ORIGINAL PAPER J. Schmid á J. R. Speakman Daily energy expenditure of the grey mouse lemur Microcebus murinus ): a small primate that uses torpor Accepted: 29 August 2000 Abstract We aimed to investigate the pattern of utili- sation of torpor and its impact on energy budgets in free-living grey mouse lemurs Microcebus murinus), a small nocturnal primate endemic to Madagascar. We measured daily energy expenditure DEE) and water turnover using doubly labelled water, and we used temperature-sensitive radio collars to measure skin temperature T sk ) and home range. Our results showed that male and female mouse lemurs in the wild enter torpor spontaneously over a wide range of ambient temperatures T a ) during the dry season, but not dur- ing the rainy season. Mouse lemurs remained torpid between 1.7±8.9 h with a daily mean of 3.4 h, and their T sk s fell to a minimum of 18.8 °C. Mean home ranges of mouse lemurs which remained normothermic were similar in the rainy and dry season. During the dry season, the mean home range of mouse lemurs show- ing daily torpor was signi®cantly smaller than that of animals remaining normothermic. The DEE of M. murinus remaining normothermic in the rainy sea- son 122  65.4 kJ day )1 ) was about the same of that of normothermic mouse lemurs in the dry season 115.5  27.3 kJ day )1 ). During the dry season, the mean DEE of M. murinus that utilised daily torpor was 103.4  32.7 kJ day )1 which is not signi®cantly dierent from the mean DEE of animals remaining normothermic. We found that the DEE of mouse lemurs using daily torpor was signi®cantly correlated with the mean temperature dierence between T sk and T a r 2  0.37) and with torpor bout length r 2  0.46), while none of these factors explained signi®cant amounts of variation in the DEE of the mouse lemurs remaining normothermic. The mean water ¯ux rate of mouse lemurs using daily torpor 13.0  4.1 ml day )1 ) was signi®cantly lower than that of mouse lemurs remaining normothermic 19.4  3.8 ml day )1 ), sug- gesting the lemurs conserve water by entering torpor. Thus, this ®rst study on the energy budget of free- ranging M. murinus demonstrates that torpor may not only re¯ect its impact on the daily energy demands, but involve wider adaptive implications such as water requirements. Key words Daily energy expenditure á Doubly labelled water á Grey mouse lemur á Microcebus murinus á Torpor Abbreviations BMR basal metabolic rate á DEE daily energy expenditure á ÁT mean temperature dierence á FMR ®eld metabolic rate á RMR resting metabolic rate á T a ambient temperature á T b body temperature á T sk skin temperature Introduction The high mass-speci®c metabolic rates of small endo- thermic animals have favoured the evolution of torpor, primarily to minimize energy expenditure, enabling them to cope with periods of cold and food shortage. Torpor is a regulated state of physiological dormancy during which body temperature T b ) may drop by 4±35 °C below normothermic levels, and metabolic rate may be reduced to as little as 5% of its normothermic value Geiser and Ruf 1995; Wang 1989 for reviews and references). Because relative heat loss is inversely related to body mass in endotherms Schmidt-Nielsen 1997), torpor occurs mainly in small mammals, which may be unable to store adequate amounts of energy to J Comp Physiol B 2000) 170: 633±641 Ó Springer-Verlag 2000 Communicated by: G. Heldmaier J. Schmid &) VerhaltensoÈkologie, Vor dem Kreuzberg 28, 72070 TuÈbingen, Germany e-mail: jutta.schmid@t-online.de Tel.: +49-7071-49478; Fax: +49-7071-49458 J. Schmid á J. R. Speakman Aberdeen Centre for Energy Regulation and Obesity ACERO), Department of Zoology, University of Aberdeen, Aberdeen AB24 2TZ, UK