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Body Temperature and Thermoregulation Learning Objectives To understand the distinction between endothermy and ectothermy, and between homeothermy and poikilothermy To be familiar with the contribution of metabolic rate, surface insulation and circulatory adaptations to the maintenance of body temperature in endotherms To understand how some insects and mammals use endothermy on a temporary basis Before we begin: Definitions 1. Importance of metabolism Endotherm - body temperature depends on heat production by animal metabolism Ectotherm - body temperature depends on heat acquired from the environment, and is little influenced by metabolic rate 2. Importance of Body Temperature Homeotherm - body temperature constant (usually as a result of endothermy) Poikilotherm - body temperature variable and strongly influenced by environment Why keep warm ? surface / volume ratio of large animals more “efficient” Recall: MR increases with body weight, but less than proportionately logMR = a + b . log bwt log MR 0.75 = b MR = a . bwtb aa log body weight Relationship between metabolic rate and body weight for animals maintained at 20oC Animal a b Mammals Birds Lizards Fish Crabs 3.3 3.6 0.13 0.43 0.27 0.76 0.72 0.8 0.81 0.78 body temp (oC) 38 40 20 20 20 Lizards at 38oC 0.68 0.82 38 Birds and mammals Other animals Lizards and fish better of at warmer temperatures Maximum speed Speed of passage of food through gut Mammals and birds are ‘gas guzzlers’ Exploit elevated MR to maintain stable temperature => Very costly (food, resources) => Generates activity that is independent of external conditions Birds and mammals Other animals Why aren’t lizards as efficient as birds or mammals? a. Body temperature - high and maintained in birds and mammals b. Intrinsically high metabolic rate: if lizard placed at 38oC MR will increase, but not to same level as in birds and mammals Physiology of birds and mammals ‘wired up’ to run faster than other animals Observed at level of O2 consumption Observed in organ system organisation: lungs, gut absorptive surface, design of circulatory system oxygen consumption Metabolic rate of cardinals reared at different temp below here need to use extra energy to keep warm too hot temperature, °C Summary so far A large part of basal metabolism is to keep warm Now onto: how do we minimise heat loss? Surface insulation: a barrier to loss of metabolically generated heat thermal conductivity (W m-2 oC-1) Water Air Muscle Fur Blubber (subcutaneous fat) 0.61 0.025 0.48 0.04 0.24 [smaller numbers are better] Heat lost through water more rapidly than through air. Muscles are poor at retaining heat, but subcutaneous fat is better Fur has low conductivity: terrestrial mammals can maintain a 30oC gradient between skin surface and external temperature (i.e. trapping air of low conductivity) seal v dog… external body 38oC Temp at skin surface = external temp blubber external body 38oC Temp at skin surface 38oC fur Blubber has 6x conductivity of fur: i.e. needs 6x thickness of fur to generate same insulation. Seal cross-section Blubber as surface insulation in seals Skin surface in water = water temp Skin surface in air > air temp Seals must lose heat to air to avoid overheating Keeping extremities warm? But what about the fins? countercurrent heat exchanger! Retaining and maintaining heat: the counter current heat exchanger Countercurrent dolphin flippers Hen’s feet hens’ legs 2 mm Metabolic rate declines as temperature declines, but down to 0oC, no heat loss from feet. When temperature below freezing, vasodilators open to prevent feet from freezing Keeping tuna muscle warm Most fish, water temp = body temp, as loss of heat through gills. In tuna: sustained fast swimming requires temp of ~30oC Achieved by counter current blood flow tuna heat exchanger a a v a v v 0.1 mm Summary so far A large part of basal metabolism is to keep warm Keeping warm insulation fur, blubber countercurrent heat exchangers extremities regional temperature control Now onto: avoiding overheating Seal heat loss Seals pump blood to body surface in air to achieve cooling via vasodilator. i.e. pattern of blood flow regulates heat loss Vasodilation jackrabbit ears before and after exercise above 30°C all less than 10°C air temp 6°C guanaco heat losses rate of heat loss = 1/fur length Heat loss Actually, major heat loss is by evaporation sweat panting why: evaporation uses a lot of energy: 418 J to heat water from 0 to 100 °C but 2443 J to boil it [1 g] Heat loss in a hurry Carotid artery in ungulates: when blood temp rises (during a chase to 44°) danger to brain: venous blood cooled in rete and nasal cavity (=<40°C) Heat loss in the desert man (70 kg): BMR needs 0.12 l / hr evaporation heat from sun = 1.2 l / hr camel (400 kg): temp goes from 34 to 41 °C (≈2900kcal, 5 l water) store heat until night reduce difference between outside + camel evaporation 0.9 l /hr fur reduces heat inflow (shearing doubles evaporation) Summary so far A large part of basal metabolism is to keep warm Keeping warm insulation fur, blubber countercurrent heat exchangers Heat loss by evaporation Now onto: Facultative endotherms Facultative endotherms If there are so many advantages to endothermy, why aren’t all species endotherms? a) costly on resources, especially food to maintain high metabolic rate b) costly for small animals with high specific metabolic rates Small animals can gain the best of both worlds by employing endothermy only when needed (facultative) Facultative endothermy in Insects: Early season bumble bees Night moths Achieved by synchronized muscle activity (with no movement) countercurrent co-contraction up/down alternate Summary so far A large part of basal metabolism is to keep warm Keeping warm insulation fur, blubber countercurrent heat exchangers Heat loss by evaporation Endothermy Facultative in insects Now onto: Torpor Torpor: some birds and mammals exhibit torpor/adaptive Hypothermia Reduces the metabolic rate in response to: low external temperature And low food availability Torpor is under physiological control Characteristics of Torpor: Reduced metabolic rate, but maintenance of control (avoids freezing, i.e. during hibernation) Reduced motor and sensory function, more comatose than Sleeping (low heart rate, low respiratory rate) Can display arousal and return to ‘normal’ body temperature metabolic rate or endogenous heat production Characteristics of Torpor (cont): Generally small animals, small mammals, birds, rodents hummingbirds: due to? Energetic cost of maintaining high body temp for small animals? Costs of arousal, costly for large animals However: bears in winter dormancy: reduce MR by 50%, body temp by 5oC Torpor in birds Eulampis Torpor in bats Torpor in mammals (marmot) use fat rather than glucose Daily torpor in mammals ?? energy saved Torpor in a pocket-mouse Summary to end A large part of basal metabolism is to keep warm Insulation and its control Endothermy Facultative in insects Torpor energy saving Reading … PowerPoints on VLE or at http://biolpc22.york.ac.uk/303/ Schmidt-Nielsen, K (1997) Animal Physiology CUP