Sprint velocity of the lizard Sceloporus occidentalis was maximal at preferred body temperature (Tb, 35°C). Mean running velocity (VR) and stride frequency (f) at this temperature were 3·23 ± 0·7 (mean ± S.E.M.) ms−1 and 15·6 ± 0·3 s−1, respectively. VR and f did not change significantly when Tb was raised to 40°C. At Tb values between 25 and 35°C the thermal dependencies of VR (Q10= 1·23) and f (Q10= 1·12) were quite low. At Tb values below 25°C the thermal dependence of these factors increased markedly. Stride length (Ls) was independent of Tb from 15 to 40°C. Lizards with a Tb of 10°C were largely incapacitated, and VR, f and Ls were all greatly reduced. Comparisons with measurements of the contractile properties of skeletal muscle of this species suggest that stride frequency is limited by the twitch contraction time at temperatures below 23°C. At higher temperatures, sprint performance is nearly independent of the thermal effects on the muscles.
Variable body temperatures of ectothermic vertebrates influence most aspects of their biology, including locomotor performance. In recent years research on thermal effects on locomotion has shifted emphasis from an examination of extreme temperatures that cause locomotor incapacitation (Brattstrom, 1965; Huey, 1982) to an investigation of the thermal effects on performance at intermediate temperatures that allow relatively normal locomotor activity (Huey & Stevenson, 1979; Bennett, 1980; Huey, 1982, 1983; Marsh & Bennett, 1985). Lizards have provided a particularly useful group of animals in which to examine the thermal effects on performance. This usefulness stems both from their ecological diversity, which provides the opportunity to relate laboratory measurements to ecological specialization (Huey, 1983), and from the relative ease with which repeatable measurements of both sustainable and burst activity can be obtained (see Bennett, 1980).
Depending on the activity performed and the temperatures experienced, loco-motion may be influenced by thermal effects on a variety of systems, e.g. cardio-vascular, respiratory, nervous and/or skeletomuscular. The investigations described here emphasize the possible relationships between thermal effects on the time-dependent properties of skeletal muscle (Marsh & Bennett, 1986) and locomotor performance in short burst runs (sprints) by an iguanid lizard Sceloporus occi-dentalis. In iguanids the skeletal muscles of the limbs appear to be primarily specialized for this type of locomotion, for approximately 80-90% of the fibres in most muscles are of the fast-twitch glycolytic fibre type (Putnam, Gleeson & Bennett, 1980; Gleeson & Harrison, 1986).
In previous studies (Bennett, 1980; Marsh & Bennett, 1985), we have shown that sprint running performance of the iguanid Dipsosaurus dorsalis is at least partially uncoupled from thermal effects on the contractile speed of skeletal muscle at body temperatures between 25 and 40°C. This conclusion was reached because of the substantially lower Q10 of running velocity and stride frequency as compared to the Q10 values of the time-dependent isometric or isotonic contractile properties. We suggested that, over this range of temperatures, stride frequency is determined by some other factor with a low thermal dependence, such as elastic storage of energy. At body temperatures below 25 °C, however, Dipsosaurus has a high thermal dependence of locomotor performance (Q10> 3). This greater thermal dependence at low temperatures appeared to be primarily due to a prolongation of the state of activation of the muscle fibres. Consequently, the time taken to contract and relax the skeletal muscles may directly limit the maximum stride frequency attainable at low body temperatures. Such findings, if substantiated for other species, are important both for what they reveal about the mechanisms determining performance during high-speed running, and also for their ecological significance. The existence of an inflection in the temperature versus performance curves would be a major deter-minant of the thermal performance breadth (Huey & Stevenson, 1979; Huey, 1982) for sprint locomotion.
MATERIALS AND METHODS
Adult Sceloporus occidentalis of both sexes were collected in Orange County, CA, under a California scientific collector’s permit to AFB and maintained in the laboratory for less than 1 week before use. Prior to the experiments they were housed in cages allowing behavioural thermoregulation and fed crickets and mealworms ad libitum. Lizards were collected in early July and had a mean body mass of 12·3 ± 0·8g and snout-vent length of 67·3 ± 1·3 mm (mean ± S.E.M., N = 10).
Sprint running performance was assessed using essentially the methods of Marsh & Bennett (1985). During runs of approximately 3 m animals were filmed from above with a super-8 movie camera. Framing rates were measured on each roll of film by filming a stopwatch and were approximately 65 s−1 when the lizards’ body temp-eratures were 20°C and above and 34 s−1 at lower body temperatures. Following each run, body temperature was measured cloacally with a Schultheis small animal thermometer. Animals were run in the morning and afternoon on six successive days. Disturbance of the animals prior to the runs was minimized by housing them in individual containers in a controlled temperature cabinet. The approximate body temperatures (Tb) used in successive experiments were 35, 15, 30, 10, 25, 30, 25, 20, 35, 40 and 5°C.
Because the animals varied in how well they ‘cooperated’ during these measure-ments, we rated the runs subjectively at the time of filming. Runs were classified as: excellent, animal ran the entire length of the track with no stimulation; good, animal required some stimulation during the run but covered large segments of the track apparently at full speed; fair, animal stopped and started many times, often hitting the sides of the track, but usually progressed the entire length of the track; poor, animal refused to walk or run without continual stimulation and often appeared not to be taking full strides. One animal refused to run in two of the trials and was judged to be ‘fair’ or ‘poor’ in all of the other trials. Another individual was judged to be ‘fair’ or ‘poor’ in 7 of 10 trials. These two animals were excluded from the statistical analysis presented in Results. Inclusion of the data from these animals lowers the mean running velocity and increases the variance for all parameters, but does not substantially alter any of the analyses. The other eight animals performed in the ‘good’ or ‘excellent’ categories almost exclusively except when running with a Tb of 10°C. Maximal running velocity (VR) was estimated from frame counts and the marks on the track and was defined as the fastest velocity maintained for at least three strides. The stride frequency (f) and stride length (Lg) reported are the average values maintained over the same portion of the run.
Means reported in the text are presented ±1 S.E.M. Because the performance data were recorded on the same animals at the different temperatures, we analysed these data with a two-way analysis of variance (see Sokal & Rohlf, 1981, p. 348). This method allows the variance due to the effect of temperature to be partitioned from that due to variation among individuals.
In analysing data on the running performance of Sceloporus occidentals, we concentrated on the data from eight animals with runs judged almost exclusively to be ‘excellent’ or ‘good’ (see Materials and Methods), as we consider these runs to be a better representation of physiological capacities at various Tb values. The inclusion of data from two additional animals that behaved poorly resulted in a small change in the mean values of VR, f and LS (Fig. 1), but did not change substantially any of the analyses or conclusions presented below. At 10°C, no runs were judged to be ‘good’ or ‘excellent’. This qualitative change in performance appears to be due to the incapacitating effects of low Tb on locomotion.
In two-way analysis of variance, variation among individuals accounted for a significant portion of the variance for all parameters measured, but a far greater portion of the variance was attributable to the effect of temperature (Table 1). As an aid to interpreting the variation in performance among the various temperatures, we calculated the minimum significant differences (MSD) using the T-method (Sokal & Rohlf, 1981). Comparison intervals based on this statistic are plotted in Fig. 1. To group means that were not significantly different, we used the SS-STP method (Sokal & Rohlf, 1981). Mean Ls of animals with Tb values from 15 to 40°C averaged 0·196 ± 0·007 m and did not vary significantly. This parameter dropped significantly at a Tb of 10 °C compared to the mean value at any other temperature (P < 0·01), corresponding to the markedly abnormal running behaviour at the lower Tb. Mean VR and f were highest at 35°C, 3·23 ± 0·22ms−1 and 15·6 ± 0·3 s−1, respectively (Fig. 1). The thermal dependence of these parameters is quite low over the range of Tb from 25 to 40°C (Table 2). Using analysis of variance, no significant effect of temperature on VR or f is demonstrable at 25 °C and above. Temperature has a significant effect on performance below 25 °C, and the calculated Qio value increases to T9 or above (Table 2).
The performance of the animals proved to be very repeatable. Duplicate measure-ments on separate days at Tb values of 25 and 35 °C resulted in similar values for VR, f and Ls (Fig. 1).
In short burst runs (sprints) Sceloporus attains the highest mean velocity when its Tb is 35°C, which is the preferred Tb of this species (Brattstrom, 1965). As in other small lizards examined to date (Bennett, 1980; Christian & Tracy, 1981; Hertz, Huey & Nevo, 1982, 1983; Marsh & Bennett, 1985), Sceloporus shows a low thermal dependence of running performance over a broad range of temperatures above and below the temperature at which performance is optimal. At 25°C, VR is 80% of the value at 35°C. Below 25°C, the thermal dependence of sprint velocity increases, a phenomenon also noted in Dipsosaurus (Marsh & Bennett, 1985).
The question addressed here is the extent to which the thermal dependence of locomotor performance derives from the effects of temperature on the muscles used in running. The hindlimbs of iguanid lizards are primarily composed of fast-twitch glycolytic fibres (Putnam et al. 1980; Gleeson & Harrison, 1986), and presumably these fibres power primarily the burst type of locomotion examined in the present study. Evidence from previous studies of isometric contractile properties indicates that the iliofibularis has similar contractile properties to the larger locomotor muscles such as the gastrocnemius (Putnam & Bennett, 1982). We assume, therefore, that the properties of the fast glycolytic fibres from this muscle are representative of the muscles used by the animals in our behavioural experiments. In a companion study (Marsh & Bennett, 1986), we have examined the contractile properties of the fast-twitch glycolytic region of the iliofibularis muscle from Sceloporus occidentalis.
The low thermal dependence of running performance of Sceloporus above 25 °C does not have a parallel in either the isometric or isotonic contractile properties (Marsh & Bennett, 1986). The velocity of shortening has a Q10 of 1·8 from 15 to 35 °C (Fig. 2). Due to the small influence of temperature on tetanic tension, the predicted power output has a similar thermal dependence over most of this temperature range (Fig. 2). Clearly, as was the case with the lizard Dipsosaurus (Marsh & Bennett, 1985), some process operable in vivo during running must serve to mitigate the effects of temperature on the contractile properties of the muscles. We concluded in this earlier study that the low thermal dependence of sprint running is probably not determined by temperature-induced changes in fibre recruitment, as has been suggested for low-speed walking in lizards (Rome, 1982) and slow swimming in fish (Rome, Loughna & Goldspink, 1985). We suggested instead that elastic storage and recovery of energy in muscles and/or tendons is a likely candidate for a thermally independent process occurring during sprint running. According to this mechanism, energy stored in elastic elements in one portion of the stride could be recovered and provide part of the power required to maintain velocity. In a running animal, the integrated value of the impulsive ground-reaction force determines the contact time and take-off velocity, which help determine stride frequency. The peak force possible during ground contact is presumably almost thermally independent due to the low temperature dependence of maximal muscle stress (Marsh & Bennett, 1985, 1986). If the presence of elastic elements in series with the muscles blunts the effects of temperature on the shape of the ground-reaction impulse, the result should be a small effect of temperature on the stride. Perhaps arguing against this hypothesis is evidence that elastic storage and recovery of energy is insignificant in small mammals (Biewener, Alexander & Heglund, 1981), and, by analogy, might not be expected to play a large role during running in small lizards. An additional possible mechanism for enhancing locomotor performance is suggested by the data of Cavagna, Mazzanti, Heglund & Citterio (1985), who describe a non-elastic potentiation of muscle performance following stretch. This mechanism may have been a factor in earlier experiments that documented a low thermal dependence of power output during post-stretch releases of frog skeletal muscle (Cavagna, Citterio & Jacini, 1981). Further understanding of the mechanistic basis for the low thermal dependence of lizard running may have to await a more complete knowledge of the patterns of muscle contraction during running, and effective modelling of these contractions in vitro.
Although the isotonic or isometric properties of the muscles do not determine directly the zone of low thermal dependence, the lower limit of this zone may be set by thermal effects on the muscles. We suggested previously (Marsh & Bennett, 1985) that this limit was set in Dipsosaurus by the effects of temperature on the duration of the state of activation following a single stimulus. The time course of twitch contractions appears to be primarily related to how fast the state of activation declines following a single stimulus (see Hoyle, 1983). Based on evidence from earlier studies on frog muscle (Jewell & Wilkie, 1960) and our data on lizard muscles, we used the time from onset of force to 50 % relaxation (tc) as an approximate measure of the duration of the state of activation. During running a lizard must contract and relax its muscles in the time required for half of a stride . Otherwise there would be interference between the antagonistic muscles that power the propulsive and return strokes of the stride. Fig. 3 shows tc and for Sceloporus plotted as a function of temperature. At high temperatures, tc is considerably shorter than its and the muscles could be activated in short tetani without limiting ts-As temperature is lowered the difference between the times lessens due to the high thermal dependence of tc-The curves approach one another at approximately 23°C, below which the thermal dependence of ts increases. These data are fully consistent with those collected on Dipsosaurus and strengthen the conclusion that the time course of the twitch response limits stride frequency and hence sprint speed at low Tb.
Huey and coworkers have emphasized the use of thermal performance breadth in assessing the adaptation of ectotherms to temperature (Huey & Stevenson, 1979; Huey, 1982). Thermal performance breadth is defined as the range of temperatures over which an animal can maintain a certain percentage (e.g. 80%) of its maximal performance. Assuming that a temperature-independent capacity for rapid escape is important for lizards under natural conditions, a wide thermal performance breadth would be favoured by natural selection. Such wide performance breadths exist in iguanid lizards but are limited by the effects of low temperature on the kinetics of twitch contractions. Further exploration of the role of muscle kinetics in determining locomotor performance of ectotherms will aid the formulation of predictions re-garding the limits of thermally related adaptations.
We thank H. B. John-Alder for assistance during the filming. This study was supported by NSF grants PCM-8102331 and DCB-8502218 and NIH grant K04 AM00351 to AFB, and by NSF grant DCB-8409585 to RLM.