Thermoregulatory Competence and Behavioral Expression in The

DEV (WILEJ) RIGHT INTERACTIVE

Mark S. Blumberg Greta Sokoloff Department of Psychology Thermoregulatory Competence The University of Iowa Iowa City, Iowa 52242 and Behavioral Expression in the Young of Altricial Species—Revisited

Received 24 July 1997; accepted 27 October 1997

ABSTRACT: The behavioral and physiological thermoregulatory capabilities of newborn and infant mammals have been studied for over half a century. Psychobiologists have noted that the infants of altricial species (e.g., rats) have physical and physiological limitations such that heat loss overwhelms heat production, thus forcing a reliance on behavioral thermoregulation for the maintenance of body temperature. Recent evidence, however, suggests that a modi®- cation of this view is justi®ed. Speci®cally, throughout a range of moderately cold air temperatures, nonshivering thermogenesis by brown adipose tissue contributes signi®cantly to the infant rat's behavioral and physiological adaptations to cold challenge. Given the prominent use of altricial species for the study of infant behavior, increased understanding of the infant's physiological responses to cold and the effect of thermal factors on behavior is warranted. ᭧ 1998 John Wiley & Sons, Inc. Dev Psychobiol 33: 107±123, 1998

Keywords: rat; hamster; thermoregulation; brown adipose tissue; homeostasis; development

INTRODUCTION protecting the brain from overheating, including the venous shunts of horned lizards (Heath, 1966) and the Like oxygen and gravity, temperature is a basic feature rete mirabile of many mammals (Baker, 1979). of the environment that shapes and constrains biolog- Threats of extreme cooling have led to the evolution ical activity. Temperature in¯uences cellular metabo- of similarly inventive solutions, such as a brain heater lism (Schmidt-Nielsen, 1990) and muscular contrac- tissue in sword®sh (Carey, 1982) and ªantifreezeº in tion (Bennett, 1984), it modulates many aspects of the blood of coldwater ®shes (DeVries, 1982). neuronal function from conduction velocity to refrac- Temperature is an especially important factor dur- tory period (e.g., Swadlow, Waxman, & Weyand, ing development. During embryonic development, and 1981), and it even guides sexual differentiation in regardless of whether we are considering a chick in- many reptiles (Deeming & Ferguson, 1989). The sen- side its egg or a human infant in utero, the thermal sitivity of neural tissue to high temperatures has made environment is maintained within narrow limits by the necessary the evolution of elaborate mechanisms for parent while the embryos are passive recipients of the heat supplied to them (Leon, 1986). With hatching or birth, however, young emerge from their thermally se- Correspondence to: M. S. Blumberg Contract grant sponsor: NIMH cure environments into a world where heat loss is a Contract grant number: MH 50701 constant threat to survival. Suddenly, they must now short ᭧ 1998 John Wiley & Sons, Inc. CCC 0012-1630/98/020107-17 standard long DEV (WILEJ) LEFT INTERACTIVE

108 Blumberg and Sokoloff become active participants in the regulation of their within which they were working. As was mentioned body temperature. earlier, there has been a tendency to assess thermoreg- The thermoregulatory capabilities of newborn and ulatory capabilities based on the maintenance of a sta- infant mammals have been the subject of experimental ble body temperature in the cold. But ªcoldº is a rel- research for over 50 years. As we will see, our under- ative concept, and any investigation of a regulatory standing and appreciation of those capabilities have system requires the choice of appropriate experimental been in¯uenced profoundly by our expectations as to stimuli. This is exactly what Conklin and Heggeness what constitutes ªsuccessfulº thermoregulation. In (1971) understood when, in discussing their experi- part, our expectations arise from the idea that there ments, they wrote that they exposed their infant rats exists a single core body temperature (e.g., rectal tem- to air temperatures ªthat would evoke, but not over- perature) that (a) is representative of all relevant body whelm, regulatory functionsº (p. 333). Therefore, test- temperatures and (b) will be maintained within a range ing infants at an extreme air temperature is not always of adultlike values if the infant is thermoregulating an effective way to assess thermoregulatory capabili- successfully. These expectations, however, have led us tiesÐif the mechanisms are overwhelmed, then one astray. Therefore, this article represents an attempt to cannot properly evaluate their control, ef®cacy, or de- reassess infant thermoregulation, especially that of al- velopment. tricial species such as the rat, based on recent physi- Taylor (1960) contributed to the development of a ological and behavioral data. revised perspective by showing that newborn rats increase oxygen consumption signi®cantly when exposed to an air temperature of 29ЊC. Taylor was A BRIEF HISTORY unique in cautioning against super®cial comparisons between the thermogenic capabilities of young rats of A common strategy in developmental studies of infant different agesÐdifferences in size and insulation are thermoregulation entails exposing pups to a very low intimately connected with age and cannot be ignored. air temperature and, by measuring rectal temperature, In effect, Taylor argued against the relatively simplis- monitoring the ªdevelopment of homeothermyº (e.g., tic approach of assessing thermoregulatory compe- Brody, 1943; Hahn, 1956). When tested in this way, tence by exposing pups to a single, arbitrary air tem- developing rat pups are increasingly able to maintain perature and measuring rectal temperature. rectal temperature in the cold from birth to 3 weeks of The determinants of heat loss in infants and adults age. This general experimental approach to studying were well known by the early 1960s: Surface-to-vol- the development of homeothermy has been popular ume ratio, insulation, and vasomotor activity all con- over the years, despite its considerable methodological tribute, passively or actively, to the promotion or re- and conceptual pitfalls. tardation of heat loss. The mechanisms of heat gain, In the experiment of Brody (1943), 1- to 22-day- however, were somewhat more obscure at that time. old pups were placed in a container maintained at an Speci®cally, while the importance of shivering ther- air temperature of 15ЊC and rectal temperature was mogenesis for adults was not in dispute, the ability of monitored. He concluded that thermoregulatory com- curarized animals to produce heat in the cold argued petence developed between the 2nd and 3rd week for a second form of heat production that was referred postpartum. Similarly, Hahn (1956) exposed young to as chemical or nonshivering thermogenesis (Hsieh, rats to an air temperature of 10ЊC and demonstrated Carlson, & Gray, 1957). This form of thermogenesis the thermoregulatory importance of the development was known to be stimulated by noradrenaline, but the of fur and the control of piloerection. Based on Hahn's site of action had not yet been identi®ed. Then, in the results, it was concluded that ªphysical thermoregu- early 1960s, Smith hypothesized that brown adipose latory mechanisms develop between the 14th and 18th tissue (BAT) is one source of nonshivering thermo- dayº postpartum (p. 430). In both of these experi- genesis in adult mammals (Smith, 1964). The role of ments, the particular choice of air temperature is not BAT as a thermogenic organ was quickly extended to discussed, and one must wonder to what extent these apply to heat production in newborn rabbits and other choices in¯uenced the conclusions of the experiments. mammalian species (e.g., Dawkins & Hull, 1964; Hull For example, would Brody's 22-day-old pups have ex- & Segall, 1965). By the end of the decade, the ther- hibited homeothermy had they been challenged at an mogenic function of BAT seemed irrefutable (Smith air temperature of 0ЊC? And would the 1-day-old pups & Horwitz, 1969). have exhibited homeothermy had they been chal- In reviewing the work of comparative physiologists lenged at an air temperature of only 30ЊC? following the discovery of the thermogenic function short That earlier investigators did not ask these ques- of BAT, BruÈck and Hinckel (1996) suggest that neo- standard tions provides insight into the conceptual framework nates can be divided into three categories. First, there long DEV (WILEJ) RIGHT INTERACTIVE

Infant Thermoregulation 109 are the precocial newborns (e.g., guinea pig, lamb, afforded by the favorable changes in surface-to-vol- full-term human) that respond vigorously to cold chal- ume relations provides a signi®cant metabolic savings lenge and are able to maintain body temperature at a to the individual pups. He concluded that ªthrough stable level in the cold, albeit only within a narrow individual competitive adjustments the huddle be- range of air temperatures. Second, there are the altri- haves as a self-regulating unit which provides warmth cial newborns (e.g., rabbit, dog, rat) whose thermo- and insulation to all its membersº (Alberts, 1978, p. genic responses, although present shortly after birth, 231). are insuf®cient to retard heat loss so that their ªbody The elegance and power of Alberts' (1978) analy- temperature drops on exposure to environmental tem- ses crystallized for psychobiologists the signi®cance peratures slightly less than thermal neutralityº (BruÈck of behavior for infants facing the problem of rapid heat & Hinckel, 1996, p. 603). Finally, there are those an- loss. This research has been so in¯uential among psy- imals (e.g., hamster, ground squirrel) that do not ex- chobiologists that the neonate's physiological means hibit any thermogenic responses to cold for days or of heat production has faded in importance. For ex- weeks after birth (Blumberg, 1997; Hissa, 1968). ample, Satinoff (1996), who has been instrumental in When mammalian neonates are categorized in the promoting the importance of behavioral mechanisms way suggested by BruÈck and Hinckel, it appears that for thermoregulation, has written that ªalthough infant physiological mechanisms of thermoregulation are rodents can increase their metabolic rate from birth, only effectively available to the precocial species. this capability is nearly useless because they cannot These animals, equipped with shivering and/or non- conserve heat and have practically no insulationº (p. shivering thermogenesis as well as control over heat 494). This view, that endogenous heat production by loss, resemble smaller and somewhat limited versions infant rodents is ineffectual, is widely accepted, as ev- of their adult forms. Furthermore, according to this idenced by its adoption (in one form or another) into perspective, neonates in the other two categories can- popular textbooks in physiological psychology. For not thermoregulate independently using physiological example, Rosenzweig, Leiman, and Breedlove (1996) mechanisms alone; their limitations can only be over- write that ªnewborn rats are not capable of endother- come by using behavior, a subject to which we now mic regulationº (p. 460) and Carlson (1994) writes that turn. newborn rats and mice ªare poikilothermous [sic] (ªcold-bloodedº); their brain is not yet developed enough to regulate body temperatureº (p. 324). THE IMPORTANCE OF BEHAVIOR Despite a long history, the study of thermoregulation in individual infants has never properly addressed Because isolated newborn rats were thought to be the function or regulatory control of BAT thermoge- physiologically incapable of successfully combating nesis at moderate air temperatures. Moreover, the de- even moderate cold exposure, it became acceptable to velopment of thermoregulation has been viewed too refer to them as poikilotherms (e.g., Fowler & Kel- narrowly as a process aimed toward adult capabilities logg, 1975; see Nedergaard, Connolly, & Cannon, rather than a process in which pups have evolved so- 1986), primarily dependent on behavioral mechanisms lutions that are unique to their physiological needs for the maintenance of body temperature (Johanson, during development. Therefore, what is needed is an 1979; Kleitman & Satinoff, 1982). Poikilothermy has approach that addresses the regulatory principles that been de®ned as ªlarge variability of body temperature govern infant thermogenesis and the consequences of as a function of ambient temperature in organisms that thermogenesis for the pup's physiological and be- without effective autonomic temperature regulationº havioral development. Such an approach, and the em- (IUPS Thermal Commission, 1987, p. 577). Based on pirical data that support it, will be detailed later in this this de®nition, newborn rats, with their insuf®cient article. Next, we review recent research in a variety of heat gain mechanisms and their uncontrollable heat species that provides an alternative perspective for as- loss, do indeed appear to exhibit the signs of poikil- sessing the thermoregulatory capabilities of infant rats. othermy. The apparent poikilothermy of an isolated rat pup can be contrasted with the thermoregulatory compe- COMPARTMENTALIZATION IN tence of a huddle of rat pups. In a series of now-classic THERMOREGULATORY SYSTEMS FROM experiments, Alberts (1978) monitored the oxygen INSECTS TO MAMMALS consumption and rectal temperature of pups allowed to huddle with littermates. He found that the huddle Over the past three decades, evidence has accrued that short expands and contracts as air temperature increases and suggests a general theme: Some thermoregulatory sys- standard decreases, respectively, and that the heat conservation tems have evolved to protect particular organ systems long DEV (WILEJ) LEFT INTERACTIVE

110 Blumberg and Sokoloff from excessive heating or cooling. Therefore, perhaps tems that help to buffer their nervous systems against more often than not, one ®nds animals with substantial the thermal ¯uctuations normally encountered in their heterogeneity in the distribution of temperatures daily movements. throughout the body core. A brief review of this lit- erature follows. Selective Brain Cooling in Reptiles, Birds, Insects and Mammals Until recently, these small, primitive creatures were Because the brain is more easily damaged by high tem- considered classic illustrations of poikilothermy, a peratures than other bodily organs (Heikkila & Brown, designation that once impeded the study of insect ther- 1979; Millan, Murdock, Bleier, & Siegel, 1979), a moregulation. As Heinrich (1993) points out, such number of species have evolved physiological mech- designations can get in the way of productive science: anisms that selectively cool the brain during exercise and during exposure to high air temperatures. For ex- From a limited perspective it would be possible to clas- ample, Crawford (1972) studied a lizard species (Sau- sify different insects in any of a variety of termsÐas poi- romalus obesus) that, when panting, can maintain kilotherms, cold-blooded, warm-blooded, heterotherms, brain temperature nearly 3ЊC below an air temperature endotherms, heliotherms, and even homeotherms. Terms of 45ЊC. tend to categorize, however, and categorization is not helpful In birds and mammals, complex vascular arrange- in understanding thermal mechanisms in insects because ments combine with evaporative cooling mechanisms many assumptions are associated with the terms. (p. 9) to yield impressive abilities to protect the brain from overheating (Baker, 1979; Kilgore, Bernstein, & Hud- Placing such assumptions aside has led to the dis- son, 1976). For example, gazelles can maintain brain covery of a number of interesting behavioral and phys- temperature as much as 3ЊC below body temperature iological mechanisms by which insects solve their during an extended runÐblood is cooled evapora- thermal problems. For example, moths exhibit a pre- tively in the nasal passages that, in turn, cools arterial ¯ight shivering response that raises muscle tempera- blood ¯owing to the brain (Taylor & Lyman, 1972). ture to a level adequate to sustain ¯ight (Heinrich, Flying birds maintain body temperature at a higher 1987). Because these ¯ight muscles are located in the level than non¯ying birds and mammals (Caputa, thorax, thoracic temperature in ¯ying moths is higher 1984). A high body temperature may be an adaptation than abdominal temperature. In fact, across a range of for continual readiness for ¯ight, but this readiness air temperatures from 0±25ЊC, the difference between comes with the constant risk of hyperthermia. There- thoracic and abdominal temperatures ranges from 25± fore, birds have evolved an array of body and brain 10ЊC, respectively, an enormous thermal gradient cooling mechanisms, including panting, gular ¯utter, within an animal that weighs only 2±3 g and therefore and an esophageal heat exchanger (Dawson, 1982; has a large surface-to-volume ratio. Gaunt, 1980). It has also been shown in pigeons that cooling of blood ¯owing through the cornea enhances selective brain cooling during ¯ight (Pinshow, Bern- Fish stein, Lopez, & Kleinhaus, 1982). Like insects, ®sh have been considered prototypical All of the above examples illustrate how thermo- ectotherms, their body temperatures increasing and de- regulatory systems have evolved to warm or cool bod- creasing with the temperature of the surrounding wa- ily compartments, from the muscle-rich thorax of ter. There are a number of ®sh species, however, that moths to the heat-sensitive brain tissue of mammals. do not ®t with this traditional view. For example, blue- When we recognize the functional signi®cance of ®n tuna regulate the temperatures of muscle, eye, and these particular compartments, we are led to ask ex- brain by conserving metabolic heat using a counter- perimental questions that otherwise would have been current heat exchange system (Linthicum & Carey, overlooked. Moreover, these examples show us that 1972). Sword®sh, bill®shes, and some sharks have there are many body temperatures, not just one, and added to this vascular heat exchange system a mass of the experimenter's choice of where to measure body thermogenic tissue, derived from muscle, that selec- temperature (e.g., rectum, brain, heart, eye, skin) can tively warms the eye and brain (Block, 1986; Block have important theoretical implications. Therefore, us- & Carey, 1985; Carey, 1982). Therefore, it appears ing the above examples as guideposts, we now turn to that large pelagic predators that rely heavily on visual the question of the functional signi®cance of BAT short tracking of prey have evolved thermoregulatory sys- thermogenesis in infant rats. standard long DEV (WILEJ) RIGHT INTERACTIVE

Infant Thermoregulation 111

THE PHYSIOLOGICAL AND BEHAVIORAL parts of the body, such as the cervical and thoracic SIGNIFICANCE OF BAT THERMOGENESIS spinal cord and the heart and lungs, that would most seriously affect survival in the cold? In addition, is it In order to advance our understanding of thermoreg- not interesting that two of the largest BAT depots, that ulation in altricial neonates, the following questions is, the interscapular and cervical BAT pads (Smith, must be addressed: (a) What are the appropriate mea- 1964), are ideally suited for the delivery of heat either sures of body temperature? (b) Is BAT thermogenesis directly (i.e., conductively) or indirectly (i.e., convec- regulated? (c) Does BAT thermogenesis contribute tively via the blood) to those vital structures? signi®cantly to the infant's behavioral and physiolog- Although this argument may seem reasonable, it ical adaptation to cold challenge, even in isolated in- nonetheless contradicts the common assumption that dividuals? These questions are very closely related, as BAT thermogenesis provides warmth to the body as a the following discussion will demonstrate. whole and its ef®cacy can be measured as a high and Choosing an appropriate measure of body temper- stable rectal temperature. In contrast, Smith and Rob- ature, like any experimental measure, re¯ects a bal- erts (1964) seem to argue that heat produced by BAT ance between theory and method. As we have seen, is compartmentalized or, at least, is directed primarily the search by early investigators for the age at which toward certain vital structures. This basic idea pro- infants attain homeothermy required a measure that vided a foundation for the experiments that will now had meaning from infancy to adulthood. Skin temper- be described. ature would not be appropriate because of the sub- In a series of studies of 2-day-old and week-old stantial changes in size and insulation during devel- rats, we have monitored BAT thermogenesis over a opment. Maintenance of deep body temperature, on well-de®ned range of air temperatures and, building the other hand, has been considered the primary goal on this information, have assessed a series of physio- of the thermoregulatory system, and no measure of logical and behavioral variables (see Table 1 for a core body temperature is as easily attained as rectal summary of the experiments described below). The temperature. Therefore, with respect to the appropriate physiological variables represent systems that in¯u- measure of body temperature, theory and method ap- ence or are in¯uenced by BAT thermogenesis during parently complement one another. cold exposureÐfor example, the respiratory and car- The discovery that nonshivering thermogenesis is diovascular systems. The behavioral variables include attributable to BAT has had little impact on the as- a measure of active sleep (i.e., myoclonic twitching of sumption that rectal temperature is the valid measure the distal limbs and tail; Gramsbergen, Schwartze, & of thermoregulatory success. But even the earliest ar- Prechtl, 1970) and a vocalization that is emitted at the ticles detailing the vascular arrangement of BAT pro- ultrasonic frequency of 40 kHz. Altogether, these stud- vided well-reasoned arguments for questioning this as- ies begin to provide a coherent picture of the organi- sumption. For example, Smith and Roberts (1964) zation of and the interactions between physiological wrote: and behavioral responding in the infants of this altricial species. Considered a priori, the absolutely small amount of heat Figure 1 is an illustration adapted from an infrared produced by brown fat tissue could be of no intrinsic sig- thermograph of the dorsal surface of a week-old rat ni®cance relative to total body heat production unless this pup in a cold environment. The innermost circle in the heat were in some way brought to bear upon a relatively interscapular area, which overlies a large depot of isolated heat sink of small size and perhaps of high sensitiv- BAT, indicates the region of highest skin temperature, ity to temperature. Of organs vital to survival in cold, the and it is surrounded by concentric regions of lower thoracic and cervical regions of the body might be satis®ed skin temperatures. In this ®gure, the thermal gradient by direct conductance or vascular connection of heat from such generative loci....[T]he ®nding by Sulzer in from the interscapular region to the lower back is Њ 1774...ofadirect venous connection between intersca- greater than 2 C. Therefore, using conventional meth- pular brown fat and the azygous vein could be readily ad- ods of temperature measurement, BAT thermogenesis duced as indicative of a direct convective heat transfer to the can be estimated using thermocouples placed on the heart. (p. 146) skin in the interscapular region and at a site distant from the BAT pad, such as in the lower back (i.e., While these investigators were addressing BAT lumbar region; Blumberg & Stolba, 1996). Because thermogenesis in adults, the argument can also be ap- BAT thermogenesis is an aerobic process, simulta- plied to infant animals. Would it not make sense for neous measurement of oxygen consumption provides an infant rat, facing profound problems of heat loss, con®rmatory evidence that BAT is producing heat short to direct and isolate its limited heat production to those (Heim & Hull, 1966). standard long DEV (WILEJ) LEFT INTERACTIVE

112 Blumberg and Sokoloff

Table 1. Physiological and Behavioral Responses in Unmanipulated Week-Old Rats. The Table Should be Read as if a Pup Were Being Cooled Successively From a Thermoneutral to a Moderate Air Temperature, and Then to an Extreme Air Temperature. The Air Temperatures Indicated are Approximate and Vary Depending on Age, Strain, Litter Size, Body Weight, and Other Factors Thermoneutral Moderate Air Extreme Air Air Temperatures Temperatures Temperatures (35±36ЊC) (25±34ЊC) (Ͻ25ЊC) References Interscapular Temperature High, Stable? High, Relatively Stable Low, Relatively Blumberg & Stolba, Unstable 1996; Blumberg & Sokoloff, 1997 BAT Heat Production No Progressive Increase Depression Blumberg & Stolba, 1996 Oxygen Consumption Low Progressive Increase Depression Blumberg & Stolba, 1996 Respiratory Rate Low Progressive Increase Depression Sokoloff & Blum- berg, 1997 Cardiac Rate Normal Normal Depression Blumberg, Soko- loff, & Kirby, 1997 Blood Pressure Normal Normal Normal Kirby & Blumberg, 1998 Rate of Myoclonic High High Depression Blumberg & Stolba, Twitching 1996; Sokoloff & Blumberg, 1998 Ultrasound Production No No Yes Blumberg & Stolba, 1996; Sokoloff & Blumberg, 1997

Air temperature, of course, is a primary controlling ther; for week-old rats, this air temperature of maximal parameter of BAT thermogenesis, so it is important to BAT thermogenesis is approximately 25ЊC (Sokoloff begin by de®ning the ranges of air temperatures at & Blumberg, 1997; Spiers & Adair, 1986). For the which BAT thermogenesis is initiated, increases pro- remainder of this article, air temperatures ranging from gressively, and is overwhelmed. To de®ne those 34ЊCto25ЊC will be referred to as moderate while air ranges, we measured interscapular temperature, back temperatures below 25ЊC will be referred to as ex- temperature, and oxygen consumption in pups accli- treme. (It should be stressed that these particular air mated to a thermoneutral air temperature of 35ЊC and temperature ªzonesº are speci®c to isolated week-old then exposed them to a series of cold challenges. In rats and that these values differ for pups at other ages; these experiments, we have focused primarily on 1- in other words, the thermoregulatory challenges used week-olds (6±8 days of age), although we have per- must be scaled to an infant's age, size, and insulation.) formed many of the same experiments on 2-day-olds. Graphical representations of the physiological cor- In week-old rats, we have distinguished three relates of moderate and extreme thermal challenge are ranges of air temperatures based on the thermogenic presented in Figure 2. Each plot is a state space dia- responses that they elicit. First, at air temperatures of gram for individual pups in which oxygen consump- 35±36ЊC, pups do not exhibit BAT thermogenesis; tion is plotted against interscapular temperature. Al- these warm air temperatures can be referred to as lying though the passage of time is not depicted in these within a thermoneutral zone or a zone of least ther- plots, they do provide an enhanced sense of the reg- moregulatory effort (Satinoff, 1996). Second, as air ulation of BAT thermogenesis during cold exposure. temperature is decreased, a threshold is reached below The test begins with a pup acclimated in a meta- which BAT thermogenesis is stimulated; for week-old bolic chamber in which air temperature is maintained rats, this lower critical temperature is approximately at 35ЊC. After 45 min of acclimation, the pup exhibits 34ЊC (Spiers & Adair, 1986). As air temperature is an interscapular temperature of 38ЊC and a low value decreased progressively, BAT thermogenesis in- of oxygen consumption. Air temperature is then decreases progressively. Finally, an air temperature is creased rapidly to 30ЊC (Figure 2a), and the pup re- short reached where BAT thermogenesis increases no fur- sponds initially with a decrease in interscapular tem- standard long DEV (WILEJ) RIGHT INTERACTIVE

Infant Thermoregulation 113

FIGURE 1 Illustration adapted from an infrared thermograph of the dorsal skin surface of a week-old rat in the cold. The bright spot overlying the interscapular region represents FIGURE 2 State±space diagrams for individual week-old the area of highest skin temperature, and it re¯ects thermo- rats in which oxygen consumption (VO2) is plotted against genesis by the interscapular brown fat pad located just be- interscapular temperature (Tis). Air temperature at the begin- neath the skin surface. Note the pronounced thermal gradient ning of each test was 35ЊC. Temperatures at the left of each from the interscapular area to the lower back. The numbers plot indicate the ®nal air temperature for that test. (adapted to the side of the illustration represent approximate skin tem- from Sokoloff & Blumberg, 1997). peratures in degrees Celsius.

2e) results not only in a decrease in interscapular temperature and an unchanged rate of oxygen perature but a depression of oxygen consumption as consumption. At some point, however, a change oc- well. curs: Interscapular temperature stops falling and oxy- Having clear and operationalized de®nitions of gen consumption begins to increase as a result of in- moderate and extreme thermal challenge is useful be- creased BAT thermogenesis. Within the state space, cause it allows us to begin delving deeper into the the trajectory rises vertically until it reaches a new physiological correlates of these challenges and their settling point. When a pup experiences a decrease in behavioral consequences. One immediate conceptual air temperature to 27ЊC rather than 30ЊC, it exhibits a dif®culty that is raised by the patterns of responses in similar response (Figure 2b). Figure 2 relates to the question of thermal comfort. As stated above, the transition from moderate to Speci®cally, given that rectal temperature in week-old extreme cooling is de®ned as the point at which BAT rats decreases even during moderate thermal chal- thermogenesis is maximized. When this occurs, inter- lenges (Conklin & Heggeness, 1971; Spiers & Adair, scapular temperature should exhibit greater depen- 1986; Takano, Mohri, & Nagasaka, 1979), do these dence on air temperature, and this is exactly what is pups exhibit physiological or behavioral signs of observed. Speci®cally, when air temperature is de- stress? creased to 25ЊC (Figure 2c), the trajectory is skewed To aid in answering this question and as a thought to lower interscapular temperatures. A pup exposed to experiment, consider the following scenario based on an extreme air temperature of 23ЊC (Figure 2d) shows a traditional control systems approach to thermoreg- an even greater decrease in interscapular temperature, ulation (Houk, 1988). Let's assume that it is only at a although oxygen consumption maintains a high value. thermoneutral air temperature that a pup is able to short Finally, an extreme thermal challenge of 21ЊC (Figure maintain core body temperature at the desired ªset- standard long DEV (WILEJ) LEFT INTERACTIVE

114 Blumberg and Sokoloff pointº level. It follows from this assumption that a pup exposed to a subthermoneutral air temperature would detect a discrepancy between set-point temperature and falling body temperature and would in turn activate BAT thermogenesis. However, as many have found, BAT thermogenesis is not capable of compen- sating for heat loss and thus core body temperature decreases (Conklin & Heggeness, 1971; Spiers & Adair, 1986; Takano, et al., 1979). Therefore, to counter this situation, the pup would either have to (a) recruit other behavioral and/or physiological mechanisms to raise core body temperature back to the set- point level or (b) decrease set-point to erase the discrepancy with body temperature (see Mrosovsky, 1990). For the remainder of this section, we will assess the plausibility of the above scenario by citing the available experimental evidence. It will be shown that neither the physiological nor the behavioral data support the commonly held view that isolated pups are overwhelmed by the thermal challenges posed by all subthermoneutral air temperatures (see BruÈck & Hinckel, 1996). Speci®cally, we will see that isolated pups adapt successfully to moderate thermal challenge and FIGURE 3 Mean values of interscapular temperature that they do so primarily through BAT activation. (Tis), back temperature (Tback), oxygen consumption (VO2),

As shown in Figure 2, pups arrive at settling points and air temperature (Ta) for moderately cooled 7- to 8-day- regardless of whether they are exposed to moderate or old rats experiencing either positive(n ϭ 4) or negative extreme air temperatures. To determine whether these (n ϭ 4) air temperature perturbations. The oxygen con- settling points differ with respect to (a) the ability of sumption responses to the two air temperature perturbations pups to defend them and (b) the speed and accuracy are mirror images of one another, and changes in Tis are with which they are defended (measures of relative dampened in relation to changes in Tback. Therefore, pups stability; Kelso, 1995), perturbation experiments were exhibit homeostatic thermoregulatory responses even at a performed (Blumberg & Sokoloff, 1997). Pups were subthermoneutral air temperature. (adapted from Blumberg & Sokoloff, 1997). cooled either to a moderate air temperature (30.5ЊC) or to an extreme air temperature (23ЊC). After they had settled at one of these two air temperatures, pups were exposed to a 3.7ЊC air temperature perturbation of the response to a negative air temperature pertur- in the positive or negative direction. Overall, we found bation. In other words, the system was behaving like that the pups in the moderate condition were better a prototypical homeostatic system. Surprisingly, the able to defend interscapular temperature against the system is behaving homeostatically at a subthermo- perturbation than were the pups in the extreme con- neutral air temperature (i.e., 30ЊC). Recall our thought dition, and they did so by increasing or decreasing experiment: If, during moderate cold exposure, the BAT thermogenesis. It was concluded from these data pups' body temperatures are indeed below the ther- that the settling points at a moderate air temperature moneutral set-point level, then one would expect these are relatively stable in comparison to settling points at pups to continue BAT heat production during the pos- an extreme air temperature. itive air temperature perturbation so that body tem- Perhaps the most striking result of the perturbation perature could increase as quickly as possible. That experiment was the physiological response of the the pups quickly decreased BAT thermogenesis dur- moderately challenged pups to a positive air temper- ing the positive perturbation suggests that this was not ature perturbation (Figure 3). As air temperature in- the case. creased during the positive perturbation, these pups Behavioral responses can also contribute to our as- exhibited rapid decreases in heat production that pre- sessment of the differential effects of moderate and vented interscapular temperature from increasing. Fig- extreme cooling. Because infant rats, like most mam- short ure 3 shows that this response was the mirror image mals, spend the majority of their time in active sleep, standard long DEV (WILEJ) RIGHT INTERACTIVE

Infant Thermoregulation 115 we have monitored active sleep during moderate and informative for assessing a pup's physiological state extreme thermal challenges (Blumberg & Stolba, than is rectal temperature alone. Second, we have pro- 1996; Sokoloff & Blumberg, 1998). In infant rats, a vided evidence that BAT thermogenesis, even in iso- predominant indicator of active sleep is myoclonic lated pups, is regulated during moderate cold expo- twitching of the distal limbs and tail, a behavior that sure. Finally, we have shown how BAT thermogenesis is easily and reliably scored by a trained observer contributes to the pup's behavioral adaptation to cold. (Blumberg & Lucas, 1996; Gramsbergen et al., 1970; We have not yet discussed, however, how the heat Jouvet-Mounier, Astic, & Lacote, 1970). produced by BAT contributes to the pup's physiolog- In one experiment, week-old rats were acclimated ical adaptation to cold. to a metabolic chamber at a thermoneutral air temper- To address this issue, we go back to the ®rst articles ature (35.5ЊC), followed by a fast drop in air temper- describing BAT as the source of nonshivering ther- ature to either a moderate (30ЊC) or extreme (21ЊC) mogenesis. Recall that Smith and Roberts (1964), in level (Blumberg & Stolba, 1996). The test lasted 70 noting the venous out¯ow from BAT, suggested that min (a 10-min baseline period at 35.5ЊC plus a 60-min the ªdirect venous connection between interscapular period of cooling), during which the observer scored brown fat and the azygous vein could be readily ad- the incidence of myoclonic twitching. The results induced as indicative of a direct convective heat transfer dicated that although extreme cooling signi®cantly re- to the heartº (p. 146). This observation, in conjunction duced myoclonic twitching over the course of the test, with earlier studies demonstrating the profound effects moderate cooling elicited no change in twitching. of cooling on heart rate (e.g., Fair®eld, 1948; Lyman Moreover, as already described, pups in the moderate & Blinks, 1959), suggested a possible role for BAT condition were better able to maintain and stabilize thermogenesis in the maintenance of cardiac function interscapular temperature than were pups in the ex- during cold challenge in infants. treme condition. Finally, a parallel experiment in 2- To assess the contribution of BAT thermogenesis day-olds using age-appropriate air temperatures to cardiac rate regulation, the electrocardiogram of yielded similar results (Blumberg & Stolba, 1996). week-old rats was monitored as pups were exposed to In addition to myoclonic twitching, we have also a series of moderate and extreme air temperatures analyzed the vocal responses of rat pups to cold. Fol- (Blumberg, Sokoloff, & Kirby, 1997). It was found lowing up on earlier work showing that cold is a pri- that at air temperatures where interscapular tempera- mary stimulus for eliciting ultrasound production by ture was maintained, cardiac rate was also maintained rat pups (Okon, 1971), it was noted that pups start to (Figure 4). At the extreme air temperature of 17ЊC, vocalize during cold exposure at the same time that however, when interscapular temperature fell, cardiac they initiate BAT thermogenesis (Blumberg & Al- rate fell signi®cantly. In a separate experiment, when berts, 1990). The conclusion that a relation exists be- pups were administered chlorisondamine, a ganglionic tween initiation of BAT thermogenesis and ultrasound blocker that prevents neural control of the heart as well production was modi®ed by a subsequent experiment as BAT, heart rate fell in lock-step with interscapular in which it was found that pups only vocalize during temperature. (Again, a parallel experiment with 2-day- extreme cold exposure, even though BAT thermoge- olds yielded similar results.) Finally, experiments em- nesis is initiated during moderate cold exposure ploying pharmacological activation of BAT and ma- (Blumberg & Stolba, 1996). More detailed experimen- nipulation of interscapular temperature have provided tal manipulations of air temperature supported this further support for the hypothesis that BAT thermo- ®nding (Sokoloff & Blumberg, 1997). Therefore, genesis contributes signi®cantly to the regulation of whether this vocalization is interpreted as a commun- cardiac rate during cold challenge (Sokoloff, Kirby, & icatory act designed to elicit maternal retrieval to the Blumberg, in press). nest (e.g., Hofer & Shair, 1978) or as an acoustic by- Cardiac rate is just one of two variables that deter- product of respiratory (Blumberg & Alberts, 1990) mine cardiac output (i.e., the amount of blood pumped and/or cardiovascular (Kirby & Blumberg, 1998) ma- by the heart per unit time). The second component of neuvers, the incidence of ultrasound production during cardiac output is stroke volume, that is, the volume of extreme cooling, but not during moderate cooling, blood forced out during each individual contraction. points out yet another qualitative difference between In those infant mammals studied thus far, cardiac out- these two ranges of air temperature. put is primarily determined by cardiac rate due to lim- We began this section with three fundamental ques- itations in the infant's ability to modulate stroke vol- tions about infant thermoregulation. First, we have ume (e.g., Teitel et al., 1985). Therefore, the falling shown how interscapular temperature, when combined heart rate of infant rats during extreme cooling sug- short with other thermal and metabolic measures, is more gests that cardiac output is falling as well, which standard long DEV (WILEJ) LEFT INTERACTIVE

116 Blumberg and Sokoloff

oclonic twitching and inhibit ultrasound production. Therefore, from a physiological and behavioral stand- point, moderately cooled pups do not appear to be thermally stressed. Recalling the thought experiment described earlier, it is possible that pups decrease set-point during moderate cooling to remove the discrepancy between body temperature and set-point temperature. This would be an example of rheostasis, or set-point modulation (Mrosovsky, 1990). Unfortunately, this notion of set- point modulation is very dif®cult to test experimen- tally. Perhaps it is preferable to simply suggest that the thermoregulatory responses of isolated pups during moderate, but not extreme, cooling are regulated and contribute signi®cantly to the physiological and behavioral adaptations of rat pups to cold.

COMPARATIVE ASPECTS OF BEHAVIORAL AND PHYSIOLOGICAL THERMOREGULATION

In light of the ®ndings summarized in Table 1, it now FIGURE 4 Mean values of interscapular temperature seems unwarranted to dismiss as inconsequential the (T ), oxygen consumption (VO ), and cardiac rate (beats per is 2 thermoregulatory mechanisms used by rat pups iso- minute, bpm) for 7- to 8-day-old rats at various air temperatures (T ). Cardiac rate decreases signi®cantly only when lated in the cold. Next, we assess the value of BAT a thermogenesis, and test the generalizability of our the limits of BAT thermogenesis are surpassed and Tis decreases. (adapted from Blumberg, Sokoloff, & Kirby, 1997). functional hypotheses, by examining the behavioral and physiological responses of infant Syrian golden hamsters (Mesocricetus auratus) to various levels of would in turn suggest that such pups are facing related cold challenge. Hamsters provide an interesting com- hemodynamic dif®culties such as decreased arterial parison group to rats because they do not exhibit en- blood pressure. But, when blood pressure was moni- dogenous heat production during the ®rst 2 weeks after tored from the descending abdominal aorta in week- birth (Hissa, 1968). old pups during moderate and extreme cooling, no Although hamsters have been referred to as ªim- such decreases in blood pressure were detected (Kirby matureº by some investigators (e.g., Nedergaard et al., & Blumberg, 1998). As Table 1 shows, blood pres- 1986), they simply do not ®t easily along the conven- sure is the one variable measured thus far that does tional altricial/precocial continuum. With a gestation not exhibit a change across the transition from mod- of only 16 days, golden hamsters are relatively pre- erate to extreme cold exposure. The mechanisms by cocial with respect to locomotor abilities, independent which pups maintain blood pressure during extreme feeding, fur growth, and eye opening, but relatively cooling have not yet been determined, but it is likely altricial with respect to physiological mechanisms of that one such mechanism is peripheral vasoconstric- thermoregulation (Daly, 1976; Schoenfeld & Leonard, tion. 1985). Speci®cally, golden hamsters exhibit neither The results presented in Table 1 tell a consistent shivering nor nonshivering thermogenesis until ap- story regarding the physiological and behavioral ad- proximately 2 weeks postpartum (Blumberg, 1997; aptations of week-old rats during cold exposure. Dur- Hissa, 1968). With respect to behavioral thermoregu- ing moderate cold exposure, pups increase BAT ther- lation, however, newborn hamsters far outstrip new- mogenesis and, by doing so, regulate interscapular born rats in their ability to rapidly move toward temperature or a correlate. The increase in oxygen util- warmth (rats: Johanson, 1979; Kleitman & Satinoff, ization of BAT is supported by an increase in respi- 1982; hamsters: Leonard, 1974, 1982). Therefore, ratory rate (Sokoloff & Blumberg, 1997). The heat there appears to be a trade-off between behavioral and produced by BAT contributes directly to the mainte- physiological mechanisms of thermoregulation in short nance of cardiac rate, and may also help protect my- these two species. Whether this is a true trade-off in standard long DEV (WILEJ) RIGHT INTERACTIVE

Infant Thermoregulation 117 that heightened development of one system requires inhibition of the other is an open question at this time. Given the ®ndings, described earlier, relating BAT thermogenesis and cardiac rate in infant rats, the question was raised as to whether hamsters can only maintain cardiac rate in the cold when thermogenic mechanisms have matured. When this hypothesis was tested, it was found that cardiac rate fell in lock-step with interscapular temperature even during mild cooling in hamster pups 12 days of age and younger (Blumberg, 1997). In contrast, by 13 days of age when pups began exhibiting BAT thermogenesis, interscapular temperature and cardiac rate were maintained. Therefore, as in infant rats, young hamsters exhibit a strong relation between endogenous heat production and cardiac rate regulation. From the above experiments, it became apparent that the physiological responses of hamsters less than 10 days of age bear a striking resemblance to those of week-old rats that have been ganglionically blocked. Noting this resemblance, we revisited the question of whether BAT thermogenesis protects sleep-related behaviors during cold exposure by monitoring the my- FIGURE 5 Mean values of interscapular temperature (Tis) oclonic twitching of week-old untreated rats, gan- and number of 1-s time bins in which myoclonic twitching glionically blocked rats, and untreated hamsters was detected during a 15-min observation period for 6- to 8-day-old rats (circles) and Syrian golden hamsters (®lled (Figure 5; Sokoloff & Blumberg, 1998). Just as with triangles) at various air temperatures (Ta). Rat pups were cardiac rate, and as expected from our previous study either untreated (®lled circles) or pretreated with a gangli- (Blumberg & Stolba, 1996), the untreated rat pups ex- onic blocker (un®lled circles). Untreated rat pups increased hibited high levels of twitching at moderate air tem- BAT thermogenesis during moderate cooling, resulting in peratures and low levels at extreme air temperatures. the maintenance of Tis and high rates of myoclonic twitch-

In contrast, twitching in both the ganglionically ing. However, during extreme cooling, both Tis and my- blocked rats and the untreated hamsters decreased in oclonic twitching decreased substantially. In contrast, gan- lock-step with air temperature. Further experiments in glionic blockade inhibited BAT thermogenesis at all infant rats in which BAT thermogenesis was selec- subthermoneutral Tas and Tis and myoclonic twitching de- ␤ creased progressively. Hamster pups do not exhibit endog- tively activated using a 3 agonist and in which the temperature of the interscapular region was manipu- enous heat production at this age and exhibited responses that were similar to the ganstionically blocked rats. (adapted lated using a thermode provide additional support to from Sokoloff & Blumberg, 1998). the hypothesis that BAT thermogenesis protects myoclonic twitching in cold-exposed infants (Sokoloff & Blumberg, 1998). Whether these thermal effects on twitching are mediated directly by temperature-sensi- cially those working with infant rats and other altricial tive neural elements (for example, in the cervical spi- species. This is because temperature is both one of the nal cord; cf. BruÈck&WuÈnnenberg, 1970) or indirectly primary environmental concerns of developing mam- through other systems (e.g., the cardiovascular sys- mals and an in¯uential modulator of behavior (Leon, tem) remains for further study. 1986; Satinoff, 1991). As discussed previously, infant rats maintained at a thermoneutral or moderate air temperature exhibit high rates of myoclonic twitching and THERMAL CONSIDERATIONS IN THE are only signi®cantly aroused at extreme air tempera- STUDY OF INFANT BEHAVIOR tures (Blumberg & Stolba, 1996; Sokoloff & Blum- berg, 1998). This simple fact makes it all the more Although the thermoregulatory capabilities of infant important that researchers be aware of the role that the rats are of interest to a relatively small group of re- thermal environment plays in modulating infant be- searchers, the details of those capabilities should be of haviors. short concern to all developmental psychobiologists, espe- Table 2 presents age and air temperature informa- standard long DEV (WILEJ) LEFT INTERACTIVE

118 Blumberg and Sokoloff

Table 2. A Selected List of Psychobiological Studies of Isolated Infant Rats Across a Range of Behavioral Paradigms. Studies by the Present Authors are Not Included. In Some Cases, Room Temperature is Not Explicitly Stated in the Article But Could Be Inferred From Other Information; Normally, Room Temperature is Approximately 22ЊC Behavior Age (days) Air Temperature References Learning: Operant 1 33ЊC Johanson & Hall, 1979 3 30ЊC Moran et al., 1981 Classical 0±4 30ЊC Bachevalier & Blozovski, 1980 3±6 35ЊC Martin & Alberts, 1982 4,8 Room Miller et al., 1990 Habituation 6,12,18 32ЊC Swithers-Mulvey et al., 1991 Ultrasound Production 1±28 Room Noirot, 1968 0±19 10ЊC Naito & Tonoue, 1987 9±10 25ЊC Goodwin et al., 1994 3,10,18 22ЊC, 30ЊC² Carden et al., 1994 Motor Behaviors 3±10 Room Bignall, 1974 1±21 21±24.5ЊC Altman & Sudarshan, 1975 0±21 Room Weber & Stelzner, 1977 Sleep Behaviors 0±17 Not clear Jouvet-Mounier et al., 1969 2±30 26±33ЊC³ Gramsbergen et al., 1970 1±17 Room LaPointe & Nosal, 1979 10±25 27±30ЊC Van Someren et al., 1990

²Colder air temperature used for youngest age. ³Air temperature adjusted appropriately to the animal's age.

tion for a representative selection of behavioral studies that this lack of control was a ªprobable source of of infant rats. The studies presented entail a variety of variabilityº in their results (p. 897). In all of their tests, behavioral paradigms, from learning to motor behav- pups were tested at room temperature which would ior, and none of these studies is concerned directly constitute an extreme thermal challenge for the pups with the thermal modulation of behavior. It is apparent less than one week of age but would gradually become that some studies test pups of a single age while others only a moderate challenge for older pups. Before test- examine developmental changes across ages. But what ing, the pups were removed from the home cage and is perhaps most striking is the wide range of air tem- ªcarried to the testing laboratory, identi®ed individu- peratures, from thermoneutral to extreme, used to ally with a felt-tip marker, and weighedº (p. 896). Be- study these various behaviors. Moreover, this single cause infants cool very quickly at room temperature dimension of air temperature does not fully capture (Blumberg, E®mova, & Alberts, 1992b), it is highly the ways in which thermal factors differ between psy- likely that the body temperatures of the different-aged chobiological studies. For example, in some studies subjects in Altman and Sudarshan's experiments were pups are tested after a period of acclimation to a spe- very different when the pups were tested, and thus it ci®c air temperature, while in others pups are tested is possible that some of Altman and Sudarshan's de- immediately upon transfer from the nest. velopmental ®ndings were in¯uenced by interactions The air temperatures used in various behavioral between behavior, body temperature, and physiologi- paradigms do not appear to be random (Table 2). For cal activation. example, most learning studies involve the testing of As shown in Table 2, many investigators of learn- pups at air temperatures above 30ЊC, while studies of ing in infants test their subjects in air temperatures of motor behavior are typically done at room tempera- 30±35ЊC. For example, Hall and his colleagues, in ture. In this regard, consider Altman and Sudarshan's their investigations of ingestive behaviors, typically (1975) examination of the development of motor be- test their newborn and infant rats in an incubator at an haviors in rats. Pups, aged 1±21 days, were given a air temperature of approximately 32ЊC (e.g., Johanson battery of tests that assessed a number of behaviors & Hall, 1979; Swithers-Mulvey, Miller, & Hall, including walking, righting, rearing, and climbing. 1991). The choice of this air temperature for testing These investigators did not control the environmental derives in part from ®ndings demonstrating that the short conditions of their experiments, and they acknowledge behaviors of food-deprived pups in response to food standard long DEV (WILEJ) RIGHT INTERACTIVE

Infant Thermoregulation 119

(e.g., milk intake, activity, mouthing, probing) are is performed, it can also be important that pups be more prevalent at warmer air temperatures (Johanson given suf®cient time to acclimate to a new thermal & Hall, 1980). Therefore, it appears that the study of environment before testing. For example, in our ex- ingestive behaviors in pups requires both food depri- perimental situation, infant rats require as much as 45 vation and testing in a relatively warm environment. min to reach a steady state at a new air temperature. It should be noted, however, that food-deprived pups There are, however, experimental questions that focus do not exhibit BAT thermogenesis at subthermoneu- on the acute behavioral activation of pups that occurs tral air temperatures (Bignall, Heggeness, & Palmer, after transfer from the nest to a novel environment 1975; Blumberg & Alberts, 1991), thus suggesting that (e.g., Noirot, 1968). As Table 2 illustrates for inves- even an air temperature of 32ЊC, although relatively tigations of ultrasound production in infant rats, such warm, is not moderate under experimental conditions ªtransferº experiments are characterized by the use of that entail food deprivation. a variety of air temperatures (In fact, in one study, the Another behavioral context in which temperature younger pups were exposed to the colder air temper- plays an important role is in the development of ol- ature.) Importantly, thermal factors also enter into factory preferences for ®lial huddling in infant rats these experiments through a number of other routes. (Alberts & May, 1984). It was found that during early For example, pups are typically removed from the development, rat pups associate warmth provided by home cage using bare hands (a source of conductive the mother with her characteristic odor, an association heat loss) and walked to the novel environment (which that induces olfactory preferences for the odor of lit- adds convective heat loss). In one often-cited study on termates by 15 days of age. The development of these the olfactory modulation of ultrasound (Oswalt & olfactory preferences was shown to be associated with Meier, 1975), pups in different experimental condi- the warmth provided by the mother and not with other tions were placed in metal bowls that were or were factors such as suckling. Although Alberts and May not lined with wood shavings. Therefore, in these and interpret their ®ndings within a learning context in other experiments, one is observing a complex inter- which a contingency is established between warmth action between hypothermia, physiological activation, and maternal odor, it is also possible that the thermal and the focal behavior (Blumberg et al., 1992b). environment plays a permissive role in allowing ol- The effect of removing infant rats from the nesting factory preferences to develop. More work is needed environment has also been used to examine the short- to distinguish between these two possibilities. and long-term effects of maternal deprivation on cor- Changes in heart rate to psychophysical stimuli ticosterone secretion and emotional reactivity (Den- have been used to infer changes in perceptual and cog- enberg, Brumaghim, Haltmeyer, & Zarrow, 1967). As nitive processing (e.g., Dailey, Wigal, & Amsel, with ultrasound production, it was found that the ther- 1986). Hayne, Richardson, and Campbell (1992), mal challenge inherent in the early ªhandlingº para- however, make the important point that competing digms made a substantial contribution to the observed physiological demands on cardiac function may in¯u- hormonal and behavioral effects (e.g., Denenberg et ence the expression of the heart rate orienting re- al., 1967; Schaefer, Weingarten, & Towne, 1962). Not sponse, especially in neonates. Noting that air tem- all later studies, however, have adequately controlled perature modulates cardiac function, they examined for thermal and nonthermal factors. For example, the in¯uence of air temperature on the orienting re- placement of a heating pad beneath the ¯oor of a cage sponses of rat pups exposed to a novel olfactory stim- (e.g., Levine, Huchton, Wiener, & Rosenfeld, 1992) ulus. While they found that air temperature had little may not be suf®cient to provide an adequate thermal impact on the behavioral components of the orienting environment. In contrast, the use of an incubator in response to a novel odor (e.g., snif®ng), cold air tem- which a pup is exposed to a more homogeneous ther- peratures did prevent the expression of the heart rate mal environment provides more reliable information decelerations that typically accompany the orienting regarding the nonthermally mediated effects of pup response, especially at the youngest ages tested. Inter- separation from the nest (e.g., Kuhn, Pauk, & Schan- estingly, comparison of their thermal and heart rate berg, 1990; Schanberg & Field, 1987). data with ours (e.g., Blumberg et al., 1997) suggests The natural ecology of the pup also cannot be ig- that the heart rate component of the orienting response nored. The nest, huddle, and mother help to provide a may only be expressed at moderate and/or thermo- stable thermal environment for the developing pup. neutral air temperatures at which the autonomic ner- Indeed, at an air temperature of 16ЊC, huddling pups vous system and the cardiovascular system are oper- are able to maintain skin temperature at 34ЊC (Jans & ating within their normal functional range. Leon, 1983). In rats, hamsters, and gerbils, huddling short In addition to the air temperature at which testing is the primary means of thermoregulation within the standard long DEV (WILEJ) LEFT INTERACTIVE

120 Blumberg and Sokoloff nest during maternal absence (Alberts, 1978; Leonard, edges the differing developmental strategies and 1974; Schneider, Veltri, DeGraw & French, 1995). ecological niches adopted by different species (Alberts When the mother is present, she provides not only & Cramer, 1988). milk but also heat to suckling pups (Leon, Coopers- Homeostasis is an emergent property of multiple mith, Beasley & Sullivan, 1990). In golden hamsters, behavioral and physiological components, organized the mother is present almost constantly during the 1st at many levels of the neuraxis (Blumberg, Schalk, & week postpartum (Leonard, 1974), perhaps a response Sokoloff, 1995; Satinoff, 1978), acting in concert and to the lack of endothermy in the newborns of this spe- reacting to environmental demands. One current chal- cies. Therefore, the thermal dimension of the nesting lenge is to understand how homeostasis evolves as environment is a critical one, and any experimental these components and the environmental and physical manipulation of the social environment cannot ignore contexts change during development. Moreover, with its thermal implications (Blumberg, E®mova, & Al- a more complete understanding of the thermoregula- berts, 1992a). tory capabilities of individual infant rats, we are now Finally, in addition to food deprivation, there are a better equipped to explore how the individual infant number of experimental manipulations (e.g., surgery, contributes to group regulatory processes in the huddle pharmacological treatment) that can disrupt a pup's as well as how the huddle shapes the thermal responses ability to activate BAT thermogenesis in the cold (e.g., of the individual. Blumberg & Alberts, 1991). For such experimental manipulations, misinterpretation of results can occur if thermoregulatory responding is not monitored. For NOTES example, if a pharmacological or surgical manipulation has the unforeseen effect of inhibiting BAT ther- We thank Bob Kirby and Evelyn Satinoff for their helpful mogenesis, then even an air temperature normally con- comments. sidered moderate can produce pronounced hypothermia. In such cases, changes in the dependent variable between the experimental and control groups REFERENCES may arise from an indirect effect of the treatment or manipulation on the pup's ability to produce heat. Alberts, J. R. (1978). Huddling by rat pups: Group behav- Therefore, in some experiments, knowing air temper- ioral mechanisms of temperature regulation and energy ature alone may not be enough to draw reliable con- conservation. Journal of Comparative and Physiological clusions regarding an infant's physiological state. Psychology, 92, 231±245. Alberts, J. R., & Cramer, C. P. (1988). Ecology and expe- rience: Sources of means and meaning of developmental CONCLUSIONS change. In E. M. Blass (Ed.), Handbook of behavioral neurobiology, Vol. 8 (pp. 1±39). New York: Plenum Over the past 50 years, re®nements in theory and Press. methodology have clari®ed the roles that physiological Alberts, J. R., & May, B. (1984). Nonnutritive, thermotactile induction of ®lial huddling in rat pups. Developmental mechanisms and behavior play in the thermal adap- Psychobiology, 17, 161±181. tations of infant mammals. While psychobiologists Altman, J., & Sudarshan, K. (1975). Postnatal development have championed the importance of behavior as a pri- of locomotion in the laboratory rat. Animal Behaviour, 23, mary contributor to thermoregulation (Satinoff, 1996), 896±20. physiologists have elucidated the hormonal, pharma- Baker, M. A. (1979). A brain-cooling system in mammals. cological, and anatomical features of organisms that Scienti®c American, 240, 130±139. contribute to heat production and heat conservation Bechevalier, J., & Blozovski, D. (1980). Acquisition and (Nedergaard et al., 1986). From the organism's stand- retention of classical conditioning in the newborn rat. De- point, this division of labor between the two disci- velopmental Psychobiology, 13, 519±526. plines is, at best, arti®cial (Blessing, 1997); thermo- Bennett, A. F. (1984). Thermal dependence of muscle func- regulation is the product of both behavioral and tion. American Journal of Physiology, 247, R217±R229. Bignall, K. E. (1974). Ontogeny of levels of neural organi- physiological processes. Building on the contributions zation: The righting re¯ex as a model. Experimental Neu- of both groups of researchers and the insights outlined rology, 42, 566±573. in this article, we can begin assessing the interactions Bignall, K. E., Heggeness, F. W., & Palmer, J. E. (1975). between physiology and behavior during develop- Effect of neonatal decerebration on thermogenesis during ment. In addition, our assessments of infant capabili- starvation and cold exposure in the rat. Experimental Neu- short ties should be made within a context that acknowl- rology, 49, 174±188. standard long DEV (WILEJ) RIGHT INTERACTIVE

Infant Thermoregulation 121

Blessing, W. W. (1997). Inadequate framework for under- wijk (Eds.), Physiological and behavioral temperature standing bodily homeostasis. Trends in Neuroscience, 20, regulation (pp. 562±580). Spring®eld: C. G. Thomas. 235±239. Caputa, M. (1984). Some differences in mammalian versus Block, B. A. (1986). Structure of the brain and eye heater avian temperature regulation: Putative thermal adjust- tissue in marlins, sail®sh, and spear®shes. Journal of Mor- ments to ¯ight in birds. In J. R. S. Hales (Ed.), Thermal phology, 190, 169±189. physiology (pp. 413±417). New York: Raven Press. Block, B. A., & Carey, F. G. (1985). Warm brain and eye Carden, S. E., Davachi, L., & Hofer, M. A. (1994). U50,488 temperatures in sharks. Journal of Comparative Psychol- increases ultrasonic vocalizations in 3-, 10-, and 18-day- ogy B, 156, 229±236. old rat pups in isolation and the home cage. Developmen- Blumberg, M. S. (1997) Ontogeny of cardiac rate regulation tal Psychobiology, 27, 65±83. and brown fat thermogenesis in golden hamsters (Meso- Carey, F. G. (1982). A brain heater in the sword®sh. Science, cricetus auratus). Journal of Comparative Physiology B, 216, 1327±1329. 167, 552±557. Carlson, N. R. (1994). Physiology of behavior. Boston: Al- Blumberg, M. S., & Alberts, J. R. (1990). Ultrasonic vocal- lyn and Bacon. izations by rat pups in the cold: An acoustic by-product Conklin, P., & Heggeness, F. W. (1971). Maturation of tem- of laryngeal braking? Behavioral Neuroscience, 104, perature homeostasis in the rat. American Journal of 808±817. Physiology, 220, 333±336. Blumberg, M. S., & Alberts, J. R. (1991). Both hypoxia and Crawford, E. C., Jr. (1972). Brain and body temperatures in milk deprivation diminish metabolic heat production and a panting lizard. Science, 177, 431±433. ultrasound emission by rat pups during cold exposure. Dailey, W., Wigal., S. B., & Amsel, A. (1986). Effects of Behavioral Neuroscience, 105, 1030±1037. photic stimulation on heart rate of infant rats. Interna- Blumberg, M. S., E®mova, I. V., & Alberts, J. R. (1992a). tional Journal of Psychophysiology, 3, 183±203. Ultrasonic vocalizations by rats pups: The primary im- Daly, M. (1976). Behavioral development in three hamster portance of ambient temperature and the thermal signi®- species. Developmental Psychobiology, 9, 315±323. cance of contact comfort. Developmental Psychobiology, Dawkins, M. J. R., & Hull, D. (1964). Brown adipose tissue 25, 229±250. and the response of newborn rabbits to cold. Journal of Blumberg, M. S., E®mova, I. V., & Alberts, J. R. (1992b). Physiology, 172, 216±238. Thermogenesis during ultrasonic vocalization by rat pups Dawson, W. R. (1982). Evaporative losses of water by birds. isolated in a warm environment: A thermographic anal- Comparative Biochemistry and Physiology, 71A, 495± ysis. Developmental Psychobiology, 25, 497±510. 509. Deeming, C., & Ferguson, M. (1989, March 25). In the heat Blumberg, M. S., & Lucas, D. E. (1996). A developmental of the nest. New Scientist, 121, 33±38. and component analysis of active sleep. Developmental Denenberg, V. H., Brumaghim, J. T., Haltmeyer, G. C., & Psychobiology, 29, 1±22. Zarrow, M. X. (1967). Increased adrenocortical activity Blumberg, M. S., Schalk, S. L., & Sokoloff, G. (1995). Pon- in the neonatal rat following handling. Endocrinology, 81, tine and basal forebrain transections disinhibit brown fat 1047±1052. thermogenesis in neonatal rats. Brain Research, 699, DeVries, A. L. (1982). Biological antifreeze agents in cold 214±220. water ®shes. Comparative Biochemistry and Physiology, Blumberg, M. S., & Sokoloff, G. (1997). Dynamics of 73A, 627±640. brown fat thermogenesis in week-old rats: Evidence of Fair®eld, J. (1948). Effects of cold on infant rats: Body tem- relative stability during moderate cold exposure. Physio- peratures, oxygen consumption, electrocardiograms. logical Zoology, 70, 324±330. American Journal of Physiology, 155, 355±365. Blumberg M. S., & Stolba, M. A. (1996). Thermogenesis, Fowler, S. J., & Kellogg, C. (1975). Ontogeny of thermo- myoclonic twitching, and ultrasonic vocalization in neo- regulatory mechanisms in the rat. Journal of Comparative natal rats during moderate and extreme cold exposure. and Physiological Psychology, 89L, 738±746. Behavioral Neuroscience, 110, 305±314. Gaunt, S. L. L. (1980). Thermoregulation in doves (Col- Blumberg, M. S., Sokoloff, G., & Kirby, R. F. (1997). umbidae): A novel esophageal heat exchanger. Science, Brown fat thermogenesis and cardiac rate regulation dur- 210, 445±447. ing cold challenge in infant rats. American Journal of Goodwin, G. A., Molina, V. A., & Spear, L. P. (1994). Re- Physiology, 272, R1308±R1313. peated exposure of rat pups to isolation attenuates isola- Brody, E. B. (1943). Development of homeothermy in suck- tion-induced ultrasonic vocalization rates: Reversal with ling rats. American Journal of Physiology, 272, 230±232. naltrexone. Developmental Psychobiology, 27, 53±64. BruÈck, K., & Hinckel, P. (1996). Ontogenetic and adaptive Gramsbergen, A., Schwartze, P., & Prechtl, H. F. R. (1970). adjustments in the thermoregulatory system. In M. J. The postnatal development of behavioral states in the rat. Fregly & C. M. Blatteis (Eds.), Handbook of physiology Developmental Psychobiology, 3, 267±280. (pp. 597±611). Oxford: Oxford University Press. Hahn, P. (1956). The development of thermoregulation. III. BruÈck, K., & WuÈnnenberg, B. (1970). ªMeshedº control of The signi®cance of fur in the development of thermoreg- two effector systems: Nonshivering and shivering ther- ulation in rats. Physiologia Bohemoslovenica, 5, 428± short mogenesis. In J. D. Hardy, A. P. Gagge, & J. A. J. Stol- 431. standard long DEV (WILEJ) LEFT INTERACTIVE

122 Blumberg and Sokoloff

Hayne, H., Richardson, R., & Campbell, B. (1992). Devel- treme thermal challenge. Developmental Psychobiology, opmental contraints on the expression of behavioral and 32, 169±176. heart rate orienting responses: II. The role of ambient tem- Kleitman, N., & Satinoff, E. (1982). Thermoregulatory be- perature. Developmental Psychobiology, 25, 51±65. havior in rat pups from birth to weaning. Physiology & Heath, J. E. (1966). Venous shunts in the cephalic sinuses Behavior, 29, 537±541. of horned lizards. Physiological Zoology, 39, 30±35. Kuhn, C. M., Pauk, J., & Schanberg, S. M. (1990). Endo- Heikkila, J. J., & Brown, I. R. (1979). Hyperthermia and crine responses to mother±infant separation in developing disaggregation of brain polysomes induced by bacterial rats. Developmental Psychobiology, 23, 395±410. pyrogen. Life Sciences, 25, 347±352. LaPointe, G., & Nosal, G. (1979). The postnatal evolution Heim, T., & Hull, D. (1966). The blood ¯ow and oxygen of muscular twitches in the developing rat. Experientia, consumption of brown adipose tissue in the newborn rab- 35, 1070±1072. bit. Journal of Physiology, 186, 42±55. Leon, M. (1986). Development of thermoregulation. In E. Heinrich, B. (1979). Keeping a cool head: Honeybee ther- M. Blass (Ed.), Handbook of behavioral neurobiology, moregulation. Science, 205, 1269±1271. vol. 8 (pp. 297±322). New York: Plenum Press. Heinrich, B. (1987). Thermoregulation by winter-¯ying en- Leon, M., Coopersmith, R., Beasley, L. J. & Sullivan, R. M. dothermic moths. Journal of Experimental Biology, 127, (1990). Thermal aspects of parenting. In N. A. Krasnegor 313±332. & R. S. Bridges (Eds.), Mammalian parenting: Biochem- Heinrich, B. (1993). The hot-blooded insects: Strategies and ical, neurobiological and behavioral determinants (pp. mechanisms of thermoregulation. Cambridge: Harvard 400±415). New York: Oxford University Press. University Press. Leonard, C. M. (1974). Thermotaxis in golden hamster pups. Hissa, R. (1968). Postnatal development of thermoregulation Journal of Comparative and Physiological Psychology, in the Norwegian lemming and the golden hamster. An- 86, 458±469. noles Zoologici Fennici, 5, 345±383. Leonard, C. M. (1982). Shifting strategies for behavioral Hofer, M. A., & Shair, H. (1978). Ultrasonic vocalization thermoregulation in developing golden hamsters. Journal during social interaction and isolation in 2-week-old rats. of Comparative and Physiological Psychology, 96, 234± Developmental Psychobiology, 11, 495±504. 243. Houk, J. C. (1988). Control strategies in physiological sys- Levine, S., Huchton, D. M., Wiener, S. G., & Rosenfeld, P. tems. FASEB Journal, 2, 97±107. (1992). Time course of the effect of maternal deprivation Hsieh, A. C. L., Carlson, L. D., & Gray, G. (1957). Role of on the hypothalamic±pituitary±adrenal axis in the infant the sympathetic nervous system in the control of chemical rat. Developmental Psychobiology, 24, 547±558. regulation of heat production. American Journal of Phys- Linthicum, D. S., & Carey, F. G. (1972). Regulation of brain iology, 190, 247±251. and eye temperatures by the blue®n tuna. Comparative Hull, D., & Segall, M. M. (1965). The contribution of brown Biochemistry and Physiology, 43A, 425±433. adipose tissue to heat production in the newborn rabbit. Lyman, C. P., & Blinks, D. C. (1959). The effect of tem- Journal of Physiology, 181, 449±457. perature on the isolated hearts of closely related hiber- IUPS Thermal Commission. (1987). Glossary of terms for nators and nonhibernators. Journal of Cellular and Com- thermal physiology. P¯uÈgers Archive, 410, 567±587. parative Physiology, 54, 53±63. Jans, J. E., & Leon, M. (1983). Determinants of mother± Martin, L. T., & Alberts, J. R. (1982). Associative learning young contact in Norway rats. Physiology & Behavior, in neonatal rats revealed by cardiac response patterns. 30, 919±935. Journal of Comparative and Physiological Psychology, Johanson, I. B. (1979). Thermotaxis in neonatal rat pups. Physiology & Behavior, 23, 871±874. 96, 668±675. Johanson, I. B., & Hall, W. G. (1979). Appetitive learning Millan, N., Murdock, L. L., Bleier, R., & Siegel, F. L. in 1-day-old rat pups. Science, 205, 419±421. (1979). Effects of acute hyperthermia on polyribosomes Johanson, I. B., & Hall, W. G. (1980). The ontogeny of in-vivo protein synthesis and ornithine decarboxylase ac- feeding in rats: III. Thermal determinants of early inges- tivity in the neonatal rat brain. Journal of Neurochemistry, tive responding. Journal of Comparative and Physiolog- 32, 311±317. ical Psychology, 94, 977±992. Miller, J. S., Molina, J. C., & Spear, N. E. (1990). Ontoge- Jouvet-Mounier, D., Astic, L., & Lacote, D. (1970). Onto- netic differences in the expression of odor aversion learn- genesis of the states of sleep in rat, cat, and guinea pig ing in 4- and 8-day-old rats. Developmental Psychobiol- during the ®rst postnatal month. Developmental Psycho- ogy, 23, 319±330. biology, 2, 216±239. Moran, T. H., Lew, Mark F., & Blass, E. M. (1981). Intra- Kelso J. A. S. (1995). Dynamic patterns: The self-organi- cranial self-stimulation in 3-day-old rat pups. Science, zation of brain and behavior. Cambridge: MIT Press. 214, 1366±1368. Kilgore, D. L., Jr., Bernstein, M. H., & Hudson, D. M. Mrosovsky, N. (1990). Rheostasis: The physiology of (1976). Brain temperature in birds. Journal of Compara- change. Oxford: Oxford University Press. tive Physiology, 110, 209±215. Naito, H., & Tonoue, T. (1987). Sex difference in ultrasound Kirby, R. F., & Blumberg, M. S. (1998). Maintenance of distress call by rat pups. Behavioural Brain Research, 25, short arterial pressure in infant rats during moderate and ex- 13±21. standard long DEV (WILEJ) RIGHT INTERACTIVE

Infant Thermoregulation 123

Nedergaard, J., Connolly, E., & Cannon, B. (1986). Brown Smith, R. E., & Horwitz, B. A. (1969). Brown fat and ther- adipose tissue in the mammalian neonate. In P. Trayhurn mogenesis. Physiological Reviews, 49, 330±425. & D. G. Nicholls (Eds.), Brown adipose tissue (pp. 152± Smith, R. E., & Roberts, J. C. (1964). Thermogenesis of 213). London: Edward Arnold. brown adipose tissue in cold-acclimated rats. American Noirot, E. (1968). Ultrasounds in young rodents. II. Changes Journal of Physiology, 206, 143±148. with age in albino rats. Animal Behaviour, 16, 129±134. Sokoloff, G., & Blumberg, M. S. (1997). Thermogenic, res- Okon, E. E. (1971). The temperature relations of vocaliza- piratory, and ultrasonic responses of week-old rats across tion in infant golden hamsters and Wistar rats. Journal of the transition from moderate to extreme cold exposure. Zoology, London, 164, 227±237. Developmental Psychobiology, 30, 181±194. Oswalt, G. L., & Meier, G. W. (1975). Olfactory, thermal, Sokoloff, G., & Blumberg, M. S. (1998). Active sleep in and tactual in¯uences on infantile ultrasonic volcalization cold-exposed infant Norway rats and Syrian golden ham- in rats. Developmental Psychobiology, 8, 129±135. sters. The role of brown adipose tissue thermogenesis. Pinshow, B., Bernstein, M. H., Lopez, G. E., & Kleinhaus, Behavioral Neuroscience, 112, 695±706. S. (1982). Regulation of brain temperature in pigeons: Sokoloff, G., Kirby, R. F., & Blumberg, M. S. (in press). Effects of corneal convection. American Journal of Phys- Further evidence that BAT thermogenesis modulates iology, 242, R577±R581. cardiac rate in infant rats. American Journal of Physi- Rosenzweig, M. R., Leiman, A. L., & Breedlove, S. M. ology. (1996). Biological psychology. Sunderland, MA: Sinauer. Spiers, D. E., & Adair, E. R. (1986). Ontogeny of homeo- Satinoff E. (1978). Neural organization and evolution of thermy in the immature rat: Metabolic and thermal thermal regulation in mammals. Science, 201, 16±22. responses. Journal of Applied Physiology, 60, 1190± Satinoff, E. (1991). Developmental aspects of behavioral 1197. and re¯exive thermoregulation. In H. N. Shair, G. A. Barr, Swadlow, H. A., Waxman, S. G., & Weyand, T. G. (1981). & M. A. Hofer (Eds.), Developmental psychobiology: Effects of variations in temperature on impulse conduc- New methods and changing concepts (pp. 169±188). Ox- tion along nonmyelinated axons in the mammalian brain. ford: Oxford University Press. Experimental Neurology, 71, 383±389. Satinoff, E. (1996). Behavioral thermoregulation in the cold. Swithers-Mulvey, S. E., Miller, G. L., & Hall, W. G. (1991). In M. J. Fregly & C. M. Blatteis (Eds.), Handbook of Habituation of oromotor responding to oral infusions in physiology (pp. 481±505). Oxford: Oxford University rat pups. Appetite, 17, 55±67. Press. Takano, N., Mohri, M., & Nagasaka, T. (1979). Body tem- Schaefer, T., Weingarten, F. S., & Towne, J. C. (1962). Tem- perature and oxygen consumption of newborn rats at var- perature change: The basic variable in the early handling ious ambient temperatures. Japanese Journal of Physiol- phenomenon? Science, 125, 41±42. ogy, 29, 173±180. Schanberg, S. M., & Field, T. M. (1987). Sensory depriva- Taylor, C. R., & Lyman, C. P. (1972). Heat storage in run- tion stress and supplemental stimulation in the rat pup and ning antelopes: Independence of brain and body temper- preterm human neonate. Child Development, 58, 1431± atures. American Journal of Physiology, 222, 114±117. 1447. Taylor P. M. (1960). Oxygen consumption in newborn rats. Schmidt-Nielsen, K. (1990). Animal physiology. Cambridge: Journal of Physiology, 154, 153±168. Cambridge University Press. Teitel, D. F., Sidi, D., Chin, T., Brett, C., Heymann, M. A., Schneider, J. R., Veltri, A. M., DeGraw, W. A. & French, & Rudolph, A. M. (1985). Developmental changes in my- J. A. (1995). Telemetered temperature in preweanling ocardial contractile reserve in the lamb. Pediatric Re- Mongolian gerbils (Meriones unguiculatus). Physiology search, 19, 948±955. & Behavior, 57, 199±203. Van Someren, E. J. W., Mirmiran, M., Bos, N. P. A., Lamur, Schoenfeld, T. A., & Leonard, C. M. (1985). Behavioral A., Kumar, A., & Molenaar, P. C. M. (1990). Quantitative development in the Syrian golden hamster. In H. I. Siegel analysis of eye movements during REM-sleep in devel- (Ed.), The hamster: Reproduction and behavior (pp. 289± oping rats. Developmental Psychobiology, 23, 55±61. 321). New York: Plenum Press. Weber, E. D., & Stelzner, D. J. (1977). Behavioral effects Smith, R. E. (1964). Thermoregulatory and adaptive behav- of spinal cord transection in the developing rat. Brain Re- ior of brown adipose tissue. Science, 146, 1686±1689. search, 125, 241±255.

short standard long DEV (WILEJ) BLANK