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I am an A level student currently reading around the subject of thermoregulation and the effect of temperature on metabolic rates. Everything I read tells me that larger organisms, for example the Ostrich, has high thermal inertia and metabolic rates about 15 times LOWER than smaller organisms - the hummingbird for example. But I also come across graphs which show an increasing trend for metabolic rates as temperature increases. If an animal is capable of high thermal inertia, why would its metabolic rates be 15 times lower?
Look forward to hearing your response.
This isn't exactly my area, but if I got it right, high termal inertia is often inversely proportional to the metabolic rate (of a warm blooded animal). Graphs that display that metabolic rates rise when temperature rises is a bit another thing. Thermal inertia, afaik, is a simple physical trait that tells how much thermal energy and in what time a mass of some material can store/release (or so). This means a big mass of stuff (like an ostrich) has high termal inertia. However, high body size causes the metabolic heat to accumulate more efficiently (since a large body has a low surface area to volume ratio), and thus large animals would "overheat" if they had high metabolic rates. I think there is even a theoretical maximum size that an organism can become before overheating becomes a problem. Some paleontologists claim largest dinosaurs were close to this kind of maximum size for terrestial animals.
Metabolic rate, on the other hand, is a sum of many things, not only the body size. Temperature is one important factor: the higher the temperature, the higher the activity of many enzymes, higher the energy and oxygen consmption and thus higher the metabolic rate itself. Small mammals/birds generally have high metabolic rates and high body temperatures, on part in order to maintain their body heat, on part to allow their active life style. Large warm blooded animals generally have lower body temperatures (despite their size and heating related to that) and lower metabolic rates, because they can maintain their body temperature easier (thanks to high termal inertia) and store more energy, thus permitting often more passive life style.
This is of course quite a general view, there is lots of variation among species. E.g. some small animals (like bats and opossums) can lower their metabolism for varying time periods. And I know my answer probably wasn't exactly top notch, so if there is someone who's expert in this are, please correct me
Thanks for your reply. I understand everything you say; so the difference in body temperature between large animals [eg. an elephant] and small animals [a doormouse for example], is accurately proportional to its difference in metabolic activity?
I suppose that increasing body size is a factor responsible for positive feedback during thermoregulation - when an animal's metabolic activity is impaired due to illness, am I right? I'm not entirely sure how illness affects an organism's metabolic rate, but maybe you can answer this too
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