AMER. ZOOL., 13:505-512 (1973). The Effects of Temperature on Respiration in the Amphibia WALTER G. WHITFORD Department of Biology, New Mexico Stale University, Las Cruces, New Mexico 88003 SYNOPSIS. The effects of temporal me on respiration in amphibians are primarily effects on gas exchange patterns and rate of oxygen consumption (Qo 2 ) in major groups of amphibians. In temperate zone amphibians except plethodontid salamanders, pulmonary oxygen uptake increases with temperature. In plethodontid salamanders cutaneous gas exchange predominates and increases at higher temperatures. Aquatic salamanders are characterized by a lower Q o than temperate amphibians at all temperatures. Tropical anurans have a Q o equivalent to temperate amphibians at a tempcrarure 10 C greater. The ability of amphibians to supply their tissues with oxygen and the effects of temperature on amphibian respiratory parameters are suggested as probable factors causing these relationships. Recent evidence for temperature independent reduction in Q o in fossorial amphibians and species differences in metabolic scope is discussed. An analysis of the effects of temperature on respiration in the Amphibia is complicated by the combinations of respiratory surfaces involved in amphibian gas exchange. Aquatic forms may use combinations of gills, skin, lungs, and buccopharyngeal surfaces in gas exchange (Guimond and Hutchison, 1972). Terrestrial amphibians exchange respiratory gases via the skin, lungs, and buccopharynx, but in some forms (family Plethodontidae and representatives in two other families: Salamandridae and Ambystomatidae) lungs have been lost and nearly all respiratory exchange is cutaneous (Whitford and Hutchison, 1965). Therefore, some of the most interesting and important effects of temperature on amphibians are on gas exchange patterns. The relative roles of skin and lung-buccopharyngeal respiration have been examined in 14 species of salamanders representing three families and in 21 species of anurans representing eight families. Two patterns of gas exchange emerged when these data were examined. The gas exchange patterns in lungless salamanders, family Plethodontidae, at different temperatures are shown in Figure 1 (data from I thank Robert Guimond, Victor Hutchison, and Roger Seymour for providing me with manuscripts and unpublished data. Fenton Kay assisted with computer analysis of data. Victor Hutchison and my graduate students critically reviewed the manuscript. 505 Whitford and Hutchison, 1965). Salamanders of the family Plethodontidae apparently lost their lungs as an adaptation to life in or adjacent to rapidly flowing mountain streams (Dunn, 1926). In Plethodontid salamanders between 82% and 95% of the carbon dioxide elimination is through the skin, and from 83% to 93% of the oxygen uptake is through the skin. Figure 1 includes data from only one completely terrestrial plethodontid, Plethodon glutinosus. Whitford and Hutchison (1965) found that this species obtained significantly more oxygen via the buccopharyngeal surfaces than the other species. Based on data for P. glutinosus at one temperature, 15 C, it is probable that variation with temperature in the gas exchange pattern of terrestrial plethodontids would not be dissimilar to that shown in Figure 1 except that the buccopharyngeal surfaces might account for a slightly greater percentage of the total oxygen consumption. In salamanders with lungs and in anurans (exclusive of tropical forms), the variances in gas exchange values due to body size partly account for the differences in responses of the gas exchange patterns to different temperatures. While the other factors causing slight differences in gas exchange patterns between species may be of interest, the general pattern of gas exchange response to temperature (see Fig. 2) is most instructive. Most of the carbon dioxide exchange is cutaneous at all tem-
506 WALTER G. WHITFORD E Pul O, O Cut O, A Pul CO, A Cut CO, 5 10 15 20 26 30 TEMPERATUREt FIG. 1. The effect of temperature on cutaneous and buccopharyngeal gas exchange in plethodontid salamanders. The 15 C points are means for four species. At 5 C and 25 C data were available only for Desmognothus quadramaculalus (Whitford and Hutchison, 1965). peratures, varying from 81% at 5 C to 76% at 25 C. The lungs supply from 32% of the oxygen consumed at 5 C to 68% at 25 C. The increases in pulmonary oxygen uptake and CO 2 release are directly related to changes in breathing rates and tidal volumes. Amphibians exhibit two distinct types of breathing or pulsations of the buccal floor: buccal oscillations which move air back and forth across the buccopharyngeal mucosa, but do not result in lung ventilation; and large buccal pulsations accompanied by opening and closing of nares and glottis which result in lung ventilation (Whitford and Hutchison, 1963, 1965; Hutchison et al., 1968; Gans and Dejongh, 1969). The linear increase in pulmonary oxygen uptake (Fig. 2) is the result of increases in tidal volumes and breathing rates. The rate and volume of lung ventilation increase linearly with temperature (Hutchison et al., 1968), but changes in rate of buccopharyngeal oscillations between species are variable. The contribution of buccal oscillations to gas exchange in amphibians in problematical. This air exchange is the only means of air movement available to plethodontids and accounts for a small per cent of the total gas exchange (between 15% and 25%) (Whitford and Hutchison, 1965); however, in amphibians with lungs, the contribution of this movement of air to respiratory exchange has not been objectively assessed. The rate of buccopharyngeal movements in salamanders and frogs increases as a function of temperature which suggests that this mode of ventilation contributes to the increased pulmonary gas exchange. Das and Srivastava (1957) proposed that the ratio between lung ventilatory movements and buccopharyngeal oscillations was a constant (K) in various species of amphibians. However, K varies considerably in the same species at different temperatures (Hutchison et al., 1968; Guimond and Hutchison, 1968). If buccopharyngeal oscillations serve primarily an olfactory function as suggested by Matthes (1927), Vos (1936), and Elkan (1955), it is not like- E -a Pul Oj O Cul O, k Pul CO, A Cul CO, * 5 W 15 20 25 30 TEMPERATURE t FIG. 2. The effect of temperatuie on gas exchange patterns of salamanders with lungs and anurans. Each point represents the average of published gas exchange values (Whitford and Hutchison, 1963, 1965; 1966; Hutchison et al., 1968; Vinegar and Hutchison, 1965; Guimond and Hutchison, 1968).
TABLE 1. Pulmonary efficiencies in a variety of amphibian species at different temperatures.* Species Beference 10 15 20 25 Ambysioma maculatum Tarica granulosa Desmognathus quadramaculatus Bufo americanus Bufo ooreas Bufo cognatus Bttfo marinus Bufo terrestris Hyla versicolor Rana catesbeiana Jtana sylvatica Xenopus laevis Ceratophrys calcurntta Kana pipiens Whitford and Hutchison (1963) Whitford and Hutchison (1965) Whitford and Hutchison (1965) Hutchison et al. (1968) Guimond and Hutchison (1968) 2.24 1 0.14 0.92 0.36 0.38 0.10 1.00 0.21 1.60 1.43.81 0.93 0.79 0.73 1 0.31 1.20 1.00 1.00 0.21 0.38 1.00 0.75 1.27 0.86 0.97 0.31 1 0.72 * Pulmonary efficiency is the volume of oxygen removed by the pulmonary surfaces divided by the volume of oxygen inspired by both lung and buceopharyngeal ventilation. Volume of oxygen inspired is calculated from ventilatory rates and tidal volumes. 1 Data on buccopharyngeal ventilation only. 0.52 1.07 ly that the rate of these movements would respond to temperature in a linear fashion. Although ventilatory rates and tidal volumes increase at higher temperatures in amphibians with lungs, the per cent oxygen removed from inspired air by the pulmonary and buccopharyngeal surface does not increase (Table 1). In some species pulmonary efficiency decreases gradually at higher temperatures, but in some frogs the efficiency at 15 C is higher than at 5 C. The low efficiencies at all temperatures strongly support the contention that the forced pump ventilatory system in amphibians results in poor mixing in the lungs (Gans, 1970). The variances in efficiencies could be due to a combination of measurement technique and/or the varying role of the buccopharynx in respiration in different species. Since respiratory surface area and effective volume of the buccopharyngeal cavity used in pumping air into the lungs vary as a function of body size expressed by KW h, where b has a value less than 1, much of the variation in gas exchange values may be explained by variation in body size (Whitford and Hutchison, 1967; Hutchison et al., 1968). When body size is eliminated as a variable, several generalizations concerning the effect of temperature on respiration in amphibians are apparent (Figs. 3, 4). Relationships of plethodontids and tropical anurans at other temperatures are missing because of insufficient data. Summarized, these generalizations are: (1) Temperate zone anurans and salamanders with lungs have higher rates of oxygen consumption (Qoo) than lungless salamanders (Plethodontidae) at the same temperature. (2) The oxygen consumption of tropical anurans is equivalent to that of temperate anurans at a temperature 10 C less than TEMPERATE ZONE ANUHANS AND SALAMANDERS PLETHODONTIDAE TROPICAL ANURANS AT 25*C SALAMANDERS AT 1S C GRAMS 40 «0 SO FIG. 3. A comparison of the effects of temperature on oxygen consumption in several groups of amphibians. The lines represent the best fit linear regression lines computed by least squares. Data are from Dunlap (1971), Fitzpatrick et al. (1971, 1972), Guimond and Hutchison (1968, 1972), Hutchison et al. (1968), Jameson et al. (1970), Morris et al. (1963), Packard (1971), Tashian and Ray (1957), Vinegar and Hutchison (1965), Whitford and Hutchison (1963, 1965, 1967), Whitford and Sherman (1968), Whitford (1968), Wood and Orr (1969), and Wood (1972).
508 WALTER G. WHITFORD E O 80 FIG. 4. The effects of temperature and body size on several groups of amphibians. Data from sources cited in Figure 3. the temperature of the tropical frogs. (3) The oxygen consumption of aquatic salamanders is significantly lower that that of terrestrial salamanders at all temperatures. (4) There is no significant difference in oxygen consumption between temperate zone frogs and salamanders with lungs. The effect of temperature on these groups of amphibians is predicted by the following equations where O 2 is oxygen consumption in microliters per gram per hour and T is temperature in degrees Celsius: aquatic salamanders O s = 1.8 + 1.05 T Plethodontidae O 2 = 14.6 + 2.5 T temperate lunged amphibians O 2 = 8.9 + 5.3 T tropical anurans O 2 = 20.4 + 5.2 T The variance in these data is largely due to variations in body size that are not eliminated when oxygen consumption is expressed on a unit weight basis. Therefore, the same data were used to compute regression equations of log 10 oxygen consumption and log ]0 body weight which are plotted in Figure 3. These equations, where O 2 = oxygen consumption in cc per hr and W = weight in grams, are: temperate amphibians 5C:logO 2 =.97 +.65 log W. 15C:logO 2 =.81 +.78 log W. 25C:log0 2 =.45 +.67 log W. tropical anuraiis 25C:logO. =.714 +.71 log W. Plothodontidae 15C:logO 2 = 1.36 + 1.17 log W. aquatic salamanders 5C: 1.72+.761ogTF. 15 C: 1.69 +.93 1ogTF. 25 C: 1.65 + 1.00 log W. This analysis demonstrated that for the groups with sufficiently large sample sizes (all except aquatic salamanders) most of the variance in oxygen consumption at a given temperature was due to body size (r 2 values between.84 and.97). The lack of difference in Q 02 in temperate zone frogs and salamanders at different temperatures reflects similarities in gas exchange parameters in these species and does not support the contention (Salthe, 1965) that frogs generally have higher respiratory rates than salamanders. The rate of oxygen consumption in amphibians may be primarily a function of the ability of the animal to obtain oxygen from its environment. The allometry of body surface and buccopharyngeal volume have been shown to be probable determinants of rates of oxygen consumption (Whitford and Hutchison, 1967; Hutchison et al., 1968). The loss of lungs in the plethodontidae, as an adaptation to rapid flowing stream habitats, eliminated a mode of varying the oxygen supply at higher temperatures by lung ventilation, thus relegating these amphibians to environments characterized by temperatures below 30 C (Brattstrom, 1963). Although over 100 species of plethodontid salamanders occur in the neotropical region in a variety of forest habitats, from lowlands to high paramo (Brame and Wake, 1963), no physiological data are available for any of these forms, and consequently, generalizations concerning plethodontids must await data on neotropical plethodontids. In frogs as well as in salamanders, the positive pressure ventilation system results in ineffective respira-
TEMPERATURE AND RESPIRATION IN AMPHIBIA 509 tory exchange (Gans, 1970). The rate of cutaneous oxygen exchange is a function of the PO 2 difference between skin capillaries and the air. With changes in heart rate and cardiac output as the only means available for establishing a more favorable gradient, changes in the rate of cutaneous oxygen uptake at higher temperatures decrease at higher temperatures. Thus, the ability to supply oxygen to the tissues appears to be limited in lunged forms as well as in plethodontids. Reduced oxygen consumption in tropical anurans when compared with temperate forms reflects shifts in temperature optima of enzyme systems. The Q 02 response curves are similar to those of temperate forms indicating that the similar gas exchange parameters are involved, but that these have been adjusted in the evolution of tropical species to respond to a higher and more constant thermal environment. Aquatic salamanders have lower rates of oxygen consumption at all temperatures than any other group of amphibians (Fig. 3). This relationship holds for very small (Norris et al., 1963), intermediate (Whitford and Sherman, 1968), and large salamanders (Guimond and Hutchison, 1972). In arid environments metamorphosed salamanders are forced to spend extended periods of time in an aquatic environment. Under these circumstances, Qo 2 is reduced below that in air, but the rate of surfacing and pulmonary gas exchange increases with temperature (Whitford and Sherman, 1968). Wood (1972) also showed that the oxygen consumption of transformed Dicamptodon ensatus measured in a water-air system, while significantly higher than in larval animals of the same size, was lower than predicted for a land dwelling salamander. This suggests that the metabolic rates of aquatic amphibians have been evolutionally adjusted to accommodate the lower availability of oxygen in their environment. These data also support the contention of Norris et al. (1963) that amphibians are partial metabolic conformers with respect to oxygen tension. At lower temperatures (15 C and below) gas exchange in aquatic salamanders appears to be primarily cutaneous (Whitford. and Sherman, 1968; Guimond and Hutchison, 1972). Guimond and Hutchison (1972) reported that, in Necturus maculosus at higher temperatures or when excited, the branchial surface assume the dominant role in gas exchange. Wood (1972) reported that in Dicamptodon ensatus larvae, gill ventilation rate increased from 9 per min at 10 C to 48 per min at 20 C, and at 20 C the larvae surfaced to gulp air. Thus, higher environmental temperatures appear to require active participation of respiratory surfaces other than the skin in aquatic salamanders as well as in terrestrial amphibians. Submerged frogs also exhibit reductions in oxygen consumption (Jones, 1967, 1972) which accommodate bradycardia. However, this reduction in Q 02 appears to be dependent on oxygen tension because Jones (1967) showed that R. pipiens submerged in 100% oxygenated water showed no reduction in Q 02 and only slight bradycardia. Data on oxygen consumption in water in aquatic frogs such as Pipa pipa, Xenopus laevis, or Ascaphus truei would be of value in determining if all groups of aquatic amphibians exhibit reduced Q 02. Variables other than differences in gas exchange surfaces influence the interpretation of the effects of temperature on respiration in the amphibia. Season of the year, photoperiod, and time of day have been shown to affect gas exchange patterns (Bohr, 1900; Krogh, 1904; Dolk and Postma, 1927; Long and Johnson, 1952; Vernberg, 1952; Fromm and Johnson, 1955; Whitford and Hutchison, 1965; Vinegar and Hutchison, 1965; Guimond and Hutchison, 1968, 1972). In temperate zone frogs, oxygen consumption peaked in the spring, had a slight rise in the fall, and fell to low levels in the winter. Photoperiod effects on amphibian gas exchange patterns are difficult to evaluate. Whitford and Hutchison (1965) reported that at 15 C Ambystoma maculatum had a significantly higher Q o, when acclimated to a 16-hr photoperiod than at an 8-hr light period. Guimond and Hutchison (1963) found that in Rana pipiens photoperiod resulted in
510 WALTER G. WHITFORD elevated Q 02 at 15 C, and Vinegar and Hutchinson (1965) reported that photoperiod affected Q O2 in Rana clamitans only at 5 C. Guimond and Hutchison (1972) reported that photoperiod had no effect on oxygen consumption or gas exchange patterns in the aquatic salamander, Nee turns maculosus. In addition, there was no evidence in these studies that photoperiod affected the role of skin and lungs in respiration at different temperatures. Based on the limited data in these studies, it appears that photoperiod is a minor variable in comparison with temperature as a factor affecting respiration in amphibians. The seasonal changes are more difficult to evaluate because of complications of temperature acclimatization, photoperiod, and the time of day at which measurements were made. In the only report dealing with daily cycles and photoperiod acclimation in amphibians, Guimond and Hutchison (1968) found that Q O2 was elevated at the beginning of the dark cycle and that differences in photoperiod altered the onset of maximum and minimum Q o.,. Standard or resting measurements of a physiological process in response to an environmental parameter provides only a partial picture of the effects of an environmental variable. There are scant data on respiration in ectotherms during activity, and data on respiration during activity in amphibia are limited to a single report by Seymour (1973«). He found that the metabolic scopes (active oxygen consumption minus resting oxygen consumption) in Scaphiopus hammondi and Biifo cognatus increased greatly at successively higher temperatures and were considerably greater than those for two species of Rana studied (Fig. 5). The greater metabolic scope in toads is apparently related to their fossorial habits. Seymour (1973a) calculated that spadefoot toads have an elevated oxygen consumption during digging and shortly thereafter to repay an oxygen debt and pointed out that burrowing activity and movement in the soil is characteristic of these animals. Thus, a high metabolic scope is advantageous in an ectotherm that engages in digging. He also calculated that 8 oe y 2 Scaphiopus hammondii ' Birfo cognatus Rana caiesbeiana - * Rana ptptens TEMPERATURE FIG. 5. The metabolic scope in four species of anurans at different temperatures. Metabolic scope is the active oxygen consumption minus the resting oxygen consumption. (Redrawn from Seymour, 1973a.) the metabolic rate of active spadefoot toads is 95% of the predicted basal metabolic rate of a mammal. These studies demonstrate that both the response to temperature and the magnitude of metabolic scope differ in species of amphibians having the same resting gas exchange pattern and resting respiratory rate. Since the species studied occupy different habitats and have different habits, generalizations concerning the significance of these differences must await further studies. It is evident from the preceding discussion that respiration in amphibians is primarily a temperature-dependent process. However, there are species of amphibians in which there are temperature-independent changes in respiration. There is a large body of literature dealing with temperature acclimation in amphibians which results in some degree of temperature compensation and, thus, temperature independence depending on acclimation status and temperatures at which Q o. 2 was measured. (See Fitzpatrick et al., 1971, 1972, and Dunlap, 1971 for a review of the litera- X
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