zyxwvu zyxw zyxwv zyxw QUANTITATIVE GENETIC ANALYSIS OF TEMPERATURE REGULATION IN MUS MUSCULUS. I. PARTITIONING O F VARIANCE zyxwvuts ROBERT C. LACY1 AND CAROL BECKER LYNCH2 Deparment of Biology, Wesleyan University, Middletown, Connecticut 06457 Manuscript received November 14, 1977 Revised copy received October 31, 1978 ABSTRACT Heritabilities (from parent-offspring regression) and intraclass correlations of full sibs for a variety of traits were estimated from 225 litters of a heterogeneous stock (HS/Ibg) of laboratory mice. Initial variance partitioning suggested different adaptive functions for physiological, morphological and behavioral adjustments with respect to their thermoregulatory significance. Metabolic heat-production mechanisms appear to have reached their genetic limits, with little additive genetic variance remaining. This study provided no genetic evidence that body size has a close directional association with fitness in cold eiivironments, since heritability estimates for weight gain and adult weight were similar and high, whether or not the animals were exposed to cold. Behavioral heat conservation mechanisms also displayed considerable amounts of genetic variability. However, due tc strong evidence from numerous other studies that behavior serves a n important adaptive role for temperature regulation in small mammals, we suggest that fluctuating selection pressures may have acted to maintain heritable variation in these traits. EVOLUTIONARY adaptation involves not simply the acquisition of advantageous traits, but the development of an adaptive strategy composed of an array of phenotypic modifications that interact with one another. Genetic studies 1956) should, of adaptive trait “complexes” or “syndromes” (DOBZHANSKY therefore, yield more information about the nature of evolutionary change than 1975b). Regarding adaptawould studies of isolated traits (BERRYand JAKOBSON tion to a specific environmental variable, temperature regulation represents a definable adaptive syndrome. The behavioral and physiological mechanisms exploited by homeotherms f o r cold-weather survival have been described, and several reviews are available (HART1957, 1971; IRVING 1964; CHAFFEE and ROBERTS 1971; CABANAC 1975). Homeotherms normally maintain body temperature within a narrow range around 37”, and this regulation requires a balance between heat production and heat loss, Heat production can be achieved by muscular contractions (shivering or gross activity) or by nonshivering thermogenesis (all metabolic heat production not coupled to muscular contraction). Heat loss can be reduced by vasomotor adjustments or increased insulation (fur and sub- ‘ Present address Section of Ecology and Systematics, Langmuir Laboratory, Come11 University, Ithaca, New York 14853 * To whom reprint requests should be addressed. Genetics 91 : 743-753 Apnl, 1979. 744 zyxwvuts zyxwv R. C. LACY A N D C. B. L Y N C H cutaneous fat). Small mammals, however, have large area to mass ratios, but cannot support the thick layer of fur or fat required to reduce heat loss substantially. Therefore, they must rely on behavioral modifications such as microhabitat selection, burrowing, nest-building, and huddling to minimize heat loss. Physiological studies of temperature regulation have been largely restricted to observation of specific changes induced by acute or chronic cold exposure. These studies have provided descriptions of the range of potential effectiveness for individual physiological components of the thermoregulatory syndrome. A clearer understanding of the adaptive integration of these components may be obtained by employing genetic analysis to gain insight into the evolutionary history of adaptation to low ambient temperatures by small mammals. Comparisons between species have formed the traditional approach to understanding the effects of prior natural selection on temperature regulation. In contrast, natural selection acts on genetic variation within species. Quantitative genetic analysis of thermoregulatory abilities in a single species potentially provides the most relevant information about evolutionary adaptation. At minimum, such analysis can reveal the nature and extent of genetic variation present in extant populations for these continuously varying traits. The house mouse (Mw musculus) is the ideal organism for quantitative genetic analysis of temperature regulation since the traits involved have been thoroughly described for small rodents (HART1971), a considerable amount is known about the effects of the environment on evoking these responses (CHAFFEE and ROBERTS1971), and many genetically diverse stocks are available. Our present limited knowledge of intraspecific genetic variation for individual thermoregulatory traits has been obtained from inbred strains of Mus musculus. Observations of strain differences indicated some additive genetic variance for nest building (LYNCHand HEGMANN 1972), thermal preference and body temperature (SILCOCK and PARSONS 1973) , and cold-temperature reproduction and survival (BARNETTand MANLY1956). Also in these studies, comparisons of F, hybrids with inbred parent strains revealed differing amounts of heterosis among the traits. We have used a variety of quantitative genetic analyses in laboratory stocks of Mus musculus to partition both the variance of several thermoregulatory traits and the covariance among them into environmental, additive and nonadditive genetic causal components. This paper deals with partitioning the phenotypic variance by a.~alysisof covariance among relatives for traits associated with temperature regulation, in a genetically heterogeneous stock of Mus musculus. zyxwv zyxwv zyxwvut MATERIALS A N D METHODS Subjects zyxwv Experimental animals came from a heterogeneous stock of laboratory Mus musculus, HS/Ibg, originally derived from an eight-way cross among inbred strains (MCCLEARN, WILSON and MEREDITH 1970) and maintained with the condition that no mice with a common grandparent be mated. Three generations consisting of 249 mice from the 21st generation (HS21), 498 mice from the 22nd generation (HS22) and 424 mice from the 23rd generation (HS23) were used in this study. zy zyxwv zyxwv zyxw zy GENETICS O F T E M P E R A T U R E REGULATION 745 Mice were housed in polypropylene small animal cages provided with about 500cc of wood shavings. Food (Wayne Lab Blox) and water were available ad libitum, and the photoperiod was maintained at 16L:8D. Mice were reared at a temperature of 21 k I", and weaned a t 25 days of age. Litters representing HS22 and HS23 were maintained for a week after birth in cotton nests built by their mothers, and were culled tu two males and two females per litter a t weaning. Testing procedures Eight traits associated with temperature regulation were measured, requiring about eight to nine weeks of testing. Nesting: At 50 days of age, mice were housed singly and a preweighed amount of cotton batting placed i n the Iood hopper of the cage lid. The cotton remaining was weighed each day, and the old nest removed. More cotton was added to the hopper if necessary. Nesting was indexed as the total weight of cotton pulled from the hopper across four days. Animals were then earpunched for identification, housed four per cage, and moved to a cold room ( 4 f I " ) where they remained for the duration of testing. Following three weeks of cold acclimation, the remaining tests were completed. Thermal preference: The temperature preference apparatus consisted of four covered alleys, each 130 x 7 cm, separated by opaque plexiglass. Because some animals prefer resting in corners, small partitions were placed at 5 cm intervals along the length of each alley (see LYNCHet d. 1976, for a picture of the apparatus). Floor temperatures were maintained in an approximately linear gradient from 24, to 42" by adjusting current flow through a heating tape beneath the aluminum floor of the board. Mice were allowed three hours to adjust to the alleys, after which the temperature ccjrresponding to the position of each mouse was recorded at ten minute intervals €or one hour. All observations during which the mouse was not moving were averaged to provide an index of thermal preference. Basil metaboZic rate: Mice were fasted for three hours, then placed in metabolic chambers and allowed an additional hour to adjust. Metabolic rate was indexed i n closed chambers a t 32" a s the time required for an animal to consume 3 cc of oxygen. Eight o r nine successive measurements were taken for each mouse, and the three contiguous slowest times were averaged. Expired carbon dioxide and water vapor were adsorbed by a mixture of Ascarite and Drierite. All measurements were made between noon and 5:OO P.M. and were expressed as cc 0, per gram body weight per hour. Nonshivering thermogenesis: This was assessed immediately following establishment of basal rate by the norepinephrine test (BRUCK1970; MEJSNARand JANSKP1971). Each mouse was injected with 0.6 mg norepinephrine (Levophed) per kg body weight, and 0, consumption measured immediately as described above. Nine or ten measurements were taken and the three contiguous fastest times were averaged. Extent of nonshivering thermogenesis was indexed a s the maximum increase in oxygen consumption above basal levels. Food consumption: Mice were allowed approximately one week following metabolic testing before food consumption was measured (at 4"). Mice were housed singly and food i n the hopper was weighed on day zero and again four days later. Measurements were expressed as weight of food consumed over the four days per gram body weight. Body weight: Body weight was measured on a small animal balance several times during the experiment. Of most interest were measurements taken a t 50 days of age (before cold acclimaticn), and at 110 days of age (after 8.5 weeks of cold acclimation). Body temperature: Animals were killed by cervical dislocation, and rectal body temperatures measured immediately with a #402 small animal probe (inserted 3.5 cm) connected to a telethermometer (Yellow Springs Instruments). All body temperatures were taken between 2:OO PM and 5:OO P.M. Brown adipose tzssue: The interscapular brown fat pad was removed, the lipid component extracted in three daily changes of a 2: 1 chloroform: methanol solution, and the remaining tissue dried for 4.5 hoars i n a vacuum oven at 90". The resulting lipid-free dry weight of the brown zyxwvutsr zyxwvuts zy zyxwvuts 746 zyxwvuts R. C. LACY A N D C. B. L Y N C H zyxwvutsrq zyxwvuts zyxw zyxwvutsr zyxw zy zyxw adipose tissue was expressed as mg per gram body weight. CWAFFEE and ROBERTS (1971) reported that lipid-free dry weight most accurately indexes this organ’s thermogenic capacity in small rodents. Females were saved for mating and were not measured for body temperature or brown adipose tissue. Data analysis Summary statistics, regressions of litter means on single parents (weighted by litter size), and intraclass correlations of full sibs were calculated for each variable. Since there were significant sex differences in nearly all traits, statistics were calculated separately for males and females (e.g., regression of females on mothers) and then pooled across sex. Heritability estimates for each variable were obtained by doubling the regression of offspring on parent scores, and standard errors were calculated according to FALCONER (1960). The intraclass correlation of full sibs was used to provide a rough estimate of combined nonadditive and common environmental variance. Full sibs share on average 1/4 of their dominance conibinations and a common preweaning environment, as well as 1/2 of their alleles, and some epistatic effects (due to interlocus interactions). Parents and offspring share only 1/2 of their alleles and consequently less epistatic effects than do full sibs (FALCONER 1960). We estimated a parameter that we have called “dominance determination” (dz) from four times the difference between the intraclass correlation of full sibs and the parent-offspring regression: d2 = 4 ( t F s- bop). The parameter d2 is then equivalent to (V,, 4VE,,)/Vp, ignoring any component due to epistasis. This provides a crude estimate of the importance of dominance variance for traits in which both epistasis and common environmental effects are minimal. + RESULTS Descriptiue statistics Means and standard errors for all variables, along with sample sizes, are listed in Table 1. Occasional problems with equipment obviated accurate measurements of some variables on one of the three generations tested. In these cases, the scores for that variable were omitted and the corresponding space on Table 1 left blank. Sample sizes differ since accurate measurements were not always possible and some animals died during the course of the experiment. For several of the variables, significant differences appeared between the generations, and a few of these intergeneration differences can be accounted for. For example, HS21 mice were not raised in nests, while HS22 and HS23 were (due to testing of the mothers for maternal nesting), and it is possible that this difference in preweaning environment may have permanently altered the behavior of the adult animals (LYNCH,STURGIS and POSSIDENTE 1977). Another known difference was that male mice of HS2l and HS22 were mated prior to being killed, while mice of HS23 were never used for mating. Heritability Estimates of heritability and their standard errors for each variable pooled across generations are listed in Table 2. For no variable was the heritability estimated from females significantly different from that of males. Adult body weight had the highest heritability (h2= 0.41), although heritability of weight gain during cold acclimation was somewhat lower (hZ= 0.19). Nesting, thermal preference and food consumption all had intermediate herita- s 5 h a 2 s3 bn E zy zyxwvut zyxwvu 747 GENETICS O F T E M P E R A T U R E REGULATION zyxwvu zyxw zyxwvuts zyxwv . I U Y $ 2 a" $ 3 ?$ 2 % S? . ? I '3 2 -34 2 Q ;. $ w il $.k. t-r 22 U - $ &$ p v) h s -22 7 % C O e$ .$a zyxw 3; g & U $1 L * 2 'v a g s 2a 4 ."2 zy 4 v) E .e 0 m Lu Fc P 5 2 * zyx 748 zyxwvutsr zyxwv zyxwv z R. C. LACY AND C. B. L Y N C H TABLE 2 zyxw zyx Heritabilities (twice parent-offspring regression) and intraclass correlations of full-sibs (f standard errors) for variables associated with temperature regulation, pooled (where possible) across three generations of a heterogeneous stock of Mus musculus Variables Nesting-score? Thermal preference+ Basal metabolic rate* Nonshivering thermogenesis* Body weight I* Body weight I1 Weight gain’ Food consumption Body temperature$ Brown adipose tissue3 ?/d Heritabilities Pooled 0.32 f 0.18 0.31 f 0.15 0.29 t 0.21 0.37 t- 0.19 0.02 t 0.05 0.11 & 0.09 0.01 f 0.15 0.23 f 0.17 0.44 t 0.18 0.19 & 0.18 0.43 t 0.08 0.M & 0.08 0.05 -C 0.17 0.33 f 0.20 0.30 t 0.07 0.24 f 0.07 -0.20 f 0.14 0.08 f 0.06 0.31 f 0.08 0.33 t 0.10 0.08 t 0.06 Intraclass correlations Pooled ?/d 0.23 2 0.06 0.29 f 0.06 0.26 10.06 0.16 f 0.07 0.27 t 0.07 0.08 t 0.08 0.28 f 0.08 0.08 I: 0.08 0.56 .t 0.03 0.57 1 0.02 0.50 & 0.05 0.57 1: 0.04 0.27 3- 0.03 0.48 t 0.02 0.28 2 0.06 0.25 t 0.06 0.19 2 0.07 0.08 -+ 0.07 0.26 -C 0.04 0.21 t 0.05 0.18 t 0.05 zyxwvu 0.09 f 0.08 0.30 f 0.09 0.41 & 0.05 0.19 f 0.09 0.28 t 0.04 0.19 % 0.05 0.57 t 0.01 0.54 t 0.03 0.37 t 0.01 0 27 ‘-t 0.04 and HS22 only. +3* HS21 HS22 and HS23 only. Males only. bilities of approximately 0.3. The two metabolic variables, basal metabolic rate and nonshivering thermogenesis, had low heritabilities just slightly different from zero. The heritability estimate for amount of brown adipose tissue was imprecise and i1ot significantly greater than zero, while that of body temperature had a slightly negative empirical value. These last two variables showed large intergeneration mean differences (Table 1),indicating an influence of unknown environmental fluctuations, which may have biased the regressions. Nonadditive and common environmental variance Intraclass correlations of full sibs pooled across generation for each variable are also listed in Table 2, with standard errors calculated according to FALCONER (1960). Weight gain and body weight had the highest intraclass correlations; the behaviors of nesting, thermal preference and food consumption all had cor- zyx relations around 0.25 ;basal metabolic rate, nonshivering thermogenesis and body temperature had intraclass correlations just under 0.20; and amount of brown adipose tissue had a correlation of less than 0.10. Estimates of “dominance determination” are listed in Table 3. Body weight had an estimate of d’greater than the theoretical upper limit of 1.O, undoubtedly due to a considerable common environmental component to the phenotypic vari- zyxw zyxwv zyxwvuts zyxwvuts zyxwvu zy 749 GENETICS O F T E M P E R A T U R E REGULATION TABLE 3 Estimates of “dominance determination’’ (d2) from four times the differencebetween the intraclass correlation of full sibs and the parent-offspring regression for variables associated with temperature regulation in a heterogeneous stock of Mus musculus e Variables Nesting Thermal preference Basal metabolic rate Nonshivering thermogenesis Body weight I Body weight I1 Weight gain Food consumption Body temperature Brown adipose tissue * Assuming heritability=O. 0.42 0.18 0.56 0.58 1.68 1.36 0.66 0.52 0.76; 0.16 zyx ance (MONTEJRO and FALCONER 1966). Body temperature had a d2 value around 0.75, while nesting, basal metabolic rate, nonshivering thermogenesis, and food consumption all had values around 0.5, indicating substantial dominance (nonadditive) and/or common environmental variance influencing those traits. Thermal preference and amount of brown adipose tissue had the lowest estimates of d 2 . DISCUSSION The information obtained from partitioning of covariance among relatives can be examined in terms of the nature of genetic variance predicted from considerations of evolutionary theory. In his “fundamental theorem of natural selection,” FISHER(1930) showed that the rate of change in fitness is proportional to the additive genetic variance in fitness. Thus, additive genetic variance of fitness itself should approach zero as a population reaches equilibrium. This theorem implies that traits that are closely and directionally related to fitness will exhibit low heritabilities (ROBERTSON 1955). However, traits with intermediate optima may either display considerable additive genetic variance, with the intermediate heterozygotes favored, or may have reduced additive genetic variance if genes for plus and minus phenotypes have become fixed at random, resulting in an intermediate phenotype for polygenically determined traits (ROBERTSON1955). This theorem also assumes continued directional selection. Characters closely related to fitness are also expected to show high levels of dominance variance. This may be due either to the selection for genes that will reduce the effect of deleterious alleles when in a heterozygous state (ROBERTSON 1955) or to an inherent advantage in heterozygosity (LERNER 1954), perhaps mediated by a buffering against environmental fluctuations. In any event, heterosis and its converse, inbreeding depression, are typically found associated with fitness characters (FALCONER 1960). 75 0 R. C. LACY A N D C. B. LYNCH zyxwv Estimates of genetic variance from long-established laboratory populations may not reflect the effects of natural selection on wild populations of the species. We have assumed that traits involved in temperature regulation have not been exposed to strong directional selection during the process of domestication, and laboratory stocks may reflect at least the relative ordering of trait heritabilities in wild populations. On this basis, we found that the traits measured fell into three rather distinct categories with respect to genetic influences. The three behaviors, nesting, thermal preference, and food consumption all had intermediate heritabilities (Table 2). Nesting and food consumption both exhibited considerable amounts of dominance variance, by our crude method of estimation (Table 3 ) . Since physiological studies have demonstrated the importance of behavioral responses f o r cold-weather survival of small mammals (cf., SEALANDER 1952, HAYWARD 1965, GLASERand LUSTICK1975), we assume that these behaviors have either been selected for intermediate optima (highly likely for thermal preference) or have been subjected to fluctuating selection pressures. Clearly, the ideal size for an individual thermoregulatory nest should vary with season, and possibly even from year to year. Metabolic demands associated with activity during winter would also result in differing levels of food consumption from that appropriate during the summer. One report exists of seasonal fluctuations in gene frequencies in a feral population of Mus musculus (BERRYand JAKOBSON1975a). and scme evidence for fluctuating gene frequencies in response to alternating selection pressures has been found for voles (MYERSand KREBS1971; GAINESand KREBS1971). In light of the large seasonal changes in much of the world, one should not assume that species with short generation times have been brought steadily toward genetic equilibrium by directional selection. Thus, the use of mathematical models that assume genetic equilibria, whether selectionist or neutralist, may have limitations for explaining the presence of multiple alleles in wild populations. I n fact, several recent studies have shown that multiple, non-neutral alleles may be maintained in a population when only one or two forms enjoy a selective advan1970; BRYANT tage over the others during each generation (MAYNARD-SMITH 1974; GILLESPIE 1977). This hypothesis for the maintenance of heritable variation by fluctuating selection pressures must be tested on wild populations. LYNCH(1 977) showed that wild-trapped Mus musculus suffered extensive inbreeding depression in nesting and production characters, but obtained no estimates of additive genetic variance. The genetic architecture of other thermoregulatory traits in wild populations has not been investigated. The physiological traits of basal metabolic rate, nonshivering thermogenesis, amount of interscapular brown adipose tissue, and body temperature showed a different genetic profile, with little or no additive genetic variance and varying amounts of dominance variance (Tables 2 and 3 ) . It appears that the physiological heat-producing capabilities of Mus have been under intense natural selection, eliminating virtually all additive genetic variance from the population. Likewise, if we assume that the actual heritability of body temperature is near zero, there zyxwvut zy zyxwvutsr zyxw GENETICS O F TEMPERATURE REGULATION zyxw 75 1 must have been strong selection pressure against any deviation from the normal body temperature for the species. Body weight (whether measured b e h e or after cold acclimation) fits a third model exhibiting both a fairly high heritability and a very high intraclass correlation (Table 2). The heritability agrees with values reported by FALCONER (1973) from parent off-springregressions on a different heterogeneous stock and from the average realized heritability of six replicates of bidirectional selection. The relatively larger intraclass correlation and the lower heritability of the body weight of younger animals indicate that there is a greater expression of maternal influences at this age, resulting in the higher d2value. The increase in body weight during the six weeks of cold acclimation had a lower heritability, with a high intraclass correlation (Table 2). Other investigators have obtained similar heritability estimates for postweaning weight gain under standard laboratory temperatures, both from parent-off spring regression (DUNNINGTON, WHITEand VINSON1976) and in response to selection (LASALLE, WHITE and VINSON1974). The high heritabilities and close agreement with estimates obtained a t warmer temperatures lend credence to the argument that body size is of limited importance in temperature regulation ( SCHOLANDER 19%). Similarly, BAKERand COCKREM (1970) obtained a realized heritability for body weight in the cold that was actually greater, but not significantly so, than that obtained at room temperature. However, mice selected in the cold failed to show the expected correlated response in tail length, suggesting the existence of genes that act on body weight because oI their influence on thermoregulation independently of genes influencing other components of body size. I n support of a directional association of body weight and thermoregulation, BIGHAMand COCKREM (1969) found that large mice had superior thermoregulatory abilities in the cold; they maintained higher body temperatures and more survived. BARNETT et al. (1975) demonstrated that wild-derived mice maintained in the cold for ten generations were genetically heavier than mice kept at room temperature. Clearly the issue of the relationship between body weight and adaptation to cold has not been settled. Measurement of multiple traits allowed for an initial intra-experimental comparison of variance components, and we concluded that behavioral, physiological and morphological traits have been under somewhat different selection pressures with respect to their roles in temperature regulation. Such comparisons are possible only when all traits are measured on the same population under standardized conditions, although tests of the generality of our conclusions with respect to natural selection will require further studies on wild populations. The major question raised by these data is why the behavioral components of the adaptive syndrome of thermoregulation showed considerable heritable variation, while most of the genetic variance in metabolic adjustments had been exhausted. I n addition to the hypothesis of fluctuating selection pressures, the explanation could lie in the greater inherent flexibility of behaviors that can be turned on and off rapidly, as opposed to metabolic modifications that may take days or weeks to complete. zyxw zyxwv zyxw zyxw zy 752 zyxwvutsr zyxwv R. C. LACY AND C. B. L Y N C H zyxwvutsrq zyxwvutsr Part of this work was from a thesis submitted by the senior author in partial fulfillment of the requirements for an M.A. degree at Wesleyan University. The research was supported by Public Health Service Grant GM 21993. We thank LAURATENNEY for excellent technical assistance, G. ROBERTLYNCHfor consultation regarding physiological measurements, and R. L. BAKERand R. C. ROBERTS for commenting on a draft of the paper. LITERATURE CITED B.~RF.R, R. L. and F. R. M. COCKREM, 1970 Selection for body weight in the mouse at three temperatures and the correlated response in tail length. Genetics 65: 505-523. BARNETT, S. A. and B. M. MANLY,1956 Reproduction and growth of mice of three strains, after transfer to -3". J. Exp. Biol. 33: 325-329. BARNETT, S. A., K. M. H. MUNRO,J. L. SMARTand R. C. STODDART, 1975 House mice bred for many generations in two environments. J. Zml. 177: 153-169. BERRY,R. J. and M. E. JAKOBSON, 1975a Ecological genetics of an island population of the House mouse (Mus musculus). J. Zool. (Lond.) 175: 523-540. -, 1975b Adaptation and adaptability in wild-living House mice (Mus musculus). J. Zool. (Lond.) 176: 391-402. BIGHAM,M. L. and F. COCKREM, 1969 Body weights, tail lengths, body temperatures, food intakes, and some slaughter data for four strains of mice reared at three different environmental temperatures. New Zealand J. Agric. Res. 12: 658-668. BRUCK,K. 1970 Nonshivering thermogenesis and brown adipose tissue in relation to age, and their integration in the thermoregulatory system. pp. 117-154. In: Brown Adipose Tissue. Edited by 0. LINDBERG. American Elsevier, New York. BRYANT, E. H., 1974 On the adaptive significance of enzyme polymorphisms in relation to environmental variability. Am. Naturalist 108: 1-19. CABANAC, M.,1975 Temperature regulation. Ann. Rev. Physiol. 37: 415-439. CHAFFEE,R. R. J. and J. C. ROBERTS, 1971 Temperature acclimation in birds and mammals. Ann. Rev. Physiol. 33: 155-197. DOBZHANSRY, TH.,1956 What is an adaptive trait? Am. Naturalist 90:337-347. zyxwvu zyx zyxwvut DUNNINGTON, E. A., J. M. WHITE and W. E. VINSON,1977 Genetic parameters of serum cholesterol levels, activity and growth in mice. Genetics 85: 659-668. FALCONER, D. S., 1960 Introduction to Quantitative Genetics. Ronald Press, New York. 1973 Replicated selection for body weight in mice. Genet. Res. 22: 291-321. -, FISHER, R. A., 1930 The Genetical Theory of Natural Selection. Clarendon Press, Oxford. GAINES,M. and C. J. KREBS,1971 Genetic changes in fluctuating vole populations. Evolution 25: 702-723. GILLESPIC, J. H., 1977 A general model to account for enzyme variation in natural populations. Evolution 31 : 85-90. GLASER, H.,and S. LUSTICK,1975 Energetics and nesting behavior of the northern white-footed mouse, Peromyscus leucopus noveboracensis. Physiol. Zool. 48:105-113. HART,J. S., 1957 Climatic and temperature induced changes in the energetics of homeotherms. Revue Can. de Biol. 16: 133-174. -- , 1971 Rodents. pp. 1-151. In: Camprrrative Physiology of Thermoregulation, vol. 11: Mammals. Edited by G. C. WHITTOW. Academic Press, New York. HAYWARD, J. S., 1965 Microclimate temperature and its adaptive significance in six geographic races of Peromyscus. Can. J. Zool. 43: 341-350. zyx zy zyxwvutsr zyxwvu zy zyxwvu zy GENETICS O F TEMPERATURE REGULATION 753 IRVING, L., 1964 Terrestrial animals in cold: birds and mammals. pp. 361-377. In: Handbook oj Physiology, Sect. 4 : Adaptation to the Environment. Edited by D. B. DILL. Amer. Physiol. Soc., Washington, D.C. LASALLE, T. J., J. M. WHITEand W. E. VINSON,1974 Direct and correlated responses to selection for increased postweaning gain in mice. Theoret. Appl Genet. 39: 306-314. LERNER, I. M., 1954 Genetic Homeostasis. John Wiley and Sons, Inc. New York. LYNCH,C. B., 1977 Inbreeding effects upon animals derived from a wild population of Mus musculus. Evolution 31 : 526537. LYNCH,C. B. and J. P. HEGMANN, 1972 Genetic differences influencing behavioral temperature regulation in small mammals. I. Nesting by Mus musculus. Behav. Genet. 2: 43-53. LYNCH,C. B., G. H. STURGIS and B. P. POSSIDENTE, 1977 Genetic and maternal influences on thermoregulatory and maternal nest-building in Mus musculus. Behav. Genet. 7:76. LYNCH,G. R., C. B. LYNCH,M. DUBEand C. ALLEN,1976 Early cold exposure: Effect on behavioral and physiological thermoregulation in the house mouse, Mus musculus. Physiol. Zool. 49: 191-199. MAYNARD-SMITH, J., 1970 Genetic polymorphism in a varied environment. Am. Naturalist 104: 487-490. MCCLEARN, G. E., J. R. WILSONand W. MEREDITH, 1970 The use of isogenic and heterogenic mouse stocks in behavioral research. pp. 3-22. In: Contribution to Behavior-Genetic Analysis: T h e Mouse as a Prototype. Edited by G. LINDZEY and D. D. THIESSEN. AppletonCentury-Crofts, New York. MEJSNAR, J. and L. JANSK+,1971 Methods f o r estimating of nonshivering thermogenesis. pp. 27-38. In: Nonshivering Thermogenesis. Edited by L. JANSKP.Academia, Prague. MONTEIRO, L. S . and D. S. FALCONER, 1966 Compensatory growth and sexual maturity in mice. Anim. Prod. 8: 179-192. MYERS,J. H. and C. J. KREBS,1971 Genetic, behavioral and reproductive attributes of dispersing field voles Microtus pennsylvanicus and Microtus ochrogaster. Ecol. Monogr. 41: 53-78. ROBERTSON, A., 1955 Selection in animals: Synthesis. Cold Spring Harbor Symp. Quant. Biol. 20 :225-229. SCHOLANDER, P. F., 1955 Evolution of climatic adaptation in homeotherms. Evolution 9: 15-26. SEALANDER, J. A., 1952 The relationship of nest protection and huddling to survival of Peromyscus at low temperature. Ecology 33: 67-71. SILCOCK, M. and P. A. PARSONS, 1973 Temperature preference differences between strains of Mus musculus, associated variables, and ecological implications. Oecologia 12 : 147-160. zyxwvuts Corresponding editor: R. W. ALLARD