Southern discomfort

NEWS AND VIEWS HIBERNATION---------------------------------- Southern discomfort available (S. Nicol, Univ. Thsmania). In some mammals, the period immediately preceding hibernation is characterized by a massive increase in food intake Andrew R. Cossins and Brian Barnes and a doubling of body weight as fat is WE all know that winter hibernation, when from southern zoogeographical zones also accumulated to survive the long period of body temperature plunges from the normal have low basal metabolic rates (B. Love- torpor. Administration of leptin (the newly 37 °C to as low as -2 °C, allows some grove, Univ. Natal, Scotsville, South discovered satiety hormone produced by specialized mammalian species to survive Africa). This, and the increased prevalence white fat) to arctic ground squirrels during particularly severe winter conditions. Met- of torpor in southern mammals, is now this fattening phase curbs their appetite abolism is depressed to low, energy-saving attributed to climatic unpredictability, and significantly reduces body weight and levels until the spring, when activation of especially the periodic dry spells caused by fat deposits (B. B. Boyer, Inst. Arctic Biolthe animal's brown adipose tissue (the El Nifto ocean-circulation events. ogy, Fairbanks). This pronounced seasonal heat-producing fat cells found in cycle of extreme weight gain and loss offers a potentially tractable small mammals and in the young of larger mammals) stokes up body model for a more general understanding of how the brain's regulatemperature in time for mating. But tion of feeding behaviour balances this traditional northern hemifood intake with energy expendispheric view was up-ended at a ture over the longer term. Interestconference held in Australia last ingly, hibernation is sensitive to the month*, when it was acknowledged quality of lipid diet, with polythat a surprisingly large proportion unsaturated fats enhancing both the of mammals from down under, length and depth of torpor (C. including a monotreme (the echidFrank, Fordham Univ., New York), na) and various marsupial and plaand involves substantial changes in cental species, routinely enter daily the expression of lipid biosynthetic torpor and seasonal hibernation, enzymes (G. Florant, Colorado often under surprisingly mild conState Univ., Ft Collins; R. Coleditions. What is more, when the man, Univ. North Carolina, Chapel marsupials arouse and rewarm, it is apparently without the benefit of Hill). The monotreme and marsupial brown adipose tissue. , - - - Darkness-----, 40 hibernators show all the features The number of species known to of classical seasonal hibernation, undergo torpor and hibernation is 35 including the dramatic reduction in being constantly revised upwards G' heat production on entry into torwith the continuing improvements ~ 30 T Ql a por and the periodic arousals to in remote radiosensing as a tech:i normal body temperatures (S. nique for measuring body temperart, 25 Nicol, Univ. Tasmania; G. Grigg, ture. Newly discovered heteroai E 20 Univ. Queensland, Brisbane; P. D. therms include the mountain pigmy ~ Rismiller, Univ. Adelaide). Howpossum, which has an extended 15 ever, they do not possess the brown five-month hibernation in the Ausadipose tissue of small placentals tralian Alps at a body temperature 10 which, being under the control of that goes down to 2 °C (F. Geiser, 24:00 4:00 8:00 12:00 16:00 16:00 20:00 the autonomic nervous system, Univ. New England, Armidale, Time of day produces heat during arousal. Australia), and two species of This highlights a general lack of mouse lemur from Madagascar, not Top, the western rufous mouse lemur, Microcebus myoxinus, a climate known for extreme cold, believed to be extinct but recently rediscovered in forests of understanding of thermoregulatory which show a pronounced daily tor- Madagascar. Bottom, the changes in ambient temperature (Ta) heat production in tissues other por during the seasonally dry winter and body temperature (Tb) of lemurs over a 24-hour period, dur- than brown adipose, and species period (see figure), with body tem- ing which t orpor was evident. Tb was kept high during the dark without this specialized brown fat feeding period, despite a progressive drop in T8 • Tb dropped in offer good models for exploring peratures of less than 10 °C (G. the morning t o 20 °c and then recovered by mid-afternoon. The Heldmaier, Univ. Marburg, Ger- early phase of warming appears to rely on the increasing day- alternative methods of thermomany). No doubt this list will grow time ambient temperature, the response giving a considerable genesis. Hindlimb preparations ever longer, especially as the new energetic saving over sustained homeothermy. (Dat a from from rat, chicken and a small implantable digital data-loggers S. Ortmann, J. Schmidt, U. Ganzhorn & G. Heldmaier Naturwis- wallaby-like marsupial, the bettong, increase their oxygen consumption come into routine use for accumu- senschaften, in the press.) (and heat production) in response lating the thermal experiences of Hypothermia and the reduced demand to an infusion of noradrenaline or other entirely free-ranging animals. Just why so many Southern Hemisphere for energy improves survival during these vasoconstrictors by switching on capillary species show torpor is not clear. Marsupials stressful periods of reduced food availabil- perfusion through metabolically active and monotremes have lower basal meta- ity. So the commonly accepted view that regions (M. G. Clark, Univ. Tosmania). It is not yet clear whether this heating bolic rates and body temperatures than the low body temperature of marsupials typical placental (northern) mammals. and monotremes results from their primi- response has the necessary power and coldHowever, some placental mammal species tive thermoregulatory status should be re- inducibility to explain either arousal or placed by a recognition of their climatic thermoregulation in the cold. Noradrenaadaptation. Thus, the echidna is well insu- line, which mediates the autonomic activa• Adaptations to the Cold, 10th International Hibernation Symposium, Cradle Mountain Lodge, Tasmania, Aust ralia, 30 lated and copes well with cold tempera- tion of heat production in placental June- 6 July 1996. Contributions published in Adaptations to tures, but it hibernates in comparatively mammals, has no thermogenic effect in the Cold (eds Geiser, F., Hulbert, A. J. & Nicol, S. C.) (Univ. mild conditions, even when food is readily echidnas or in many marsupial groups, New England Press, Armidale, Aust ralia, 1996), 582 NATURE · VOL 382 · 15 AUGUST 1996 NEWS AND VIEWS including those that show torpor and hibernate, so the autonomic system probably does not mediate arousal in these species. Birds also lack brown adipose tissue, but produce thermoregulatory heat in the cold by increased tone of antagonistic muscles. Activation of increasing numbers of skeletal muscle fibres matches heat prod (...truncated)