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Ecology© 2006 The Authors Journal compilation Ecological Society Blackwell Publishing Ltd Huddling in groups leads to daily energy savings in free-living African Four-Striped Grass Mice, M. SCANTLEBURY,*  N. C. BENNETT,* J. R. SPEAKMAN,à N. PILLAY¤ M. Scantlebury © 2006 The Authors Journal compilation Ecological Society, Functional Ecologydifferent temperatures. Temperatures of 5, 10, 15, 20,C were used with different individualgroups of mice of group sizes one, two, three, four, four,six and eight mice. The sexes for these groups were:one male; one male and one female; two males andone female; four females; four females; four males andtwo females; and six males and two females, respec-tively. The temperatures selected spanned a largepart of the natural daily variation in ambient tem-perature experienced by the mice, during the winter,at the time of study (Cowling approximately the minimal expected nest temperature(C. Schradin and N. Pillay, unpublished data) andC is just below the thermoneutral zone (Haim& Fourie 1980), beyond which thermoregulatory bene-Þts of huddling cease. Measurements took placeduring the night (approx. 20.00Ð03.00 h) when micewould normally be sleeping. We used a dim red light(lux), which allowed us to work during the nightbut did not disturb the mice (Haim & Rubal 1994).Groups were placed in the respirometry chamber for atleast 30 min before any measurements took place inorder for them to settle down (Speakman After this initial period, beginning at the lowest tem-perature (5 C), we took measurements of oxygenconsumption every 30 s for at least 20 min (or longerif the animals did not settle). We then increased thetemperature of the water bath by 5 C (i.e. to 10 from 5 C) and allowed the mice to settle for a further30 min. Measurements were then again taken every30 s for another 20 min. These procedures wererepeated for all the required temperatures. At temperature, the mean of the lowest 10 readings ofoxygen consumption (ml O) was taken, whenanimals were seen to be at rest (Bennett open circuit respirometry system (Depocas & HartHill 1972) was used in which a metabolic chamber) was immersed in a temperature-controlledwater bath (Labotec, Lauda, Kšnigshofen, Germany).The same chamber was used for all measurements (i.e.for all group sizes of mice and for all temperatures).Dried air was pumped into the chamber at a variablerate (min 500 ml min; max 4000 ml min) deter-mined by the number of mice (we used approximately for each mouse that was in so that depressions in oxygen concentration weremaintained at 0á25Ð0á4%. The air passed throughapproximately 4 m of copper coil that was submergedin the water before it entered the chamber. Thisensured the temperature of air that entered thechamber was the same as the water bath. The ßow ofair into the chamber was controlled by a ßow regulator(Omega FMA-A2310, Stamford, CT) placed upstream.Measurements of were taken using an oxygenanalyser (S-2 A Applied Electrochemistry, AEITechnologies, Inc., Naperville, IL, USA). The analyserwas calibrated to an upper value in dry air (20á95% Oprior to the measurement of each animal and to alower value (0% O in N gas, Afrox, Germiston, SouthAfrica) prior to initial measurements. Results werecorrected to standard temperature and pressure.
    RMR was determined as the minimal oxygen con-), using the apparatus describedabove for individual mice after they had been capturedfrom the Þeld (after the second recapture, see DLWmethods below). The temperature of the water bathwas maintained at 31 C, which is within thethermoneutral zone (Haim & Fourie 1980). Readingswere taken when animals were seen to be at rest, afteran initial hour in which they were allowed to settle inthe respirometry chamber (Speakman before, measurements of oxygen consumption weretaken every 30 s for 20 min and the mean of the lowest10 readings of oxygen consumption (ml O) wastaken, when animals were seen to be at rest (Bennett      The daily energy expenditures (DEE, kJ day) of nineadult mice (eight females and one male from ninedifferent groups, with group sizes of two, three, four,four, four, four, six, six and six individuals) weremeasured using the doubly labelled water (DLW)technique (Lifson & McClintock 1966; Speakman1997). One mouse per group was used. Because maleswere frequently Þtted with radio-transmitters, butfemales were not, we decided to concentrate onmeasuring the DEE of females. This had the additionaladvantage of omitting intersexual variation in DEE.However, for one of the groups of mice we did notcapture a female. Instead, we captured a male with atransmitter. Since the DEE of this one male did notdiffer statistically from that of all the other females 0á1), we decided to include this one male in theanalysis. On day 1 (the Þrst day of capture), the animalswere weighed (0á1g) and a 0á1-ml blood sample wasobtained from the tail to estimate the backgroundisotope enrichments of O. Blood sampleswere immediately heat sealed into 50-l glass capillaries.Afterwards, a known mass of DLW [100 g 95% APE-O water (Rotem Industries Ltd, Beer Sheva,Israel) and 50 g 99á9% APE-enriched H water (IsotecInc. Miamisburg OH) mixed with 342 g O] wasadministered (IP, 0á3 g/100 g body weight). Syringeswere weighed before and after administration0á0001 g, Sartorius balance) to calculate the mass ofDLW injected. Blood samples were taken after 1 h toestimate initial isotope enrichments. Mice weretrapped 2Ð3 days later (Speakman & Racey 1988) toestimate isotope elimination rates. Half of the miceof each group were then temporarily removed andhoused together in cages in the outdoor enclosure, M. Scantlebury © 2006 The Authors Journal compilation Ecological Society, Functional EcologyMice weighed on average 41á7 8á9 g ( = 9) uponinitial capture. There was a signiÞcant positive effectof body mass on DEE ( = 0á048 and = 0á005 for control and reduced groups,respectively), but there was no effect of group size = 0á164 and = 0á779 forcontrol and reduced groups, respectively) on DEE.Therefore, it was not the case that mice in naturallysmaller group sizes had signiÞcantly higher DEEvalues. DEE values increased by a mean of 19%(10á8 kJ day) when group sizes were reduced byone-half (70á0 18á1 and 59á1 13á2 kJ dayrespectively, two-sample = 0á03, Fig. 2a).There was no effect of body mass or group size onthe increase in DEE when groups were reduced = 0á107 for body mass, = 0á230 for group size). Therefore, it was also not thecase that the increase of DEE (upon reduction ofgroup size) was correlated with original group size orthe body mass of the measured individual. We wereinterested in whether we could detect any effects ofnatural variation in group size on DEE. Since includ-ing too many covariates in a model with small samplesize diminishes statistical power (MacCallum, Browne& Sugawara 1996), we reanalysed the DEE dataslightly differently by using each covariate (body massand group size) separately, using GLM analyses. In thecontrol group, we found signiÞcant positive andnegative effects for mass ( = 0á01) andgroup size ( = 0á046), respectively. Bycomparison, although there was a signiÞcant effect ofmass when groups had been reduced ( = 0á001), there was no signiÞcant effect of group size = 0á09). These results suggest that theremight have been effects of natural group size on DEEbut that our sample size was too small ( = 9 groups)to detect signiÞcant differences when both covariateswere included in the same model. SusMS averaged 0á19 in natural group sizes. There were noeffects of body mass or group size on SusMS = 0á49 for body mass and = 0á18 for group size), or any signiÞcant change inSusMS when group size was reduced.Assuming that mice gain the beneÞts of huddling forthe duration that they are in the nest (. 14 h day inthe winter and 10á5 h day in the summer: C. Schradin,personal observation), we calculated the averageexpected increase in DEE for mice in the free-livinggroup sizes that we measured using the derivedrelationship between temperature and group size on (Equation 3). We calculated the difference in for the range of free-living group sizes that weremeasured for DEE and for those that had beenreduced by half. We then converted the differencebetween these two values of to kJ day using afactor of 20á51 kJ l (Hardy 1972). As this Þgurewould indicate the energy increase due to a reductionin group size for 24 h, we converted this into anequivalent increase in energy expenditure for a 14-hperiod, which was the length of time that mice were intheir nests (huddling) at night. Using this method, thecalculated average increase in DEE for the free-livingmice for which group size had been reduced by one-halfwas 9á3 3á0 kJ dayWTO averaged 7á70 2á57 ml day and WEI averaged in natural group sizes (Fig. 2b).There were no effects of body mass or group size on = 0á662 for body mass and = 0á763 for group size) or WEI = 0á625 for body mass and = 0á942 for group size). However, WTO signiÞcantlyincreased and WEI signiÞcantly decreased when groupsizes were reduced (two-sample = 0á016 for or WEI).Many laboratory studies have shown that smallmammals in large groups have lower individual energyexpenditures than those in small groups or singleindividuals (e.g. Contreras 1984; Hayes Daily energy expenditure (DEE, kJ daywater turnover (WTO, ml day) of free-living individualsfrom unmanipulated group sizes (natural) and of the sameindividuals 2 days later when group sizes have been reducedby one-half (reduced). Each line and Þlled circle denotes asingle individual. M. Scantlebury © 2006 The Authors Journal compilation Ecological Society, Functional Ecology(although see Anava . 2001; Scantlebury 2002). In some circumstances, thermoregulatorysavings due to huddling are suggested to be onepossible mechanism driving the evolution of groupliving (Beauchamp 1999). Four-Striped Grass Miceare unusual in that they live in groups in a xeric habitat,whereas they are solitary in mesic grasslands (Schradin& Pillay 2003, 2004). One obvious difference betweenthe two environments is that the variations in ambi-ent temperature (both daily and annually) and foodresources are much greater in the desert. Thus, micestand to gain more energetic and water-conservingbeneÞts from huddling in the desert than in thegrasslands Ð which could be one of the possible reasonsfor the observed differences between the social structuresbetween populations of Four-Striped Grass Mice inIn conclusion, we show for the Þrst time that, infree-living mammals, DEE and WTO values are lowerin natural group sizes than in group sizes that havebeen artiÞcially reduced. Differences in DEE are con-sistent with measured energy savings in the laboratoryof the same population of mice under controlledconditions. Observed differences in free-living DEE aretherefore likely to be produced by differences in ther-moregulation. Thermoregulatory savings are onepossible reason that mice live in groups in desertswhereas they are solitary in grasslands.AcknowledgementsWe would like to thank Northern Cape Department ofAgriculture, Land Reform and Environment and thestaff at Goegap Nature Reserve for their assistance.We thank the Universities of Pretoria and the Wit-watersrand, the National Research Foundation RSA(Grants 2053801 and 2053514) and the British logical Society (SEPG) for Þnancial support to N.C.B.,N.P. and M.S. C.S. was funded by the Fonds zurFšrderung des akademischen Nachwuchses (FAN) ofthe University of Zurich and the Claude Harris LeonFoundation. We would also like to thank FredrikDalerum for providing comments on an earlier versionof the manuscript and Brigitte Britz, Christina Keller,Carola Schneider, Melanie Schubert and PhilippWidmann for assistance in the Þeld. 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