Talk:Sauropod/DRAFTmetabolism

Metabolism
For the discussion of sauropod metabolism a number of basic terms are important. These are defined here for the context of this page following Sander et al. (2010) :

Typically, a bradymetabolic animal has a basal metabolic rate an order of magnitude lower than a tachymetaboloic animal of comparable body mass. In extant terrestrial animal, endothermy is usually coupled with tachymetabolism and homoiothermy (mammals and birds), while ectothermy is coupled with bradymetabolism and poikilothermy.
 * ectothermy: an animal acquires the heat necessary for the organism to function from the environment
 * endothermy: an animal generates the heat necessary for the organism to function metabolically
 * poikilothermy: an animal's body temperature tracks ambient temperature
 * homoiothermy: an animal's body temperature is constant and elevated against the environment
 * bradymetabolic: an animal has the low basal metabolic rate of most extant reptiles (∼30 kJ/kg bodymass0.75)
 * tachymetabolic: an animal has an elevated basal metabolic rate as seen in modern placental mammals (289 kJ/kg body mass0.75)

An important factor with regards to heat flow in an animal is its surface to volume ratio. The surface area determines the amount of heat lost, while the volume determines the amount of heat prodcued metabolically. With increasing size, surface grosw by the square, while volume grows by the cube. Therefore, all other things being equal, small animals tend to have a heat loss problem (they must ensure not to loose too much heat), while large animals have a heat retention problem (they may overheat). This is why small endotherms usually have fur or feathers as insulation, while large animals (elephants, rhinoceroses) have little hair. Elephants use their large ears for cooling, too.

Theories
Many attempts have been made to explain the metabolism of sauropods, because their huge size presents a number of problems. Initially, sauropods were generally seen as giant lizards, and thus thought to be ectotherms, poikilotherms, and bradymetabolic. Modelling of the heat exchange of sauropods appeared to support this interpretation, because a tachymetabolic adult sauropod would overheat. However, in Edwin Harris Colbert pointed out that sauropods had a much more favorable surface to volume ration than e.g., elephants, because of their long necks and tails. Also, ectotherms grow slowly and can be incapacitated by cold spells, so that huge sauropods would have to have reached hundreds of years of age, and might have fallen into extended torpor during the winter.

An often favored theory is called gigantothermy. At small and medium size, ectothermy and endothermy are very distinct. The scaling of surface to volume ratio means that at very large size the strategies converge. A very large ectotherm thus retains a high and relatively constant body core temperature because it produces and radiates heat very slowly. An adult sauropod would not be affected by the temperature fluctuations between day and night, even if it was bradymetabolic. However, this does not solve the problem of slow growth (and resulting enormous age), or that of extended torpor during winter for juveniles and babies.

Other researchers have advocated mammalian- and bird-style tachymetabolic endothermy for sauropods, leading to homoiothermy, based on the mammal-like posture of sauropods with their fully erect stance and gait, and the requirements for high blood pressure and a four-chamberes heart resulting from this. Bob Bakker noticed that the predator-prey ratios of dinosaur faunas are similar to that of living endothermic mammals, and not of ecotherms. The microcstructure of dinosaurian bone shows fibrolamellar bone Haversian canals, a combination which is only seen in large mammals and birds today. The rates of dinosaurs were much higher than in ectotherms, which is indicated by the fibrolamellar bone and by growth mark (LAG = line of arrested growth) counts. Also, in ectotherms the isotopes of oyxgen built into the bones vary much more than in endothermic mammals, and this low intra-bone oxygen isotope variability was also found in sauropod bones. Similarly, the latitude an animal lives in influences the oxygen isotope composition of its teeth, with ecotherm crocodiles and turtles showing a similar composition in warm climates to endotherms, but a different composition in higher latitudes. There are latitude-dependent differences in enamel oxygen isotope compositions between ectotherms (crocodiles and turtles) and saurischian dinosaurs from the same environments, indicating.