Hibernation and Daily Torpor in Two Pygmy Possums (Cercartetus Spp., Marsupialia) Author(S): Fritz Geiser Reviewed Work(S): Source: Physiological Zoology, Vol

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Hibernation and Daily Torpor in Two Pygmy Possums (Cercartetus Spp., Marsupialia) Author(s): Fritz Geiser Reviewed work(s): Source: Physiological Zoology, Vol. 60, No. 1 (Jan. - Feb., 1987), pp. 93-102 Published by: The University of Chicago Press Stable URL: http://www.jstor.org/stable/30158631 . Accessed: 16/04/2012 20:11

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http://www.jstor.org HIBERNATION AND DAILY TORPOR IN TWO PYGMY POSSUMS (CERCARTETUS SPP., MARSUPIALIA)'

FRITZ GEISER2 Schoolof BiologicalSciences, Flinders University, Adelaide, South Australia 5042, Australia (Accepted6/24/86) The physiologyof torporwas investigatedin the pygmypossums Cercartetus concinnus(18 g) and C. lepidus(12 g). Body temperatures(Tb) during torpor remained within 1 C of air temperature(Ta) and showed a minimum of about 5 C in both species.Oxygen consumption (Vo2) duringtorpor was reducedto about 0.05 liters 02/kg h, which amountsto only 1%of the rate of normothermicanimals. Below Ta of 5 C, the metabolismduring torpor increased with a furtherdecrease in Ta,while Tbwas regulatedat about 5 C. Ratesof arousalwere fasterthan predictedfor endo- thermicvertebrates of comparablesize. Durationof torporbouts rangedfrom less thana day to a week,with the longestbouts occurring at Ta< 10 C. The physiological characteristicsof torporin these two marsupialspecies are qualitativelyand quanti- tativelysimilar to those of placentalhibernators.

INTRODUCTION nounced reduction of body temperature Pygmy possums, small (<50-g) marsu- and mass-specific oxygen consumption pials of the family Burramyidae, are found during torpor (Bartholomew and Hudson in Australiaand New Guinea. Seven species 1962), a shorter duration of torpor (Hick- are recognized, with Distoechuruspennatus man and Hickman 1960; Strahan 1983), occurring only in New Guinea. The taxo- and a slower rewarming from torpor (Bar- nomic membership of the two feathertail tholomew and Hudson 1962; Fleming possums (Acrobates pygmaeus and Disto- 1985a, 1985b) than in placental mammals. echurus pennatus) to burramyids is cur- Furthermore, there are no experimental rently under review (Archer 1984), and the data on the thermoregulatory ability of remaining species belong to the genera pygmy possums during torpor and thus no Cercartetus and Burramys. indicationthat the reductionin Tbis a con- Torpor has been observed in all Austra- trolled process. Some of these quantitative lian burramyids and is qualitatively similar differencesled to the conclusion that torpor to hibernation in placental mammals (Bar- in burramyid marsupials may be less "re- tholomew and Hudson 1962). It has been fined" than in the "perfect" model of hi- suggested, however, that quantitative dif- bernating rodents (Lyman 1982). ferences do exist between the two groups. Physiological data are available for three Such differences appear to be a less pro- of the six Australian burramyids (Bartho- lomew and Hudson 1962; Fleming 1985a, 'I wishto thankMeredith Smith for herloan of 1985b). Because large interspecific differ- severalpygmy possums and John Coventry for allowing ences in the pattern of thermoregulation me to use hispitfall traps in the BigDesert and partake have been observed in rodent families that of his dinkumbush chooks. Bronwyn McAllan helped contain hibernators and me to set up and checktrap lines. RussellBaudinette may "deep" spe- providedmany helpfulsuggestions during the exper- cies that enter shallow, daily torpor (Lyman imentalperiod, and SaraHiebert, Jim Kenagy,and 1982), it was of interest to investigate two anonymousreferees constructively criticized the whether such specific differencesin the manuscript.The study was supportedby a Flinders thermalphysiology also occur in the bur- UniversityResearch Scholarship and the paperwas compiled while the author held an Alexandervon ramyids.I also examinedin greaterdetail Humboldt-LynenResearch Fellowship at the Uni- the suggesteddifferences in the physiology versityof Washington. between placental hibernators and the 2 Present address: Department of Zoology, Univer- marsupialfamily Burramyidae by studying sity of Washington, Seattle, Washington 98195. temporaland thermalaspects of torporin Cercartetus the western Physiol. Zool. 60(1):93-102. 1987. concinnus, pygmy - 1987 by The University of Chicago. All possum, and C. lepidus (syn. Eudromicia rightsreserved. 0031-935X/87/6001-8636$02.00 lepida), the little pygmy possum. 93 94 F. GEISER MATERIALAND METHODS certain nights. Food was exchanged daily in the afternoon. THE ANIMALS Cercartetusconcinnus is found in South EXPERIMENTALDETAILS Australiaand WesternAustralia and occurs Diurnal fluctuations in oxygen con- in mallee heath and dry sclerophyll forest. sumption (V02) were determined at Ta 6- It is nocturnal, uses tree hollows as daytime 31 C for 30 periods of 21.1 - 2.1 h (range shelters,eats insects, pollen, and nectar, and 15.5-29.25 h, C. concinnus) and for 20 pe- storesfat in its prehensiletail (Smith 1983). riods of 22.0 - 2.3 h (range 17.25-25.75 h, Three males and two females were live C. lepidus). These experiments began in the trapped at various locations on the south- late afternoon. Food and water were not eastern coast of South Australia and the available during measurements of Vo2. To Eyre Peninsula and kept in the laboratory determine whether burramyids regulate at Ta 22 C under natural photoperiod. In their Tbduring periods of torpor at a specific autumn (March) they were transferredto a "set point," torpid animals were cooled constant Ta of 19 C with a photoperiod of (<0.05 C/min), and the Vo2 was continu- 12L:12D, a condition similar to the natural ously monitored. Additional measurements photoperiod at that time of the year, and during shorter periods were used to deter- held for 3 wk before the experiments com- mine the V02 Of postabsorptive,normo- menced. All experiments were performed thermic, inactive animals at T, around and in autumn between March and May 1984 above thermoneutrality. The Vo2 of one C. and 1985. The mean body mass during the lepidus was measured during prolonged experimental period was 18.6 - 1.8 g (SD) torpor over a 7-day period. Flow rates of (range 16-22 g). air in the open flow system were between Cercartetus lepidus occurs in Tasmania 0.15 and 0.30 liters/min and were adjusted and two areas of southeastern Australia. It and measured with calibrated rotameters. is the smallestof all possums.It is mainly Measurements of V02 were used to deter- nocturnal and has been found sheltering in mine the metabolic rate of normothermic, hollow logs and birds' nests (Green 1983). inactive animals measuredwhen a variation It appearsto feed primarilyon invertebrates of less than 5%over at least 15 min occurred and stores fat in its tail. One male and one within an inactivity period of at least 30 female were live trapped in the Big Desert, min; the metabolic rate of torpid animals Victoria, in September and transferred to was determined at times of constant Vo2 the laboratory, where they were kept at Ta over at least 30 min. Furthermore, the du- 19 C and a photoperiod of 12L:12D (similar ration of torpor and the time that was re- to natural photoperiod at the time of cap- quired to arouse from torpor to normo- ture) for 5 wk before measurements com- thermia were determined from these mea- menced. Experiments with these two in- surements (the Vo2 peak during arousalwas dividuals were conducted in spring and assumed to be the end point of the arousal early summer (October to beginning of De- period). In some instances the displacement cember) 1984. Another male was caught at of sawdust from the animal's back was used Messent Conservation Park, South Aus- to determine the duration of torpor. All tralia, and was L.eptin the laboratory at Ta measurements were conducted in a quiet 22 C and natural photoperiod. In autumn controlled-temperature room (Ta + 0.5 C) (March) it was transferred to Ta 19 C and that was acoustically insulated from the re- a photoperiod of 12L:12D and was kept cording equipment. A video camera was under these conditions for 3 wk before the used to observe the animals during the ex- experiments began. The mean body mass periments. was 12.6 - 1.9 g (range 10-15 g). A Servomex Model OA 184 paramag- The animals were fed with Heinz canned netic oxygen analyzer was used for the Vo2 baby food (fruits), a mixture of Heinz high- measurements together with a Rikadenki protein cereal and honey, and apples. Be- potentiometric recorder. The Vo2 was de- cause pygmy possums held in captivity termined from the difference between the become extremely fat, their weight was oxygen content in two parallel open-flow controlled by feeding them only apples on circuits, one being a room air reference and TORPOR IN PYGMY POSSUMS 95 the other containing the animal. All gas tendency to enter torpor spontaneously volumes were correctedto dry standard than the largerC. concinnus. temperature and pressure (STPD).The Ta Most entriesinto torporin both species was measuredcontinuously in the respi- occurredbetween 2400 and 0600 hours, ratorychamber (volume 3 liters)with cal- beforethe onset of light. The time of day ibratedthermocouples. The Tbwas mea- when torpor commenced was not related sured with calibrated 0.5-mm diameter to Ta. The nocturnalpeaks of Vo2 during thermocouplesinserted 20 mm into the activityof C. concinnusoccurred between rectum. Esophageal temperatureduring 1845 and 0130 hours,with a mean of 21.1 torpor was also determinedoccasionally. + 2.1 h, anddid not appearto be dependent During induced arousal the rate of re- on Ta. In C. lepidus,the nocturnalpeaks warming was determined by taping the of oxygenconsumption associated with ac- therrmocouplewire of the rectallyinserted tivity occurred between 1845 and 2200 thermocoupleprobe to the animal'stail. hours(with two exceptions). A Student'st-test was used for compar- At temperaturesabove 15 C, bothspecies isons of pairedobservations. Straight lines usuallyentered torpor on a daily basisand werefitted by linearregression analysis. Dif- aroused spontaneouslyin the afternoon. ferencesin slope and elevationwere deter- The longest torpor bouts of C. concinnus mined using the derived t- and F-values. were 4 days at Ta 12 C and 8 days at Ta8 Means of samples are expressed-SD. N C. In C. lepidus,torpor lasted for a maxi- = number of individuals, n = number of mum of 2 daysat Ta19 C (sawdustmethod) determinations. and about 6 days at Ta 8.7 C (Vo2 measurement;fig. 1). RESULTS Periodsof apnealasting for about 5 min Both Cercartetusconcinnus and C. lep- at Ta 10 C were observedin torpidC. con- idus showeda high tendencyto enter tor- cinnusand longerthan 30 min at Ta8.7 C por.In all but one experiment,C. concinnus in C. lepidus. These periods were punc- enteredtorpor during measurements of Vo2 tuatedby markedpolypnea. when no food andwater were provided and Body temperaturesat restwere stable at Tawas less than 20 C. All C. lepidusentered Ta 3-32 C with a mean Tb of 34.4 + 0.5 C torporunder these conditions when Tawas (C. concinnus; N = 5, n = 17) and 33.7 less than 27 C. Apartfrom these induced + 0.8 C (C. lepidus; N = 3, n = 14) (fig. 2). torporperiods, the animals frequentlyen- Higher Tb'swere observedat Ta's> 32 C. tered torpor spontaneouslyin the labora- Duringtorpor, Tbdecreased with decreas- torywhen food andwater were freely avail- ing Ta. The lowest AT values (Tb - Ta) Of able (table 1). Spontaneoustorpor at Ta 19 0.3-2.5 C occurredbetween Ta 5 and 20 C, C was observedin the morning;in the af- and most A T were less than 1.0 C in both ternoon animals usuallywere normother- species. Below Ta4 C (C. concinnus)and mic. Cercartetuslepidus showed a greater Ta5 C (C. lepidus),an increasein AT was

TABLE1

SPONTANEOUSTORPOR IN Cercartetus concinnus AND C. lepidus

Ta Species (-C) n Torpid Normothermic %Torpid C. concinnus (N= 5) 38 19 ... 19 19 50 11-13 60 42 18 70 8 24 21 3 88 C. lepidus (N=3) 19 72 51 ... 21 71 NOTE.-Food and waterwere availablead lib. Observationswere made between 0800 and 1000 hours. N = numberof individuals;n = numberof observations. 96 F. GEISER 10 Cercartetus Lepidus 9-

8 - Tb 3520C 7

6 r r 5

o .> 3

/Tb9.20C Ok 12 21 12 21 12 21 12 21 12 21 12 21 12 21. 12 TIMEOF DAY (h) FIG.1.-The fluctuationin the rateof oxygenconsumption (0o2) of an individualCercartetus lepidus measured over a 7-day periodat T, 8.7 + 0.7 C. The abscissarepresents the local time, and the darkbars indicatethe periodsof darkness.Body temperatures (Tb) are indicatedat the end of the measurement. observed.The lowest individualTb's mea- indistinguishable from values measured suredduring torpor were 4.7 C in C. con- rectally in C. concinnus. In torpid C. lepi- cinnusand 5.9 C in C. lepidus.Esophageal dus, the esophageal temperatures were at temperaturesduring torpor at Ta10 C were most 1.4 C above the rectal temperatures

.0 40 Cercartetus concinnus Cercartetus lepidus 0 35 0 0 0 0 8 o 0 00 o0 o -o 0oo o o

25 a * 30 2020 .*0

15 15

* 15 10

0 5 0 10

0 35 0 5 10 15 20 25 30 35 5 10 15 20 25 30

(C) Ta (OC) Ta ) FIG.2.-Body temperatures(Tb) Of normothermic, inactive (0), and torpid(e) Cercartetusconcinnus and C. lepidusas a functionof air temperature(T,). The solid lines representTb = T,. TORPOR IN PYGMY POSSUMS 97 below Ta's of 10 C; at Ta 19 C both tem- these valuesextrapolated to the abscissaat peratures were within 0.2 C. 35.0 C (C. concinnus)and 36.5 C (C. lepi- The Tb Of torpid pygmy possums was dus). These interceptswere 0.6 and 2.8 C, regulated above a specific minimum. In the respectively,above the mean resting Tb. typicalcase, shown in figure3, Ta was de- The lowercritical temperature of the ther- creased from 10.5 C, the temperature at moneutralzone was between28 and 30 C which the C. concinnus had entered torpor. in both species.The mean metabolicrate As Ta decreased to 4.1 C, Vo2 remained at Ta 28.8-31 C was 1.20 - 0.3 liters 02/ stable at a level of about 0.06 liters 02/kg kg h (N = 4; C. concinnus) and 1.49 + 0.2 h. At Ta 3.6 C, Vo2 increased to a new pla- liters 02/kg h (N = 3, n = 8; C. lepidus), teau of 0.24 liters02/kg h. An increasein and both values were slightly greater than Tato 4.2 C resultedin a decreasein Vo2 to the predicted standard metabolic rate for a 0.16 liters 02/kg h, and Tbwas 4.7 C. Ma- marsupial of the respective body mass nipulations during measurement of Tb at (Dawson and Hulbert 1970). 1445 hours induced arousal. The mean Ta For the daily Vo2 minima (fig. 4), a at which an increasein Vo2 followedcool- steady decrease with Tawas observed above ing was 4.4 - 0.4 C (N = 3) in torpid C. Ta5 C. The Vo2 of torpid C. concinnus was concinnusand 5.3 + 0.6 C (N = 3, n = 4) 0.046 - 0.02 liters 02/kg h (N = 5, n = 16; in C. lepidus. In all but one experiment, Ta 5-13 C and 0.052 + 0.01 liters 02/kg h wherecooling inducedarousal, the pygmy (C. lepidus, N = 3, n = 17; Ta 6-11 C). possums remainedtorpid but had an in- Below Ta 5 C, a linear increase in Vo2 dur- creasedmetabolic rate. ing torpor occurred in both species. Ex- Reductionin Taresulted in a parallelin- trapolation of these lines intersected the creasein Vo2 duringrest and torporat Ta abscissa at 4.9 C (C. concinnus) and 5.6 C belowthe minimum Tb(fig. 4). The Vo2 of (C. lepidus). The Vo2 during torpor at the normothermic, inactive animals below Tabelow which an increase in metabolism thermoneutralityincreased linearly as Ta was observed during torpor was 0.7% and decreasedin both species;the lines relating 0.8%, respectively, of the values observed

0.4 Cercartetus concinnus

i" 0.3 N

0.2

11 .1 oC 10

I.-. 6 5-

9 10 11 12 13 14 15

TIME OF DAY FIG.3.-The rate of oxygenconsumption (Vo2) Of an individualtorpid Cercartetus concinnus exposed to changingair temperature (Ta). The body temperature (Tb) is indicated;handling during this measurement induced arousal.The abscissarepresents the local time. 98 F. GEISER

8 Cercartetusconcinnus S CercartetusLepidus

\ 7 7 o

o cu 6 cm o 2 o5 N 0 o 5 oc 0000 0

o"o 3 00 o-\o o8 3 o

2 000 2% . 0 1. 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 Ta (-C) T, (-C) FIG.4.-Rates of oxygenconsumption (Vo2) Of Cercartetus concinnus and C. lepidusas a functionof ambient temperature(T,). The symbolsindicate: normothermic inactive (0), and the Vo2 minimaat constantT0 (e) or after T, had been lowered(U). The equationsfor the lines are:C. concinnus,normothermic inactive: y = 7.5 - 0.21x, r = -.98; in torpor:y = 0.83 - 0.17x, r = -.93. Cercartetuslepidus, normothermic inactive: y = 7.7 - 0.21x, r = -.97; in torpor:y = 0.98 - 0.17x, r = -.64.

in normothermic,resting animals and only DISCUSSION 3.8%and 3.5%when comparedwith the The presentstudy suggests that torpor in basalmetabolic rates. marsupialsis underprecise control and that The relativeweight loss was 4.6 - 1.5 their thermoregulationis not inferior to mg/g h (C. concinnus)and 4.9 + 1.6 mg/g their placentalcounterparts. There appear h (C. lepidus)for animalsthat had entered to be no generalphysiological differences torpor.Weight loss (WL, mg/g h) was in- in the characteristicsof torporand hiber- verselyrelated to torporduration (TD, in nation in placentaland marsupialmam- hours)in both species(WL = 6.4-0.21 TD, mals.Both groups contain species that show r = -.60, C. concinnus;WL = 5.6-0.04 TD, profoundtorpor while others enter shallow, r = -.69, C. lepidus)(data not shown). daily torpor. The rateof increasein Tbduring arousal Minimum Tb'Saround 5 C as in pygmy was directlyrelated to Ta in both species possums (table 3) have been reportedin (table2). The fastestoverall arousal rate was many placental "deep" hibernators(e.g., 0.65 C/min in C. concinnus and 0.81 Strumwasser1960; Pengelley1964; Wang C/min in C. lepidus.The fastestarousal rate and Hudson 1971). However, the mini- determinedover a period of 10 min was mum Tb's in the burramyidsare much 0.79 and 0.90 C/min, respectively.The lowerthan those observedin marsupialsof availabilityof food during the night pre- otherfamilies (Morrison and McNab 1962; cedingarousal did not influencethe arousal Wallis 1976;Geiser 1985a, 1986). rates.Initial differences (0.2-1.4 C) between The metabolicrate of torpidpygmy pos- esophagealand rectal temperaturein C. sums was reducedto less than 1%of the lepidusobserved at low Tawere gradually restingvalues (table 3). Duringactivity of reducedduring arousal. Cercartetusconcinnus and C. lepidusat Ta's The time requiredto arousefrom torpor below 10 C, oxygen consumptionwas in- to normothermiaincreased exponentially creasedabout 200-fold in comparisonto with decreasingTa (fig. 5). No significant the torporvalues. The low metabolicrate differencesbetween induced and sponta- of pygmy possums was accompaniedby neous arousalwere observedin eitherspe- A T's thatwere usually less than 1 C (Flem- cies. ing 1985a, 1985b;present study). Hiber- TORPOR IN PYGMY POSSUMS 99 nating placentals show a similar metabolic Guinea (for review, Geiser 1985b).In hi- rate and AT during torpor (Kayser 1961; bernatingrodents rhythmical arousal also Lyman 1982; French 1985). occursabout every week or two (Pengelley Body temperatures during torpor ofbur- and Fisher 1961;Pohl 1961;Kenagy 1981; ramyids are regulatedto remain at or above French 1982). However, in contrast to a specific minimum similar to dasyurids many placentalhibernators, some pygmy (Geiser et al. 1986). However, the ther- possumsappear to forageand feed during moregulatory increase in the metabolic rate winter(Coventry and Dixon 1984). A less occurred at a much lower Tain burramyids distinctseasonality in the torporpattern of (at Ta = 5 C) than in dasyurids (Ta > 11 most pygmy possums is not surprising, C). The regulation of Tb at a specific min- consideringthe relativelymild wintersin imum during torpor and the sensitivity to many parts of Australia. However, the changes in Ta of less than 0.5 C show that mountainpygmy possum, Burramyspar- torpor in pygmy possums is a controlled vus, which is exposed to strong seasonal process. fluctuations,appears not to emergeduring In burramyid marsupials, the longest winter(Mansergh 1984); nor do femaleC. torpor bouts last from 2 to 13 days, with nanusin partsof Victoria(A. K. Lee 1985, the possible exception of C. caudatus, which personal communication). is foundin Queenslandrainforests and New At high Ta, both C. concinnus and C.

TABLE2 AROUSAL RATESOF Cercartetusconcinnus AND C. lepidus

Tb Tb Initial End OverallArousal FastestArousal Ta (-C) (-C) (oC) (-C/min) (-C/min) Food

C. concinnus (N = 5)

4.1 6.0 32.4 .11 .28 - 4.4 4.7 33.9 .13 ND - 6.0 7.2 34.7 .18 .34 - 9.0 10.9 36.0 .28 .44 + 9.8 10.0 36.9 .30 .55 + 10.6 12.7 35.0 .31 .39 + 12.0 13.7 34.8 .33 .48 - 17.0 18.1 36.5 .46 .48 + 19.0 20.0 33.4 .56 .58 + 19.0 21.3 34.5 .60 .64 + 26.9 28.9 34.5 .59 ND - 5 - 23 7.0 35.0 .56 .71 - 19 - 22 19.4 35.0 .65 .79 +

C.lepidus (N = 3)

6.0 6.9 35.1 .19 .47 - 8.7 9.2 35.2 .34 .62 - 18.3 18.5 34.8 .71 .77 + 18.5 18.9 36.0 .81 .88 + 19.0 19.5 36.0 .71 .80 + 24.0 25.2 35.5 .81 .90 - 3.4 - 20 7.7 35.9 .67 .83 - 19-* 23 19.3 36.0 .73 .80 +

NOTE.-Thefastest arousal rates were determined over time periods of at least 10 min at Tb'sabove T,. Arrows indicatethat the animalswere transferred from the T, theyhad entered torpor to a differentTa at whichrewarming was measured.ND: not determined.Food duringthe nightprior to the measurementwas eitheravailable (+) or not available (-). 100 F. GEISER 500 Cercartetus concinnus Cercartetus lepidus 200 200 100

S100 o 50 .E E 50 sos L C~ 20 o ' 20 10 10 5

0 5 10 15 20 25 30 0 5 10 15 20 25 30 Ta(oC) Ta (OC) FIG.5.-Semilogarithmic plot of the arousaltimes of Cercartetusconcinnus and C. lepidusas a functionof air temperature(T,). The symbolsindicate induced (0) and spontaneous(0) arousal.No significantdifferences betweenthe two could be detected.The equationsare: C. concinnus,induced: log y = 2.65 - 0.060x, r = -.97, spontaneous:log y = 2.64 - 0.053x,r = -.98; C. lepidus,induced: log y = 2.35 - 0.054x, r = -.99, spontaneous: log y = 2.48 - 0.062x, r = -.998. lepidusentered torpor on a dailybasis; and for mammalsand birds(Heinrich and Bar- it appearsthat the torporbouts increased tholomew1971). This does not supportthe with decreasingTa, as in groundsquirrels view that marsupialspossess a lower ther- (French1982). This suggeststhat prolonged mogeniccapacity than placentals(Fleming torporof pygmypossums in the wildoccurs 1985a). The absenceof a lag of the rectal during cold weather. In dasyuridmarsu- temperaturebehind that of the anteriorpart pials, prolongedtorpor has not been ob- of the body duringarousal is most likely a servedeven at low Ta(Wallis 1976;Geiser reflectionof the small body size of pygmy and Baudinette1985). possums. Arousal in C. concinnus and C. lepidus The tendency to enter torpor sponta- was rapid, with the fastest arousal rates neously was high in both burramyidsin- about20% above the predictedarousal rates vestigated.Torpor could be inducedin au-

TABLE3 SUMMARYOF PHYSIOLOGICAL DATAOF AUSTRALIAN BURRAMYID MARSUPIALS

Vo2 INTORPOR TORPOR BMR BODY TbMIN Min Mean DURATIONTb REST (liter 02/ SPECIES MASS0(g) (oC) (liter02/kg h) (days) (OC) kg h) SOURCES

Cercartetusnanus 24 (35-90) 6 .05 .19 13 34.9b .86 1, 2 C. concinnus 13 (18.6) 4.7 .023 .046 11 34.4 1.20 3, * C. lepidus >8 (12.6) 5.9 .032 .052 6 33.7 1.49 * C. caudatus...... 30 <1 4 Burramysparvus 42(44.3) 6 .022 .063 7 36.1 .83 5 Acrobatespygmaeus 12 (14) 7 .07" 2 34.9 1.08 6

SOURCES.-1,Bartholomew and Hudson 1962;2, Dimpeland Calaby1972; 3, Wakefield1970; 4, Atherton and Haffenden1982; 5, Fleming1985b; 6, Fleming1985a; *, presentstudy. "Body masseswere taken from Strahan (1983); C. lepidusfrom Coventry and Dixon (1984). Body massesin parenthesesshow the valuesfor the animalsused in the Vo2and Tbmeasurements. Tb min and Vo2min represent the minimarecorded for eachspecies. The torporduration is the maximumrecorded for eachspecies. The other valuesrepresent means. b"Active" animals. C Calculatedfrom percent of Vo2at rest. TORPOR IN PYGMY POSSUMS 101 tumn, spring,and summerby exposingan- than to seasonal temperaturestress and imals to low Tain a similarmanner as in food shortages. the rodentSicista betulina(Johansen and The physiologicalvariables associated Krog 1959).Prolonged and deep torporin with torpor in burramyidsare strikingly pygmypossums could be inducedwithin 1 similarbetween the various species (table day of cold exposurewhen food was with- 3). Thisis in contrastto cricetidand sciurid drawn,and therewas no evidencethat cold rodents,which show a largerange of ther- acclimationwas necessaryfor the process. moregulatorymodes and contain species However,it is likely that cold acclimati- that are strict homeotherms,species that zation would lowerthe minimum Tbas in entershallow, daily torpor, and "deep"hi- dasyuridmarsupials (Geiser et al. 1986). bernators (Lyman 1982). Among those The abilityto enter torporat any time of placentalspecies that enter prolongedtor- the year suggeststhat Cercartetusspp. do por, thereare greatdifferences in the min- not possessa strongendogenous seasonal imum Tb,metabolic rate, durationof tor- cycleof torporand activity.Torpor in these por,and rateofrewarming; and the pattern marsupialsappears to be an opportunistic of torporin burramyidmarsupials is within responseto unpredictableconditions rather this rangeof variations.

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