Temperature selection by the bullfrog, Rana catesbeiana

Temperature selection by the bullfrog, Rana catesbeiana

Camp. Biochm. Physiol., 1971, Vol. 4OA, pp. 213 to 227. Pergamon Press. Printed in Great Britain TEMPERATURE SELECTION BY THE BULLFROG, RANA CATE...

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Camp. Biochm.

Physiol., 1971, Vol. 4OA, pp. 213 to 227. Pergamon Press. Printed in Great Britain






Department of Zoology, University of California, Los Angeles, California 90024 (Received

24 February


Abstract-l. Behavioral responses of juvenile bullfrogs, both in an aquatic temperature gradient and in an apparatus which offers a choice of discrete water temperatures, demonstrate a capability for maintaining specific levels of nearly constant body temperature by selection of an appropriate thermal environment. 2. Previous thermal history alters the level of preferred body temperature, whereas feeding and photoperiod do not. Hypothalamic lesions interfere with normal thermoregulatory behavior. 3. Statistical parameters of preferred body temperatures maintained by bullfrogs under controlled laboratory conditions are generally similar to those of body temperatures recorded from bullfrogs under natural conditions. Small discrepancies between the means of preferred body temperatures maintained in the laboratory and body temperatures recorded under natural conditions can be explained by consideration of limitations of experimental apparatus and/or ecological factors. 4. It is concluded that behavioral responses solely to temperature can account for body temperatures of bullfrogs observed in nature. INTRODUCTION

THE ROLE of temperature

as a determinant of amphibian behavior is little understood, particularly within an ecological framework. Temperature selection under controlled conditions has been demonstrated for a number of amphibians (Workman & Fisher, 1941; Rosenthal, 1957; Herreid & Kinney, 1967; Licht & Brown, 1967; Lucas & Reynolds, 1967 ; de Vlaming & Bury, 1970), but no data are available for metamorphosed anurans, even though they offer attractive possibilities for such studies. Many anurans bask, and there is a substantial amount of knowledge concerning their body temperatures under natural conditions. In a previous communication (Lillywhite, 1970) I have reported that bullfrogs (Rana catesbeiana) regulate body temperature by behavior. The present investigation extends earlier studies and attempts to ascertain the level and precision of temperature selection by bullfrogs in artificial thermal gradients. Studies of other vertebrates show the need for laboratory investigations to evaluate inherent preferences and to confirm that behavioral responses to temperature can indeed explain body temperatures which are maintained in nature. The influence of * Present address: Department of Zoology, University of California, Berkeley, California, 94720. As of 1 October 1971: Division of Biological Sciences, The University of Kansas, Lawrence, Kansas, 66044. 213



thermal history, photoperiod, digestive ature selection are also investigated. MATERIALS

state, and hypothalamic


lesions on temper-


Aquatic gradient A trough 244 cm long, 31 cm wide, and 61 cm deep was constructed from galvanized metal. Copper tubing soldered to the underside and carrying a refrigerant cooled one end of the trough, while infra-red heat lamps directed against the underside, which was blackened with non-reflecting paint, heated the other end. The bottom of the trough was covered with 2 cm of water in which temperatures, measured at a depth of 1 cm, normally ranged from 35-4O”C at one end to 4-10°C at the opposite end. The trough was kept in a room having a controlled, constant temperature (22°C) and a controlled photoperiod supplied by diffuse fluorescent lighting (25 ft-c). Dim light (




thermocouples) brain and deep body temperatures from a frog in a separate 8-hr test. Maximum brain-body temperature differences of 1*7”C were recorded, and for 70 per cent of the duration of recording, brain-body temperature differences were 1 ‘C or less. Specific temperature apparatus In a second series of tests, frogs similar in size to those tested in gradients were independently offered a choice of five discrete temperatures (17,21,25,29, and 34*C) when placed into a circular container herein referred to as a “specific temperature apparatus”. The container was constructed by placing thin galvanized metal so as to form a circuIar enclosure 91 cm in diameter within a wooden box 91 cm by 91 cm square and 46 cm high. Pea gravel covered the floor, and five circular metal pans, 22 cm in diameter and 3) cm deep, were symmetrically arranged in a circle concentric with the container. Each pan was painted white with a green number in its center and filled with water, the surface of which was level with the pea gravel. Water temperatures were controlled either by thermostatically controlled heating devices taped to the underside of a pan, or by copper coils carrying a refrigerant and secured similarly, Vertical stratification of temperature was less than 1 “C in the pan with the coolest water and was virtually non-existent in all others. The entire apparatus was kept within a room having a constant temperature of 24°C and was visible from an adjacent room through one-way glass. View of the floor of the apparatus was rendered possible by a circular mirror positioned over it, and frogs could be observed without disturbance.




I -

t I 25



f IM)




I 175



I 250

, cm

FIG. 1. Temperature

profiIe of an aquatic temperature gradient. Circles include ranges of vertical temperatures at beginning of a 24-hr period; depth of water, 2.4 cm. Triangles include ranges of vertical temperatures at end of 24-hr period; depth of water, 1.5 cm. Four groups of frogs (N = 10-14 in each) were tested in this apparatus. An initial group was composed of individuals which had previously been kept within a greenhouse in an enclosure offering both aquatic and terrestrial substrata. These frogs were fed occasionally on Tene6rio larvae. A second group from the greenhouse was identical except that the frogs were fed abdomens of the moth ~a~duca sexta just prior to testing. These experiments were performed during summer, and air temperatures in the greenhouse ranged from approximately 15-35°C. The third group was acclimated to a temperatum of 29°C (LD 9 : 15) for a period of at least two weeks in a manner similar to that described above for frogs tested in the aquatic gradient. The fourth group was acclimated to a temperature




of 4°C for at least 2 weeks and were kept between moist sand and burlap saturated with water within an aquarium. These were kept in a cold room of constant darkness except for irregular daily periods of brief duration when the room was entered for maintenance. RESULTS

Behavior of frogs in gradient

Frogs were able either to sit in contact with the metal trough substrate or to lie partially afloat in the water. The frogs moved along the gradient either by hopping or by a combination of swimming and crawling. When first introduced into the aquatic gradient, most of the unoperated frogs from all acclimation groups explored the gradient, moving back and forth in the trough, but usually avoiding the extreme ends. Body temperatures greater than approximately 35°C and less than approximately 10°C were normally avoided.







FIG. 2.

Individual records of body temperatures during the initial 20 hr of recording from four juvenile bullfrogs in aquatic temperature gradients. Records were selected to illustrate characteristic patterns of behavioral responses (see text) and extremes in levels of body temperature. Bars on abscissa represent periods of darkness. (A) Individual acclimated to 10°C; LD 8 : 16. (B) Individual acclimated to 25°C; LD 12 : 12. (C) Individual acclimated to 25°C; LD 12 : 12; fed prior to experiment. (D) Individual acclimated identically to (C).

This behaviour resulted in a sinusoidal temperature record which usually did not persist beyond the first 30 min (see Fig. ZC). In a few cases, frogs would begin to move to one end of the gradient and then reverse their movement, selecting a position without having explored the opposite end (Fig. 2A). Occasionally, frogs selected a position upon first encountering it (Fig. 2B). In most cases frogs were inactive after selecting a temperature, and the records of body temperature showed relatively few fluctuations.





Some frogs maintained a virtually constant body temperature (Figs. ZA, C), while others showed occasional sinusoidal fluctuations of body temperature similar to those resulting from initial exploratory movements (Fig. 2B). A third pattern was for frogs to remain for a long period at one temperature and then change location and remain for a long period at a new temperature (Fig. 2D). There were no obvious differences between the temperatures selected during light and dark periods, regardless of previous acclimation. That frogs were responding to temperature and not to location was tested by comparing observations of frogs placed in the trough under conditions of uniform water temperature with those of frogs exposed to a temperature gradient. The former tended to distribute themselves randomly along the length of the trough, with some tendency to aggregate in corners. Frogs exposed to a gradient of temperature oriented to specific locations near the center of the trough. Frogs with hypothalamic lesions appeared alert and would jump when prodded, but were quiescent when left undisturbed. It was found that these frogs generally did not move from their original position following introduction into the gradient and consequently did not experience the range of temperatures available to them. Therefore, they were introduced into the gradient at various locations along its length. Without exception, lesioned frogs remained at or very near the location of introIn several cases lesioned frogs at warmer duction, regardless of its temperature. temperatures ( > 35°C) died, presumably from overheating. Body temperatures of all frogs tested in aquatic gradients were taken from the temperature records at 30 min intervals for the duration of measurements, except for the first hour. These temperatures were then grouped according to acclimation history, and the results are shown as frequency distributions in Fig. 3. Unoperated frogs from all acclimation groups selected temperatures within considerably narrower ranges than lesioned frogs. Frogs with hypothalamic lesions remained at all temperatures along the entire length of the gradient ; body temperatures above approximately 35°C were presumably recorded after an animal had died. Body temperatures were fairly uniform among all groups of unoperated frogs, except for those acclimated to 10°C (Table 1; Fig. 3). Body temperatures recorded from these frogs were significantly lower (t = 5.07 ; P < O*OOl)*than those recorded from frogs acclimated to 25°C and identical conditions of photoperiod. Differences in body temperatures recorded from fed and unfed frogs acclimated to identical conditions of temperature and photoperiod were not statistically significant. The mean body temperature of frogs in both groups acclimated to a cycle of temperature, where body temperatures were permitted to drop during the dark period, were lower than the mean body temperature of frogs acclimated to a constant 25°C. However, differences were only statistically significant (t= 2.66; P-c O-02) between groups which were fed. * In performing t-tests, the means of all body temperatures recorded for each individual were compared: thus, each frog had but one datum, preserving the independence of measurements.

25 25 Cycle :

12 11 10


LD 12: 12* LD16:8 LD 12: 12

LD 12 : 12 LD 12: 8:1612






523 491 450

528 398 536

No. of observations




28.1 27.1 25.3

22.3 27.6 26.7




28 26 26

21 26 28





17+--33.6 14~5-34.5 18-3-32.4

13.S-30.4 16+35.0 17.5-35.2




2.75 2.68 2.88

3.53 3.43 340

Standard deviation

Body temperature (“C)


* Denotes animals which were fed immediately prior to experimentation. 7 Data from Lillywhite (1970).

26 16 L D Cycle : LD 12: 12* 26 L 16 D From field?

10 25 2.5

12 9 12


Temp. (“C)









25.3-30.9 24+29.8 224-28.2

18,8-25.8 24-2-3 1.0 23.3-30~1

Central 68 T/o




p !? F




Behavior of frogs in specijic temperatureapparatus The behavior of frogs in this apparatus paralleled that of frogs in aquatic gradients. Most frogs initially explored the area available to them and entered most or all pans of water. After varying lengths of time, however, a single pan was selected in which the animal would sit either constantly or for long periods which


60 40 20

l~j::-:,,~,,, ~~ IO




Body temperature





, ‘c

FIG. 3. Frequency distributions of body temperatures taken from records of of juvenile bullfrogs in aquatic temperature gradients every 30 min for duration _. 24-hr recording period, except for the first hour, and grouped according to the following acclimation histories: (A) lO”C, LD 8 : 16; (B) 25”C, LD 8 : 16; (C) 25”C, LD 12 : 12; (D) 25”C, LD 12 : 12, fed; (E) 25”C, LD 16 : 8; (F) cycie: 26% L, 16°C D, LD 12: 12; (G) cycle: 26°C L, 16°C D, LD 12: 12, fed; (H) animals with h~othal~ic lesions. Vertical fines are for the purpose of assisting reading of the abscissa.




were occasionally interrupted by brief visits to rock. Some individuals preferred rock to water. Frogs sat in water with the greater portion of the body submerged in a position similar to that used in aquatic gradients. Temperature selection was quantified by recording the water temperature of the pan in which the frog was located at hourly intervals for 5 consecutive hours beginning approximately 19 hr after the animal was introduced into the apparatus. If a frog was found to be positioned in the same water pan for at least three consecutive scores, that water temperature was recorded as the selected temperature. In cases where this condition was not satisfied, either the frog was observed again for five consecutive hours on the following day, or no datum was obtained for that particular test. In over 80 per cent of the cases where scores were obtained, frogs were scored in the same pan for all 5 consecutive hours. To be certain that animals were selecting pans according to temperature rather than location, the behavior of both individuals and groups of frogs was recorded by time-lapse photography. Under conditions of uniform water temperatures, frogs occupied all water pans. When up to five frogs were present in the apparatus at the same time, they were usually dispersed and occupied all pans when water temperatures in the pans were uniform. The establishment of water temperature differences among pans by activating the heating and cooling elements caused these same animals to aggregate in three, two and even one pan.




29 34 Selected water temperature, OC

FIG. 4. Frequency distributions of water temperatures selected (see text) by juvenile bullfrogs in a specific temperature apparatus, and grouped according to the following acclimation histories: (A) 4”C, near-constant dark; (B) 29”C, LD 9 : 15 ; (C) animals from greenhouse, occasionally fed throughout acclimation period; (D) animals from greenhouse, fed just prior to experiment.

Frogs which were acclimated to 4°C selected temperatures which did not appear different from those selected by frogs acclimated to 10°C and tested in the gradient (Figs. 3,4A). While modal temperatures selected by frogs acclimated to 29°C were lower than those of frogs acclimated to 4°C (Figs. 4A, B), the respective distributions of selected temperatures were not significantly different (MannWhitney U-test). Frogs taken from the more natural situation of the greenhouse selected temperatures similar to those acclimated to 25°C and tested in the aquatic





gradient (Figs. 3,4C, D). Differences in selected temperatures between fed and unfed frogs from the greenhouse were not statistically significant (Mann-Whitney U-test). DISCUSSION

The behavior of bullfrogs in an aquatic temperature gradient clearly demonstrates a capability for maintaining near-constant body temperature by selection Temperature records and observations of an appropriate thermal environment. of animals indicate that maintenance of a given level of body temperature results principally from frogs orienting to a particular location along the length of the gradient and remaining there (Fig. 2), rather than from a succession of avoidance reactions to contrasting temperatures encountered in the course of random movements. The latter type of behaviour would appear to play a major role in determining body temperatures reported for various other ectothermic vertebrates, including an amphibian (newt), in experimental gradients (e.g. see Doudoroff, 1938; Licht & Brown, 1967). A mean of temperatures resulting from the type of behaviour described here for bullfrogs probably provides a more reliable indication of “preference” than does a mean of temperatures which result from a succession of random movements over a wide range of temperature; the latter is more likely to depend on the physical characteristics of the gradient. The utility of the term “preferred temperature” has recently been questioned (see Regal, 1966, 1968; Templeton, 1970). I have chosen to follow Licht et al. (1966) in retaining the term for reference only to temperature maintained in laboratory gradients. Use of this term to describe a temperature about which an ectotherm regulates its core temperature under natural conditions may have little significance, since ectotherms may “prefer” different temperatures at different times, depending upon their state of physiology and motivation (Hadfield, 1966; Regal, 1966, 1968). The level of temperature selection

While the modes of distributions of body temperatures of bullfrogs from field populations are identical to those of some 25”C-acclimated laboratory groups, the means of body temperatures maintained by the latter are from 1.5 to 4*3”C lower than that of the former (see Table 1). Similar discrepancies between body temperatures recorded in the field and those maintained in laboratory gradients have been reported elsewhere, both for fishes (e.g. Doudoroff, 1938) and for reptiles (Dewitt, 1967a, b; Licht et al., 1966). While several suggestions as to why these differences should occur have been offered by the various authors, the actual relationships remain obscure. However, discrepancies between body temperatures maintained in the laboratory and body temperatures maintained under natural conditions should not be particularly surprising in view of the numerous factors which might be involved in causing the differences. Both Dewitt (1967a) and Licht et al. (1966) have advanced the idea that differences in field and laboratory temperatures measured in lizards may result from limitations imposed by the thermal environment to which these animals are




exposed in nature: thermoregulation about a preferred temperature in the field, it is argued, is either impossible or abandoned at various times. In view of a consideration of the ecology of Rana catesbeiana, a similar explanation seems tenable here, although use of the term “preferred temperature” presents a semantic problem (see above). In a previous communication (Lillywhite, 1970) I have reported that heating of littoral waters of ponds may influence bullfrogs to emerge onto banks where shade is absent and the frogs are exposed to insolation. During these “enforced” exposures to high levels of radiation and ambient temperatures, regulation of body temperature may be restricted to postural adjustments (Lillywhite, 1970) or possibly physiological mechanisms (Lillywhite, 1971); body temperatures may be restricted to a narrow and relatively high range. In certain situations, “basking” frogs utilized shade sources when offered to them, suggesting that they preferred a lower body temperature than that which was currently being maintained. Furthermore, unlike many heliothermic reptiles, bullfrogs are active and thermoregulate at night as well as during the day (Willis &al., 1956; Lillywhite, 1970). Results of the present study which demonstrate a lack of difference in thermoregulary behavior between light and dark periods complement these field observations. Obviously, maintenance of body temperatures identical to the high levels experienced during daylight are precluded at night under natural conditions prevailing in the field. Similar die1 limitations are placed upon thermoregulatory activity in other amphibians (Licht & Brown, 1967) and in reptiles (Bustard, 1967). Discussion of the inadequacies of laboratory temperature gradients as an experimental device for correlating laboratory behavior with environmental thermotaxis has been presented elsewhere (see Doudoroff, 1938; Norris, 1963; Regal, 1968) and will not be discussed extensively here. However, it should be emphasized that artificial gradients may impose limitations on thermoregulatory behavior, and explanations of discrepancies between body temperatures maintained in the field and in the laboratory based solely on ecological considerations need not be the simplest ones. In the present study it can be argued that thermal stratification may cause a displacement of body temperatures from preferred levels due to the effects of multiple sensory cues of discordant intensity acting simultaneously upon the animal. For example, a frog might maintain a body temperature in the aquatic gradient which is lower than the preferred temperature because of avoidance reactions solely to the higher temperatures which prevail at the water surface. Photoperiod by itself does not alter temperature selection, as judged from results of tests involving frogs exposed to three regimes of photoperiod at a constant acclimation temperature of 25°C (Table 1, Fig. 3). The possibility of an interaction between photoperiod and temperature cannot be discounted since acclimation to various combinations of photoperiod and temperature was not tested. Photoperiod has been reported to influence both thermal tolerance and gas exchange in other amphibians (IIutchison, 1961; Whitford & Hutchison, 1965; Vinegar & Hutchison, 1965 ; Mahoney & Hutchison, 1969). Previous thermal history clearly influences the level of temperature selection in bullfrogs. Acclimation to moderately high (25°C) and moderately low (10°C)





temperatures results in a change in the level of preferred temperature of about 4.5%; this shift is in the direction of acclimation (Fig. 3). Acclimation to 4°C does not appear to cause further reduction in the preferred temperature below that of animals acclimated to 10°C (Figs. 3 and 4). Thermoregulatory behavior of both fishes (Norris, 1963; Fry, 1964) and larval Ranapipim (Lucas & Reynolds, 1967) have been shown to be modified by thermal acclimation, whereas preferred temperatures of lizards (Licht, 1968; Mueller, 1970) and the newt Turicha riduris (Licht & Brown, 1967) were shown to be independent of previous thermal acclimation. Since these data preclude any generalization concerning the plasticity of thermal preferences in response to temperature acclimation in poikilotherms, the adaptive significance (if any) of this phenomenon remains obscure. Acclimation to a temperature of 29°C results in selected temperatures which are lower than expected on the basis of results of the other acclimation studies (Fig. 4B). This response is strikingly similar to that reported for lizards: acclimation to temperatures above their preferred levels results in a decrease in the mean preferred temperature (Wilhoft & Anderson, 1960; Licht, 1968; Mueller, 1970). The acclimation temperature 29°C is higher than preferred temperatures obtained in the present study, but lower than the mean of body temperatures obtained under natural conditions (see Table 1). Licht (1968) has suggested that “reverse” acclimation in preferred temperature may represent a pathological condition resulting from prolonged exposure to high temperatures. It should be emphasized that under field conditions bullfrogs are not continuously exposed to the high temperatures at which they regulate diurnally, but, rather, experience die1 oscillations of temperature. Acclimation of bullfrogs to a cyclic temperature regime results in preferred temperatures which are intermediate to those of frogs acclimated to high and low temperatures (Table 1). This does not result from differences in temperature selection between light and dark periods, and suggests that acclimation to a temperature cycle is a “compromise” between the high and low temperature extremes. This response is different from that obtained for critical thermal maxima (CTM’s) in amphibians, where exposure to cycled temperatures results in a CTM in response to the peak temperature (Hutchison & Ferrance, 1970; Seibel, 1970). These results suggest that thermal preference and heat resistance are not directly coupled with respect to their capacity for modification (also see Licht, 1968). Acclimation to a cycle of temperature more closely approximates the temperature regime of frogs under natural conditions than does constant temperature acclimation. The observation that the former does not result in a mean preferred temperature which is nearer to the mean of body temperatures maintained in the field suggests that differences in mean body temperatures between field and laboratory-maintained animals are due to factors other than temperature acclimatization. Although feeding of frogs resulted in apparent increases in preferred temperatures, these did not differ statistically from preferred temperatures of unfed frogs which were acclimated similarly. While orientation to higher temperatures following



feeding has been demonstrated in reptiles (Regal, 1966) and has been conjectured to occur in toads (Hadfield, 1966), this apparently has no significance in bullfrogs. The precision of temperature selection The precision with which body temperature is maintained within a preferred range can be conveniently expressed by the range within which 68 per cent of all observations occur (Dewitt, 1967b). This range includes plus and minus one standard deviation on either side of the mean of a normal distribution. Values have been calculated for all conditions of acclimation for those animals tested in gradients and are included in Table 1. Distributions of temperature were assumed to be normal. The central 68 per cent of body temperatures recorded from bullfrogs in laboratory gradients define ranges of which the absolute values are similar to that for body temperatures recorded from juvenile bullfrogs under natural conditions and show little variation among the various acclimation groups. Thus, while temperature acclimation alters the level of temperature preference, the precision with which body temperatures are maintained at either level differs little. Statements concerning the precision of temperature preference would be incomplete without a consideration of individual performance. While pooled records of body temperature from a large number of animals may include a considerable range of values, the variability present in most individual records is considerably less (see Fig. 2). A significant portion of the variance of pooled records can be attributed to the fact that some animals tend to remain at temperatures near the lower limit of the range while others tend to remain at the upper limit of the range. In either case, body temperatures of individuals may be precisely regulated by behavior, regardless of their level. Similar individual differences in the level of regulated body temperature have been reported to occur in lizards (Dewitt, 1967b). The precision with which an individual may behaviorally regulate its body temperature is substantiated by the observation that while water temperatures at loci in the gradient changed slowly over time, these changes were almost never reflected in records of body temperatures from normal, unoperated individuals because they shifted location slightly as the temperature changed. On the other hand, records from lesioned and dead animals revealed a definite drift in body temperature over time. It may be noted that some drift occurs throughout most of the temperature record of the frog depicted in Fig. 2A, while frogs depicted in Figs. 2C and 2D apparently “correct” for this. This information suggests that bullfrogs may show sensitivity to small changes of temperature, and that constancy of body temperature may have significance, either apart from, or in relation to, an absolute thermal level. These suggestions are of further interest since amphibians apparently orient to gradients of water temperature under natural conditions (Prosser, 1911; Brattstrom, 1962; Licht & Brown, 1967; Lillywhite, 1970; Whitford & Massey, 1970). The small variation in body temperatures recorded from individuals in laboratory gradients is paralleled by a stability of body temperature telemetered from adult bullfrogs under natural conditions (Lillywhite, 1970).



Because a basic similarity in statistical parameters is evident between preferred body temperatures of bullfrogs maintained under controlled conditions in the laboratory and body temperatures recorded from bullfrogs under natural conditions, it is concluded that behavioral responses solely to temperature can account for body temperatures of bullfrogs observed in nature. This substantiates earlier interpretation (Lillywhite, 1970) of the significance of active thermoregulation in determining body temperatures of bullfrogs under natural conditions. Central control of body temperature While ~ermore~lato~ behavior has been described in various amphibia, little is known concerning its neurological basis. The behavior of bullfrogs with hypothalamic lesions when placed at various positions in a temperature gradient demonstrate that a much wider range of temperatures are acceptable to these animals than to animals with the hypothalamus intact, and that relatively extreme temperatures are probably required to elicit avoidance reactions in lesioned frogs. These data suggest that central neurons are involved in controlling~e~ore~latory motor responses in bullfrogs. A surprising result was that lesioned frogs did not move from positions where temperatures rose to lethal levels. The possibility cannot be ruled out that adverse effects of high temperature on lesioned frogs resulted in their death before temperatures reached levels which might elicit avoidance behavior. Lucas & Reynolds (1967) reported that h~ophysectomy of larval R~~~~~n~ resulted in random positioning in an aquatic temperature gradient, whereas normal individuals selected definite modal temperatures. Their results also showed that treatment of larvae with propylthiouracil or thyroxine resulted in selection which was lower and less precise than in untreated larvae, but still more precise than in hypophysectomized larvae. These authors state (op. cit. p. 169) that their “ . . , method of hypophysectomy often entails damage to nearby brain regions”, and it is possible that damage was incurred to the hypothalamus or hypothalamic efferents. Hypophysectomy of metamorphosed frogs in the present study did not interfere with thermoregulatory behavior. Participation of the forebrain in thermoregulatory behavior has been demonstrated in fishes (Hammel et al., 1969), reptiles (Hammel et al., 1967) and in mammals (see Corbit, 1969). Acknowledgements-I thank Dr. George A. Bartholomew for his guidance throughout the research and writing of this study. I also thank Drs. James H. Brown, Robert C. Lasiewski, Rodolfo Ruibal, Paul Licht, and Werner Terjung for critical reading of the manuscript. This work was supported in part by USPHS 5-FOl-GM-41, 781-02 to the author and NSF GB-18744 to George A. Bartholomew.

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