthe NPK application was made at the start of rehydration, possiblybecause of accentuated uptake in the early stages of recovery.The results support the hypothesis thatP. purum has a lowernutrient requirement thanB. rutabulum and highlight the importanceof continuous hydration for the latter's more productive plantlife strategy. The data also show that considerable new growthof bryophyte tissues is possible without additional nutrientabsorption. Brachythecium rutabulum ; Pseudoscleropodium purum ; mineral nutrition; desiccation; solute leakage; plant life strategies