Oxygen consumption and activity in salamanders: Effect of body size and lunglessness

Oxygen Consumption and Activity in Salamanders: Effect of Body Size and Lunglessness

MARTIN E. FEDER ' Lkpartment of Zoology and Museum of Vertebrate Zoology, University of California, Berkeley, California 94720

ABSTRACT Oxygen consumption during and after two minutes of strenuous activity was measured a t 25°C in three groups of salamanders: temperate zone lunged, temperate zone lungless, and neotropical lungless. No increase in oxygen consumption was detected during two minutes of activity. There were significant differences among the three groups of salamanders in the effect of body size on maximum oxygen consumption and aerobic scope. Small lungless salamanders consumed more oxygen than small lunged salamanders; however, large lunged salamanders consumed more oxygen than large lungless salamanders. Neotropical lungless salamanders showed lower maximum oxygen consumption than temperate zone lungless salamanders, but the effect of body size was similar in these two groups. Small salamanders, both lunged and lungless, pay off oxygen debt a t the same relative rate as indicated by percent of scope. Large lungless salamanders pay off debt a t a slower rate than large lunged salamanders. Maximum oxygen consumption, aerobic scope, and payoff of oxygen debt are correlated with total respiratory surface area, especially a t large sizes. Large lungless salamanders show a reduced capacity to transport oxygen following activity.

The interaction between oxygen transport capacity and energetic demands is crucial in determining the level of oxygen consumption of an organism. Cellular oxygen consumption may increase above basal levels only if the oxygen supply can be augmented. One major factor affecting oxygen exchange capacity is respiratory surface area. Total respiratory surface area (RSA) and the rate of oxygen consumption (Voz) are highly correlated in many taxa (Hemmingsen, '60; Ultsch, '73). Many factors affect energy demand, including activity. This study examines the inter-relationship of RSA and activity in the determination of Vo, in salamanders.

The interplay between oxygen supply and demand is especially intriguing in salamanders because RSA differs between lunged and lungless salamanders. Above 0.44 g body size, lunged salamanders have larger RSAs than lungless salamanders of the same weight (Ultsch, '74) ; and this discrepancy increases with body size (Whitford and Hutchison, '67). If a simple quantitative relationship exists between RSA and VO,, lunged salamanders

J. EXP. ZOOL., 202: 403-414.

should exhibit higher rates of oxygen consumption than lungless salamanders of the same size. The expected difference is evident in salamanders in hypoxic or aquatic media (Beckenbach, '75; Ultsch, '76). Nevertheless, in atmospheric air (the normal respiratory medium of most salamanders), Voz is similar for resting lunged and lungless salamanders under standard conditions (Feder, '76a). This similarity may be due to the low standard metabolic rate of all salamanders, high concentrations of oxygen surrounding extra-pulmonary gas exchange surfaces, or compen- satory changes in the gas exchange system (discussed in Feder, '76a). In any case, the decreased RSA of lungless salamanders does not necessarily result in a reduction of standard

In salamanders, RSA might be more important in the determination of activity metabolism than standard metabolism. On a theoretical basis, the effect of RSA, on Vo, should be more pronounced a t high VO,, as during or

VO,.

' Present address: Department of Anatomy, University of Chicago, 1025 E. 57th Street, Chicago, Illinois 60637.

403

404 MARTIN E

after activity, than a t low VO, (Ultsch, '73, '76; Beckenbach, '75). Bennett and Licht ('73) observed greater maximum VoZ following activity in a lunged anuran than in a lungless salamander. Also, Whitford and Hutchison ('67) found a high correlation between Vo2 and RSA in apparently stressed or active salamanders (discussed in Feder, '76a). Additional measurements of active Voz under standard conditions are needed to understand the RSA- Vo2 interaction in salamanders. This study presents data on Voz during and after activity in lunged temperate zone salamanders plus temperate zone and neotropical lungless salamanders.

METHODS A N D MATERIALS

Scaling of V,

equation:

or its logarithmic (base 10) equivalent:

where VO, is in units of p l OJhr, W in grams, and A and b are constants. On log-log plots of VO, against W, bis the slope of the line of best fit and A (termed "intercept") is Vo, a t unit body size.

RSA scales as an allometric function of W in salamanders. However, RSA scales as Wo 865 in lunged salamanders and as Wo 735 in lungless salamanders (Ultsch, '74). These relationships indicate that (1) the proportion RSAIW decreases as body size increases; (2) RSAJW decreases more rapidly with increasing body size in lungless salamanders than in lunged salamanders. If Vo, bears a simple quantitative relationship to RSA, there should be a similar correspondence between Vo2 and W as between RSA and W. According- ly slopes (bl of Vo2 against W should be lower for lungless salamander than for lunged salamanders, and decreases in slope may signify increasing RSA-related restriction of metabolism. This formulation assumes that no other components of the gas exchange system limit oxygen uptake. Also, i t , is assumed that weight-specific scaling of Vo, is similar for all species within each group of salamanders.

Experimental procedure

Neotropical salamanders were captured in Guatemala or Mexico in August-September,

Vok is related to body weight (W) by the

Vo, = AWb

log Vo, = log A + blog W

5. FEDER

1975 and July, 1976. Voucher specimens from the same localities have been deposited in the Museum of Vertebrate Zoology, University of California, Berkeley, California. Species and elevation of capture are: Bolitoglossa occiden- talk (1,050 m), B. rostrata (3,000 m), Pseudo- eurycea bellii (2,760-3,320 m), P. gadovii (3,250 m), P. srnithi (2,960 m), and P. unguidentis (2,960 m). Temperate zone species were captured by colleagues or purchased from commercial suppliers.

All salamanders were acclimated for a t least one week prior to measurement a t 12'- 13"C, unfed, under a photoperiod of LD 14110 centered a t 1400 PDT. Measurements of VoZ were made a t 25.0' rtr: 0.1'C to ensure high levels of oxygen consumption. Animals were not acclimated a t the measurement temperature because not all species examined in this study can withstand 25'C for long periods.

VOl was measured during and after two minutes of vigorous activity using the procedure and apparatus of Bennett and Licht ('73). Except for the presence of shock grids, the apparatus was identical to that used by Feder ('76b) for determination of standard metabolic rates. Each animal was weighed and placed individually upon a copper wire grid within a glass jar connected to a War- burg manometer. Each metabolic chamber was in turn placed in a water bath regulated a t the measurement temperature. After several hours for thermal equilibration, each chamber was calibrated and sealed. During the following 2.5 minutes, salamanders were administered 5-15 volts of electrical stimulation a t irregular intervals through the copper wire grid for two minutes. All animals exhib- ited vigorous activity during stimulation. After stimulation ceased, animals became quiescent. Oxygen consumption was measured for 75 minutes following activity.

The following modifications were made in the procedure of Bennett and Licht ('73): Larger jars (10.5 cm high X 4 cm diameter) were used as metabolic chambers to accommo- date larger salamanders. Inert materials (par- affin, moist paper towelling) were placed in the bottom of jars in which smaller animals were measured to decrease volume. It was assumed that salamanders had reached standard metabolic rates after thermal equilib- rium but prior to stimulation; accordingly VO, was not determined before stimulation. Be- cause there is a diurnal rhythm in VOl in

OXYGEN CONSUMPTION AND ACTIVITY IN SALAMANDERS

x x

405

I I L: , 1000.0

500

Neotropicol Lungless Temperate Zone Lunged Temperate Zone Lungless

some amphibians (Turney and Hutchison, ’741, all measurements were made between

Statistical procedures All values of Vex, corrected to standard tem-

perature and pressure, are reported in p1 0,. Analyses of covariance follow the scheme of Kim and Kohout (‘75). In these analyses, the factor “group” refers to the three groups of salamanders: temperate zone lunged, temperate zone lungless, and neotropical lungless. Aerobic metabolic scope (designated “scope”), defined as the difference between maximum and standard rates of oxygen consumption (Fry, ’47; Bennett and Licht, ’72) was calculated for each animal using standard metabolic rate equations from Feder (‘76a,b). Lines were fitted for log Vo, and log scope against log W using the method of least squares. Poly- nomial regression produced no appreciable improvement in fit. Species means for max- imal Vo, were corrected for weight by dividing Vo2 in p l/hr by W using values of b from table 2. Sample means were compared through one way analysis of variance.

1300-2100 PDT.

RESULTS

Pattern of Vo2 increase and activity Salamanders responded to 2-minute electri-

cal stimulation- by moving rapidly. Activity was continuous and apparently maximum; increases in either frequency or voltage of shocks elicited no greater activity. Vo, for nearly all animals was so low during this pe- riod as to be undetectable by the manometric system. Following cessation of stimulation, animals became quiescent almost immediate- ly. No individuals were obviously moving, although minor or brief movements may have escaped notice.

In the 75 minutes following activity, Vo, for individual salamanders increased and then decreased. The maximum Vop during this pe- riod (designated “Vo,m,x”), aerobic metabolic scope, and rate of repayment of oxygen debt were determined from these measurements; and constitute the dependent variables in the following analyses. These variables are related to body size, species, and group (neotropical lungless, temperate zone lungless, and temperate zone lunged).

406 MARTIN E. FEDER

TABLE 1

Effect ofgroup and log body size on log Votmm and log aerobic, metabolic scope

Dependent variable

Source of variation V0,max Scope

Mean Mean square F square F df

(1) Group and log weight, saturated model 2.09 213.09 2.09 154.96 5 (2) Group and log weight, additive model 3.43 350.22 2.33 172.52 3

12a) Group, adjusted for log weight 0.14 14.55 1.07 78.95 2 i2b) Log weight, adjusted for group 9.32 952.67 4.03 298.46 1

(3) Lack of homogeneity of slopes 0.07 7.39 1.74 128.62 2 (4) Residual 0.010 - 0.167 - 93

Analysis follows scheme of Kim and Kohout ('75). For all values of F, p < 0.01.

TABLE 2

Coefiients for log-log regressions of maximum rate of oxygen consumption tVO,max) and aerobic scope against body weight in salamanders at 25°C

Maximum oxygen consumption Aerobic scope Group

r b log A A r b log A A

Temperate zone lunged 24 0.959 0.81620.051 2.35120.041 224.4 0.890 0.93920.102 1.96820.081 92.9 Temperate zone lungless 39 0.964 0.63420.029 2.428t0.019 267.9 0.784 0.45620.059 2.251t0.040 178.2 Neotropical lungless 36 0.941 0.63120.039 2.3291.0.035 213.3 0.844 0.56420.129 2.111t0.077 129.0

Regression equation log 0, consumption = log A + blog weight, where weight IS in grams, and O2 consumption and Aare ins1 O,/(hr) Log Aand b are given rt one standard error A IS the oxygen consumed by a 1-g salamander

Vognax

In figure 1 VOzmax is plotted against body weight. Analysis of covariance (table 1) indicates that body weight, group, and interaction between these two factors each account for significant portions (p < 0.01) of the variation in Vo2max.

V O , , ~ ~ shows the same relationship to body size in salamanders as does standard metabolic rate to body size in most groups of animals: VoZmax increases with body size in an allometric manner (table 2). Large animals show greater absolute V O , ~ ~ ~ than small animals, but small animals exceed large animals on a weight-specific basis. Weight alone may account for 92% of the variation in VoZmax within a group (table 2).

Linear regressions for each group of salamanders show significant variation in the effect of body size upon Voamax; this interaction is related to, the presence of lungs. The slope (b) of the VOZmax regression for lunged salamanders is about 30% higher than the slope for either temperate zone or neotropical lungless salamanders (table 2). Also, slopes for VoZmax in lungless salamanders, both temperate zone and neotropical, are well below the slope for standard Vo2, 0.8 (Feder, '76a,b).

However, the slope for VoZmax is greater than that for standard VO, in lunged salamanders. Because slopes are correlated with RSA and potentially oxygen exchange capacity (METH- ODS AND MATERIALS), the observed slopes suggest that lunglessness reduces VO,,~,. Within- group comparisons of large and small salamanders are the basis for this conclusion, as some lungless salamanders show higher V O ~ ~ ~ ~ than lunged salamanders of the same size (fig. 1).

Two factors are responsible for the effect of group: neotropical or temperate zone range; presence or absence of lungs. Neotropical lungless salamanders show lower VOzm,, than temperate zone lungless salamanders. The intercept of the regression for neotropical salamanders is lower (table 2), and, with few exceptions, absolute metabolic rates are lower as well (fig. 1).

Although large lunged salamanders clearly exceed large lungless salamanders in VO,,,,, lungless salamanders have a higher VoZmax than lunged salamanders over a wide range of body sizes. At small body sizes, lungless salamanders greatly exceed lunged salamanders in VO~,,, (fig. 1). The intercept for temperate zone lungless salamanders is 20% higher than the intercept for lunged salamanders.

OXYGEN CONSUMPTION AND

TABLE 3

Slopes (b) of loglogregressions of Va against body size during 75 minutes following activity

ACTIVITY IN SALAMANDERS 407

Time interval Temperate Temperate after activity zone zone Neotropical

( m i d lunged lungless lungless

0-5 5-10 10-15 15-20 20-25 25-30 30-40

50-60 60-75

40-50

0.774t 0.082 0.451 t 0.057 0.823-C 0.091 0.554-C 0.048 0.840-CO.079 0.614t0.044 0.806t0.063 0.612t0.036 0.82OtO.075 0.70220.043 0.808t 0.063 0.762t 0.051 0.825t0.058 0.707-CO.042 0.861 2 0.057 0.776t 0.046 0.807-CO.061 0.777-CO.051 0.78610.079 0.77720.055

0.56720.052 0.6092 0.066 0.607t0.065 0.5962 0.071 0.693t0.073 0.6842 0.064 0.725-CO.062 0.73420.054 0.7102 0.057 0.6692 0.048

Correlation coefficients (r) were highly signdieant for all regressions (p < 0 01) Slopes are given * one standard error

Aerobic metabolic scope VoZmax is a physiological indicator of the

maximum amount of oxygen a salamander's respiratory system can transport. However, VO,,,, is the sum of two components, standard metabolic rate and VO, associated with recovery after activity. Because variation in standard metabolic rate, that i s unrelated to activity will affect VO~,,,, VoZmax may be a poor indicator of aerobic recovery after activity per se. By contrast, aerobic metabolic scope (defined as the difference between VoZmax and standard metabolic rate) reflects only recovery after activity.

In this study aerobic metabolic scope was calculated for each individual by subtracting from VOzmax the standard metabolic rate pre- dicted by regression equations. Because these equations mask much of the variation in standard VO,, this procedure may also distort variation in scope and may nullify some statistical analyses.

Comparisons of scope indicate major differences in accord with those observed for VoZmax. Body size, group, and interaction of body size and group each explain significant (p < 0.01) portions of the variation in scope (table 1). For all groups scope increases with weight in an allometric fashion ( b< 1) (table 2) ; weight- specific scope decreases with body size. How- ever, the slope of the scope-weight regression for lunged salamanders is much higher (ca. 80%) than the slopes for lungless salamanders, neotropical and temperate zone (table 2). Yet intercepts of the regressions for lungless salamanders are 40-90% higher than intercepts for lunged salamanders. Thus small lungless salamanders have larger scopes than

small lunged salamanders, but large lunged salamanders have larger scopes than large lungless salamanders.

Oxygen debt Elevation of VO, after the cessation of

activity (termed "oxygen debt") supposedly is coupled with the restoration of pre-activity levels of metabolites (Pernow and Saltin, '71). Accordingly, repayment of oxygen debt may indicate the aerobic component of recovery. Lunged and lungless salamanders show major differences in the time course of oxygen debt repayment.

For each group, separate regressions of Vo2 against W were computed for the ten time intervals during which Vo, was measured (table 3). The slopes of these regressions indicate any changes in the effect of body size on Val. Before activity, the slope is approximately 0.8 in each case (Feder, '76a'b). For temperate zone lunged salamanders, the slope remains relatively constant during the 75 minutes following activity. However, for temperate zone lungless salamanders, the slope declines to 0.45 during zero to five minutes after activity; the slope slowly increases and returns to approximately 0.8. For neotropical lungless salamanders, the decline in slope is similar to that for temperate zone lungless salamanders. However, the slope did not return to 0.8 in this group. In summary, the effect of body size on Vo, remains constant after activity in lunged salamanders, but in lungless salamanders it first increases and then returns to resting levels as oxygen debt is paid off. These differences among groups indicate size-related (and hence potentially RSA-related) restriction of oxygen uptake in lungless salamanders.

To allow more direct comparison among groups of the time course of oxygen debt repayment, Vo, was expressed as percent of aerobic scope. Regressions from tables 2-3 plus Feder ('76a,b) were used to compute percent scope according to the following formula:

%scope = . x 100%

Percent scope is plotted as a function of time after activity and body size in figures 2 and 3. Because percent scope is calculated from regressions, i t reflects the best fit for the data rather than raw VO2. Accordingly, some values may fall outside 0-100% scope, and all body sizes may not reach 100% scope. Also, % scope

Actual Vo, -Standard Vo, VoZmax -Standard Vo2

408 MARTIN E. FEDER

Fig. 2 Effect of body size and time after activity on percent of scope in temperate zone lunged salamanders at 25T. Body size is plotted on a log,, scale. Arrows point in the direction of increasing body size and time after activity. The numerical value for percent scope of a 30-g salamander 75 minutes after activity is indicated alongside the plotted value. See text for further description of figure.

Fig. 3 Effect of body size and time after activity on percent of scope in temperate zone lungless salamanders at 25°C. The data are plotted as in figure 2.

OXYGEN CONSUMPTION AND ACTIVITY IN SALAMANDERS 409

TABLE 4

Oxygen consumption following activity in salamander species

VO,max x Consump

(&I 0,l Aerobic tion after Species

N Weight (g) X (gb.hri scope activity ' Temperate zone lunged

Ambystoma gracile 3 22.3523.19 125.627.0 219.927.5 73.425.3 82.2213.2 Ambystomajeffersonianum 2 5.662 1.08 208.32 5.7 284.522.3 133.62 1.9 131.52 23.0 Ambystoma macrodactylum 6 2.4420.45 281.02 19.5 324.6223.1 185.52 18.6 190.8220.1 Notophthalmus viridescens 5 1.0420.77 170.72 10.4 171.52 10.4 55.12 10.2 123.22 11.5 Tarieha torosa 8 10.2221.80 132.927.7 194.327.8 63.825.4 105.324.8

Batrachoseps attenuatus 5 0.7920.05 250.7223.6 226.92 17.2 151.0222.8 163.0215.9 Desmognathus quadramaculatus 3 19.5520.81 76.327.4 239.9221.2 23.627.1 56.022.7 Ensatina eschscholtzii 5 2.3520.68 194.1214.3 257.3220.3 111.4212.7 152.327.2 Eurycea longicauda 2 1.2620.03 271.321.4 296.121.8 180.420.8 200.8224.8 Gyrinophilusporphyriticus 6 6.8420.32 131.02 5.7 274.329.9 66.025.4 110.023.7 Hydromantesplatycephalus 5 2.3220.40 240.0230.0 316.02 17.5 158.2227.0 177.3227.3 Plethodon glutinosus 8 3.8320.69 207.9222.2 320.02 15.9 132.42 19.2 170.62 15.2 Pseudotriton ru ber 4 10.1421.67 115.728.4 278.9217.9 55.227.7 96.1210.3

Bolitoglossa occidentalis 8 0.7220.03 222.42 17.5 196.3216.4 136.92 17.5 162.8219.9 Bolitoglossa rostrata 5 2.7620.32 169.6212.8 243.72 15.2 103.02 12.2 122.3212.1 Pseudoeurycea bellii 6 15.8621.61 67.629.8 111.9217.0 10.4210.5 53.329.0 Pseudoeurycea gadovii 5 1.5120.06 184.02 15.9 213.32 15.5 109.82 15.5 145.0220.4 Pseudoeurycea smithi 7 5.1420.54 134.127.8 240.728.6 74.626.9 100.323.9 Pseudoeurycea unguidentis 5 3.3420.29 155.428.0 240.426.8 91.227.2 130.328.0

Temperate zone lungless

Neotropical lungless

Means are given It: one standard error and in units ofpl O,/(g.hri unless otherwise indicated. Values of bare taken from table 2. ' Calculated for 75 minutes following 2 minutes of vigorous activity.

for a given body size and time,reflects both synchrony and absolute level of VO,. If an individual salamander shows an increase from the first to the last time interval, and a second salamander of the same size shows exactly the opposite pattern, % scope might average only 50% for each time interval. If both showed the same time course, % scope might range from 0 to 100%.

Figure 2 shows % scope plotted as a function of body size (on a log,, scale) and time after activity for temperate zone lunged salamanders a t 25°C. Both independent variables increase in value from the origin in the fore- ground. For any given body size, a transect along the Y (time) axis yields the typical pattern of oxygen debt: an increase after activity followed by a gradual decrease to resting levels. Both large and small animals reach the same peak % scope (about 70%), and all sizes decline to approximately the same level after 75 minutes (about 30%). However, large lunged salamanders peak noticeably later than small lunged salamanders. In large individuals percent scope does not reach a maximum until 10 to 15 minutes, and remains elevated for about 45 minutes after that time.

Temperate zone lungless salamanders show

even more asynchrony between large and small body sizes (fig. 3). Small lungless salamanders show approximately the same magnitude and sequence of oxygen debt payoff as lunged salamanders. By contrast, large lungless salamanders actually show a reduction in Vo, (relative to resting levels previously determined by Feder, '76b) after activity. Per- cent scope rises slowly after activity, and does not peak until 25 to 30 minutes. Moreover, large lungless salamanders have only suc- ceeded in reducing scope to 86% by the time 75 minutes have passed. Also, the high levels of % scope for large lungless salamanders suggest a high degree of synchrony. Salamanders other than large lungless show some variation in the timing of oxygen debt payoff, as indicated by % scopes well below 100%. In contrast, large lungless salamanders seem locked in a pattern of late synchronous payoff a t high % scope.

Neotropical lungless salamanders resemble temperate zone lungless salamanders in the time course of oxygen debt payoff.

If recovery from activity can be judged by the rate a t which % scope returns to resting levels, then salamanders with the smallest weight-specific RSA's show delayed recovery.

410 MARTIN E. FEDER

However, if the total amount of oxygen consumed is a better index, large lungless salamanders may not be a t a disadvantage; they maintain higher % scopes for longer periods than other salamanders. This problem can be resolved by direct comparison of the total amount of oxygen consumption during the 75 minutes after activity. Analysis of covariance indicates that group, body size, and interaction of group and body size each had significant effects on total oxygen consumption (p < 0.01). These three factors had similar effects upon the difference between total and standard Vo, during the 75 minutes after activity. Regressions of total Vo, and total minus standard Vo, for the three groups dupli- cate the pattern of figure 1. Regressions for lungless salamanders have lower slopes but higher intercepts than regressions for lunged salamanders. Thus, absolute measures of total oxygen consumption corroborate the pattern seen for % scope.

Several sources of error may invalidate conclusions based on total VO, and especially total minus standard Val. Because small salamanders return to about 30% scope rather than 0% scope following recovery (figs. 2, 31, total Voi for such animals is inflated by this excess metabolism. Moreover, overestimation is proportional to speed of recovery. Large lungless animals have not completed recovery by the end of 75 minutes, so their oxygen debt is underestimated. Consequently, total VO, measurements must be regarded with cau- tion; salamanders of different body sizes m,ay be more equitable in total minus standard VO, than otherwise indicated.

Interspecific variation Table 4 presents mean values for post-activ-

ity Vo, of species comprising the three groups of salamanders. There was significant interspecific, variation (p < 0.01) in weight-corrected Volm,, within temperate zone lunged and temperate zone lungless groups. In each group, the most active species (as judged by movement during stimulation) tend to have the highest Volm,, and scope.

DISCUSSION

The reduced RSA in large lungless salamanders may result in reduced oxygen exchange capacity relative to lunged salamanders. Slopes of regressions of Vo, against body size are lower in lungless salamanders than in lunged salamanders. Small lungless salaman-

ders, with RSAs nearly equal to RSAs of lunged salamanders, have equivalent or supe- rior abilities to exchange oxygen. Yet large lungless salamanders, with low RSAs relative to lunged salamanders, are unable to raise VO, as high or pay off oxygen debt as rapidly as large lunged salamanders. Further- more, neotropical lungless salamanders do not show any marked superiority over temperate zone lungless salamanders in oxygen exchange capacity, even though the neotropical forms encounter higher temperatures (Feder, '76b) and reach larger maximum sizes (Dunn, '26).

Considerable controversy surrounds the question of whether the control of metabolism is exerted primarily a t the cellular or whole animal (e.g., RSA) level (see Ultsch, '73, '76 for a more complete discussion of this controversy). Measurements of VO, for amphibians have failed to support either alternative con- clusively. One major reason for this ambi- guity is that the above question consists of two questions i n reality: In what ways does RSA regulate VO,? In what ways does Vo, regulate metabolism?

The answer to the first question is now clear for salamanders; RSA places an upper limit on VO,, When the level of Voz is sufficiently low, RSA has little effect upon the weight-specific scaling or absolute level of VOZ. In air, resting lungless salamanders (Feder, '76a,b) or anurans with pulmonary respiration removed (Bentley and Shield, '73) show the same Vol as comparable animals with pulmonary gas exchange included. Such animals probably do not approach the maximum oxygen exchange capacity of their gas exchangers. However, when Vo2 is sufficiently elevated or when oxygen concentrations in the respiratory medium are sufficiently ,reduced, RSA exerts a marked effect upon Voz. In water (Shield and Bentley, '73; Ultsch, '73, '74, '76; Heath, '76; Wakeman and Ultsch, '76) and hypoxic aerial media (Beckenbach, '751, functionally lungless salamanders show patterns of gas exchange similar to those of the active lungless salamanders in this study. I t remains to be seen whether RSA sets an absolute upper limit to VO? or only exerts a relative effect upon the weight-specific scaling

In this light, the extensive studies of Whit- ford and Hutchison ('63, '65, '66, '67) on gas exchange and its partitioning in air-breathing salamanders assume new importance. Whit-

of VO,.

OXYGEN CONSUMPTION AND ACTIVITY IN SALAMANDERS 411

ford and Hutchison sewed plastic masks onto salamanders to enable separate simultaneous measurement of pulmonary and cutaneous gas exchange. However, in doing so they apparently elevated Voz above resting levels (Guimond and Hutchison, '68; Hutchison et al., '77). The regressions of masked salamanders for VO, (Whitford and Hutchison, '67) correspond closely to regressions for active un-restrained salamanders (table 2). Hence the conclusions of Whitford and Hutchison may apply primarily to active salamanders. In summary, these findings are: (1) Lungs account for far more oxygen uptake than extra- pulmonary respiration; (2) The percentage of total oxygen uptake due to lungs increases with temperature; (3) Vo, for lunged salamanders exceeds that for lungless salamanders, especially a t high temperatures (e.g., 25°C) and large body size. Lungs provide the great- est compensation in precisely the situations where lunglessness is most restrictive of VO,: large size, high temperature, and activity. These patterns correspond to findings for rep- tiles that pulmonary gas exchange is of pri- mary significance to active Vo, rather than resting Vo, (Bennett, '73; Ruben, '76).

Before concluding that lunglessness places significant restrictions on the ability of salamanders to be active, it is necessary to exam- ine in what ways VO, is linked to activity metabolism. In vertebrate endotherms, energy production during sustained activity is closely coupled to consumption of oxygen (Bartholomew, '72). However, amphibians generate the vast majority of energy during strenuous or burst activity through anaerobic pathways (Bennett and Licht, '73; Turney and Hutchison, '74; Hutchison et al., '77). Lactate production may account for up to 96% of all energy generated during activity (Ben- nett and Licht, '73). Hence, access to oxygen is not necessary for high levels of activity. Accordingly, i t is inaccurate to conclude that lunged and lungless salamanders have different capacities to be active because they show different VOZmax. In fact, in neither group does Vo, increase above resting levels until activity has ceased.

After activity excess lactate must be me- tabolized or excreted, and levels of oxygen stores plus high energy compounds restored. Traditionally, oxygen debt is associated, with these goals (Karpovich, '65); return of VO? to resting levels signifies that recovery is complete. If this is the case, the slower payoff of

oxygen debt in lungless salamanders has considerable physiological significance.

However, the supposed correlation between oxygen debt payoff and recovery may be spurious to a large extent. Following activity in endotherms, elevated tissue temperatures and consequent partial uncoupling of oxida- tive phosphorylation may result in an increase in Vo2 after activity not associated with lactate removal (Brooks et al., '71a,b). After activity in amphibians, Voz returns to resting levels well before lactate is removed (Bennett and Licht, '73); also, there is poor correlation between lactate production during activity and oxygen debt. Although Hutch- ison et al. ('77) report high quantitative correlation between oxygen debt and lactate production, they find considerable asynchrony among oxygen debt payoff, lactate removal, and restoration of normal levels of associated metabolites. In addition, several vertebrates remove lactate after activity without any increase in oxygen consumption (Irving et al., '42; Kramer et al., '71; Minaire, '73). Because of these inconsistencies, one cannot conclude that delayed payoff of oxygen debt in lungless salamanders has any effect on their capacity for activity.

Nevertheless it appears likely (if for reasons of evolutionary parsimony alone) that oxygen debt has some function. Bennett and Licht ('73) suggest that oxygen debt in amphibians is related primarily to replenishment of oxygen and energy stores, while lactate removal occurs by other means. Also, oxygen consumption or debt may be more important in connection with moderate or slight levels of activity. For these reasons, it would be prema- ture to dismiss differences in Vo, between lunged and lungless salamanders as inconse- quential.

The foregoing discussion assumes a quantitative relationship between RSA and oxygen exchange capacity, and that no other components of the gas exchange system besides RSA limit oxygen uptake. These assumptions are not entirely valid. For example, small lungless salamanders show higher VOzmax, scope, and total VOl than small lunged salamanders with greater RSA. Apparently, levels of key enzymes in the metabolic pathways may account for such discrepancies. Among ecto- therms that show the same general patterns of anaerobiosis and oxygen debt as salamanders, those species with high levels of burst activity and high metabolic scopes also show

412 MARTIN E. FEDER

the highest concentrations of lactate dehydro- genase and phosphofructokinase (Bennett, '72, '74; Ruben, '76). In addition, normal body temperature and geographic range may in part determine Val. Neotropical lungless salamanders show lower VoP,,, and scope than temperate zone lungless salamanders (fig. 1 ; table 2). Standard Vo2 is lower as well (Feder, '76b). Because neotropical salamanders ex- perience higher average temperatures than temperate zone salamanders, the difference in Vo2 between these groups may represent compensation for temperature (Bullock, '55) rather than a difference in RSA or enzyme levels.

Furthermore, there may be qualitative differences in efficacy of lungs and skin as gas exchangers. Ambient pOz and oxygen diffu- sion rates through lungs and skin may be different (Lenfant and Johansen, '67; Lenfant et al., '70; Piiper and Scheid, '75). Also, ventilation affects lungs but not skin. Alternatively, other components of the gas exchange system may compensate for differences between lungs and skin in PO,, ventilation, and perfusion (Feder, '76a). Hence RSA, though important, is not the only factor that determines oxygen exchange capacity.

What is the ulimate limit to metabolism in salamanders? There is no one limiting factor to energy production and use in urodeles. Rather, the interaction of RSA, cellular control, activity, and a number of other factors determine maximum Vo,.

The magnitude of some differences between lunged and lungless salamanders is dependent upon temperature (Feder, '77). Slopes of log aerobic scope-log body size regressions for lunged and lungless salamanders become more disparate with increasing temperature. Also, large lungless salamanders pay oxygen debts more slowly relative to lunged salamanders as temperature increases. The combined effects of lunglessness, body size, and temperature upon post-activity Vo,constitute an appealing but inadequate physiological ex- planation for the small size and cool body temperatures of temperate zone lungless salamanders. Maximum Vo, of lunged and lungless salamanders are far more similar to one another than to maximum Vo, of other vertebrate groups (Feder, '77). Also, neotropical lungless salamanders live a t relatively high temperatures and large body sizes (Feder, '76b) despite the effect of lunglessness upon post-activity Vo, in this group. Accordingly,

the physiological effects of lunglessness may have no serious ecological consequences for salamanders.

ACKNOWLEDGMENTS

I thank J. H. Feder, T. J. Papenfuss, and F. H. Pough for collecting salamanders for this study. Doctors A. F. Bennett, V. H. Hutchison, P. Licht, F. H. Pough, G. R. U1- tsch, and D. B. Wake criticized earlier ver- sions of the manuscript. Research was sup- ported by NSF Grant BMS-75-16138 to P. Licht, NSF Grant BMS-75-20922 to D. B. Waka, a University of California Chancellor's Patent Fund Grant to the author, and a NSF Graduate Fellowship to the author.

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Oxygen consumption and activity in salamanders: Effect of body size and lunglessness