The critical thermal maximum of the iguanid lizard Urosaurus ......Bogert (194% considerable attention has been given to the subject of thermoregulation in reptiles and in other coldblooded

The critical thermal maximum of theiguanid lizard Urosaurus ornatus

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Authors Tremor, John William, 1932-

Publisher The University of Arizona.

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THE. CRITICAL THERMAL MAXIMUM OE THE IGUAN3D

LIZARD UROSAURUS ORNATUS

by .

John W® T rem or

A Thesis Submitted-, to the Faculty of the

DEFARTMEMT OF ZOOLOGY

In P a rtia l Fulfillm ent of the Requirements For .the Degree Of

; DOCTOR OF PHILOSOPHY

In the Graduate C ollege'

THE U W ERSITY OF ARIZONA

1962.

THE UNIVERSITY OF ARIZONA

GRADUATE COLLEGE

I hereby recommend that this dissertation prepared under mydirection by _____ John W» Tremor_______________________________entitled THE CRITICAL THERMAL MAXIMUM OF THE IGUANID LIZARD

UROSAURUS ORNATUS________________________________

be accepted as fulfilling the dissertation requirement of the degree of Doctor of Philosophy_____________________________

3c, /9& 2-Dissertation Director Date

After inspection of the dissertation, the following members of the Final Examination Committee concur in its approval and recommend its acceptance:*

GrK-'uli. 3 i m

l-Q .nn.G u.*

*This approval and acceptance is contingent on the candidate's adequate performance and defense of this dissertation at the final oral examination. The inclusion of this sheet bound into the library copy of the dissertation is evidence of satisfactory performance at the final examination.

STATEMENT BY AUTHOR

This thesis has been submitted in partia l fulfillment of r e ­quirements for an advanced degree at The University of Arizona and is deposited in The University Library to be made available to bor­rowers under ru les of the Library.

Brief quotations from this d issertation are allowable without special perm ission, provided that accurate acknowledgement of source is made. Requests for perm ission for extended quotation from or reproduction of this m anuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in their judgement the proposed use of the m aterial is in the in terests of scholarship. In all other instances, however, perm ission must be obtained from the autnor.

SIGNED:

ABBtMAQT

The c ritica l therm al maximum of the tre e lizard U rosaum s

ornatns linearis (Baird and Girard) wag determ ined a f te r seasonal ■.

acclim atioa and therm al acclim ation in the laboratory. Laboratory

aoolim atioa eraslstocl of exposure fo r seven days # various constant

and ctiHraaily^Gyoleci t e n p o r a t a r e s * . ■

. f h ree responses to h e # s tr e s s w ere used to m easure the effeet

Of therm al acclimation,' /'These# in./the o rd e r o f th e ir occurrence^ a re •

(1) gape# (2) lo ss of rightiiig: response '(the c r itic a l the rm al maximum)#

and (3) death. Significant, correlations- were found between all th ree

th e rm al responses,, •

- - , Acclimation at li^C# 20*C# and 30*0 resu lts in increasing

c r itic a l therm al maxima (45,31^0# 46,. 39^0#. and 46*98*0 respectively).

Acclimation a t 36*0# n ear- eccritic temperature# seem s to be s tre ssfu l

since the OTM .falls to 46,86*0, The CTM values for diurnally-cycled .

anim als . (8*“'2T*0#. 16*-36*0# and, 29®«4©#C) a re significantly different

from corresponding"' values for consfant-tem perature acclim ation,

' In. th is iguanid# the hom w static function of behavioral therm o^

regulatiQn appears re la ted to c r itic a l th e rm al responses# inasmuch as

,th e re is a behavioral control of physiological compensation to tem perature

' i s . ' . . '

which, m asks the effect of seasonal tem perature change*

The effects' of ra te s of heating? and of repeated exposure to

high tem perature? on the c ritica l therm al responses w ere investigated

and a re briefly reported*

ACKNOWLEDGEMENT

The author is indebted to Dr. Charles H. Lowe, J r . whose

encouragement and advice concerning the problem he originally

suggested made possible this paper. The suggestions and help of

other professors and graduate students are also deeply appreciated.

Of particular benefit was the aid of Gerald O. Gates, Wallace G. Heath,

P ete r J. Lardner and Jam es R. Walker. The investigation was

conducted in the University of Arizona Arid Lands Program , supported

in part by a grant from the Rockefeller Foundation.

TABLE OF CONTENTS

U C q >e- vb .xct » , » a -P' i» - » 49 »• # j o » o o . & ^ -a ■*>'&■& a 1

& <b *> S> o o x> O 43 -e> 49 O O 4 0 4 > - ® « - © P - O p - O p « Q - O - O O Q P O O 0 j 9 4 ^ 4 > 0 0 0 » 0 p -O t i 2

REsiJLTfs a n d d i s c trssioN $ & <> <? -® ^.»»«©• .e P ^ ^ o ^ »* i s

]mTEEUkL.Fm IS

EXPERIMENTAL CONSTAWM^ACC 9 (> A & -a -® -® O ■& J0 -O Q o O « -O -O rO -0 .0 6= O fi> » P -O -0 -® p -© -0 -C3 -© -0 29

E X P E R m E N m L CYCLED-TEM PERATm ' '^ ^ .C z -a -o o -o a -e -p ^ o -o ho -o o p -®. -o & p p -o -® -o P p P o 9 -e 0 0 -® o o « -o 3 9

RELATION OF ACCLIMATION AND RESPONSE 43

THERMAL STRESS &-p. * * * -0 9-0 © * p -© & © © ^ © © © © © © © ©©©©©. © © © 56

ACCLIMATION AND WEIGHT CHANGE ©©© .©©p©©©©© 62

> P P -o # p. p. -f © -#. © 4 -d 9 <> © -0 , p © 9 © © © p p. e © © 9-6

EFFECT OF BATE OF HEATING * 7 2

SUMMARY AND CONCLUSIONS ......... 75

LITERATURE CI7? ED # -q a» #«»»^»-o ^«o ct»»»<>»9 < #» o -» 80

Page

X, Season of year (1961) and tem peratu re CC) ofoccurrence of the rm al response points » . 4 , , , 1 4

. 24 Comparison of spring (1961) and fa ll (1961)thermal response poxnts * -e o c o ® » & & & v ® © <, © © © © © ©»«© © © 18

3, One-factor analysis of variance for seasonaltherm al r^esponse ©©©'©©k©©©©©©© ©© ©©©©©©©© ©©©©©© »© ©©©© ©©© 19

4© R egressions and correlations of therm al responsepoints and season (.19 61 ©©©©©©.©©©©©.©©©©;©©©©©©©© ©«© © © © © ©

5© Tem peratures (°C) of therm al response points,according to season^ after acclim ation a t 30°C ©..»,«©©©.©» 24

6© T em peratures (% ) of therm al r esponse points^according to season, a f te r acclim ation a t 35°C . ©»© © ©»© © © © 25

7© Constant-tem perature acclim ation and. tem p era tu re ''■(0C) of occurrence of therm al response points ©©©©©©,»,©©, 32

8© Cycled-tem perature acclim ation and tem perature(0C) of occurrence of therm al response points ..,»©©© © © © © © © 40

9© Values for reg ression and correlation of theacclim ation index resulting from controlled(constant and cycled) tem perature acclim ation ©©,©©.„©»»©, 46

10© Acclimation index and mean temperature data forthe seven days preceding field collections and' thermal response determinations ©©©©»»©©©©©»©»,»©©»©©©©, 48

11© Mean body tem peratures recorded upon collectionin sum m er (May, 1952) and fall (Sept©, and Oct©, 1952)© ,©, 51

12© Regression and correlation coefficients for CTM andfor death on gape response following varyingconditions of acclim ation © © © © © © © © © © ©©©©©©©©© © © ©. © © © © © © © © © 55

TABLES (continued)

13*. R egression and correlation coefficients fo r CTM (n - 200) and fo r death (n « 1Y2) on gape response fo r field, constant-tem perature and cycled-tem pera- tu re acclim ation data *»********** * $ * * * * * * *»* * * # p & *»* 3 7

14, Comparison of the regression coefficients for gape r esponse v ersus CTM and gape response v e r s u s death by the method of least squares (b) and Bartlett*s method (B) * * * <* * * -* * ,»-p * * * * -»#»* * * ***,-* * * # * * * * * * * *, * * * 59

IS* Tem perature of gape response following alternating subjections to tem perature increase .and re tu rn to*

■ acclim ation tem peratu re (1#^C) * * * , * * * , , , , , , , , 60

16* Percent weight difference (loss o r gain) after sevendays acclim ation at GonsWnW emperature *,* ** . 63

17* Analysis for CTM diff erences in m ales and fem alesfollowing therm al acclim ation * * * * * * $ * *.*, * »• # * § * * -p * * * * * 66

IB, Snout-vent length (mm))and sex fo r the data inTable 17 ****** * * :*■ * * * * * p. * * * * * * p * * ******** * * * * ® * * ****** 67

19 * Regression of CTM on snout-vent length andcorrelations fo r cum ulative' data' sum m arized in iTables 17 and IB * * * * * * * * .* *, * * ************ * * * * * * * * * * * #. * 6B

20* T herm al response points and ra te of tem perature ■ -in c rease (after 30oG acclimation) ********************* * * 73

w i i

FIGURES

FIGURE

1, Chamber a i r tem perature record of 8* - 27®C cycled-tem perature experiment „ , -O Q ■© s » P @ -4> » p -© P 9 P P O -

2, Apparatus used in determining c ritica l therm al responses. Left# w ater foatli with testing cham bers.Right, te letherm om eter with probes 6

3, Urosaurus ornatns (male) in heat s tre s s chamber# removed from w ater bath (see Fig* 2)s showing body tem perature probe leading out from under tail# andthe a ir tem perature and surface tem perature probes*«, * 8

4, Seasonal differences in gape response and CTM .> , „ , 15

5„ Seasonal diff erences in acclim ation index andtem perature of d e a t h @ *. . * o . 18

6» Regressions of gape response and CTM on tim e 'of year > • . . . . . . . p. . Q. -o * . q f p * p. . . #. p *: 2.0

7« Regression of acclim ation index and tem peratureof death onfxme of year p . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.

8. Seasonal differences M and CTM.when acclim ated fo r seven days to 30°C and 3 5&C . . . . . . ■■ 26

9. Seasonal differences, in acclim ation index and ‘ tem perature of death when'acclim ated for sevendays a t 30 and 3 5* _*C.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7

10.. Regression of c r itic a l therm al maximum - on gap e *response for field acclim ation . . . . . . . . . . . . . » . . . . . . . . . 30

.11. Regression, of tem perature of death;on/gape 'response for field acclimation . . . . . . . . . . . „>,....». * . . . . . 31

12. Gape response and CTM as influenced by constant- ;tem peratu re acclmxat^on-. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

. v l l i ;'

FIGURES (continued)

FIGURE

IK Acclimatioii index and death a s influenced by■ constant-tem perature acclimation » * ^ ^ »„ * * #«*

: 14-p Regression of c ritic a l therm al maximum on gape . ■response fo r constant-tem perature acclim ation , #»,,»»«

15, R egression o f tem peratu re of death on gape response ■ fo r constant-tem perature acclim ation

1% Gape response and CTM as influenced'by cycled’-tem perature acclxmatron «* * .* «* #»-* <? # # * #■ * # # # •# p

17, Acclimation index and tem perature of death as influenced by cycled-tem perature acclim atien

18, R egressionof CTM on gaperesponse for ■cycled-' • tem perature acclxniation »0», ® e. <? a p», © -©© -©# © © © .©©©©© © © © ©

10© Regressiou of tem peratu re of death on gape response • for- cycled-tem perature acclim ation ©. © ©$ # ©,, © © ©#

20© R egression of acclim ation index on constant-tem pera- tu re and cycled-tem perature acclimation© Means of environmental air- tem peratures and the re sp e c tiv e : ■ . acclimation index plotted ©»©»© © * # .*■• © © #» »© © © © © © # »■ * © © © ©

21© Morning body tem peratures recorded upon, capture ■ in Sabino Canyon in May, 1052 and Sept© -Oct© * 1952© ©»„

22© R egressions of CTM and tem perature of death on gape response by method of least squares and B artlett’s method including a ll field, constant- tem peratu re and cycled-tem perature acclim ation

FIGURES (continued)v

FIGURE

23, Succeeding gape responses with body tem perature increases alternating with decreases to the acclim ating tem perature of 15°C, (a) Increase to tem perature of preceding gape response, (b) Increaseto 2*C above preceding gape tem p era tu re ,, , , , , . * *«, *. *, * ■ 61

24, Percent weight lo ss a fte r 7 days acclimation a tdifferent acclim ation t empera tu res , , , , , , , * , , . , , , , , , * . , , * 64

25, Comparison of fem ales and m ales in relation to CTMand snout-vent length to r seasonal acclim ation, , 69

26, Comparison of fem ales and m ales in relation to CTM and snout-yent length for constant-tem perature acclim ation ,, ^ ^ , 70

27* Comparison of f em ales and m ales in relation to CTM and snout-vent length to r cycled-tem peratur e a c c l i m a t i o n , , 71

28, Response to heating after 30*0 acclim ation a t a tem perature increase of 0, 6^C/min, and of 0 ,2 C! / m m , , , , , , *, *, * , , , , , * * , , , , , , , , , , , , , , * * * 74

a

INTRODUCTION

Urosaurus ornatus (Baird and Girard) is a sm all iguanid tre e

lizard that is abundant in parts of Arizona# neighboring sta tes and in

northern Mexico. Many aspects of its general and physiological ecology

have been under investigation in the Southwest during recen t years . The

present study was undertaken to investigate c ritica l therm al phenomena

in a natu ra l population of th is species and to com pare the findings for

na tu ra l seasonal ra te levels with those fo r experim ental tem perature

controlled acclim ation.

It was established by Lowe and Vance (1955) that the c ritica l

therm al maximum of th is diurnal rep tile responded significantly to

experim ental therm al acclim ation. Thus the present work was designed

for (1) a determ ination and evaluation of the effect of natural seasonal

events on the c r itic a l therm al maximum of this species 5 and (2) fu rth er

clarification of the nature of the upper therm al response points as well

a s the natu ra l sequence of th e ir occurrence under experim ental con­

ditions.

Since M osauerts (1936) early investigations of reptilian therm al

behavior and the m ore extensive and well-known work of Cowles and

Bogert (194% considerable attention has been given to the subject of

therm oregulation in rep tiles and in o ther coldblooded organism s.

Several investigators have pointed to the evolutionary implications of

therm oregulation in lizards (Bogert, 1949; N orris, 1953; Vance, 1953;

Pearson, 1954; Cowles, 1958; Stebbins, 1958; Inger, 1959; Larson,

1961) . Cyclic activity and therm oregulation in rep tiles was recently

reviewed by Saint Girons and Saint Girons (1956).

Recent review s of therm al acclim ation and compensation have

been given by Bullock (1955), F ry (1958), and P ro sse r (1958, 1961).

With the early work of F ry (1942), indicating the significance of tem pera­

tu re acclimation in the physiology and ecology of fishes, m ore such

studies became concerned with coldblooded vertebrates in generaL Yet,

i t was but relatively recen t that experim ental tem perature acclimation

was brought to bear specifically on the problem of rep tilian therm al

relations (Vance, 1953; Lowe and Vance, 1955; Dawson and Bartholomew,

1958; Dawson, 1960).

METHODS

Specimens of the subspecies Urosaurus ornatus linearis Baird

w ere collected from a population occurring in lower Sabine Canyon,

Santa Catalina Mountains, Pim a County, Arizona. This locality is nine

airline m iles (13. 8 m iles, by road) from the laboratory a t the University

of Arizona, in Tucson. The canyon stream supports a riparian woodland

of ash, oak, sycam ore, cottonwood, m esquite and willow with adjacent

slope covering of climax desert vegetation dominated prim arily by

foothill paloverde, sahnaro, and associated plants of the Arizona Upland

section of the Sonoran D esert, The anim als w ere taken from 2800 to

3400 feet; and along the edge of the canyon stream w here desert and

riparian vegetation interm ingle.

A to ta l of 312 anim als was collected by the thread-noose method.

This number includes those used in prelim inary experim ents but not

reported on here; data for 253 individuals a re reduced and analyzed.

Total body weights of the anim als ranged from 0.47 to 5.20 gram s.

Both m ales and fem ales were represen ted in each experim ental sam ple.

Upon collection the lizards w ere brought to the laboratory,

weighed, and either immediately tested for therm al tolerance o r placed

under controlled environmental conditions. F o r the la tte r, the anim als

were identified by m arking with fingernail polish after weighing, and

were then placed in battery ja rs in lots of six o r le ss . Before the ja rs

were placed in the acclimation cham ber, they were supplied With w ater-

filled watch g lasses and a libera l number of mealworm la rvae (Tenebrlo

m olito r) which were constantly available to a ll experim ental anim als,

^Precision" Low Tem perature Cabinets (B, O.D, boxes), with

a tem perature control of plus or minus 0.3*0, and rela tive humidities

ranging from 45% to 60%, were used for m ost of the constant tem perature

acclim ation experim ents. F o r som e sam ples an Aminco-Aire Humidity-

Tem peratur e Control Apparatus (’’acclim atizer'r) was used,. This had

an aecuracy of 0« l'°C and rela tive humidity was se t a t 55%,

Experim entally-cycled tem perature acclimation,, with simu­

lation of the therm al component of the natu ral environment* was accom ­

plished by th e u se of a walk-in incubator in which tem peratu re was

cycled front' the low experim ental extrem e to the high over a 2.4 hour

p e r io d .' The curve for 8* - 27°C (Fig, 1) is representative of tem perature

change in a ll such experimentsj the data fo r program m ing w ere derived

.from a ir tem perature recordings (Tempscribe* 7 days) a t the study a rea .

These tem peratu re cycles w ere experimentally controlled by a Partlow

program m er and w ere accurate to within 3 ,0°C .

The lizards were tem perature-acclim ated for a period of seven

days (except in one specified case) in a ll of the experiments using

controlled environments, A four watt fluorescent tube was placed within

a foot of the animal-containing jar* and light was supplied for the num ber

of daily hours corresponding to the daylight hours of the season of

collection.

A fter collection o r acclimation* the critica l the rm al maxima

and other therm al response points w ere determ ined by subj eeting the

anim als to heat stress* The experim ental apparatus (Fig, 2) consisted

Fig. 1. Chamber a ir tem perature record of 8° - 27°C cycled tem perature experiments

6

Fig. 2. Apparatus used in determining critical therm al responses. Left, water bath with testing cham bers. Right, teletherm om eter with probes.

of two battery ja rs j each lined with standard plant p ress cardboard,

im m ersed in a 1000 watt P recision Scientific Co* metabolic water bath.

The therm istor-sensing end of a sm all animal probe (Yellow Springs

teletherm om eter interchangeable probe^ No, 402) w as inserted into the

cloaca and past the pubis j the probe was fastened to the ta il by adhesive

tape, thereby allowing for freedom of body movement (Fig. 3). The

constant u se of this probe, m easuring directly the deep body tem perature,

elim inates experim ental e r ro r inherent in the procedure of assuming

the body tem perature to be that of the environment, o r in the procedure

of removing the anim al from the stressing environment to determ ine the

c r itic a l the rm al response point . One o r the other of these difficulties

has complicated sev era i previous studies and th e ir re su lts .

Also taped within the chamber was a Yellow Springs surface

tem perature probe (No* 409) to m easure substratum tem perature, and

an a ir tem perature probe (No. 405). A ll tem peratures w ere read from

m ulti-channel teletherm om eters (Y, S. model 46TUC) accurate to within

1% of range (11. 0#C), i. e . , ± 0. 055*C.

Heat s tre s s was imposed by allowing the body tem perature to

r is e , from that of the environmental tem perature, a t a ra te of approxi­

m ately 0* 6°C per minute to the tem perature of death. This heat r ise

closely followed the a ir tem perature increase and fell somewhat below

the substratum ra te (0. 7°C /m in .). Although the ra te of tem perature

8

Fig. 3. Urosaurus ornatus (male) in heat s tre ss chamber, removed from water bath (see Fig. 2), showing body tem perature probe leading out from under tail, and the a ir tem perature and surface tem perature probes.

increase of both the a ir tem perature and the substratum tem perature

tend to fa ll off somewhat a t the experim ental extrem es (ca„ 48° - 50°C)?

body tem perature continues to r is e linearly and #onstantly» This pro­

cedure of allowing the body tem perature to closely approxim ate the ra te

of tem perature increase of the environment has been followed since 1956

in this laboratory and since I960 in the present study* It is the experi­

m ental condition which Hutchison (1961) also maintained for the determ i­

nation of the c r itica l therm al maximum in salam anders; and as he sta tes,

the anim al is properly ’’heated, from a previous acclim ation tem perature

at a constant ra te just fa s t enough to allow deep body tem perature to

follow environmental te s t tem peratures without a significant tim e lag, ”

The tem peratures of occurrence of seven c ritica l therm al

response points w ere observed and recorded. A fter a certain increase

in the anim al's activity, as the tem perature begins to r is e from the

in itial environmental level, the f irs t notable response is that of gaping.

The animal suddenly opens wide its mouth and resp ira to ry movements

notably increase . The evaporative cooling function of this mechanism

seem s obvious but little definite inform ation supports the supposition

(Templeton, 1960). This response has the advantage of an objectively

and very accurately determ ined tem perature of occurrence. Hind leg

paralysis follows (ca. 3* 5°C higher), leaving the anim al capable of now

desperate movement only by its still-coordinated front legs. It is between

the points of hind leg and front leg paralysis that the loss of righting

response occurs- This point is determ ined by turning the animal on

its back and recording the tem perature a t which it can no longer righ t

itself- This is taken a s the c ritica l therm al maximum which herein­

after will be re fe rre d to also as the CTM- Cowles® definition (Cowles

and Bogertj 1944) of the c r itic a l therm al minimum and maximum and

its la te r modification by Lowe and Vance (19 55) applies well to this

in terpr etation since ju st beyond th is point the anim al can no longer

"escape from conditions that will promptly lead to its death-,f More

closely conforming to this definition would be, perhaps, the therm al

response point following the loss of righting response, i- e , , front leg

paralysis. This point is , however, m ore difficult of m easurem ent and

is only recorded in 70% of the experim ents w hereas the loss of righting

response is recorded in virtually 100% of the cases-

The point occurring afte r front leg paralysis (ca, 0 ,2aC higher)

is a response of spasmodic movements, thrashings of the body and sharp

la te ra l movements of the head. This iS apparently the c r itica l therm al

response point that has been taken as th e CTM by m ost of the investigators

in this field (Cowles and Bogert, 1944; S tebbins,' 1954; Lowe and Vance,

1955; M cFarland, 1955; Rosenthal, 1957; Hutchison, 1961; Larson, 1961),

The anim al, when rem oved immediately a t th is point, will recover; its

chances of survival decrease the longer i t is left a t th is and higher

tem peratures* Although this reaction does not always take place a t the

tem peratu res expected* as has been noted also fo r salam anders (see

Hutchison* 1961)* if has been designated by others as the GTM not only

for its ecological significance but also fo r its ease of observation under

conditions w here the anim al could not be manipulated* and manipulation

is necessary to determ ine the loss of righting response and som etim es

front leg paralysis*

In th is species loss of righting response and onset of spasmodic

movements a r e points that bracket the fron t leg paralysis response*

'The fo rm er is considered the m ore appropriate for the GTM since* a t

onset of body spasms* fo r p ractica l purposes the anim al has already

lost its ability to escape from its lethal environment* Thus the GTM

here employed is the loss of righting response; th is occurs a fte r hind

leg paralysis and on the o rd er of 9 ,2°C lower than the f i r s t front leg

para ly sis .

About (X above initial spasmodic movements, strong whole-

body convulsions a re evident in about 50% of the cases. Death follows

within. 0„ 5o-l„,0oC and is determ ined consistently by the la s t movement

of throat o r eyelid. This is an a rb itra rily selected point that is*

nevertheless* a physiologically im portant one which became possible

to recognise a fte r the f i r s t few experim ents w ere conducted.

The c ritica l therm al r esponse points m ost consistently recorded

and given sta tis tica l consideration a re three; namely* gaping* less M

righting response (CTM)* and death. The strong correlation between

gape and CTM perm its calculation of an acclimation index* which is

the weighted mean of these two points* This combines the m ore

objective and consequently m ore p rec ise determ ination of the fo rm er

with the necessarily m ore subj ectively determined latter* The index

to some extent compensates fo r the natu ra l variability of the relation

between the responses* and possibly allows a m ore accurate com parison

between responses to differing conditions of acclimation* Although

Simply trea ted here as another th e rm al response* its ab strac t and

a rtif ic ia l natu re is c learly understood*

Statistical M e t h o d s Lowe and Vance (1955) modified Cowles1

and Bogertfs (1944) definition of the c r itica l therm al minimum or

maximum to the extent tha t it ,4may be visualized as a value tha t is

the arithm etic m ean of the collective the rm al points a t which locomotory

activity becomes disorganized and the anim al loses i ts ability to escape

from conditions that w ill promptly lead to its death*n Such means and

th e ir standard e rro rs have been determined* and they have been graphed

using th e method of S ic e and L eraas (1936)* Standard e r ro rs (±) follow

the values fo r means in. the Tables; the standard deviations a re not

tabulated* F o r determ ining values ©ft* the appropriate form ula given

in Simpson* Roe and Lewontin (1.960) was used*

Methods gf calculating reg ression and correlation coefficients^

and th e ir te s ts ? a re also taken from Simpson# Roe and Lewoniin (X960)#

and from others. Gape response is not an independent variable but is

determ ined by tem perature of acclim ation, B a rtle tt 's B (Bartlett#

1949) provides here a line of best fit and is used in determining the

relation between gape response# GTM# and death. F or purposes of

prediction# regressions by the method of least squares a re also calcu­

lated,

F ra tio s determ ined from one-factor designs w ere also used in

ascertaining the effect of season on c r itica l therm al responses. Spe­

cifically# te sts w ere made fo r the effect of the spring# summer# and

fa ll natural environmental conditions on the responses of gape# CTM#

and death# and on the acclim ation index.

F o r calculation and comparison# the means of the cycled

experim ental tem peratures and of the environmental field tem peratures

were used ,

RESULTS AND DISCUSSION

N atural F ield Acclimation

The tem peratures of therm al response points as influenced by

season a r e given in Table 1 and graphed in F igures 4 and 5, In a ll

Table 1

SEASON OF YEAR (1961) AND.TEMPERATURE (°C) OF OCCURRENCE OF THERMAL RESPONSE POINTS, DATA GRAPHED IN FIGS, | AND 5.

RESPONSE AND SEASON N MEAN RANGE

GapeSpring 23 42, 70 ± 0,15 41, 65 - 44, 60Summer 26 43,02 *■ 0,14 41,95 - 44, 70F a ll 10 43, 50 ± 0,28 41, 50 44, 50

CTMSpring 23 46,43 ± 0,10 45,30 - 47, 55Summer 26 46, 47 d: 0,11. 45,00 -4 7 ,5 5F a ll 10 46,35 ± 0e 16 45,20 - 47,20

Acclimation IndexSpring 23 44, 56 iWO, 10 43, 63 - 45, 60Summer 26 44, 75 A 0,10 43,60 - 45, 78F a ll 10 44,90 A 0,15 43, 85 - 45, 65

DeathSpring ' 13 48, 30 A 0,12 47,25 - 49,05Summer 26 48,37 a 0,12 47,15 - 49,80F a ll 10 48, 70 a 0,14 47, 60 - 49,30

GAPE

(°

C)

CT

M48 -

O 47 o

46 1

45

44

43

42

A A

41

SPRING SUMMER FALLT I M E ( S E A S O N , 1961 )

Fig. 4. Seasonal differences in gape response and CTM. Data in Table 1.

AC

CL

IMA

TIO

N

IND

EX

(°C

) D

EA

TH

5 0

Oo 49

48

47

46

45

4 444

43

SPRING SUMMERT I M E ( S E A S O N ,

FALL1961 )

Fig. 5. Seasonal differences in accli­mation index and temperature of death. Data in Table 1.

17responses but CTM there is a trend over spring (Mar, - A p r,), sum m er

(June - July) and fa ll (Oct, ) ■ toward a higher therm al tolerance. This is

best shown by the tem peratures of gaping where the difference between

spring and fall is 0 ,80% which is significant a t the 1% level, It should -

be noted, however, that in no case does there appear a significant differ­

ence in therm al response between spring and sum m er and between

sum m er and fall. Probabilities for the differences in the c ritic a l therm al

response points a re given in Table 2,

The th ree seasons studied (spring, sum m er, fall) w ere also

tested by analysis of variance, F ra tio s (Table 3) derived from one-

fac to r designs indicate a seasonal effect on gaping response ( P < 0 ol)

but show no such effect on the other responses. The lowest of these F

values is for the c ritica l therm al maximum (F 0.34),

When plotted on daily increm ents, and the reg ression lines

fitted, a trend in seasonal increase in therm al response levels is shown,

and again with the exception of CTM (Figs, 6 and 7), Regressions and

correlations a re calculated fo r a l l raw data, with n between 50 and 60;

the means of the c ritica l therm al responses, however, a re graphed for

a one to four day period. The probability values for the correlation

coeffecients a re given in Table 4,

The evidence cited above fo r an actual seasonal effect on

physiological adjustm ent is made m ore c lear by m easurem ent a fte r

18

Table 2

COMPARISON OF SPRING (1961) AND FALL (1961) THERMAL RESPONSE POINTS. DATA IN TABLE 1.

RESPONSE t P

Gape 3.5024 <0.01

CTM 0.4224 < 0 .7

Acclimation Index 1. 8074 < 0 .1

Death 2.1246 < 0 .0 5

19

Table 3

ONE-FACTOR ANALYSIS OF VARIANCE FOR SEASONAL THERMAL RESPONSES.

ESPONSE SOURCE SUM OF SQUARES D .F .

MEANSQUARE F P

ape Levels 4.46 2 2.23 3.84 c 0 . 10Deviations 32. 52 56 0.58

TM Levels 0.21 2 0.10 0.34 c 0 . 50Deviations 16.27 56 0.29

cclimation Levels 0.89 2 0.45 0.45 < 0 . 50idex Deviations 12.38 56 0.22

eath Levels 1.11 2 0.55 1. 77 CO. 50Deviations 14.50 46 0.31

UA

4 2

64 120- J 1----------- 1— 1—

MARCH APRIL JUNE JULYT I M E ( S E A S O N AND D A Y S :

190 1 rOCTOBER

Fig. 6. Regression of gape response and CTM on time of year. Data in Table 4.

AC

CL

IMA

TIO

N

IND

EX

(°C

) D

EA

TH

2150Oo

49

48 - o

47

46

45

44

4364 120 90

MARCH APRIL JUNE JULY OCTOBERT I M E ( S E A S O N AND D A Y S )

Fig. 7. Regression of acclimation index and temperature of death on time of year. Data in Table 4.

22

Table 4

REGRESSIONS AND CORRELATIONS OF THERMAL RESPONSE POINTS AND SEASON (1961). DATA GRAPHED IN FIGS. 6 AND 7.

IESPONSE b r P N

>ape 0.0031 0.306 <0.02 59

0.0003 0.043 < 0 .2 59

Acclimation Index 0.0007 0.156 < 0 .2 59

)eath 0.0025 0.230 < 0 .1 49

experim ental acclim ation a t a constant tem perature. The data a re given

in Tables 5 and 6. After acclimation a t both ;30QC and 35°C all of the

therm al response points w ere higher in the fall of 1961 than in the sum m er

of the sam e year (Eigs, 8 and 9). Again the la rger differences appear in ■

gape response. Here it m ust be stated that the 3 5°C acclim ated anim als

of the fa ll of 1961 w ere exposed accidentally to a rela tive humidity of

10%, The data^ although calculated here for purposes of comparison,

must be in terpreted with this in mind. Although data w ere gathered in

the fall (only) of I960, and tabulated and graphed, no com parative

seasonal data for that year is available.

Many w orkers have found m ore c lear-cu t seasonal differences

in some physiological responses of coldblooded verteb ra tes. Riddle

(1909) found ra te of digestion of a standardized object to change by about

30% in certa in months of the year in fish, frogs and tu rtle s . C arter

(1933) reported the tem perature-pulse ra te curve of the excised frog

heart to vary with the season, A photoperiodic, and thus seasonal,

influence on gonadal development was reported by Bartholomew (1953)

in Xantusia v ig ilis. Hoar (1955) found the goldfish to have m ore heat

resistance after exposure to a longer photoperiod. B essauer (1955)

showed a seasonal variation in size of liver* te stis and ovary in Anolls

carolinensis and Wilhoft (19 58) found the height of thyroid epithelium in

geeloporus oceidentalis to change over the seasons. Oxygen consumption

Table 5

TEMPERATURES (°G) OF THERMAE RESPONSE POINTS, ACCORDING TO SEASON, AFTER ACCLIMATION AT 30*C9

, DATA GRAPHED IN FIGS, 8 AND 9.

RESPONSE AND SEASON N MEAN RANGE

GapeF all, 1960 9 43.21 ±0*29 41. 70 - 44, 00Summer, 1961 9 43,25 ± 0,22 41.90 - 43, 85F all, 1961. 11 44.24 ± 0,19 43.00 45,40

CTM .F all, 1960 8 47.33 ± 0.13 46. 75 - 47, 90Summer, 1961 9 46,75 ± 0,12 46,00 - 47,20F all, 1961 10 46. 89 ± 0,15 46,00 - 47. 50

Acclimation IndexF a ll, 1960 8 45.39 ± 0,14 44.93 - 46.10Summer, 1961 9 45.00 ± 0.14 44,40 - 45,43

■ Fall, 1961 10 45, 58 ± 0 ,1 6 44, 60 - 46.25

Death .Fall, 1960 9 48.08 ± 0 ,1 8 47.00 - 48.80Summer, 1961 9 48.25 ± 0 ,1 6 47.10 » 49. 00F all, 1961 11 48,80 ± 0,17 47. 50 m 49,30

Table 6

TEMPERATURES (°G) OF THERMAL RESPONSE POINTS, ACCORDING TO SEASON, AFTER ACCLIMATION AT 3®^C,

DATA GRAPHED IN FIGS, 8 AND 9.

RESPONSE AND SEASON N MEAN RANGE

Gape ■ FaU, 1960 • 9 42*91 £ 0 ,2 4 41 .90 « 44, 50 .

Summer, 196:1 9 42,68 ± 0,39 41,35 - 45,25Fall, 1961 10 44*29 £ 0,37 42, 6 0 - 46.70

CTMF all, 1960 9 47, 58 £ 0,20 46. 75 - 48, 50Summer, 1961 9 46,14 £ 0,20 4 5 .2 0 -4 7 .3 0FaH, 1961 10 46,41 £ 0 .1 6 45. 70 - 47.20

Acclimation Index F an , 1960 8 4 5 .2 4 -fc 0,15 44, 65 - 46.25Summer, 1961 9 44,46 £ 0,29 43.28 - 46.27F a ll, 1961 10 45,39 £ 0.23 44. 65 — 46,30

Death , , . F all, I960 9 48,45 £ 0.21 4 7 ,3 0 - 49.00Summer, 1961 9 48*12 £ 0,20 . . 47,10 - 49,20F an , 1961 10 49, 46 £ 0,15 48,90 - 50, 60

GAPE

(°

C)

CTM

(°C

)

26

4 8

47

46

45

44

43

42

4 I J______ I__LFALL, I 960 SUMMER, 1961 FALL, 196

T I M E ( S E A S O N )

Fig. 8. Seasonal differences in gape response and CTM when acclimated for seven days to 30°C (open) and 35°C (solid). Data in Tables 5 and 6.

AC

CL

IMA

TIO

N

IND

EX

(°C

) D

EA

TH

27

5 0Oo

49

48

47

46

45

4 4

4 3FALL, I960 SUMMER, 1961

T I M E ( S E A S O N )FALL, 196

Fig. 9. Seasonal differences in acclimation index and temperature of death when acclimated for seven days at 30°C (open) and 35°C (solid). Data in Tables 5 and 6.

of the cunner was discovered by Haugaard and Irving (1943) to be higher,

when m easured a t 15^C, in the winter than in the sum m er„ At l^G, the

rev erse was found to be tru e fo r the salam ander Plethodon cinereus

(Vernberg, 1952),

In his extensive study of the c ritica l therm al maximum of

salam anders, Hutchison (1961) found the CTM of Diemictylus viridescens

to increase from spring to fall and decrease over the w in ter These

CTM were determ ined a fte r acclim ation a t 20°C fo r one month. These

findings a re in agreem ent with those of th is work, in showing what he

te rm s a lag in response to the higher sum m er tem peratu res. Such

re su lts may indicate a differential compensatory adjustm ent to the

acclimation tem perature over the seasons. This concept is discussed

in m ore detail below,

Larson (1961) found the fence lizard Sceloporus occidentalis

collected in the sum m er, in the C entral Valley of California, to have

a higher CTM than those collected a t the sam e locality in the spring,

Vance (1953), however, could dem onstrate no difference in oxygen

consumption in sum m er-collected U rcsaurus ornatus as compared with

anim als collected in the fall. This finding for resp ira to ry metabolism

in P rosaurus is the sam e as that found in th is study of c r itica l therm al

responses in the sam e species from the sam e locality .

Figures 10 and 11 show the regressions of CTM on gape

response and of death on gape response^ respectively* fo r a ll field data;

correlation coefficients a re given in Table 12, These correlations a re

Significant and indicate the consistency of the o rder in which the c ritica l

therm al r esponses occur and th e ir dependence on the tem perature of

acclim ation. This relatively sm all dependence of both CTM and tem p er-

a ture of death on acclimation tem peratu re for field animals# com pared

With the re su lts of experim ents involving constant-tem perature and cycled-

iem perature acclimation# is discussed below.

Experim ental C onstant- Tem perature Acclimation

Table 7 provides data# graphed in Figures 12 and 13# for the

tem peratures of occurrence of the c ritic a l therm al responses as

influenced by the experimentally imposed constant tem peratures of 15*0*

25*0 and 30°Ct The CTM (45, 31GC# 4 6 ,39°C# and 4 6 .98°C# respectively)

r is e s linearly.

Such dependency of CTM on acclim ation tem perature was

reported as early as 1895 by Davenport who worked with the tadpoles of

Bufo te r r e s t r i s , Hathaway (1927) was one of the ea rlie r workers who

investigated such tem perature to lerance a fte r acclim ation in fish.

Doudoroff (1942) found the heat resis tan ce of certain m arine fish to depend

CT

M

30

48

47 •o

46

45o SPRING • SUMMER A FALL

4 4454 442 43414 0

GAPE ( ° C )

Fi g. 10. Regression (solid line - method of least squares; broken line - Bartlettfs B) of critical thermal maximum on gape response for field acclimation. Data in Table 12.

DE

AT

H

31

50

49

•• o

48

47o SPRING • SUMMER a FALL

46

42 434 0 44 4541GAPE ( ° C )

Fig. 11. Regression (so1 id line - method of least squares; broken line - Bartlett’s B) of temperature of death on gape response for field acclimation. Data in Table 12.

T ab le 7

GONSTANT-TEMPERATURE ACCLIMATION AND TEMPERATURE f C) OF OCCURRENCE OF THERMAL. RESPONSE POINTS.

DATA GRAPHED IN FIGS. 12 AND IS;

END POINT AND ACCLBIATION TEMPERATURE N MEAN RANGE

Gape15% 19 42» 03 * 0. 23 40,55 - 44, 0525% 21 42.97 ± 0» 09 42 a 00 - 44a 0030% 29 4 3 .6 1 A 0.15 : 41a 70 - 45,4035% 18 42. 79 ± 0,22 41a 35 ** 45.25

GTM15% 19 4 5 .3 1 A 0.30 42.20 - 47.0025% 21 46-39 ± 0 .1 0 45.30 - 47.2030% 27 46.98 ± 0 .0 9 46.00 *< 47a 9035% 18 46.86 ± 0 .2 1 45.20 " 48, 50

Acclimatioii Index 15% 19 43. 67 ± 0.24 41. 52 - 45,3325% 21 44.68 ± 0P. 06 44,05 - 45.4030% 27 45.33 ± 0 ,1 0 44.28 - 46,1035% 18 44,85 ± 0 * 19 4 3 .2 8 - 46.27

Death .15% 14 47,49 ± 0 .2 3 44. 85 - 48,2025% 20 48.38 ± 0,07 47, 80 - 49,0030^C 29 48,41 ± 0.11 47.00 - 49.3035% T7 48.27 ± 0 ,1 5 4 7 ,1 0 - 49,20

48

47

Oo

46

O

45

44

43

Uo

LUCL<O

42

41

401_L

- I

J_J.

33

15 2 5 30 35A C C L I M A T I O N T E M P E R A T U R E - C O N S T A N T ( ° C )

Fig. 12. Gape response and CTM as influenced by constant-temperature acclimation. Data in Table 7.

4 9

OL 48

XH<UJ 47 Q

46

Oo 45

XUJo7 44

h-<

43

O 42 O <

4 I

■ i

34

- I

J_L15 25

A C C L I M A T I O N T E M P E R A T U R E30 35C O N S T A N T ( ° C

Fig. 13. Acclimation index and death as influenced by constant-temperature acclimation. Data in Table 7.

on acclim ation tem perature and Mellanby (1940) reported tem perature

acclimation-dependency of activity in Rana tem poraria and Salamandra

salam andra.;'

It was notj however, until the work by F ry (1942) and

co-w orkers on therm al acclim ation and its effect on fishes tha t the

effectiveness and significance of such acclimation became m ore gener­

ally recognized. Since then a good deal of information on therm al

requirem ents and acclim ation in fishes, amphibians and rep tiles has

accumulated.

Over a se r ie s of acclim ation tem peratures^ and derived high

and low lethal tem peratures, B rett (1944) describes "to lerance polygons"

for certain freshw ater fishes within which the anim als survive, Doudoroff

(1942) found certain m arine fishes to acclim ate to a high tem perature

relatively quickly, losing the tolerance gained slowly a t lower tem pera­

tu re s , I t may be noted h ere that the seven days laboratory acclimation

used in th is study of Urosaurus ornatus was chosen, not after a rig id

investigation of the tim e necessary for acclimation a t each tem perature,

but as a reasonable, convenient and unvarying length of tim e to be used

in a ll of the acclim ation experim ents. The general, desirability of a

much m ore detailed investigation of acclim ation therm operiods is, of

course, definitely indicated fo r m ost if not a ll anim al groups.

M cFarland (19 55) found therm al tolerances in Taricha tom sa

to be dependent on acclim ation temperature.. Both Zweifel (1955) and.

Hutchison (1961) listed a general relationship between the acclimation^

dependent CTM and the habitats of the salam anders investigated,

Lowe and Vance (1955) w ere the f irs t to show the effect of

therm al acclim ation on the CTM in a rep tile (U rosaurus ornatus),

Larson (1961) also dem onstrated th is effect in gceloporiis occidentalism

It is noted in Table 7 and in F igures 12 and 13 that the therm al response

points for U rosaurus a f te r 35°C acclim ation fa ll significantly below those

fo r 30°C acclim ation. It would seem that the constantly imposed n ea r-

eccritic tem perature of 35*0 (eccritic mean s 35, 5*0) is of a s tressfu l

nature. This conclusion is borne out by the work of Wilhoft (1958) who,

in Sceloporus occidentalism, ascertained the need of " re s t" periods from

prolonged exposure to "optimum" tem perature. When exposed for a

long period of tim e to th is tem perature, the height of the thyroid

epithelium increased^ and death eventually ensued. F reem an (1950)

found a drop in oxygen consumption of goldfish a t higher acclimation

tem peratures; the la tte r had a sim ila r effect on the CTM of certain

salam anders (Hutchison, 1961),

F igures 14 and 15 show the reg ressions of CTM on gape response

fo r a ll constant-tem perature acclim ation data. Here is shown, particu­

la rly by the method of B artle tt5s B fo r regression , the g rea te r

49

48

47

O 46o

5

O 4 5

4 4

43

42

37

* 1 5 ° C• 25° C A 30° C a 35° C

4 0 41 42 4 3 4 4 45GAPE ( ° C )

Fig. 14. Regression (solid line - method of least squares; broken line - Bartlett's B) of critical thermal maximum on gape response for constant-temperature acclimation. Data in Table 12.

DE

AT

H

38

49 • A A## A ^ A ^

48

47

46

45

444543 4 44240 41

GAPE ( ° C )

Fig. 15. Regression (soUd 1ine - method of least squares; broken line - Bartlett’s B) of temperature of death on gape response for constant-temperature acclimation. Data in Table 12.

independence of the tem perature of death in relation to acclimation

temperature-, as compared with the CTM» This rela tively g rea ter

independence of the point of death is also illustrated in F igure 11. The

therm al point of death is apparently genetically determ ined in such a

way as to be non-adjustable.

Experim ental Cycled-Tem perature Acclimation

When the therm al environment of TJrosaurus is cycled diurnally

from 8* to 27QG3 16* to 36tiC? and 20* to 40*GS c ritic a l therm al responses

resu lt as given in Table 8 and in F igures 16 and 17. Insofar as is known*

this is the f irs t attem pt to determ ine therm al response points fo r a cold­

blooded vertebra te a f te r acclimation to a tem perature-sim ulated natu ra l

environment. These tem perature cycles were program m ed according

to the natu ra l daily change from one extrem e to the other* and were

selected to represen t the tem peratures prevailing in the animals^ natural

environment during the spring* sum m er and fall* When therm al response

points as influenced by the cycled tem peratures (means) a re compared

with those resulting from corresponding constant-tem perature acclimation*

no differences a r e found (Tables 7 and. 8| F igs. 12, 13, 16, and 17).

Therefore* i t may be concluded that the means of these laboratory-cycled

.tem peratures conditioned the therm al response points.

Table 8

CYCLED-TEMPERATURE ACCLIMATION AND TEMPERATURE (*C) OF OCCURRENCE OF THERMAL RESPONSE POINTS.

DATA. GRAPHED IN FIGS. 16 AND 17,

................. r ■ ' '/END POINT AND ACCLIMATION TEMPERATURE - CYCLED (*G) N MEAN RANGE

Gape8* ~ 27°C 12 42, 50 ± 0P 22 41, 60 - 44, 50

16* * 36*C 22 42.8! 6 0,15 41,55 - 44.0520° - 40#C ■ 9 43, 37 * 0.12 4 2 ,80 - 43, 80

CTM.8° - 27°C 11 45,93 * 0,23 44,40 - 46,65

16* ~ 36*0 22 46,83 * 0,08 46,00 - 47, 4520* ~ 40*C 9 47,15 * 0,14 46.35 *- 47,65

Acclimation Index8* # 27 °C 11 44*23 * 0,19 43,00 45* 58

16* - 36*C 22 4 4 ,82 * 0,10 4 4 . 00 - 45* 7220* * -40*0 9. 4 5 ,2 6 * 0*09 44, 73 * 45* 60

Death8* - 27*C 9 4 7 ,5 7 * 0 ,1 9 4 6 . 40 - 4 8 . 10

'16* - 36*0 22 48* 24 * 0.11 47,10 — 48* 8020* r 4Q°C 7 48,47 * 0,15 48* 00 * 4 9 , 00

------ ---------.... ..... ........ ...— -------------- -

4 8

Oo

47

I -O

4 6

45

44

Oo

LUQ_<(£>

43

42

4 IJ—L

i

8 - 2 7 16-36 2 0 - 4 0(x = 17.5) ( x = 26) ( x= 30 )

A C C L I M A T I O N T E M P E R A T U R E - C Y C L E D ( °

Fig. 16. Gape response and CTM as influ­enced by cycled-temperature acclimation. Data in Table 8.

oo

4 9

Xh- 48 <LUQ

47

Oo

XLUO

46

45

I—< 44

OO 43

J__ L

I

1__L8 - 2 7 16-36 2 0 - 4 0

(x = 17.5) (x = 26) (x= 30 )A C C L I M A T I O N T E M P E R A T U R E - C Y C L E D ( °

Fig. 17. Acclimation index and temperature of death as influenced by cycled-temperature accli­mation. Data in Table 8.

Figures: 18 and 19 (data in Table 12) show the regression of

CTM and tem perature of death* respectivelya on gape response fo r the

cycled-tem perature experim ents. The sam e conclusion may be drawn

here as that for the data graphed in F igures 14 and 15*

Relation of Acclimation and Response

Knowing the extrem es of a tem peratu re cycle (data presented

in preceding section) one can theoretically predict? from the regression

of c ritica l the rm al response on controlled acclim ation tem perature

(Table 9, Fig, 20), the various therm al ■response points; o r, knowing

one of the extrem es and the tem peratu re of a therm al response point,

the other extrem e can be predicted. This relationship may be expected

in, and is indeed shown by, a l l of the c r itic a l therm al responses but is

evidenced best by the acclim ation index, chosen here fo r the reasons

given above in the section on methods; see Table 10, Fig, 20 (in which

i t is plotted for a l l conditions of acclim ation). The data show, however,

th a t when m ean tem peratures fo r the seven days preceding each

collection in spring, sum m er and fa ll a re considered as influencing the

seasonal acclimation indices and these responses a re com pared with the

corresponding values fo r cycled-tem perature and constant-tem perature

acclimation, the tem peratures of these seasonal acclim ation indices (and

CT

M

a

49

48

O 47*o

45

4 4 -

43 4 4 45424 0 41GAPE ( ° C )

Fig. 18. Regression (solid line - method of least squares; broken line - Bartlett’s B) of CTM on gape response for cycled- temperature acclimation. Data in Table 12.

k5

o

Xh-<LUQ

49

48

o*47

• 16 - 3 6 ° C a 2 0 - 40°C

46

4 0 4241 43 4 4 45GAPE ( ° C )

Fig. 19. Regression (soUd Une - method of least squares; broken line - Bartlett’s B) of temperature of death on gape response for cycled-temperature acclimation. Data in Table 12.

T ab le 9

VALUES FOR REGRESSION AND CORRELATION OF THE ACCLIMATION INDEX RESULTING FROM CONTROLLED (CONSTANT AND C YCLED)

TEMPERATURE ACCLIMATION. DATA GRAPHED IN FIG.

s r P N

0.0982 0.9656 0,01 6

AC

CL

IMA

TIO

N

IND

EX

45.5Oo

45.0

44.5

44.0

43.5

FAl L A ^ no

C H - a S UMM E R

15 20 25 30 35ACCLI MATI ON T E M P E R A T U R E - C O N S T A N T

AND M E A N S OF E X T R E M E S ( 0 C)

Fig. 20. Regression of aeeHmation index on constant- temperature ( o ) and eye!ed-temperature ( o ) acclimation (35°C stress value plotted but not included in regression). Seasonal means of environmental air temperatures ( a ), calculated from low and high air temperature extremes, are plotted for the acclimation indices of the seasonally collected lizards. The squares (□ ) represent the same seasonal acclimation indices (y axis) plotted against seasonal body temperatures (the body temperature means plotted are for the mean difference between the diurnal eccritic body temperature and the nocturnal low body temperature extreme, which is taken as the low nocturnal air extreme); arrows indicate the same direction, in each case toward the regression line, for the plotted squares. Interpre­tation of the greater departure of summer values from the regression line is discussed in text.

fa b le 10

ACCLIMATION INDEX AND MEAN TEMPERATURE DATA FOR THE SEVEN DAYS PRECEDING FIELD COLLECTIONS AND THERMAL RESPONSE DETERMINATIONS* DATA GRAPHED IN FIG, 20,

FIELDt e m p e r a t u r e s SEASON TEMPERATURE (°C)

MAXIMUM MINIMUM m e a n ACCLIMATIONINDEX

Environm ental Spring 26,0 10,0 18,0 44.56(Air) Summer 40,0 23,0 31. 5 44, 75

F all 32,0 18,0 25,0 44,90

Body Spring . 35,5 10,0 23, 7 44,56Summer '36,8 23,0 29.9 44. 75F a ll 35,5 18.0 26 ,7 44.90

the other c r itic a l the rm al r esponses as well) a re hot a t a ll as predicted.

The points for seasonal acclim ation as determ ined by imm ediate environ­

m ental therm al h istory fa ll considerably outside of the 9 5% confidence

lim its fo r reg ression of acclim ation index on cycled-tem perature and

constant-tem perature acclim ation.

This inability to predict a seasonal therm al response point from

knowledge of natu ra l environmental tem perature may not be surprising

in view of the now well-known mechanism of behavioral therm oregulation

(Cowles and Bogert, 1944). When one substitutes eccritic body tem pera­

tu re m eans (as determ ined for U rosaurus by Vance, 1953) fo r the upper

environmental extrem es of spring, fa ll and sum m er, one obtains the

different values (see-.field body tem peratures; Table 10) as the mean

influencing tem peratu res. This calculated eccritic mean (34 .5*C fo r

spring and. fall, 3 6 ,8#C fo r summer) is the upper body tem perature

reached and controlled behaviorally by Urosaurus and, to an im portant

extent, is independent of the environmental tem perature. Assuming the

low extrem e body tem perature to be that of the environment (an a s ­

sumption made with little evidence) one finds the new m eans to bring the

values of the acclim ation indices fo r spring, sum m er and fa ll c loser to

the line of reg ressio n for controlled acclim ation (Fig, 20). Indeed, the

spring and fa ll values fa ll virtually on th is line. The sum m er value

lies s ti ll somewhat removed and may be a reflection of this very sm a ll

an im a ls lessened ability to control its body tem perature in the intense

sum m er heat above the ground surface. Its sm all size and consequent

rap id heat gain o r loss makes behavioral regulation m ore difficult and

eventually less effective. Vance's data (Table 11, Fig. 21) show this

difference in seasonal regulation. The possibility exists that the higher

tem perature extrem es of the sum m er may resu lt in a lower than expected

therm al response point, as evidenced by the lower response elicited by

35#C acclimation, plotted in F igure 20 for reference, but, of course,

not included in the controlled-acclim ation regression .

The proximity of a ll the seasonal body tem perature means

(behaviorally controlled over the seasons) and the proximity of a ll of

these experim entally derived c ritic a l therm al responses Is indicative

of the homeostatic function of behavioral therm oregulation in this species.

This is an im portant form of compensatory physiologic adjustment.

The subject of compensation and the limiting factors involved

continues to rece ive increasing attention. Much of th is work regards

compensation of ra te processes in poikilotherms to tem perature, as

recently reviewed by Bullock (1955). P rech t (1958) describes five m ajor

types of compensation ranging from none to complete; complete compen­

sation being the maintenance of the sam e ra te function afte r a change in

the influencing factor. F ry (1958) points out that compensation, as a

rule, is only partia lly effective.

5 1

Table 11

MEAN BODY TEMPERATURE RECORDED UPON COLLECTION IN SUMMER (MAY, 1952) AND FALL (SEPT, AND OCT. * 1952)2 DATA FROM VANCE: (1953); DATA GRAPHED IN FIG, 21,

SEASON m e a n b o d y t e m p e r a t u r e s (°c ).(TIME, A .M .)

8 ^ 9 9 - 1 0 10 - 11 i i - i2N

Ssm m er : 35.0 " 3 0 .5 : 37,5 ■ ; 36,4

'Fall / 32,2 34,4 34,9 : 35,4

$2

3 8

37

36 UiQC3H 35

LaJCL

y jG—

34

>- 33QOCD

32

9o

10o

8 - 9

IIo

16o

9-10TIME

7o

13o

JL

10-11

14o

II-I2N

Fig. 21. Morning body temperatures recorded upon capture in Sabino Canyon in May, 1952 (open) and Sept.fOct.,1952 (solid). Numbers above points indicate sample size. Data from Vance, 1953.

Concerning such physiological compensation to tem perature in

the coMMooded vertebrate., there is relatively little known. Wells

(1935) was among the f irs t to rep o rt what we now regard as partia l

compensation. He reported mud suckers acclim ated a t 33°C and tested

a t 2QeC and 22°C to have an oxygen cbnsumption lower than those main­

tained a t 11°C» Vance (1953) found that U rosaurus ornatus acclim ated

a t 35^0 had significantly lower ra te s of oxygen consumption a t 15*0

than those maintained a t 8*0, Dawson and Bartholomew (1956) sim ilarly

found that geeloporus occidentalis a t 16*0 had a higher ra te of oxygen

consumption when tested a t 33*0 than those acclim ated a t the la tter

tem peratu re, 3h contrast* Gelineo and Gelinee (1956) repo rt what

amounts to an alm ost complete metabolic compensation in two species

of Lacerta,

A behavioral control of physiological compensation to tem pera­

tu re in coldblooded vertebrates has been dem onstrated in p referred -

tem perature work, Garside and T ail (1958) found rainbow trout* after

acclim ation a t 5°C? 10*C# 150C and 20*CS to pref er the tem peratures of

16*C* 15*G, 13*0* and 11*0* respectively, Wilhoft and Anderson (1960)

found gceloporus occidentalis to p re fe r a lower body tem perature (30,1*0)

a fte r acclim ation a t 35*0 than afte r 12*0 and 25*0 acclim ation (33, 7*0

and 33.2*0 respectively). The authors speculate that th is lower p re fe r-

endum accompanies o r is responsible fo r reduced oxygen consumption

(as reported for th is species by Dawson and Bartholomew, 1956) to

prevent "burning o u t.n

In th is study perhaps the higher tem perature values of therm al

response points obtained a fte r constant-tem peratnre acclimation in the

fa ll as com pared to sum m er (Tables 5 and 6; F igs. 8 and 9) rep resen t

a physiological compensation to the differing environmental tem peratures

experienced over these two seasons* On the other hand, the lower accli­

mation index of sum m er anim als, m easured upon collection, compared

with fall (Table 5- Fig. 5) may be indicative of a behavioral compensatory

adjustment. In any case? Vance's (19S3) finding of no difference in

oxygen consumption of fa ll ITrosaurus as compared with sum m er, taken

into consideration with the acclim ation data of this study , points to the

compensatory natu re of rep tilian behavioral therm oregulation. If we

modify the following statem ent by Edwards (1943) to include the concept

of behavioral therm oregulation, then we have a situation in the sand crab

s im ila r to that in U rosaurus; "There a re indications that the effect of

tem perature may vary with season, and in ternal (behaviorally induced?)

changes in the anim al may thus offset the environmental changes in

tem perature in the direction of preserv ing stable m etabolism through the

seasons, ” Ther e is little question that Bullock (1955) is co rr ect in the

assum ption that the wide distribution of natural acclim ation and rela ted

ra te compensation point to th e ir la rge scale ro le in the ecology and

evolution of poikilotherm s.

55

Table 12

REGRESSION AND CORRELATION COEFFICIENTS FOR CTM AND FOR DEATH ON GAPE RESPONSE FOLLOWING VARYING CONDITIONS OF ACCLIMATION, DATA GRAPHED IN FIGS* 10y 11, 14, 15, 18 AND 19 „

ACCLIMATIONCONDITION

METHOD OF LEAST SQUARES BARTLETT’S

N b • . £ P B

F ield

CTMX

Gaping75 0,2795 0,2139 0,1 0.2151

DeathX

Gaping

Constant tem p,

CTM

59 0,2657 0, 8615 0,01 0,2171

XGaping

IS 0,6213 0.5844 0.001 0,5311

' Death ■X

Gaping

Cycled tem p.

78 0,6129 0,8578 0,001 0.2626

CTMX

Gaping42 0,41.50 0,4470 0,01 1.1730

DeathX

Gaping38 0,3529 0,4559 0.01 0.4693

Table 13 and Figure 22 sum up the regressions of CTM and

tem perature of death for field* constant-tenaperature and eycled-temper*-

a tu re acclim ation. A t te s t fo r the difference in the regressions of

CTM vs. gape (B r 0.4139) and death vs* gape (B = 0* 3101), calculated

by the method of Bartletfc^s B, is significant a t the 1% level (Table 14)*

This g rea te r dependence of CTM on acclim ation tem perature may be of

survival value, as compared with the relative tem perature independence

of death. The environmentally influenced CTM, then, increases di­

rec tly with rising, potentially lethal tem perature.

T herm al S tress

\Table 15 and F igure 23 show the resu lts of experim ents designed

to determ ine effects of heat s tre s s . Animals were acclim ated a t 15°C

for four days in an attem pt to lower the tem perature of the gape response.

This was done to m ore readily allow detection of a r is e in the therm al

level of th is response, as influenced by ensuing experim ental conditions.

A fter initially determining the tem perature of the gape response* the

lizards were alternately heated (at the ra te of 0 .6*C per minute) to the

tem perature of the immediately preceding gape r esponse and cooled to

the 15*C acclim ating tem perature a t ca, 0 ,8*C per minute. There w ere

no significant differences in gape response after such treatm ent (Table 15,

Table 13

REGRESSION AND CORRELATION COEFFICIENTS FOR CTM (N=200) AND FOR DEATH #=172) ON GAPE RESPONSE® FOR FIELD, . CONSTANT-TEMPERATURE AND C YCLED-TEMPERATURE

ACCLIMATION DATA. DATA GRAPHED IN FIG« 22,

METHOD OF LEAST SQUARES BARTLETT'S

N b r P B

CTMX

Gape

Death

201 0,4625 0,4752 0,001 0,4139

X. Gape

176 '0,3267 - 0,4443 ! 0,001 0,3101

58

4 54 2 4 44 34 0 41

Fig; 22. Regressions of CTM and tem perature of death on gape response by method of least squares (solid lines) and B artlett’s method (broken lines), including all field, constant-tem perature and cycled-tem perature acclim ation data. Data in Table 13.

Table 1.4

COMPARISON OF THE REGRESSION COEFFICIENTS FOR. GAPE RESPONSE VERSUS CTM AND GAPE RESPONSE VERSUS DEATH BY THE METHOD OF LEAST SQUARES

<fo) AND BARTLETTfS METHOD (B)„DATA SUMMARIZED IN TABLE 13.

. REGRESSION COEFFICIENT t P

b 1.4925 < 0 .2

. B

.......... ■■■ ------------- ---------------------------------1--------:

3,1242 <0 ,01

T able 15

t e m p e r a t u r e o f g a p e r e s p o n s e f o l l o w in g a l t e r n a t in g SUBJECTIONS TOTEMPERATURE INCREASE AND RETURN TO ACCLIMATION TEMPERATURE (15*0),

PATA GRAPHED IN FIG, 23, ,

MAXIMUM BODY TEMPERATURE CG)

DETERMINATIONNO, 1 MEAN RANGE

Tem perature of preceding 1 (Initial) 8 42, 75 ± 0,39 41,35 r- 44.80gaping response 2 8 42,64 ± 0,41 41.30 - 45.05

3 8 42, 26 ± 0 , 32 41,05 * 43. 704 8 42,15 ± 0 ,3 8 40, 55 i- 43, 40

Tem perature of preceding 1 (Initial) 8 43,02 ± 0 ,3 4 41.45 - 44, 65gaping response plus 2*C 2 7 43,96 ± 0,49 42.00 - 45. 70

3 6 42. 76 ± 0 ,2 3 41, 70 - 43. 654 5 42.41 ± 0, 30 41.60 - 43. 50

o 44

43

42

41

4 0

LUCL<O

n m

1 2 3 4T E M P E R A T U R E I N C R E A S E S

(a.)

46

^ 4 5Oo 44

43 -LU

< 4 2O

41

4 0

T E M P E R A T U R E( b . )

3 4I N C R E A S E S

Fig. 23. Succeeding gape responses with body temperature increases alternating with decreases to the acclimating temperature of 15°C. (a) Increase to temperature of pre­ceding gape response, (b) Increase to 2°C above preceding gape temperature. Data in Table 15.

Fig* 23a), although a downward trend is evident. In th is experim ent a l l

the anim als survived. Since U rosaurus is often active a t body tem pera­

tu res close to these levels* these resu lts a r e not surprising .

If, however, the body tem perature each tim e is ra ised to a

point 2*G above the gape tem perature, L e* * to the level of the predicted

CTM* quite different resu lts a re obtained. F igure 23b shows an imm edi­

a te r is e in response a fte r the in itial determination* and a m ore signifi­

cant drop after the second heating period. Also* the anim als die a t a

ra te approaching the LDgg afte r four exposures to the tem perature of

the CTM (Table 15), The resu lts re c a ll Selye?s (1950* and elsewhere)

concept of the alarm reaction and sequential stages of what he term ed

the G eneral Adaptation Syndrome,

It would appear, then, that the tem perature at which gaping

usually occurs does not ac t as a significant ’’s tre s so r” whereas the

somewhat higher tem perature of the c ritica l therm al maximum definitely

does. These resu lts a re quite s im ila r to those for salam anders as

reported by Hutchison (1961), ,

Acclimation and Weight Change

Table 16 a n d Figure 2 4 provide data concerning weight gained

or lost during acclim ation a t the constant tem peratures indicated. As

63

Table 16

PER GEET WEIGHT DIFFEREMGE (LOSS OR GAIN) AFTER SEVEN PAYS ACCLIMATION AT CONSTANT TEMPERATURE.

BATA GRAPHED JN FIG. 2:4, '

ACCLIMATIONTEMPERATURE: E m e a n

f '. ' ' .

RAKGE

15% 13 ~ 9 15 d: 09 86 5a 43 -Mo 53

25% 21 4,0 8 5 db 1< 47 — 8P 68 — —IS *85

36% 31 8.0 84 i: lo 52 -Ho 11 -26.17

35% 19 ^ 1 4 .7 8 * 1 ,9 6 : — Op 82. — —2.8, 41

WE

IGH

T LO

SS

(%)

64

2 0

10 -

5 -

i

015 25 30 35

ACCLI MATI ON T E M P E R A T U R E - C O N S T A N T ( ° C )

Fig. 24. Percent weight loss after 7 days acclimation at different acclimation temperatures. Data in Table 16.

previously stated, anim als were provided with mealworms and water

during the acclim ation periods. In one experiment the anim als w ere fed

before being placed at the acclim ation tem perature of 15°C ; these were

excluded from this determ ination as was a sam ple of anim als acclim ated

to 35°C in a cabinet where re la tiv e humidity dropped to 10% during the

acclim ation period e

It is c lear that the anim als held a t 15PC lost m ore weight than

those acclim ated a t 25#C because of poor eating, or fasting^ a t the lower

tem perature. Otherwise; the increased loss of weight with increased

tem perature of acclim ation reflec ts the increased ra te of metabolism,

and, in th is case, i t is an alm ost linear relationship.

Sex and Body Size

Tables 17, IB and 19 and F igures 2 5, 26 and 27 show the

relationships between body size, sex and c ritica l therm al maximum.

A ll of the acclim ation experiments a re included in these determ inations,

except for the 20® - 4Q®C cycled tem perature group where only two

fem ales were represented in the sample^ In a ll cases but one (spring,

1961 acclimation) the fem ales a re found to have a higher CTM than the

males* But in every sam ple, the mean snout*-vent length of the fem ales

is shorter than that of the m ales, a situation reflective of the population

Table 17

ANALYSIS FOR CTM DIFFERENCES IN MALES AND FEMALES FOLLOWING. THERMAL ACCLIMATION*DATA GRAPHED IN FIGS, 25, 26 AND 27.

CRITICAL THERMAL MAXIMUM

ACCLIMATION CONDITION SEX N MEAN RANGE

FieldFall, ?60 &*61 fem ale

m ale9

1547* 03 ± 0 ,14 46*48 ± 0 .1 8

46.20 - 47,85 4 5 .2 0 " 47,80

Spring, r61 femalem ale

7IS

46.26 ± 0 .1 8 ' 46* 53 ± 0.12

45. 30 - 46. 60 45, 60 - 47, 55

Summer, f61 fem alem ale

' 13 46. 66 ± 0.18 . 46. 31 ± 0,10

45.00 - 47,55 45. 50 ~ 46. 90

Constant tem perature 15*G fem ale

m ale8

1146.07 ± 0. 23 44.76 ± 0 .4 2

45.30 - 47,00 42,20 — 46, 60

25^C femalem ale

813

46.40 ± 0 .1 2 46.38 ± 0,16

46,10 - 47.15 45,30 - 47.20

30eC fem alem ale

1512

47.25 ± 0,07 46, 65 ± 0 .1 2

46.70 - 47.85 46.00 - 47.20

35°C femalemale

1018

47.02 ± 0.17 46. 52 ± 0,23

46.10 - 48,10 4 5 ,2 0 - 47,95

Cycled tem perature 8* - 2 T C female

m ale5 ;6

46,19 ± 0 .3 2 45. 72 ± 0.30

45.00 - 46. 65 44.40 - 46, 50

16^ *- 36^0 fem alem ale

9 . 13

46. 94 ± 0. 10 46,76 ± 0. 12

46. 55 - 47. 40 46.00 - 47,45

67

Table 18

SNOUT-VENT LENGTH (mm) AND SEX FOE THE DATA IN TABLE 17.. DATA GRAFHED M FIGS, 25, 26 AND 27, •

SNOUT-VENT LENGTH

ACCLIMATION CONDITION BEX I MEANi

RANGE

F ie ldF all, * 60 & *61 fem ale 9 40, 22 ± 2*02 35 - 53

male 15 43,73 ± 1, 74 33 - 57Spring, *61 fem ale 7 41,14 ± 1, 98 34 - 48

male 15 46,07A 1,35 37 ~ 52Summer, *61 fem ale 13 49.31 ± 0 ,7 4 46 - 54

male 13 51.08 ± 0.67 4 8 - 56

Constant tem peratur e 15*C fem ale 8 44, 62 ± 2 .2 5 34 *• 50

m ale 11 47.82 ± 1.28 41 - 5325°C fem ale 8 48, 50 ± 1.07 42 - 51

m ale 13 51.23 ± 0 . 69 47 - 5530*C female 15 43. 67 ± 1. 79 34 — 52

m ale 12 45, 50 ± 2. 38 33 - 5535% fem ale 10 42.30 ± 2,06 35 - 51

m ale 18 44.39 ± 1 . 51 30 - 53

Cycled tem perature 8° - 27% fem ale 5 41.20 ± 2 , 50 3 5 - 4 7

m ale 6 48.83 ± 1 , 30 45 - 53

16* - 36% fem ale 9 47 .00± 1,02 43 - 51m ale 13 51,15 ± 0 ,8 6 45 - 55

68

Table 19

REGRESSIONS OF CTM ON SNOUT-VENT LENGTH AND CORRELATIONS FOR CUMULATIVE DATA SUMMARIZED IN TABLES IT AND 18V ■

SEX b E

t--- — -----‘ ’P'-"'

Fem ales -0,0229 0.2167 . ' 0.05

Males Oo0008 0.0059 ; 0.1

Fem ales and Males -0,0148 0.1179 0.1

CTM

(°

C)

S-V

LEN

GTH

(M

M.)

55

50

45

40

35

30

■A69

48

47

464

45

1 -

4 4

FALL, '60 8 '61 SPRING, '61 SUMMER,*6SEASON ( I 9 6 0 , 1961)

Fig. 25. Comparison of females (open) and males (solid) in relation to CTM (below) and snout-vent length (above) for seasonal acclimation. Data in Tables 17 and 18.

CTM

(°

C)

S-V

LEN

GTH

(M

M.)

55

50

45

40

35

30

- 470

- i

48

47

46

45

4 415 25 30 35

ACCLI MATI ON T E M P E R A T U R E - C O N S T A N T ( ° C )

Fig. 26. Comparison of females (open) and males (solid) in relation to CTM (below) and snout-vent length (above) for constant- temperature acclimation. Range for CTM of 15°C males extends to 42. 20°C. Data in Tables 17 and 18.

CTM

(°

C)

S-V

LEN

GTH

(M

M.)

55

50

45

40

35

30

■ . 71

48

47

46

45

448 - 2 7

A C C L I M A T I O N T E M P E R A T U R E16—36

CY CL E D ( ° C )

Fig. 27. Comparison of females (open) and males (solid) in relation to CTM (below) and snout-vent length (above) for cycled-temperature acclimation. Data in Tables 17 and 18.

72as a whole, Snout-vent length was taken as the p referred size m easure­

ment ra th e r than weight since., in a ll the controlled acclim ation experi­

ments ? weight changed variously during the period of acclim ation. The

correlation coefficient of length on weight fo r Urosaurus ornatus is

0. 991 ± 0.006 (Vance, 1953). This relationship is given by the equation

log W = log 0.001 - 3.290 log L.

That the differences in CTM evident here a re not due to sex but

ra ther to body size is suggested by the significant (negative) reg ression

of CTM on snout-vent length in fem ales (Table 19) . This finding alone,

however, may not completely ru le out the possibility of a true sex

difference.

In only two cases (Fig. 27) a re the differences in size between

the sexes significant. And in only two other cases (Fig. 26) a re there

sex differences in CTM, T herefore, it Is assum ed that combining the

data for the sexes, fbr quantitative analysis, is valid.

Eff ect of Rate of Heating

A fter acclimation a t 30°C, c ritica l therm al responses were

determ ined a t the usual ra te of tem perature increase (0 .6°C /m in.) and

compared with those elicited by rheostat-controlled heat increase a t a

slow er ra te of 0 .2°C per minute. The resu lts a re given in Table 20

T able 2 B

THERMAL RESPONSE POINTS AND RATE OF TEMPERATHRE INCREASE (AFTER 30*C ACCLIMATION)^

DATA GRAPHED IN FIG, 28.

RESPONSE AND RATE OF TEMPERATURE INCREASE E MEAN RANGE

Gape0, G C /min« 11 44.23 d: Oo 19 43.00 - 45.40O^B^C/min. 9 43. 74 ± 0 ,2 6 42. 90 - 46. 90

CTM ■0» @®C/min« 10 4.6* 89 * 0*15 46* 00 - 46.900 .20C /m in, 9 46.34 ± 0* 09 46*00- 46*90

Acclimation Index9»6#C/mibou 10 • 45.58 ± 0*16 44*60 - 46.25 "0»2#C/m im 9 45* 04 db 0P16 44* 50 - 45* 85

Death0 ,6*C/min» : IT 48* 80 ± 0*17 47* 50 - 49*30

9 48* 15 ± 0* 19 47. 00 - 49* 10

BODY

T

EM

PE

RA

TU

RE

5 0

4 9

Oo 4 8

4 7

4 6

4 5

4 4

4 3

74

E

GAPE NORIGHTING

DEATH ACCLIMATIONINDEX

T H E R M A L RESPONSE P O I N T S

Fig. 28. Response to heating after 30°C acclimation; at a temperature increase of 0. 6°C/min. (open) and of 0. 2°C/min. (solid). Data in Table 20.

75

and plotted in F igure 28, The fa s te r ra te of heating yields higher values

in each case. Although the Reverse might be expected (e. g . , see Fry*

1942; Hutchison? 1961) due to a longer tim e allowed fo r some acclim ation

a t the slower rate? the lower responses actually found might resu lt from

a decreased tolerance to the higher tem peratures when exposed to them

for a longer period of tim e. Moreover? this is reasonable when i t is

recalled that acclim ation to 35*C yields therm al response points lower

than those obtained by acclim ation at 30*0 (see Table 7), The continuing

need fo r increased standardization of techniques in th is field is obvious.

Summary and Conclusions

The c r itic a l therm al maximum of the tre e lizard TJrosaurus

ornatus linearis (Baird and Girard) was determined upon collection in

the field in southern Arizona during spring? sum m er and fall? and afte r

therm al acclim ation in the laboratory. Animals w ere acclim ated fo r

periods of seven days to the constant tem peratures of 15*0? 25*0? 30*0

and 35*0? and to the diurnally cycled tem peratures of 8* ~ 27*0?

16* - 36*0: and. 20* - 40*0, A to ta l of 312 animals w ere tested? 253

reported on here.

Seven acclimation-conditioned responses to increased tem pera­

tu re w ere studied. T hree of these w ere eventually used to evaluate the

effects of field and experimentally imposed environmental conditions;

in the o rder of the ir natu ral occurrence, these a re (1) gape, (2) loss

of righting response (the c ritica l therm al maximum)j and (3) death. An

acclim ation index, the weighted mean of the f irs t two responses above,

has also been found to be useful as a m easure of acclim ation.

Information concerning the effect on the c ritica l therm al

maximum (CTM) of ra te s of heating and of repeated exposure to high

tem perature was also obtained* as was com parative weight loss data

over the period of acclim ation.

The homeostatic function of behavioral therm oregulation in

Urosaurus ornatus was found to be m easureable by c ritica l therm al

response. Data a re presented to show that such therm oregulation m asks

the effect of seasonal tem perature change. J r fall and spring, c ritica l

therm al responses as influenced by m ean field diurnal body tem peratures

a re found to closely approximate those experimentally controlled. T her­

moregulation, for reasons discussed, appears to operate m ore efficiently

in these seasons than in the sum m er.

Seasonal differences after acclim ation a t 30°C and 35*C a re

shown and the ir possible compensatory nature is speculated upon.

Animals acclim ated a t 15*C, 25*0 and 30*0 respond in a linear

m anner in CTM, yielding values Of 45.31*0, 46* 39*0 and 46.98*0

respectively. At 35*0 acclim ation the CTM fa lls to 46. 86*0, significantly

77below the value fo r 30°C» Thus the n ear-eco ritic body tem perature of

35^0, when imposed over a number of days, appears to be stressfuL

When anim als a re dium ally cycled for seven days over the

ranges of 8° - 27*0, 16* - 36*0 and 20* ~ 40*0, the c r itic a l therm al

responses a re found to differ insignificantly from the corresponding

values of constant-tem perature acclim ation. This relationship, in regard

to field tem perature re su lts , is discussed.

Significant correlations between the in itial c ritica l therm al

point and CTM and death wer e found under a ll conditions of acclim ation.

When the tem peratures of the c ritica l therm al maxima and of

death a re trea ted as variables plotted upon the tem perature of gaping,

no significant difference is found between these two reg ressions. When,

however, a ll of these therm al response points a re plotted on tem perature

of acclim ation, the regression (by the method of B artleW s B) of death

on gape is significantly lower than that of CTM on gape. The apparent

acelim atability of the lower, ecologically im portant c r itic a l therm al

points (gape, loss of righting response) and the rela tive tem perature

independence of the physiologically significant death point a re discussed.

As m easured by the successive tem peratures of occurrence of

gaping, a fte r repeated body tem perature increase to tha t point, tem pera­

tu re a t gape (after 15*C acclimation) does not appear to unduly s tre s s

the anim al. Repeated exposure to the tem peratures of the critica l

therm al maximum* however* does appear to elicit an "a larm " reaction.

Under sim ila r conditions of rela tive humidity and feeding*

anim als acclim ating a t 35^C lost m ore weight than anim als acclimating

a t 30°G and the la tte r lost m ore weight than those a t 25#C, On the other

hand* 15*C acclim ating Urosaurus lost m ore weight than those a t 25*0

since* a t the lower tem perature* they presum ably ate le ss (or nothing

a t all) .

Mo correlation of body size and CTM was found, in m ales, A

Significant negative regression of CTM on Size in fem ales was found.

This provides some evidence that the higher CTM for f emales* as com­

pared with those of males* reflects the consistently sm alle r sizes of

the fem ales collected fo r each experiments

C ritica l therm al maxima were determ ined fo r 30*0 acclim ated

anim als by raising the body'tem perature 0,2*0 p e r minute. This was

compared to the determ ination made over a 0,6*0 per minute ra te of

heating (the standard ra te used throughout th is study) of identically

acclim ated lizards. The CTM of the sam ple m ore slowly heated a t 0.2*0

per minute was 46,34*0 which is significantly lower than the 46,89*0

fo r the sample heated a t the faste r ra te . The rev e rse might be expected

on the basis of e a rlie r assum ptions fo r fishes and amphibians; i. e. *

longer heating a t a slower ra te might yield a higher CTM. However* the

g reater amount of tim e spent a t the higher and m ore s tressfu l tem pera-

tu res (which do yield lower CTM; see Table 7, Fig. 12) apparently

offsets any acclim ation resulting from slow er heating.

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