Natural Daily Temperature Stress, Dehydration, and Acclimation in Juvenile Ambystoma Maculatum (Shaw) (Amphibia: Caudata)

Division of Comparative Physiology and Biochemistry, Society for Integrative andComparative Biology

Natural Daily Temperature Stress, Dehydration, and Acclimation in Juvenile AmbystomaMaculatum (Shaw) (Amphibia: Caudata)Author(s): F. Harvey Pough and Richard E. WilsonSource: Physiological Zoology, Vol. 43, No. 3 (Jul., 1970), pp. 194-205Published by: The University of Chicago Press. Sponsored by the Division of ComparativePhysiology and Biochemistry, Society for Integrative and Comparative BiologyStable URL: http://www.jstor.org/stable/30155529 .

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NATURAL DAILY TEMPERATURE STRESS, DEHYDRATION, AND ACCLIMATION IN JUVENILE AMBYSTOMA MACULATUM

(SHAW) (AMPHIBIA: CAUDATA)1

F. HARVEY POUGH AND RICHARD E. WILSON

Division of Biological Sciences, Section of Ecology and Systematics, Langmuir Laboratory, Cornell University, Ithaca, New York 14850

INTRODUCTION

Rapid acclimation by salamanders to increased temperatures in laboratory experiments has been described by Hutchison (1961). He found that No- tophthalmus (Diemictylus) viridescens transferred from 19.5 to 32 C completed 30% of their maximum acclimation in 3 hours, 61% in 9 hours, and full acclimation in 24 hours. The ecological significance of such rapid acclimation has been unclear because field studies of body temperatures of salamanders have generally supported Bogert's (1952) conclusion that "most terrestrial salamanders live in areas where the temperature of their surroundings rarely exceeds 20 C."

Our interest in the problem was stim- ulated by the discovery of a natural situation where newly transformed Am- bystoma maculatum (Shaw) emerging from a pond were apparently subjected to severe temperature stress and dehydration. This paper discusses the behavioral and physiological responses of the salamanders in natural and laboratory situations.

1 Supported by PHS grant no. 1-SO4 FR 06002 from the General Research Support Branch, Divi- sion of Research Facilities and Resources. Dr. W. N. McFarland read the manuscript and of- fered helpful suggestions.

MATERIAL AND METHODS

THEORY

Preliminary observations suggested that salamanders taken from under rocks on the afternoon of a sunny day might have higher critical thermal maxima (CTM) than salamanders collected early in the morning. This situation provided a unique opportunity to study acclimation under natural conditions, the first time to our knowledge that this has been possible. In design- ing our investigation we tried whenever possible to devise control experiments in the field. Only when this was impos- sible (as, for example, in looking for a daily rhythm under constant conditions) did we use laboratory experiments. Our goal was to identify the critical environmental factors producing acclimation in the field rather than to study acclimation, per se. We asked the following questions: (1) In a homogeneous sample of newly emerged salamanders, does the CTM increase significantly from morning to evening on a sunny day? (2) Is there a similar increase in CTM on a cloudy or rainy day when salamanders are not exposed to high temperatures? (3) Does the increased CTM persist over night? (4) Under constant temperature in the laboratory is there a daily rhythm in CTM? (5) Are salamanders collected in the afternoon dehydrated, and does

194



NATURAL DAILY TEMPERATURE ACCLIMATION IN AMBYSTOMA 195

this affect their CTM? (6) Do salamanders actively select retreats on the basis of temperature or passively en- dure the temperatures to which they are subjected?

STUDY AREA

An unnamed pond on Connecticut Hill, Tompkins County, New York was the site of field observations and col- lections of animals for laboratory experiments. In early spring when Ambystoma maculatum breed the pond is about 50 m in diameter and 1.5 m deep. During the period of the study (July 29-September 15, 1969), the pond had shrunk to 20 X 30 m with a maximum depth of 0.5 m. The exposed shoreline was covered with flat rocks

ranging in size from pebbles 2 cm in diameter and 0.5 cm thick to large rocks 25 or 30 cm long, 10-15 cm wide, and up to 10 cm thick. Juvenile salamanders emerging from the pond at night sought diurnal shelter under these rocks.

Temperatures under the rocks were measured with calibrated Schultheis quick-recording thermometers or with a YSI telethermometer. Temperatures under rocks with salamanders (salamander temperatures) were recorded by placing the thermometer bulb beneath the animal. Bogert (1952) found that a temperature measured in this way is the same as the salamander's body temperature. Temperatures under rocks without salamanders (sub-rock temperatures) were measured in the deepest part of the depression beneath a rock. Day-long recordings of environmental temperatures were made on August 3, a sunny day. Less extensive observations were made on eight other occasions during the study. On July 29 an area containing 25 rocks was se-

lected and the temperature under every rock recorded, whether there was a salamander under it or not. On some other occasions only the temperatures of rocks with animals under them were recorded.

CRITICAL THERMAL MAXIMUM

Animals used in CTM studies were collected from 0800-093o (morning samples) or 1530-18oo00 (afternoon samples). They were removed from under rocks and placed immediately in a plastic box with a few millimeters of water. This box in turn was placed in an in- sulated container within 1 C of the temperature at which the animals had been collected. It took about 15 minutes to collect enough animals for a day's experiment. The animals were brought to the laboratory and CTM determinations begun within 30 minutes after collection of the last animal in a series. The salamanders were kept in shallow water in a lighted room within 1 C of their collection temperature until the CTM was measured. The maximum time between capture and the final CTM determination in the largest sample was about 4 hours.

Determinations of CTM were made by heating the salamanders in a beaker of water at 1 C per minute, starting from the acclimation temperature. Hut- chison (1961) found that, at this rate of heating, the deep body temperature is the same as the water temperature. At faster rates the body temperature may lag behind the water temperature, and at slower rates some acclimation occurs during the heating period. The CTM temperatures in this study are water temperatures measured beside a salamander.

Practically every investigator has used different criteria to determine the



196 F. HARVEY POUGH AND RICHARD E. WILSON

CTM. In an ecological sense, the critical temperature is the temperature at which an animal loses its ability to escape from conditions that will promptly to its death (Cowles and Bogert 1944). The choice of degree of incapacitation considered necessary to doom an animal has varied with the investigator, and the situation is complicated by species differences in responses to rising temperature. Zweifel (1957) chose the point where swimming movements first became spasmodic as the CTM. Brooks and Sassman (1965) used loss of righting response as their criterion. Hutchi- son (1961) used the "abrupt onset of spasms" as the CTM, stating that " [the onset of spasms] marks the complete loss of any ability of the animal to escape." Brattstrom (1968) suggested the use of the onset of spasms as the CTM for salamanders and loss of righting response for anurans.

We chose three behavioral responses that we consider to represent distinct stages of incapacitation: (a) the development of a "list" in resting animals, (b) turning over on the back, and (c) the appearance of rigor. We did not use the onset of spasms because we found that about 25 %o of our animals emerged from spasms and swam briefly before entering rigor. Thus for our animals the onset of spasms did not nec- essarily signal the loss of all ability to escape.

FIELD EXPERIMENTS

To obtain a homogeneous sample of newly emerged salamanders, we marked with paint all the rocks found around half the circumference of the pond on the afternoon of August 2, and collected all the salamanders under them. We considered that the animals we found under these rocks the next morning and afternoon had emerged from the pond

during the night. The August 4 morning sample was also collected from these rocks and represents primarily newly emerged animals.

We were able to determine the effect of exposure to high temperatures on sunny days by making morning and afternoon tests of CTM on salamanders collected on a cloudy day (August 9) and a rainy day (August 10). On both days, sub-rock temperatures remained near their nighttime levels.

In order to determine whether acclimation developed during daytime was maintained overnight, we located animals under rocks between 1700-1730 hours on August 5 and placed a metal barrier around each rock to prevent the salamander's escape. These animals were collected the next morning and their CTM's determined.

LABORATORY EXPERIMENTS

To discover whether a daily rhythm of CTM could account for the field results, we kept animals in 2-4 mm of water at 20 C in a constant temperature room with a 14-hour photoperiod coin- cident with the natural photoperiod. Animals used in the daily rhythm experiments were acclimated to these conditions for 90-114 hours before their CTMs were measured. These animals were not fed; salamanders being kept for other experiments were given brine shrimp ad lib.

In the field, we found that salamanders would move from under rocks that were becoming too hot. In the laboratory we attempted to measure the temperatures at which animals emerged. Three inches of damp sand were placed in the bottom of a 3 X 6-foot box.

X-shaped depressions were made in the sand and flat rocks, similar to those around the pond, were placed over them. The arms of the X's extended




beyond the edges of the rocks, ensuring that the salamanders had a clear path for emergence. A thermistor was placed in the middle of each X, under the center of the covering rock, and the temperature read on a YSI Model 47 telethermometer. Two to seven salamanders were placed in the central depression under each rock at 20-21 C. The rocks were heated from above by 250-w infrared bulbs. Each time a salamander emerged the temperature under the rock was recorded and the temperature of the sand onto which it emerged was measured. Twenty-seven salamanders, acclimated as described above for 72-120 hours, were used in the experiment. Each animal was tested twice- once in dim light from a north window (36 ft-c measured with a Weston me- ter), and once in bright light (seven 30-w fluorescent bulbs 4 feet above the sand-165 ft-c). These experiments were run from 1400-1600 hours, the time of day at which salamanders are exposed to the highest environmental temperatures in the field.

DEHYDRATION AND REHYDRATION

Salamanders were dehydrated in still air in the laboratory at 21 C until they no longer righted themselves when placed on their backs. Rehydration was accomplished by placing the animals in plastic dishes with a paper towel and 2-4 mm of tap water. They were weighed on a Mettler H15 balance to the nearest 0.1 mg, after being blotted dry between two layers of paper towel. On the afternoon of September 15 (a sunny day), 22 salamanders were collected and returned to the laboratory. Half of them, selected at random, were weighed and allowed to rehydrate in the constant-temperature room. The remaining 11 were placed in dry con- tainers and left overnight in the

constant-temperature room with the intention of determining their CTMs in the morning. The effect of dehydration on the CTM was also studied by using 11 animals acclimated for 72 hours to 21 C and a natural photoperiod before being dehydrated to the point of being unable to right themselves.

RESULTS

ENVIRONMENTAL STUDIES

The salamanders were newly metamorphosed at the time of our experiments. Snout-vent lengths ranged from 27.0 mm to 31.0 mm (mean 29.3 mm, N -19). Body weights of fully hydrated animals ranged from 0.68 g to 0.90 g (mean = 0.78 g, N 11).

From 0800-090o, temperatures under rocks were 17-21 C. (Except as noted, rock temperatures and salamander temperatures are assumed to be the same). On August 3, a sunny day, maximum temperatures under 10 rocks were recorded at 1500-15a0 hours, and ranged from 27.0 to 38.9 C. The lowest temperature was recorded 12 cm beneath the surface under two rocks piled on top of each other; there were four salamanders under these rocks. The highest temperature was observed beneath a rock 17 X 12 X 4 cm. There was a salamander under this rock at 0900, but not at 15"5 when it was checked again. This was our first intimation that salamanders move away from rocks that get too hot.

A thermistor resting on the pond bottom in 5 cm of water registered a maximum of 33.5 C at 1500 hours.

On July 29 from 1430 to 1515 hours, we measured the temperatures of all the rocks in four discrete groups, a total of 25 rocks (fig. la). Nine of these rocks had salamanders under them; the highest salamander tempera-




ture was 32.0 C. Only six rocks with temperatures of 32 C or less did not have salamanders under them. On other occasions we measured temperatures less systematically. The combined measurements-25 rocks with 34 salamanders and 24 rocks without salamanders -are shown in fig. lb. Sub-rock temperatures ranged up to 42.5 C, but we found no salamanders at temperatures above 32.0 C. All measurements were made from 143o-18oo00 hours.

In the early morning, when sub-rock temperatures were 17-21 C, salamanders appeared lively and moist. When there were two or more salamanders under the same rock they were not together. In the afternoon salamanders were dry and sluggish; bits of soil adhered to their bodies and many had difficulty righting themselves when placed on their backs. In these afternoon samples, when there were two or more salamanders under a rock they

0. 29 JULY Solomonder Present FIELD TEMPERATURES Rock without

5 -Salamander

S b. COMBINED FIELD o TEMPERATURES

O 5-

EMERGENCE 5-

25 30 35 40 45

Temperature oC

FIG. 1.-Salamander and sub-rock temperatures (a) temperatures measured under 25 rocks on July 29, 1969, (b) all field temperature measurements combined, (c) sub-rock temperatures when salamanders emerged in laboratory experiments. Low and high light levels combined (see text).




were usually in contact with each other. We found one aggregation of five individuals, one of four, two of three, and four of two-a total of 23 of 56 individuals were in aggregations (41%). The animals collected on the afternoon of September 15 were found to have been dehydrated 4.9%-36.2% (average 14.4%) relative to their weights after 38 hours of rehydration.

CTM DETERMINATIONS

The responses of salamanders to in- creasing temperature in the CTM determinations were generally similar to those described by Hutchison (1961). Upon being placed in the beaker of water, the animals were usually quies- cent, resting either at the surface of the water or on the bottom of the beaker. As the temperature increased the animals became increasingly active. The first sign of loss of coordination was a rolling motion of the trunk as the animal swam. This was quickly followed by the development of a list to one side as the animal rested at the surface. Animals which rested on the bottom of the beaker did not show this list. The next stage of incapacitation was turning over. Unlike Hutchison's (1961) salamanders, our Ambystoma recovered spontaneously after a period of 1-30 seconds and resumed swimming mo- tions. These two manifestations of incapacitation were not observed in all animals. Occasionally an animal turned over before it began to list. The CTM was unmistakable: the animal began to tremble, the trembling increased in severity until the animal arched in rigor, its mouth opened, and a bubble of air was released. At this point the salamander usually sank to the bottom of the beaker. The temperature at which rigor appeared was called the CTM. About 25% of the animals

briefly resumed swimming after the appearance of the first spasms. Animals removed from the water bath within 30 seconds of the appearance of rigor recovered. Some were left in the water for 1-3 minutes to see if the rigor would disappear as has been described in liz- ards (Licht, Dawson, and Shoemaker 1966). Rigor did not disappear and these animals did not recover when they were removed from the bath.

Values obtained for the three stages of incapacitation are shown in fig. 2. Morning CTM values, for animals collected at body temperatures of 17-21 C were 38.6 + 0.09 C and 38.4 + 0.08 C (mean + SE) in two samples on suc- cessive days. The CTM of animals collected on the afternoon of August 3 had risen to 39.7 + 0.11 C. The increase is statistically significant (table 1). Sala- manders located in the late afternoon and confined by barriers during the night had reacclimated to low temperatures by the following morning. The CTM for these animals was 38.2 + 0.12 C. Salamanders collected on the afternoon of August 10, a rainy day, had body temperatures of 20.2-21.2 C. The CTM for these animals was the same as for morning animals (38.4 + 0.13 C). A sample collected on the afternoon of a cloudy day (August 9) with body temperatures about 23 C had a slightly elevated CTM (38.8 + 0.08 C).

DAILY CYCLES

Two sets of animals were acclimated to 20 C to see if a daily rhythm in CTM contributed to the change observed in the field. Animals collected for this experiment (4 days in the laboratory) had CTMs of 38.5 + 0.07 C and 38.6

-+ 0.19 C at 0600 and 1800 hours, re- spectively. Animals that had been in the laboratory, unfed, for 10 days prior




30 35 40

Listing First Morning Sample (13) Second Morning Sample (14) Sunny Afternoon Sample(17) Overnight Sample (13) I Rainy Afternoon Sample (3) Cloudy Afternoon Sample (10)

Turn Over First Morning Sample (12) Second Morning Sample (13) Sunny Afternoon Sample (23) Overnight Sample (16) Rainy Afternoon Sample (7) Cloudy Afternoon Sample (10)

CTM First Morning Sample 3 August (13) Second Morning Sample 4 August (16) Sunny Afternoon Sample 3 August (24) Overnight Sample 5-6 August (18) Rainy Afternoon Sample 10 August (10)

_rk Cloudy Afternoon Sample 9 August (10) I I I I I I I I I I I I

30 35 40

Temperature 0C

FIG. 2.-Temperatures of three stages of heat incapacitation in salamanders (see text for full de- scription). Number in parentheses shows the number of animals in each sample; listing and turning over was not observed in every animal. Horizontal line = range, open rectangle = + 1 so, closed rectangle = -- 2 SE, vertical line = mean.

to acclimation (a total of 14 days in laboratory when CTMs were determined) had values of 38.2 + 0.11 C and 38.4 + 0.12 C at the same times. The 1800-hour values were not signifi-

cantly higher than the 0600-hour values for either group (P > .05, one-sided t-test) and the values for the two groups at either time are not different (P > .05, two-sided t-test).

TABLE 1 STATISTICAL COMPARISON ("t"-TEST) OF CTM VALUES IN VARIOUS SAMPLES

Second Sunny Rainy Cloudy Morning Afternoon Overnight Afternoon Afternoon Sample Sample Sample Sample Sample

(38.4 C) (39.7 C) (38.2 C) (38.4 C) (38.8 C)

First morning sample (38.6 C) ...... >.05 <.005 <.05 >.05 >.05 2-sided 1-sided 2-sided 2-sided 1-sided

Second morning sample (38.4 C) ..... <.005 >.05 >.05 <.005 1-sided 2-sided 2-sided 1-sided

Sunny afternoon sample (39.7 C) .... <.005 <.01 <.01 2-sided 2-sided 2-sided

Overnight sample (38.2 C) .......... >.05 <.005 1-sided 1-sided

Rainy afternoon sample (38.4 C) .... >.05 ... 2-sided

NOTE.-Two-sided tests indicate whether there is a difference between the means of two samples; one-sided tests give the probability that the sample in the horizontal row has a lower mean than that in the vertical column. P < .05 is taken as a significant difference.




DEHYDRATION AND CTM

Many salamanders in the afternoon samples were unable to right themselves when they were collected, but regained the ability to right themselves after being in water for a few minutes. Sala- manders were dehydrated in the laboratory until they lost the ability to right themselves at 23.2 /c-44.6 % of original body weight (mean 36.2 0). These animals rehydrated slowly in tap water requiring 4-14 hours to reach equilibrium (fig. 3). They all survived but never returned to their original weight. There was no apparent correlation between the extent to which an animal was dehydrated and the equilibrium weight after rehydration.

Our effort to acclimate dehydrated salamanders from the field overnight at 20 C and test their CTMs in the morning was frustrated when all the animals died. Instead we dehydrated 11 animals acclimated for 3 days to 21 C until they barely lost righting response (15.9%- 37.40/c, mean= 28.7% of hydrated weight) and determined their CTMs.

The value for eight animals was 38.0

- 0.8 C. Three other animals died un-

ostentatiously at lower temperatures. There was no significant difference in the initial dehydration of the dead and surviving salamanders.

HEAT STRESS EMERGENCE

Rock heating rates ranged from 0.19 to 0.39 C per minute in the heat stress emergence experiments. There was no correlation between the rate of heating and the temperature at which salamanders emerged. Animals emerged at an average sub-rock temperature of 32.1

-+ 0.5 C under 36 ft-c of light and 32.2

-+ 0.5 C under 165 ft-c. The data from the two light levels have been pooled in figure lc. Six animals started to emerge and paused for 5-10 minutes in shade at the edge of a rock. We were able to place a thermistor beneath each of these animals and record its approx- imate body temperature at the moment it finally left the rock. These temperatures were 32.0, 34.9, 35.0, 36.0, 36.0, and 36.0 C. The maximum temperature

100

500

90-

60

1 2 3 4 5 6 7 8 9 10 14 15 16 17 18 19 20 21 68

Hours of Rehydration

FIG. 3.-Rehydration curves for salamanders. Selected records representing the extremes and inter- mediates in a sample of 11 animals.




of sand onto which an animal emerged was 38.0 C, 4 degrees hotter than the temperature under the rock.

DISCUSSION

The very rapid acclimation to high temperature shown by salamanders has heretofore had no obvious adaptive significance. Field studies have indicated that salamanders seldom experience temperatures approaching their CTM (Bogert 1952; Stebbins 1954; Rosen- thal 1957; Zweifel 1957; Brattstrom 1963; Anderson 1968). Seasonal changes in temperature sensitivity have been noted in Notophthalmus viridescens (Hutchison 1961), but these could not be correlated directly with environmental temperatures. The highest CTM values for a given acclimation temperature were found in early winter, and the lowest values in spring. Brattstrom (1963) suggested that acclimation oc- curring over a period of days could be of value to Eurycea bislineata when the temperature of the streams it inhabits changes.

Our data indicate that significant acclimation can occur in the field in the course of a single day in young Ambys- toma maculatum. The primary factor in this acclimation appears to be attaining high body temperatures (27- 32 C). No acclimation occurred on a rainy day when salamanders' temperatures remained near nighttime levels and very little change in CTM was observed on a cloudy day when body temperatures rose only to 23 C.

We were surprised not to find a daily rhythm of temperature sensitivity in our salamanders. Such rhythms have been demonstrated in fish (Hoar and Robertson 1959), turtles (Kosh and Hutchison 1968), and frogs (Mahoney and Hutchison 1969). Its absence may be a function of our experimental de-

sign; our aim was to reproduce as closely as possible the conditions of our field study. Thus we used a natural photoperiod (LD 14:10) and a temperature (20C) close to the nighttime temperature the animals experienced in the field and determined CTMs at nearly the same times as on field samples. Under these conditions, and pre- sumably under field conditions, there does not appear to be a significant daily rhythm of temperature tolerance. It is quite possible that under different acclimation temperatures and photoperiods in the laboratory a daily rhythm could be demonstrated.

The ability of salamanders to acclimate to increased temperatures under laboratory conditions has been ex- pressed in terms of the increase in acclimation temperature needed to raise the CTM 1 C. Hutchison (1961) transferred Notophthalmus from 4 to 20 C and found that an increase of 6.8 C in the acclimation temperature raised the CTM 1 C. When newts were transferred from 19.5 to 32.0 C (temperatures close to those experienced by the salamanders in our experiment), an increase of 4.3 C in acclimation temperature produced a 1 C rise in CTM. For A. opacum, acclimated at temperatures of 2.5 to 31 C, an increase of 9.4 C in acclimation temperature resulted in a 1 C rise in CTM. These figures represent full acclimation following at least one month's exposure to constant temperatures. The value we obtained under very different conditions is strikingly similar to these. On August 3, an average temperature increase of 8.1 C oc- curring over a 9-hr period produced a 1.1 C increase in the CTM (1 C increase in CTM per 7.4 C change in acclimation temperature). This remark- able acclimation may be facilitated by fluctuating temperatures. Working with




fish, Heath (1963) found that temperature fluctuations on a 24-hour cycle gave the highest acclimation temperature for any given range of fluctuation. Hutchison and Ferrance (1970) found that Rana pipiens transferred from 15 C to a constant 25 C required 72 hours to achieve full acclimation of CTM. Frogs placed in a cyclic temperature regime with a 25 C maximum reached the same maximum acclimation in 48 hours. In Cyprinodon macularius, Lowe and Heath (1969) found that a daily cycle of temperature change produced a higher CTM than constant exposure to the maximum temperature of the cycle.

Hutchison (1961) suggested that salamanders exposed to daily temperature cycles could achieve a step-wise acclimation because in his experiments re- acclimation to low temperatures was slower than acclimation to high temperatures. No such cumulative effect was noted in our study. The CTM of animals known to have been exposed to high temperatures for at least one day dropped overnight below the level seen in newly emerged salamanders.

We did not carry out marking experiments to determine how long individual salamanders remained under rocks at the pond edge. We felt that the trauma of marking followed by fre- quent checking would distort the results. A number of observations suggest that individual salamanders spend several days under the rocks. On numerous occasions we found a salamander under a particular rock and none under near- by rocks at observation periods several days apart. This may indicate only that that particular rock was the only one in the area suitable for salamanders, but it could also indicate that it was the same animal each time. Some rocks sheltered the same number of indi-

viduals at two or three observations, suggesting that they were the same animals. On the basis of these observations, we feel that at least some individuals spent several days under rocks at the pond edge. Thus, a large propor- tion of the population may be exposed to high temperatures on a daily basis for several days.

The aggregation of salamanders in the late afternoon was striking. We found no aggregations in the morning, but 41% of the animals we found in the afternoon were in aggregations of two or more individuals. Gehlbach, Kimmel, and Weems (1969) found that tiger salamanders (A. tigrinum) aggre- gated when damp spots under cover became localized. This agrees with our observations; we did not find aggregations under rocks where the entire sub- strate was moist.

The dessication stress the animals in our study were subjected to was suggested by the awkwardness of animals collected in the afternoon. Many were unable to right themselves when first collected, and most had difficulty in doing so. Animals that were unable to right themselves were nonetheless able to walk; loss of righting response does not signal an ecological endpoint in this case. Dehydration, per se, is probably not a major threat to salamanders in the situation we studied. The significance of dessication may lie in its effect on the CTM which was

significantly lowered in animals dehydrated 15.9%-37.4%. This observation contrasts with Hutchison's (1961) report that dehydrating Notophthalmus 14%-21.5% increased the CTM from 35.89 to 36.96 C.

The synergistic action of temperature and dehydration places a premium on selection of relatively cool and moist microhabitats by the salamanders. In




the field, no salamander was found under a rock warmer than 32 C. Of 14 rocks without salamanders measured on July 29, only four were cooler than 32 C. Perhaps coincidentally, this figure correlates remarkably well with the results of heat stress experiments in the laboratory. In these experiments, salamanders moved out from under rocks at an average sub-rock temperature of 32.1 C. The range of movement of a salamander on the surface in daytime must be limited. Not only is it exposed to predators, but soil surface temperatures at the pond reached 38- 40 C. The laboratory experiments showed that salamanders will move onto soil warmer than the rock they are leaving, but this escape is probably a last-ditch measure rather than a normal feature of a salamander's activity. The effectiveness of the salamanders' thermoregulation is suggested by the fact that we found no more than a dozen dead salamanders under rocks during the study and some of these had clearly been crushed.

The temperature at which salamanders emerged from under rocks in the laboratory (32.1 C) is within the range where morning animals show the first signs of loss of coordination (listing). In newly emerged salamanders, one day's acclimation increased the temperature of the onset of listing 3.2 C while the CTM was raised only 1.1 C. Furthermore, there may be some accumulation of resistance to listing from day to day while there is no such step-wise acclimation in CTM. For example, in figure 2, compare the first morning sample (all newly emerged animals) to the second morning sample (some animals exposed one day to high temperatures) and the overnight sample (all animals exposed to at least 1 day of high temperature); listing oc-

curs at a higher temperature in the samples of animals which have had some previous acclimation. Studies of critical temperatures have concentrated on the CTM as an ecologically significant and relatively easily determined measurement. It may be, however, that selection has operated to prolong the period when escape is possible rather than resistance to dying, per se. This would be shown by an increase in the temperature for onset of listing. In- creased CTMs would be a reflection of this phenomenon, but not its ecologically most significant feature.

SUMMARY

Newly metamorphosed spotted salamanders (Ambystoma maculatum [Shaw]) emerged from a pond and spent several days under rocks in full sun on the shore. Temperatures under some rocks reached midafternoon highs of 42.5 C, but salamanders were found only under rocks 32 C or cooler. In the laboratory, salamanders emerged from under heated rocks when the rock temperature reached 32.1 C.

Environmental factors producing this acclimation were studied in field experiments. The CTM of newly emerged animals in the morning was 38.6 C. On a sunny afternoon the CTM had increased to 39.7 C. Attaining high body temperatures appeared to be the major cause of this increase. On a rainy day no increase in CTM was observed. No

daily rhythm of CTM was found in salamanders kept at a constant temperature in the laboratory. In the field the acclimation gained during a day was lost overnight.

Salamanders collected in the afternoon were dehydrated 4.9%-36.2% (mean -14.4%) of body weight. Some had difficulty righting them-




selves. Dehydration lowered the CTM in laboratory experiments.

Three stages of heat incapacitation were noted: listing, turning over, and rigor ( CTM). Exposure to high temperature raised the onset of listing three times as much as it raised the CTM. Furthermore, there was some

day-to-day accumulation of resistance to listing while the acclimation of CTM was lost overnight. Prolongation of the period when escape is possible (re- flected in the temperature for onset of listing) may be the selectively impor- tant feature of the salamanders' acclimation.

LITERATURE CITED

ANDERSON, J. D. 1968. Thermal histories of two populations of Ambystoma macrodactylum. Herpetologica 24:29-35.

BOGERT, C. M. 1952. Relative abundance, habi- tats, and normal thermal levels of some Vir- ginian salamanders. Ecology 33:16-30.

BRATTSTROM, B. H. 1963. A preliminary review of the thermal requirements of amphibians. Ecology 44:238-255.

S1968. Thermal acclimation in anuran amphibians as a function of latitude and altitude. Comp. Biochem. Physiol. 24:93-111.

BROOKS, G. R., JR., and J. F. SASSMAN. 1965. Critical thermal maxima of larval and adult Eurycea bislineata. Copeia 1965:251.

COWLES, R. B., and C. M. BOGERT. 1944. A preliminary study of the thermal requirements of desert reptiles. Amer. Mus. Natur. Hist., Bull. 83:261-296.

GEHLBACH, F. R., J. R. KIMMEL, and W. A. WEEMS. 1969. Aggregations and body rela- tions in tiger salamanders (Ambystoma tigrinum) from the Grand Canyon rims, Ari- zona. Physiol. Zool. 42:173-182.

HEATH, W. G. 1963. Thermoperiodism in sea-run cutthroat trout (Salmo clarki clarki). Science 142:486-488.

HOAR, W. S., and G. B. ROBERTSON. 1959. Tem- perature resistance of goldfish maintained under controlled photoperiods. Can. J. Zool. 37:419-428.

HUTCHISON, V. H. 1961. Critical thermal maxima in salamanders. Physiol. Zoil. 34:92-105.

HUTCHISON, V. H., and M. R. FERRANCE. 1970. Thermal tolerance of Rana pipiens acclimated to daily temperature cycles. Herpetologica 26:1-8.

KOSH, R. J., and V. H. HUTCHISON. 1968. Daily rhythmicity of temperature tolerance in east- ern painted turtles, Chrysemys picta. Copeia 1968:244-246.

LICHT, P., W. R. DAWSON, and V. H. SHOEMAKER. 1966. Heat resistance in some Australian liz- ards. Copeia 1966:162-169.

LOWE, C. H., and W. G. HEATH. 1969. Behavioral and physiological responses to temperature in the desert pupfish Cyprinodon macularius. Physiol. ZoSl. 42:53-59.

MAHONEY, J. J., and V. H. HUTCHISON. 1969. Photoperiod acclimation and 24-hour varia- tions in the critical thermal maxima of a tropical and a temperate frog. Oecologia 2: 143-161.

ROSENTHAL, G. M. 1957. The role of moisture and temperature in the local distribution of the plethodontid salamander Aneides lugubris. Univ. California Pub. Zool. 54:371-420.

STEBBINS, R. C. 1954. Natural history of the salamanders of the plethodontid genus Ensa- tina. Univ. California Pub. Zool. 54:47-124.

ZWEIFEL, R. G. 1957. Studies on the critical thermal maxima of salamanders. Ecology 38: 64-69.



Documents

Natural Daily Temperature Stress, Dehydration, and Acclimation in Juvenile Ambystoma Maculatum (Shaw) (Amphibia: Caudata)