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At least, why don't all animals produce no more heat than the tiny amount that's a byproduct of walking or a bonobo climbing up a vine when the outside temperature is above 0°C? They won't freeze anyway.

After all, the amount of food controls the population size at whatever size makes each individual have just enough food, so shouldn't those with a lower energy demand be more likely to get enough food to rear more children? Shouldn't mutations that make a warm blooded animal able to thrive with a lower core temperature and produce less heat be selected for because there's more likely to get enough food to rear more children?

Also, shouldn't cheetahs evolve to produce extremely little heat in the day during rest even on not very hot days so they will be less hot and can chase their prey for longer? Have they already evolved that way?

If a species did evolve to produce less heat, would that also evolve to walk slower because it takes a lower speed to minimize the energy burnt per distance because they're producing less heat? I know there there will be a nonzero speed that minimizes the absolute minimum possible energy burnt per distance possible to evolve for that speed and a lower speed will actually increase the energy burnt per distance because the animal is walking for a longer time but the rate of energy burnt per time varies less rapidly with speed at such low speeds because of the energy burnt to fight against gravity.

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  • $\begingroup$ Life on Earth is thriving, it's not just surviving on the edge. This leads to organisms with a high power to weight ratio. Energy efficiency does play a role, the less energy you waste the higher you can make the power to weight ratio for a fixed weight. But there is no need to keep the maximum power at some very low value. $\endgroup$ – Count Iblis Jul 26 '16 at 6:48
  • $\begingroup$ Having a warm temperature allows body chemistry to happen more easily, move muscles faster, etc. That would be the advantage. When it's cold, reptiles can't do much, but mammals can. When it's too hot, reptiles suffer from heat and need shelter just like mammals. $\endgroup$ – A L Jul 26 '16 at 22:16
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Cold/warm blooded are misleading categories

The terms cold blooded and warm blooded is very misleading as it gives one the sensation that animals may warm up their internal temperature for the only reason of warming it up.

The terms homeo-, poikilo-, exo- and endo- terms are much more useful in thinking about control of body temperature. Have a look at this post and this post to understand these terms. Make sure to really read these posts to correctly understand these terms and how they interact.

Reformulating your question

What people, in general mean by cold vs warm blooded animals is whether they actively control their body temperature or not. That is cold blooded would mean exotherm while warm blooded would mean endotherm. As such the question becomes "why would any animal care about being endotherm if the body temperature won't go under freezing point anyway?"

Answer

Any metabolic pathway is affected by temperature. A change of temperature of only a few degrees can drastically affect the rate of chemical reactions. Homeotherm individuals (often homeotherm thanks to endothermy) are able to keep a quasi constant level of activity despite external change in temperature. It allow them to forage during hot and cold whether for example. This is a very important advantage but it often comes at the important energy cost. It is a trade-off between the benefit of homeothermy vs the energy cost of endothermy that will define the best interest of an organism.

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    $\begingroup$ Absolutely. Also, although it's fairly obvious once you think about it, I would complete the explanation by highlighting that it's easier to achieve and maintain a relatively stable body temperature that is higher than your surroundings than one that is lower than your surroundings... $\endgroup$ – arboviral Jul 26 '16 at 15:17
  • $\begingroup$ Maybe for a lot of species, it's because being a poikilotherm would necessarily come with a costly trait because they reproduce sexually. Maybe if they reproduced sexually, they would have evolved to be poikilotherms because that would come with a much smaller costly trait. $\endgroup$ – Timothy Jan 20 '17 at 21:03

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