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Why do animals with a larger mass have a lower rate of oxygen consumption?

I know it has something to do with BMR, BMI and Metabolism- I just don't know exactly- someone explain pls?
In very broad terms;

- in absolute terms, a large animal will need and use more energy than a small one, simply because it has more mass (tissue) to supply with energy and oxygen
- however, in relative terms, (per unit mass), a large animal will burn less energy than a small one i.e. its BMR will be lower
- this is largely because a major cause of energy consumption is the need to generate amd maintain body heat. Generating body heat is done in the tissues of the body, so the more body you have, the more energy you can generate. Conversely, small animals have a larger surface area compared to their volume and so they will lose heat more rapidly.
- in other words, heat is lost as a function of body surface area (or the square of average body length), but generated as a function of body volume (or the cube of body length). By getting bigger, an animal increases its capacity to generate heat faster than its tendency to lose heat
- therefore, small animals need to burn more of their energy reserves in order to maintain the same body temperature. That's why it's impossible for a bird to be smaller than a hummingbird or a mammal to be smaller than a shrew - these animals lose heat so fast, they are right at the limit of how quickly a biological system can generate heat fast enough to replace that lost by radiation.
- oxygen consumption is a proxy for metabolic rate as it is central to ATP metabolism, so big animals will use more oxygen in absolute terms, but less per unit of body mass

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